Viral proteins and nanostructures and uses thereof

Abstract

Provided herein are recombinant polypeptides comprising an engineered ectodomain of a viral protein from enveloped viruses. Also provided herein ar two-component protein nanostructures and compositions for use in vaccinating, generating an immune response, or treating or preventing a viral infection.

Description

VIRAL PROTEINS AND NANOSTRUCTURES AND USES THEREOF CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 583,117, filed September 15, 2023, the contents of which is incorporated by reference herein in its entirety. INCORPORATION BY REFERENCE OF SEQUENCE LISTING

[0002] The instant application contains a Sequence Listing which has been submitted electronically in XML format and is hereby incorporated by reference in its entirety. Said XML copy, created on September 11, 2024, is named 061291-518001WO.xml and is 1,130 KB in size. BACKGROUND

[0003] When an enveloped virus encounters a target cell, its viral membrane fusion protein undergoes a conformational change that drives fusion of the viral envelope with the target cell’s cell membrane. This fusion process delivers the viral genome into the target cell. For many enveloped viruses, the adaptive immune response to the viral membrane fusion protein is a key source of protective immunity, in part because neutralizing antibodies may inhibit this fusion process. Hence, vaccines for enveloped viruses often include a viral membrane fusion protein as an antigen.

[0004] There is an unmet need for viral membrane fusion proteins stabilized by designed amino acid substitutions. The present disclosure provides recombinant polypeptides and related compositions and methods that address this need for Respiratory Syncytial Virus (RSV), hMPV, PIV3, PIV5, SARS-CoV-2, and Nipah virus. SUMMARY

[0005] In one aspect, the disclosure provides a recombinant polypeptide, comprising an engineered ectodomain of a trimeric pathogenic (e.g., viral) protein, wherein the ectodomain comprises a C- terminal helix-forming segment comprising one or more amino acid substitutions, relative to a native reference sequence of the pathogenic (e.g., viral) protein, selected such that the segment forms a stable alpha-helical homotrimer. In another aspect, the disclosure provides a nanostructurecomprising a trimeric component comprising a helix-forming segment as disclosed herein. In another aspect, the disclosure provides helix-forming segments as disclosed herein.

[0006] In some embodiments of the recombinant polypeptide, the C-terminal helix forming segment has improved hydrophobic packing compared to the native reference sequence. In some embodiments, the C-terminal helix forming segment comprises between about 7 and about 31 residues. In some embodiments, the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids. In some embodiments, the C-terminal helix forming segment comprises a polypeptide sequence according to any one of LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566), LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567), LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568), EKIXXAIKKAXKL (SEQ ID NO: 569), EXIXKAIKXLXXXXX (SEQ ID NO: 570), XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571), XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

[0007] In some embodiments, the C-terminal helix forming segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

[0008] In some embodiments, the segment comprises a polypeptide sequence according to any one of L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573), L V [A / T] T X2 K X2 L X2D L I X2X2L [K / E] X2L L X2K L X2X2(SEQ ID NO: 574), or L N K V K K X2V X2X2L X2 X2 X2 V X2 X2 L E K X2 L X2 (SEQ ID NO: 575), wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0009] In some embodiments, segment comprises a polypeptide sequence according any one of E K I X2X2A I K K A X2K L (SEQ ID NO: 576), E X2I X2K A I K X2L [L / X2] X2X2[X1 / X2] X2(SEQ ID NO: 577), X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2 X2 X1 X2 X2 (SEQ ID NO: 578) , or X2 X2 L K K A A X2 I X1 K K X1 L K X2 X2 (SEQ ID NO: 579), wherein X1is apolar residues selected from A, I, L, and M, and wherein X2is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid. In some embodiments, the segment comprises a polypeptide sequence listed in Table 25A or Table 25B. In some embodiments, the native reference sequence of the viral protein is any one of SEQ ID NOs: 1, 104, 327, 382, 459, 499.

[0010] In some embodiments, the recombinant polypeptide comprises an engineered ectodomain of an hMPV fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-formingsegment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 104 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix- forming segment, between about residue 470 and about residue 490 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 7 and about 21 residues.

[0011] In some embodiments, the segment comprises (1) an amino acid substitution at position Q471 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, I, Q, R, S, T, (2) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, R, S, T, Y, (3) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of A, I, L, M, Q, S, T, W, (4) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of A, D, E, I, K, L, N, Q, S, T, (5) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of A, D, E, H, K, N, Q, R, S, T, (6) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, H, I, K, L, M, N, Q, T, V, (7) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, E, I, K, L, M, N, Q, R, S, T, V, (8) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of A, D, E, K, N, Q, R, S, T, (9) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substituted with any one of A, D, E, F, I, K, L, M, N, Q, R, S, T, W, Y, (10) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of A, I, L, M, R, S, T, V, (11) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any one of D, E, I, K, L, M, N, Q, R, S, T, (12) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, K, Q, R, S, T, (13) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, F, H, I, K, L, M, N, Q, R, S, T, V, W, Y, (14) an amino acid substitution at positionA484 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, L, M, R, S, T, V, Y, (15) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of D, E, G, K, L, Q, R, S, T, (16) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of A, E, I, K, L, Q, R, S, T, (17) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of A, E, I, K, L, R, S, T, V, (18) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, I, K, L, N, Q, R, S, (19) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of A, D, E, K, S, and / or (20) any combination of (1)-(19). In some embodiments, the segment comprises a polypeptide sequence listed in Table 6B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0012] In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 16 and about 30 residues.

[0013] In some embodiments, the segment comprises (1) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of T, N, K, R, E, S, (2) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, I, V, (3) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of E, Q, L, D, (4) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of E, D, K, (5) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of Q, R, D, A, T, S, K, (6) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of I, V, L, (7) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of K, E, N, S, (8) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substituted with any one of T, D, E, S, Y, A, (9) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of L, N, (10) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, D, E, K, Q, S, N, (11) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein Sis substituted with any one of E, D, S, T, Q, K, (12) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of R, K, L, E, A, (13) an amino acid substitution at position A484 relative to SEQ ID NO: 104, wherein A is substituted with any one of V, I, M, (14) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of E, A, K, H, Q, S, N, (15) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of S, E, K, R, V, D, H, (16) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of I, L, (17) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, K, R, (18) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of K, E, S, R, Q, (19) an amino acid substitution at position S490 relative to SEQ ID NO: 104, wherein S is substituted with any one of E, V, T, R, L, (20) an amino acid substitution at position G491 relative to SEQ ID NO: 104, wherein G is substituted with any one of GL, I, V, (21) an amino acid substitution at position R492 relative to SEQ ID NO: 104, wherein R is substituted with any one of E, Q, S, A, D, (22) an amino acid substitution at position E493 relative to SEQ ID NO: 104, wherein E is substituted with any one of A, E, N, L, K, Q, S, (23) an amino acid substitution at position N494 relative to SEQ ID NO: 104, wherein N is substituted with any one of I, L, (24) an amino acid substitution at position L495 relative to SEQ ID NO: 104, wherein L is substituted with any one of K, L, T, V, I, (25) an amino acid substitution at position Y496 relative to SEQ ID NO: 104, wherein Y is substituted with any one of K, E, R, Q, (26) an amino acid substitution at position F497 relative to SEQ ID NO: 104, wherein F is substituted with any one of D, R, E, Q, (27) an amino acid substitution at position Q498 relative to SEQ ID NO: 104, wherein Q is substituted with any one of V, L, and / or (28) any combination of (1)-(27). In some embodiments, the segment comprises a polypeptide sequence listed in Table 6D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0014] In some embodiments, the ectodomain further comprises one, two, three or more amino acid substitutions at positions 63, 97, 98, 99, 100, 101, 102, 140, 147, 153, 185, 188, 219, 231, 294, 365, 368, 450, 463, or 470 relative to SEQ ID NO: 104.

[0015] In some embodiments, the recombinant polypeptide comprises an engineered ectodomain of a PIV3 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 327,comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 327 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 20 and about 28 residues.

[0016] In some embodiments, the segment comprises (1) an amino acid substitution at position L460 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, M, V, (2) an amino acid substitution at position N461 relative to SEQ ID NO: 327, wherein N is substituted with N, (3) an amino acid substitution at position K462 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R, (4) an amino acid substitution at position V463 relative to SEQ ID NO: 327, wherein V is substituted with any one of L, V, T, (5) an amino acid substitution at position K464 relative to SEQ ID NO: 327, wherein K is substituted with any one of A, K, Q, (6) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, S, (7) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of E, K, (8) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of V, L, T, (9) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, D, E, (10) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of T, Q, K, E, (11) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L, M, Y, F, W, (12) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of L, W, A, I, (13) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, E, (14) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of E, I, K, Q, (15) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of L, M, T, V, E, (16) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of S, K, R, A, (17) an amino acid substitution at positionR476 relative to SEQ ID NO: 327, wherein R is substituted with any one of K, E, S, N, (18) an amino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, D, E, and / or (19) any combination of (1)-(18).

[0017] In some embodiments, the segment comprises a polypeptide sequence listed in Table 7B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 465 and about residue 490 relative to SEQ ID NO: 327, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 14 and about 30 residues.

[0018] In some embodiments, the segment comprises (1) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of E, K, D, S, N, Q, T, R, A, (2) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of D, R, K, E, M, Q, A, S, N, (3) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of I, V, L, (4) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, S, D, R, H, T, N, A, (5) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, S, E, N, T, Q, H, D, Y, (6) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of L, D, V, I, A, N, T, (7) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of E, T, L, K, N, I, R, Q, S, (8) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, Q, S, H, R, T, (9) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of R, Q, K, E, T, S, I, N, (10) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of V, L, I, Q, T, (11) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of H, K, D, T, E, S, R, N, Q, A, (12) an amino acid substitution at position R476 relative to SEQ ID NO: 327, wherein R is substituted with any one of A, T, H, E, D, K, R, Q, S, (13) an amino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L,V, (14) an amino acid substitution at position N478 relative to SEQ ID NO: 327, wherein N is substituted with any one of E, L, K, I, R, S, Q, (15) an amino acidsubstitution at position Q479 relative to SEQ ID NO: 327, wherein Q is substituted with any one of K, H, E, N, Q, R, T, A, S, (16) an amino acid substitution at position K480 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R, E, T, S, L, A, I, V, (17) an amino acid substitution at position L481 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, V, I, (18) an amino acid substitution at position D482 relative to SEQ ID NO: 327, wherein D is substituted with any one of K, A, E, S, H, T, N, D, R, (19) an amino acid substitution at position S483 relative to SEQ ID NO: 327, wherein S is substituted with any one of Q, T, E, A, S, N, D, K, L, (20) an amino acid substitution at position I484 relative to SEQ ID NO: 327, wherein I is substituted with any one of I, L, A, V, (21) an amino acid substitution at position G485 relative to SEQ ID NO: 327, wherein G is substituted with any one of L, K, R, E, I, (22) an amino acid substitution at position S486 relative to SEQ ID NO: 327, wherein S is substituted with any one of T, A, E, R, H, D, S, and / or (23) any combination of (1)-(22). In some embodiments, the segment comprises a polypeptide sequence listed in Table 7D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0019] In some embodiments, the recombinant polypeptide comprises an engineered ectodomain of a PIV5 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 382, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 382 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 6 and about 26 residues.

[0020] In some embodiments, the segment comprises (1) an amino acid substitution at position A463 relative to SEQ ID NO: 382, wherein A is substituted with any one of L, T, V, A, (2) an amino acid substitution at position L464 relative to SEQ ID NO: 382, wherein L is substituted with any one of K, I, Q, A, W, E, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 382, wherein Q is substituted with any one of K, Q, T, E, S, R, (4) an amino acid substitutionat position H466 relative to SEQ ID NO: 382, wherein H is substituted with any one of K, A, E, L, I, W, R, Q, T, D, Y, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 382, wherein L is substituted with any one of V, I, L, M, FA, T, C, H, (6) an amino acid substitution at position A468 relative to SEQ ID NO: 382, wherein A is substituted with any one of D, T, K, L, E, R, I, N, S, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 382, wherein Q is substituted with any one of E, K, S, T, A, R, Q, D, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 382, wherein S is substituted with any one of A, K, L, I, T, S, V, H, Y, E, W, FR, Q, M, (9) an amino acid substitution at position D471 relative to SEQ ID NO: 382, wherein D is substituted with any one of T, E, V, L, S, I, A, K, Y, W, (10) an amino acid substitution at position T472 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, E, R, S, T, A, D, L, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 382, wherein Y is substituted with any one of T, K, S, R, Q, D, E, I, H, M, (12) an amino acid substitution at position L474 relative to SEQ ID NO: 382, wherein L is substituted with any one of T, S, L, A, D, W, Q, I, Y, V, K, E, (13) an amino acid substitution at position S475 relative to SEQ ID NO: 382, wherein S is substituted with any one of T, E, I, K, S, Q, A, L, R, D, (14) an amino acid substitution at position A476 relative to SEQ ID NO: 382, wherein A is substituted with any one of R, K, A, S, E, I, T, D, Q, (15) an amino acid substitution at position I477 relative to SEQ ID NO: 382, wherein I is substituted with any one of K, Q, R, D, T, E, I, Y, S, L, (16) an amino acid substitution at position T478 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, K, S, D, W, L, Q, I, T, (17) an amino acid substitution at position S479 relative to SEQ ID NO: 382, wherein S is substituted with any one of R, K, Q, S, A, D, E, (18) an amino acid substitution at position A480 relative to SEQ ID NO: 382, wherein A is substituted with any one of S, K, (19) an amino acid substitution at position T481 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, D, S, K, M, N, A, T, (20) an amino acid substitution at position T482 relative to SEQ ID NO: 382, wherein T is substituted with any one of R, S, Q, L, K, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, A, S, (22) an amino acid substitution at position S484 relative to SEQ ID NO: 382, wherein S is substituted with any one of S, E, D, Y, (23) an amino acid substitution at position V485 relative to SEQ ID NO: 382, wherein V is substituted with any one of Q, T, (24) an amino acid substitution at position L486 relative to SEQ ID NO: 382, wherein L is substituted with K, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 382, wherein S issubstituted with any one of S, K, (26) an amino acid substitution at position I488 relative to SEQ ID NO: 382, wherein I is substituted with any one of S, K, and / or (27) any combination of (1)- (26).

[0021] In some embodiments, the segment comprises a polypeptide sequence listed in Table 8B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0022] In some embodiments, the polypeptides comprises an engineered ectodomain of a SARS- CoV2 spike (S) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 459 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 25 residues.

[0023] In some embodiments, the segment comprises (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of E, D, K, (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S is substituted with any one of T, S, K, (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with A, (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any one of I, A, L, M, (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of K, S, D, R, E, (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of I, Y, K, T, R, E, (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of T, A, (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of L, I, E, T, M, V, (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of E, K, T, R, (10) an amino acid substitution at positionY1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of I, V, K, R, (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of V, A, I, Y, T, S, (12) an amino acid substitution at position K1158 relative to SEQ ID NO: 459, wherein K is substituted with any one of L, R, S, K, D, W, N, I, (13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of K, T, Q, I, R, E, (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of I, L, R, E, K, S, (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, N, I, A, S, W, Y, (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of K, S, T, R, (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, D, R, K, I, A, (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of W, S, M, D, T, I, N, (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of E, A, K, L, (20) an amino acid substitution at position D1166 relative to SEQ ID NO: 459, wherein D is substituted with any one of K, S, (21) an amino acid substitution at position L1167 relative to SEQ ID NO: 459, wherein L is substituted with any one of R, K, (22) an amino acid substitution at position G1168 relative to SEQ ID NO: 459, wherein G is substituted with any one of K, S, (23) an amino acid substitution at position D1169 relative to SEQ ID NO: 459, wherein D is substituted with S, (24) an amino acid substitution at position I1170 relative to SEQ ID NO: 459, wherein I is substituted with S, and / or (25) any combination of (1)-(24).

[0024] In some embodiments, the segment comprises a polypeptide sequence listed in Table 9B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 1145 and about residue 1175 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 12 and about 22 residues.

[0025] In some embodiments, the segment comprises (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of Q, T, E, S, N, D, K, (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S issubstituted with any one of T, K, N, R, S, A, E, (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with any one of L, T, I, V, (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any one of K, Q, R, H, S, E, (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, N, A, S, K, T, D, (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, T, V, R, K, N, (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of S, V, T, A, (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, L, E, I, V, (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of H, E, S, T, Q, (10) an amino acid substitution at position Y1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of L, E, I, T, A, S, R, (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of T, V, I, A, S, M, (12) an amino acid substitution at position K1158 relative to SEQ ID NO: 459, wherein K is substituted with any one of K, E, N, R, T, A, Q, I, (13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of T, E, A, K, Q, (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of L, M, A, E, T, Y, I, S, (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, I, (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of S, R, K, N, E, Q, (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, S, T, R, (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, M, A, (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of A, L, and / or (20) any combination of (1)-(19).

[0026] In some embodiments, the segment comprises a polypeptide sequence listed in Table 9D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0027] In some embodiments, the recombinant polypeptide comprises an engineered ectodomain of a Nipah fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stablealpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 499 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 16 and about 33 residues.

[0028] In some embodiments, the segment comprises (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of I, A, T, L, M, (2) an amino acid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, K, T, S, I, D, (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of M, L, I, V, T, (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, K, A, T, S, D, R, I, Q, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of R, S, K, T, E, Q, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of T, L, I, V, A, (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, A, W, E, L, I, (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, R, Q, E, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of W, D, K, Y, I, M, E, T, (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of I, V, M, L, A, (13) an amino acid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of T, K, M, R, E, L, A, S, (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, S, A, E, T, D, (15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of L, I, V, FT, A, M, W, K, Y, (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of I, K, A, L, E, D, S, Y, (17) an amino acid substitution atposition R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of A, S, K, R, T, L, E, (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of K, E, R, Y, T, Q, (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of W, I, V, L, E, S, Q, A, T, (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, Q, E, W, T, S, A, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of S, T, K, R, Q, (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of R, E, I, S, Y, L, K, D, (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of I, R, K, W, E, T, (24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of A, T, R, K, Q, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of K, R, T, S, (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of E, V, L, K, (27) an amino acid substitution at position I489 relative to SEQ ID NO: 499, wherein I is substituted with any one of E, Q, L, K, R, and / or (28) any combination of (1)-(27).

[0029] In some embodiments, the segment comprises a polypeptide sequence listed in Table 10B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 12 and about 26 residues.

[0030] In some embodiments, the segment comprises (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of L, I, V, (2) an amino acid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of Q, T, N, D, E, S, K, R, A, (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of I, V, L, A, (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q issubstituted with any one of S, K, T, D, E, R, Q, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of S, Q, N, E, A, D, K, T, R, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of L, A, I, V, N, (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of E, Q, S, R, K, A, T, D, L, N, (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, E, Q, D, N, S, A, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of A, S, R, T, V, E, I, K, L, D, Q, (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of L, I, V, (13) an amino acid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, H, D, T, A, S, R, Q, E, N, (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, R, S, E, A, T, H, D, (15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of A, L, I, V, (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, T, R, K, Q, S, I, (17) an amino acid substitution at position R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of K, N, E, Q, S, T, H, R, A, (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of D, L, E, K, T, R, V, I, Q, (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of L, V, I, (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of E, K, N, D, L, Q, H, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of E, K, S, Q, A, T, (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of V, L, I, (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of R, K, L, V, E, Q, I, (24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of R, E, A, S, L, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of Q, R, T, S, L, (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of L, and / or (27) any combination of (1)-(26).

[0031] In some embodiments, the segment comprises a polypeptide sequence listed in Table 10D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0032] In some embodiments, the recombinant polypeptide comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises (a) a C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer, (b) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1, (c) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1, (d) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1, (e) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1, (f) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1, (g) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1 or (h) any combination of (a)-(g).

[0033] In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 30 residues.

[0034] In some embodiments, the segment comprises substitutions relative to the reference sequence SEQ ID NO: 1 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

[0035] In some embodiments, the segment comprises (1) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with A, I, L, M, V, G, T; (2) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (3) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (4) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with K, Q, R, preferably A, V, T, I; (5) anamino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with A, I, L, M, V, F, W, Y, G, T, preferably A, I, L, M, V; (6) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with any amino acids, preferably D, E, K, N, Q, R, S, T, Y; (7) an amino acid substitution at position E511 relative to SEQ ID NO: 1, wherein E is substituted with any amino acids; (8) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with D, E, K, N, Q, R, S, T, Y , preferably A, I, L, M, V, F, W, Y, G, T; (9) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with any amino acids, preferably A, I, L, M, V, F, W, Y, G , more preferably D, E, K, N, Q, R, S, T, Y; (10) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y; (11) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with any amino acids except P, preferably A, I, L, M, V, F, W, Y, G; (12) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with A, I, L, M, V, F, W, Y, G, or T, S, K; (13) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with any amino acidexcept P, preferably D, E, K, N, Q, R, S, T, Y; (14) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with Any except P, preferably D, E, K, N, Q, R, S, T, Y; (15) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; and / or (16) any combination of (1)-(15).

[0036] In some embodiments, the segment comprises (1) an amino acid substitution at position L503 relative to SEQ ID NO: 1, wherein F is substituted with Q, V, K, R, N, L, (2) an amino acid substitution at position A504 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably S, T, L, A, Q, K, E, Y, (3) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with I, V, N, T, L, (4) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably Q, N, K, R, V, S, (5) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably A, N, K, E, D, Q, (6) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with T, M, V, R, (7) an amino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with T, I, K, Q, M, E, V, S, (8) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with S, K, N, D, E, (9) an amino acid substitution atposition E511 relative to SEQ ID NO: 1, wherein E is substituted with R, S, E, K, A, T, L, (10) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with V, N, T, L, (11) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with D, T, H, K, E, N, R, (12) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with A, N, E, S, V, K, T, D, (13) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with I, E, L, T, Q, (14) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with E, I, K, N, R, Q, (15) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with A, S, K, E, R, (16) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with K, S, Q, R, D, E, (17) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with V, L, I, (18) an amino acid substitution at position I520 relative to SEQ ID NO: 1, wherein G is substituted with K, Q, E, N, T, (19) an amino acid substitution at position P521 relative to SEQ ID NO: 1, wherein G is substituted with H, D, E, K, R, N, Q, (20) an amino acid substitution at position E522 relative to SEQ ID NO: 1, wherein G is substituted with L, R, I, V, (21) an amino acid substitution at position A523 relative to SEQ ID NO: 1, wherein G is substituted with E, V, L, K, R I, (22) an amino acid substitution at position P524 relative to SEQ ID NO: 1, wherein G is substituted with A, K, T, E, R, (23) an amino acid substitution at position R525 relative to SEQ ID NO: 1, wherein G is substituted with H, R, S, L, N, E, D, (24) an amino acid substitution at position D526 relative to SEQ ID NO: 1, wherein G is substituted with I, L, V, R, (25) an amino acid substitution at position G527 relative to SEQ ID NO: 1, wherein G is substituted with E, K, Q, D, (26) an amino acid substitution at position Q528 relative to SEQ ID NO: 1, wherein G is substituted with D, K, S, R, A, (27) an amino acid substitution at position A529 relative to SEQ ID NO: 1, wherein G is substituted with T, L, (28) an amino acid substitution at position Y530 relative to SEQ ID NO: 1, wherein G is substituted with L, E, T, (29) an amino acid substitution at position V531 relative to SEQ ID NO: 1, wherein G is substituted with A, R, K, (30) an amino acid substitution at position R532 relative to SEQ ID NO: 1, wherein G is substituted with V, A, and / or (31) any combination of (1)-(30).

[0037] In some embodiments, the segment comprises a polypeptide sequence listed in Table 2B or Table 2C, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequenceNQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10) or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10).

[0038] In some embodiments, the ectodomain comprises (b) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: E487R + K498A, E487R + K498E, E487K + K498E, D486A + E487R + K498A, D486Q + E487R + K498A, D486E + E487A + D489A + T400D, D486A + E487M + K498A, E487Q, D486S, F488W + D489A + T400D + E487R + K498A, F140W + D489A + T400D + E487R + K498A, Q494I + S485I + K399A + 487R + 498A, Q494M + S485I + K399A; D486A + 487M + 498A, Q494L + S485A + K399V + D486A + 487M + 498A, Q494M + S485A + K399V + D486A + 487M + 498A, Q494A + S485F + K399V + D486A + 487M + 498Y, D489A + T400D + E487R + K498A, or D489A + T400D. In some embodiments, the ectodomain comprises the amino acid substitutions D489A, T400D, E487R, and K498A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P.

[0039] In some embodiments, the polypeptide comprises, C-terminal to the ectodomain, a heterologous multimerization domain. In some embodiments, the multimerization domain is a trimerization domain. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53- 50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64). In some embodiments, the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L.

[0040] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 6, below, optionally lacking a p27 peptide shown in bold, and in which “X” refers to sites involving an added C-terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQEL DKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFL LGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYI NNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSEL LSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWK LHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQADTCKVQSNRVFCDTMNSL TLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSLGAIVSCYGKTKCTASNKNRGI IKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEPIINYYDPLVFPSDEFD ASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 6).

[0041] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 7, below, optionally lacking a p27 peptide shown in bold, in which “X” refers to sites involving an added C- terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQEL DKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFL LGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYI DKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSEL LSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWK LHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAETCKVQSNRVFCDTMNSL TLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNKNRGI IKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEPIINFYDPLVFPSDEFD ASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 7).

[0042] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 8, below, optionally lacking a p27 peptide shown in bold:QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQEL DKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFL LGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYI NNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSEL LSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWK LHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQADTCKVQSNRVFCDTMNSL TLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSLGAIVSCYGKTKCTASNKNRGI IKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEPIINYYDPLVFPSDEFD ASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 8).

[0043] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 9, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQEL DKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFL LGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYI DKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSEL LSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWK LHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAETCKVQSNRVFCDTMNSL TLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNKNRGI IKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEPIINFYDPLVFPSDEFD ASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 9).

[0044] In some embodiments, the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1- 9.

[0045] In another aspect, the disclosure provides a trimeric protein complex comprising a polypeptide disclosed herein. In some embodiments, the thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (a)-(h). In some embodiments, the stability, assayed by storage at about 40°C, isincreased compared to a trimeric protein complex lacking modifications (a)-(h). In some embodiments, the thermal stability is increased compared to a reference RSV F protein comprising amino acid substitutions consisting essentially of S155C, S290C, S190F, and V207L (DS-Cav1). In some embodiments, a thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (a)-(g).

[0046] In some embodiments, the stability, assayed by storage at about 40°C, is increased compared to a trimeric protein complex lacking modifications (a)-(g).

[0047] In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide disclosed herein. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments, the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the RSV fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1 and a C- terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0048] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineeredectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, D489A, T400D, E487R, and K498A relative to SEQ ID NO: 1 and a C- terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0049] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and T249P relative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0050] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and D486A relative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0051] In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19. In some embodiments, the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

[0052] In another aspect, the disclosure provides a pharmaceutical composition comprising a polypeptide, a protein complex, or a nanostructure disclosed herein. In another aspect, the disclosure provides a vaccine comprising a polypeptide, a protein complex, or a nanostructure disclosed herein. In another aspect, the disclosure provides a trimeric protein complex comprising a polypeptide disclosed herein. In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide disclosed herein. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a human Metapneumovirus (hMPV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a hMPV / A fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a hMPV / B fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a human Parainfluenza virus type 3 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a humanParainfluenza virus type 5 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a SARS-CoV- 2 spike (S) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a Nipah virus fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the engineered fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the engineered spike (S) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences. In some embodiments the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

[0053] In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a method of generating an immune response in a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a method of treating or preventing a viral infection in a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a composition for use in vaccinating, generating an immune response, or treating or preventing any viral infection disease disclosed herein. In another aspect, the disclosure provides a composition, method, or use as described herein. In another aspect, the disclosure provides a method of making a composition, comprising culturing host cells modified to express one or more polypeptides as described herein.

[0054] In another aspect, the disclosure provides a recombinant polypeptide for use in displaying a molecule such as an antigen, comprising an alpha-helical segment and a multimerization domain, wherein the alpha-helical segment comprises one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the alpha-helicalsegment has improved hydrophobic packing. In some embodiments, the alpha-helical segment comprises between about 7 and about 31 residues. In some embodiments, the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids.

[0055] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to any one of LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566), LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567), LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568), EKIXXAIKKAXKL (SEQ ID NO: 569), EXIXKAIKXLXXXXX (SEQ ID NO: 570), XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571), XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

[0056] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

[0057] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to a) L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573), b) L V [A / T] T X2K X2L X2D L I X2X2L [K / E] X2L L X2K L X2X2(SEQ ID NO: 574), or c) L N K V K K X2 V X2 X2 L X2 X2 X2 V X2 X2 L E K X2 L X2 (SEQ ID NO: 575), wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0058] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according a) E K I X2 X2 A I K K A X2 K L (SEQ ID NO: 576), b) E X2 I X2 K A I K X2 L [L / X2] X2 X2 [X1 / X2] X2 (SEQ ID NO: 577), and c) X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2X2X1X2X2(SEQ ID NO: 578) , or d) X2X2L K K A A X2I X1K K X1L K X2X2(SEQ ID NO: 579) wherein X1is apolar residues selected from A, I, L, and M, and wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0059] In some embodiments, the alpha-helical segment comprises a polypeptide sequence listed in Table 25A or Table 25B or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the alpha-helical segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or the polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0060] In some embodiments, the multimerization domain is I53-50A or a variant thereof. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64). In some embodiments, the polypeptide comprises an N-terminal fusion of the alpha-helical segment to the multimerization domain via a peptide bond or polypeptide linker. In some embodiments, the polypeptide comprises, N-terminal to the alpha- helical segment, an antigen polypeptide.

[0061] In another aspect, the disclosure provides a polypeptide comprising an alpha-helical segment, comprising a polypeptide sequence listed in Table 25A or Table 25B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide described herein. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the nanostructure is a two-component nanostructure comprising a second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0062] In another aspect, the disclosure provides a pharmaceutical composition comprising a polypeptide or a nanostructure described herein. In another aspect, the disclosure provides a vaccine comprising a polypeptide or a nanostructure described herein. In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of generating an immune response or treating or preventing a viral infection in a subject, comprising administering to the subject a polypeptide or a nanostructure described herein.

[0063] In another aspect, the disclosure provides a method of making a polypeptide or a nanostructure described herein, comprising culturing host cells modified to express one or more polypeptides as described herein.

[0064] In another aspect, the disclosure privides a recombinant polypeptide, comprising an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises: (1) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1, (2) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1, (3) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1, (4) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1, (5) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1, (6) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1, or (7) any combination of (1)-(6).

[0065] In some embodiments, the ectodomain comprises one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1.

[0066] In some embodiments, the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: : E487R + K498A, E487R + K498E, E487K + K498E, D486A + E487R + K498A, D486Q + E487R + K498A, D486E + E487A + D489A + T400D, D486A + E487M + K498A, E487Q, D486S, F488W + D489A + T400D + E487R + K498A, F140W + D489A + T400D + E487R + K498A, Q494I + S485I + K399A + 487R + 498A, Q494M + S485I + K399A; D486A + 487M + 498A, Q494L + S485A + K399V + D486A + 487M + 498A, Q494M + S485A + K399V + D486A + 487M + 498A, Q494A + S485F + K399V + D486A + 487M + 498Y, D489A + T400D + E487R + K498A, or D489A + T400D.

[0067] In some embodiments, the ectodomain comprises the amino acid substitutions D489A, T400D, E487R, and K498A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P.

[0068] In some embodiments, the polypeptide comprises a heterologous multimerization domain. In some embodiments, the multimerization domain is a trimerization domain. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64). In some embodiments, the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L.

[0069] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 6, below, optionally lacking a p27 peptide shown in bold, and in which “X” refers to sites involving an added C-terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQEL DKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFL LGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYI NNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSEL LSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWK LHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQADTCKVQSNRVFCDTMNSL TLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSLGAIVSCYGKTKCTASNKNRGI IKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEPIINYYDPLVFPSDEFD ASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 6).

[0070] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 7, below, optionally lacking a p27 peptide shown in bold, in which “X” refers to sites involving an added C- terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQEL DKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFL LGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYI DKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSEL LSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWK LHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAETCKVQSNRVFCDTMNSL TLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEPIINFYDPLVFPSDEFD ASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 7).

[0071] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 8, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQELDKYKNA VTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFLLGVGSAIASGIA VCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYINNQLLPMLNRQSCRISNI ETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIV RQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDN AGSVSFFPQADTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITS LGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEP IINYYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 8).

[0072] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 9, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQELDKYKNA VTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFLLGVGSAIASGVA VCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYIDKQLLPILNKQSCSISNI ETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIV RQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDN AGSVSFFPQAETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITS LGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEP IINFYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 9).

[0073] In some embodiments, the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1-9.

[0074] In another aspect, the disclosure provides a trimeric protein complex comprising a polypeptide described herein.

[0075] In some embodiments, a thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (1)-(7).

[0076] In some embodiments, the stability, assayed by storage at about 40°C, is increased compared to a trimeric protein complex lacking modifications (1)-(7). In some embodiments, the thermal stability is increased compared to a reference RSV F protein comprising amino acid substitutions consisting essentially of S155C, S290C, S190F, and V207L (DS-Cav1).

[0077] In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide described herein. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53- 50A polypeptide. In some embodiments, the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the RSV fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide.

[0078] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1, and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0079] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an e engineeredectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, D489A, T400D, E487R, and K498A relative to SEQ ID NO: 1, and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0080] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and T249P relative to SEQ ID NO: 1, and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64). and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0081] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and D486A relative to SEQ ID NO: 1, and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0082] In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of sequence listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences.

[0083] In some embodiments, the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

[0084] In another aspect, the disclosure provides a pharmaceutical composition comprising a polypeptide, a protein complex, or a nanostructure described herein. In another aspect, the disclosure provides a vaccine composition comprising a polypeptide, a protein complex, or a nanostructure described herein. In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of generating an immune response in a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of treating or preventing RSV disease in a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a composition described herein for use in vaccinating, generating an immune response, or treating or preventing RSV disease. In another aspect, the disclosure provides a method of making a composition described herein, comprising culturing host cells modified to express one or more polypeptides as described herein. In another aspect, the disclosure provides a composition, method, or use as described herein.

[0085] Any aspect or embodiment described herein can be combined with any other aspect or embodiment as disclosed herein. Further aspects, embodiments, and advantages of the invention will be apparent from the Detailed Description that follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, and accompanying drawings, where:

[0087] FIG. 1 shows a structural model of RSV F protein in the prefusion conformation (PDB 4MMU), with stabilizing elements separated into five different spaces. Spaces 1-4 were targeted by stabilizing mutations. Space 5 refers to the C terminus of the protein.

[0088] FIG.2 shows a close-up view of the structure of C termini of RSV F protein determined by X-ray crystallography of prefusion RSV F (PDB 4MMU) before and after remodeling. Residues that are remodeled (residues 503-509) are outlined with a thicker black highlight (left) and additional structure added by remodeling is shown in black (right).

[0089] FIG.3 shows ddG scoring with representative designs highlighted.

[0090] FIG.4 shows hydrophobicity scoring of designs. Mean (solid line) and standard deviation (dashed lines), WT (dotted line).

[0091] FIG.5 shows a representative electron micrograph of a protein nanostructure as described herein.

[0092] FIG. 6A shows a structural model of a PIV5 F protein before (left) and after (right) remodelling of the C terminus. Omitted or unstructured regions (left, not shown) are predicted to adopt an alpha-helical structure (right, dark black).

[0093] FIG. 6B shows a structural model of a PIV3 F protein before (left) and after (right) remodelling of the C terminus.

[0094] FIG. 6C shows a structural model of a Nipah F protein before (left) and after (right) remodelling of the C terminus.

[0095] FIG. 6D shows a structural model of an hMPV F protein before (left) and after (right) remodelling of the C terminus.

[0096] FIG.6E shows a structural model of a SARS-CoV-2 S protein before (left) and after (right) remodelling of the C terminus.

[0097] FIG.7 shows predicted ddG for Paramyxoviridea as a function of remodel length. Topleft: PIV5, topbottom: PIV3, topright: Nipah. Dotted line represents the mean, solid line represents the WT sequence. Note that the WT sequence only includes structured residues present in the PDB.

[0098] FIG. 8 shows representative remodeled designs from HMPV using RFdiffusion. De novo regions are colored black, context from the input PDB colored white.

[0099] FIG.9 shows predicted ddG for Pneumoviridae and Coronavirdae as a function of remodel length. Top: HMPV, bottom: SARS-CoV-2. Dotted line represents the mean, solid line represents the WT sequence. Note that the WT sequence only includes structured residues present in the PDB.

[0100] FIG. 10 shows predicted hydrophobicity for Paramyxoviridea as a function of remodeled sequence position. Topleft: PIV5, topbottom: PIV3, topright: Nipah. Dotted line represents the mean, solid line represents the WT sequence. Note that the WT sequence only includes structured residues present in the PDB.

[0101] FIG.11 shows Predicted hydrophobicity for Pneumoviridae and Coronavirdae as a function of remodeled sequence position. Top: HMPV, bottom: SARS-CoV-2. Dotted line represents the mean, solid line represents the WT sequence. Note that the WT sequence only includes structured residues present in the PDB.

[0102] FIG.12 shows Principal Component Analysis of distances in group 1 (parallel) remodeled sequences.

[0103] FIG. 13 shows Principal Component Analysis of distances in group 2 (not parallel) remodeled sequences.

[0104] FIGs. 14A-14C show position specific probabilities for group 1 (parallel). Probabilities represent the likelihood of remodeled length. FIG.14A shows position specific probabilities for Clust_p2. FIG.14B shows position specific probabilities for Clust_p1. FIG.14C shows position specific probabilities for Clust_p0.

[0105] FIGs.15A-15D show position specific probabilities for group 2 (not parallel). Probabilities represent the likelihood of remodeled length. FIG.15A shows position specific probabilities for Clust_o0. FIG.15B shows position specific probabilities for Clust_o1. FIG.15C shows position specific probabilities for Clust_o3. FIG.15D shows position specific probabilities for Clust_o2.

[0106] FIGs. 16A-16G show positional weightings for each cluster. FIG. 16A shows Positional weightings for Clust_p0. FIG. 16B shows Positional weightings for Clust_p1. FIG. 16C shows Positional weightings for Clust_p2. FIG. 16D shows Positional weightings for Clust_o0. FIG. 16E shows Positional weightings for Clust_o1. FIG. 16F shows Positional weightings for Clust_o2. FIG.16G shows Positional weightings for Clust_o3.

[0107] FIG. 17 shows neutralizing titers against RSV / B (B18537 strain) elicited by various nanostructure immunogens based on RSV / B antigens.

[0108] FIG. 18 shows neuralizing titers against RSV / A (Tracy strain) elicited by various nanostructure immunogens based on RSV / A antigens.

[0109] FIG.19 shows a structural comparison of cryo-EM structures of the RSV F ectodomains of A) RSV / A.023, and B) DS-Cav1 fused to foldon (PDB 7LUE). The added C-terminal alpha-helical segment in RSV / A.023 is colored in dark gray and surrounded by a dashed box. Antibody structures were removed from the model of PDB 7LUE prior to generating images.

[0110] FIG.20 shows a structural comparison of C-terminal regions for cryo-EM structures of the RSV F ectodomains of A) RSV / A.023, and B) DS-Cav1 fused to foldon (PDB 7LUE). The added C-terminal alpha-helical segment in RSV / A.023 is colored in dark gray and surrounded by a dashed box. Antibody structures were removed from the model of PDB 7LUE prior to generating images.

[0111] FIG. 21 shows maximum binding to the monoclonal antibody 3×1 by biolayer interferometry to remodeled PIV3 F (Top) and maximum binding normalized to the anti- Component A specific antibody 16A8 maximum binding.

[0112] FIG. 22 shows maximum binding to the monoclonal antibody PIA174 by biolayer interferometry to remodeled PIV3 F (Top) and maximum binding normalized to the anti- Component A specific antibody 16A8 maximum binding.

[0113] FIG. 23 shows maximum binding to the monoclonal antibody 16A8 by biolayer interferometry.

[0114] FIG.24 shows maximum binding of PIV3 F with generic C-terminal remodel sequences to the monoclonal antibody 16A8 by biolayer interferometry.

[0115] FIG. 25 shows maximum binding to the monoclonal antibody 3×1 by biolayer interferometry to PIV3 F with generic C-terminal remodel sequences (Top) and maximum binding normalized to the anti-Component A specific antibody 16A8 maximum binding.

[0116] FIG. 26 shows maximum binding to the monoclonal antibody PIA174 by biolayer interferometry to PIV3 F with generic C-terminal remodel sequences (Top) and maximum binding normalized to the anti-Component A specific antibody 16A8.DETAILED DESCRIPTION

[0117] Before the embodiments of the disclosure are described, it is to be understood that such embodiments are provided by way of example only, and that various alternatives to the embodiments of the disclosure described herein may be employed in practicing the invention. Numerous variations, changes, and substitutions will occur to those skilled in the art and may be practiced without departing from spirit of the invention.

[0118] Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Various scientific dictionaries that include the terms included herein are well known and available to those in the art. Although any methods and materials similar or equivalent to those described herein find use in the practice or testing of the disclosure, some preferred methods and materials are described. Accordingly, the terms defined immediately below are more fully described by reference to the specification as a whole. I. Definitions

[0119] The singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0120] As used herein, the term “about” means a range of values including the specified value, which a person of ordinary skill in the art would consider reasonably similar to the specified value. For example, about means within a standard deviation using measurements generally acceptable in the art. For example, about means a range extending to + / - 10%, + / - 5%, + / - 3%, or + / - 1% of the specified value.

[0121] The term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a range having an upper limit or no upper limit, depending on the variable being defined). For example, “at least 1” means 1 or more than 1.

[0122] The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined). For example, “at most 4” means 4 or less than 4, and “at most 40%” means 40% or less than 40%. When, in this specification, a range is given as “(a first number) to (a second number)” or “(a first number)-(a second number)” thismeans a range whose lower limit is the first number and whose upper limit is the second number. For example, 25 to 100 mm means a range whose lower limit is 25 mm, and whose upper limit is 100 mm.

[0123] The term “identical” or percent “identity,” in the context of two or more nucleic acid or polypeptide sequences, refers to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence. Methods of alignment of sequences for comparison are well known in the art. Once aligned, the number of matches is determined by counting the number of positions where an identical nucleotide or amino acid residue is present in both sequences. The percent sequence identity is determined by dividing the number of matches in the alignment by the length of the reference sequence, followed by multiplying the resulting value by 100. For example, a peptide sequence that has 1166 matches when aligned with a reference sequence having 1554 amino acids is 75.0 percent identical to the test sequence (1166÷1554 * 100=75.0). As the terms are used herein, gaps in the alignment do not decrease the percent sequence identity. Unless otherwise specified, optimal alignment of sequences for comparison is conducted by the global alignment algorithm of Needleman and Wunsch, Mol. Biol. 48:443 (1970) as implemented by EMBOSS Needle (on the World Wide Web at ebi.ac.uk / Tools / psa / emboss_needle / ) (Madeira et al. Nucleic Acids Res. 50(W1):W276-W279 (2022)). Other alignment methods may be used, including without limitation those described in Devereux, et al, Nucleic Acids Res. 12:387-95 (1984); Atschul et al. J. Mo. Biol.215:403-10 (1990) (BLAST); Carrillo and Lipman Siam J. Appl. Math. 48(5) (1988); Computational Molecular Biology (Lesk, AM, ed., 1989); Biocomputing Informatics and Genome Projects, (Smith, DW, ed., 1993); Computer Analysis of Sequence Data, Part I, (Griffin and Griffin, eds., 1994); Sequence Analysis in Molecular Biology (von Heinje, 2012); Sequence Analysis Primer (Gribskov and Devereux, J., eds. 1993). Sequence identity is calculated using the implementation of the Needleman-Wunsch algorithm provided by the National Library of Medicine (on the World Wide Web at blast.ncbi.nlm.nih.gov / Blast.cgi?PAGE_TYPE=BlastSearch&BLAST_SPEC=GlobalAln).

[0124] For example, sequence identity can be determined by standard methods that are commonly used to compare the similarity of two polypeptide or two polynucleotide sequences. Using a computer program such as EMBOSS Needle or BLAST, two polypeptide or two polynucleotide sequences are aligned for optimal matching of their respective residues (either along the full lengthof one or both sequences, or along a pre-determined portion of one or both sequences). The programs provide a default opening penalty and a default gap penalty, and a scoring matrix such as PAM 250 (a standard scoring matrix; see Dayhoff et al., in Atlas of Protein Sequence and Structure, vol.5, supp.3 (1978)) that can be used in conjunction with the computer program.

[0125] As used herein, the term “helix-forming segment” refers to a portion of a protein or polypeptide that forms, or is predicted to form, an alpha-helix. An “alpha-helix” is an element of protein secondary structure stabilized by hydrogen bonds between carbonyl oxygen and the amnino group of every third residue in the helical turn. The smallest segment of a protein that is generally considered to form an alpha-helix is about 6-7 amino acid results. Accordingly, in some embodiments, a helix-forming segment comprises between about 5 and about 30 amino acid residues, between about 7 and about 14 amino acid residues, between about 7 and about 21 amino acid residues, between about 7 and about 28 amino acid residues, between about 7 and about 35 amino acid residues, between about 7 and about 42 amino acid residues, or between about 7 and about 49 amino acid residues; or any values therebetween, such as without limitation 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, or more amino acids. In some embodiments, the helix forming segment forms a parallel, three-helix bundle.

[0126] As used herein the term “alpha-helical homotrimer” refers to a three-helix bundle with helices in parallel orientation. The term excludes six-helical bundles such as those formed by assembly of three anti-parallel, two-helix bundles; i.e., the term “alpha-helical homotrimer” as used herein excludes heptad-repeat regions of gp41 or recombinant variants thereof.

[0127] As used herein, the term “stable” such as in “stable alpha-helical homotrimer” means that the protein structure (e.g., homotrimer) persists under suitable conditions. A stable protein structure may be detected by biophysical or biochemical methods known in the art—including but not limited to size exclusion chromotagraphy, dynamic light scattering, electron microscopy, analytical ultracentrifugation, X-ray crystallography, nuclear magnetic resonance spectroscopy, circular dichroism, thermal denaturation, or interaction measurements. A “stable” alpha-helical homotrimer may be distinguished from an unstable homotrimer in part by structural analysis (e.g., by X-ray crystallography, NMR, or EM), or by measuring the impact of the alpha-helical homotrimer, for example by binding studies (BLI, SPR) or biophysical studies (thermaldenaturation). In some embodiments, the stable alpha-helical homotrimer may be stable at room temperature and / or at elevated temperatures (e.g., 40℃). An alpha-helical homotrimer may either form a homotrimer in isolation, or as part of a larger trimeric protein complex (such as a trimeric antigen). In some embodiments, inclusion of the stable alpha-helical homotrimer stabilizes the trimeric protein complex by a ΔΔG of at least -10, at least -20, at least -30, at least -40, at least - 50, or at least -60, as predicted computationally or experimentally determined. In some embodiments, the stable alpha-helical homotrimer is an “obligate” homotrimer.

[0128] As used here, “conservative amino acid substitution” means that: hydrophobic amino acids (Ala, Gly, Met, Val, Ile, Leu, Phe, Thr, Trp) are substituted with other hydrophobic amino acids; hydrophobic amino acids with bulky side chains (Phe, Tyr, Trp) are substituted with other hydrophobic amino acids with bulky side chains; amino acids with positively charged side chains (Arg, His, Lys) are substituted with other amino acids with positively charged side chains; amino acids with negatively charged side chains (Asp, Glu) are substituted with other amino acids with negatively charged side chains; and polar amino acids (Cys, Ser, Thr, Asn, Gly, Tyr) are substituted with other polar amino acids. Amino Acid Three letter symbol One letter symbol Alanine Ala A

[0129] Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising,” as well as “has” or “having” and “includes” or “including,” will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps. “Consisting essentially of” or “consists essentially” indicates exclusion of elements or steps that materially affect the basic and novel characteristics of the claimed invention. II. Engineered Ectodomains

[0130] The disclosure provides an engineered ectodomain of trimeric viral proteins, including but not limited to paramyxoviridae, pneuomoviridae, rhabdoviridae, filoviridae, herpesviridae, orthomyxoviridae, coronaviridae, retroviridae, and arenviridae. Table 1 shows viral fusion protein that are designable. In some embodiments, the trimer viral protein is an enveloped viral fusion protein. Table 1. Indication Protein Order Genus Class FamilyHuman Fusion (F) Mononegavirales Respirovirus I respirovirus 1 Paramyxoviridae eeredectodomain of a trimeric viral protein, wherein the ectodomain comprises a C-terminal helix forming segment comprising one or more amino acid substitutions, relative to a native reference sequence of the viral protein, selected such that the segment forms a alpha-helical homotrimer.

[0132] In some embodiments, the C-terminal helix forming segment has improved hydrophobic packing compared to the native reference sequence. In some embodiments, the C-terminal helix forming segment comprises between about 7 and about 31 residues. In some embodiments, the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids. In some embodiments, the C-terminal helix forming segment comprises a polypeptide sequence according to any one of LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566), LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567), LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568), EKIXXAIKKAXKL (SEQ ID NO: 569), EXIXKAIKXLXXXXX (SEQ ID NO: 570), XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571), XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

[0133] In some embodiments, the C-terminal helix forming segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

[0134] In some embodiments, the segment comprises a polypeptide sequence according to any one of L X2X2T I X2X2L L X2I [V / I] X2X2L [I / L] X2X2L (SEQ ID NO: 573), L V [A / T] T X2K X2L X2 D L I X2 X2 L [K / E] X2 L L X2 K L X2 X2 (SEQ ID NO: 574), or L N K V K K X2 V X2 X2 L X2 X2 X2 V X2 X2 L E K X2 L X2 (SEQ ID NO: 575), wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0135] In some embodiments, segment comprises a polypeptide sequence according to any one of E K I X2 X2 A I K K A X2 K L (SEQ ID NO: 576), E X2 I X2 K A I K X2 L [L / X2] X2 X2 [X1 / X2] X2(SEQ ID NO: 577),X2K [X1 / T] [L / E] E [T / A] X1X2[I / X2] V X2X2[X1 / X2] [X1 / X2] X2X2X1X2X2(SEQ ID NO: 578) , or X2X2L K K A A X2I X1K K X1L K X2X2(SEQ ID NO: 579), wherein X1 is apolar residues selected from A, I, L, and M, and wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid. In some embodiments, the segment comprises a polypeptide sequence listed in Table 25A or Table 25B. In some embodiments, the native reference sequence of the viral protein is any one of SEQ ID NOs: 1, 104, 327, 382, 459, 499. Respiratory Syncytial Virus (RSV) F protein

[0136] Respiratory Syncytial Virus (RSV) F protein is a major conserved surface antigen of RSV and antibodies against it are associated with protection against disease. RSV F protein is a validated target for protection against infection by RSV as demonstrated by the clinical efficacy of palivizumab, a monoclonal antibody that binds F-antigen and leads to neutralization of the virus (Johnson et al., J Infect Dis. 1997 Nov;176(5):1215-24). RSV F protein is known to undergo a significant change in structure from prefusion to postfusion form which catalyzes viral and host membrane fusion to allow for viral entry into the cell (McLellan et al., Science. 2013;342(6158):592-8). Prefusion F protein has important epitopes that are lost during the transition to postfusion F protein (Melero et al., Vaccine. 2017;35(3):461-468). Antibody depletion studies with human sera absorbed with RSV F protein in either conformation demonstrate that the majority of the neutralizing response against RSV F protein targets the prefusion structure (Krarup et al., Nat Commun.2015;6:8143). These studies also demonstrate the potential for antibodies that bind postfusion F protein to interfere with neutralization (Ngwuta etal., Sci Transl Med.2015;7(309):309ra162). In general, high levels of antibodies against RSV F protein are associated with protection against severe disease. However, generating high-titers of neutralizing antibodies against RSV F protein remains challenging, due to the specific biochemical nature of the RSV F protein and the unpredictability of vaccine responses to RSV F. Structural model of RSV F protein in the prefusion conformation is shown in FIG. 1, with stabilizing elements separated into five different spaces. Spaces 1-4 were targeted by stabilizing mutations. Space 5 refers to the C terminus of the protein.

[0137] Illustrative sequences are shown in Table 2A. A native RSV / B F protein sequence was used for design (GenBank: WDV37446.1). The (predicted) transmembrane region is residues 527-549 and is bold / underlined. The signal peptide is underlined with italic. The approximate region surrounding the p27 peptide is bold. Table 2A. SEQ Description Sequence ID(S155C, S290C, YTINTTKNLNVSISKKRKRRFLGFLLGVG S190F, V207L) SAIASGIAVCKVLHLEGEVNKIKNALQLTTLPSEVSLCNTDIFNSKYDCKIMTSKTDI SSSVITSLGAIVSCYGKTKCTASNKNRGIStabilized NNQLLPILNQQSCRIFNIETVIEFQQKNS muation RLLEITREFSVNAGVTTPLSTYMLTNSELTNIMITTIIIVIIVILLSLIAVGLLLYCK ARSTPVTLSKDQLSGINNIAFSN 0

[0138] In some embodiments, the RSV refers RSV / A. In some embodiments, the RSV refers RSV / B.

[0139] In some embodiments, the ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 1. In some embodiments, the ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 2. In some embodiments, the ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 3. In some embodiments, the ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 4. In some embodiments, the ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 5.

[0140] In another aspect, the disclosure privides a recombinant polypeptide, comprising an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises: (a) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1, (b) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1, (c) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1, (d) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1, (e) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1, (f) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1, or (g) any combination of (a)-(f). C-terminal helix-forming segment

[0141] The C-terminal end of the ectodomain of many viral fusion proteins is, in at least some cases, known to be or predicted to be a helical bundle that interfaces with a helical transmembrane domain. The present inventors have observed that, in the RSV F protein, the C-terminal helical region of the ectodomain has suboptimal hydrophobic packing. Computational modeling (with RosettaRemodel) was used to generate artificial polypeptide sequences, each predicted to form astable alpha helix. In illustrative, non-limiting Examples provided below, the helical backbone is first optimized with side-chains represented as centroids, and then the side-chains are designed in all-atom mode. Optimal linker length can be determined by a plot of ddG as a function of linker length (Rosetta remodel), or ddG normalized to linker length (RFdiffusion). Then 6-14 additional amino acids were modeled with helical constraints.

[0142] Illustrative sequences are shown in Table 2B. Residues 500-502 of the native RSV F protein are included as NQS (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 2B. C-terminal Alpha-helical segments (Rosetta remodel) Name SequenceRemodeledLength SEQ ID NO:C-Term 26NQSKKLLDAADAVKKMLEKEKSS20 140C-Term 27NQSKKVLETIRWIETVISRQRSS20 141, ize the portion of the F protein in a helical conformation.

[0144] Illustrative sequences generated by RFdiffusion are shown in Table 2C. Residues 500-502 of the native RSV F protein are included as NQS (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified Table 2C. C-terminal Alpha-helical segments for RSV (RFdiffusion) Name SequenceRemodeledSEQ IDC-Term 3NQSKLVKKVIKETHEIKKKLEDLLK23 167C-Term 4NQSRSNKKTKNKVKSIEKQVKEIEKRLEKLER31168Position Preferred Allowed residues SEQ ID NO: L503 Polar QVKRNL580D526 Hydrophobic ILVR603G527 Polar EKQD604ut 10 and about 30 residues. In some embodiments, the segment comprises substitutions relative to the reference sequence SEQ ID NO: 1 at two or more, three or more, or four or more residues that, without being bound by theory, may generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

[0146] The computational design described herein has detailed yield information on desirable amino acid substitutions that, individually or in groups, may stabilize the RSV F protein ectodomain. Illustrative, non-limiting amino acid substitutions that may be used are described as follows. In some embodiments, the C-terminal helix-forming segment (“the segment”) comprises amino acid substitutions at one or more of positions 505-519 according to reference SEQ ID NO: 1. It will be readily understood by those skilled in the art that alignment to the reference sequence of this segment depends on preserving the helical structure of the segment, and therefore insertions and deletions in the alignment are not permitted in generating sequence alignment for this segment. The starting amino acid (e.g., F in F505) is included here for clarity only, it being understood that the modification provided herein may be used with other strains of RSV in which the starting amino acid is different from the amino acid in the RSV / B reference strain sequence SEQ ID NO: 1.

[0147] In some embodiments, the segment comprises a polypeptide sequence listed in Table 2B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises a polypeptide sequence listed in Table 2B or having 1, 2, 3, 4, 5, or more amino acid substitutions thereto. In some embodiments, the segment comprises a polypeptide sequence listed in Table 2C, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises polypeptide sequence listed in Table 2C or having 1, 2, 3, 4, 5, or more amino acid substitutions thereto.

[0148] In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10) or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10).

[0149] In some embodiments, the C-terminal helix-forming segment comprises between about 5 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 5 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 5 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 5 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 5 and about 10 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 15 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between about 15 and about 20 residues.

[0150] In another aspect, the disclosure provides an alpha-helical segment, comprising a polypeptide sequence listed in Table 25A or Table 25B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the polypeptide comprises a trimeric pathogen protein, N-terminally or C-terminally linked to the alpha-helical segment. In some embodiments, the C-terminal helix-forming segment comprises at least 5 residues. In some embodiments, the C-terminal helix-forming segment comprises at least 10 residues. In some embodiments, the C-terminal helix-forming segment comprises at least 15 residues. In some embodiments, the C-terminal helix-forming segment comprises at least 20residues. In some embodiments, the C-terminal helix-forming segment comprises at least 25 residues. Stabilizing substitutions

[0151] Computational modeling was used to identify amino acid substitutions to stabilize RSV / B F protein in the prefusion conformation. Without being bound by theory, the following amino acid substitutions are described herein as “stabilizing substitutions” because they are predicted to stabilize the RSV F protein by increasing shape complementarity within the tertiary structure of RSV F protein in the prefusion conformation. The amino acid substitutions may have other effects on structure, such as generating hydrophobic or charge-charge interactions (e.g., salt bridges) within the structure. These mutations are listed in Table 3A. Table 3A. stabilizing substitutions Space Substitutions Space 1 F140W K399A K399V T400D S485I S485A, R, , , L, ,

[0152] Embod. Table 3B. E487R + K498AQ494I + S485I + K399A + 487R + 498A Q494M + S485I + K399A, D486A + 487M + 498A

[0153] In some, p tions S155C, S290C, S190F, and V207L.

[0154] In some embodiments, the ectodomain comprises one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1.

[0155] In some embodiments, the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: E487R + K498A; E487R + K498E; E487K + K498E; D486A + E487R + K498A; D486Q + E487R + K498A; D486E + E487A + D489A + T400D; D486A + E487M + K498A; E487Q; D486S; F488W + D489A + T400D + E487R + K498A; F140W + D489A + T400D + E487R + K498A; Q494I + S485I + K399A + 487R + 498A; Q494M + S485I + K399A; D486A + 487M + 498A; Q494L + S485A + K399V + D486A + 487M + 498A; Q494M + S485A + K399V + D486A + 487M + 498A; Q494A + S485F + K399V + D486A + 487M + 498Y; D489A + T400D + E487R + K498A; or D489A + T400D. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P. Additional substitutions to stabilize the F protein in a prefusion conformation

[0156] Without being bound by theory, the following amino acid substitutions are predicted to stabilize the RSV F protein. The amino acid substitutions may have other effects on structure, such as generating hydrophobic or charge-charge interactions (e.g., salt bridges) within the structure. These mutations are listed in Table 4A. Table 4A SubstitutionsL142C, V144C, I148C, A149C, V154I, S155C, L188C, S190I, S215P, E232A, R235Y, S238C, T249P, N254C,

[0157] Ithree or more amino acid substitutions at positions 54, 55, 58, 66, 67, 88, 92, 98, 101, 103, 106, 140, 142, 144, 148, 149, 154, 155, 188, 190, 207, 215, 232, 235, 238, 249, 254, 279, 290, 296, 298, 361, 371, 399, 400, 428, 458, 485, 486, 487, 488, 489, 494, 495, or 498 relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises one, two, three or more amino acid substitutions at T54H, S55C, T58M, K66E, N67I, T67I, T67V, N88C, E92C, E92D, Q98C, Q101P, T103C, R106C, F140W, L142C, V144C, I148C, A149C, V154I, S155C, L188C, S190I, S215P, E232A, R235Y, S238C, T249P, N254C, Q279C, V296A, V296I, A298L, Q361C, N371C, K399A, T400D, N428C, Y458C, S485I, D486A, D486S, D486N, E487M, E487Q, E487R, F488W, D489A, D489S, Q494M, V495Y, or K498A relative to SEQ ID NO: 1.

[0158] Combinations of substitutions are showns in Table 4B. Table 4B. S155C + S290C + S190F + V207L S55C + L188C + L142C + N371C + T54H + V296I S

[0159] In some embod ments, t e ectodoma n compr ses t e am no ac d subst tut ons at S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, L142C, N371C, T54H, and V296I relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomaincomprises the amino acid substitutions at S55C, L188C, T54H, and S190I relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T103C, I148C, S190I, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T103C, I148C, T54H, S190I, V296I, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, T54H, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, S190I, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, T54H, S190I, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S155C, S290C, S190I, and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, L142C, N371C T54H, V296I, D486S, E487Q, and D498S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S155C, S290C, T54H, S190I, and V296I relative to SEQ ID NO: 1.

[0160] In some embodiments, a RSV F protein mutant comprises a disulfide mutation selected from the group consisting of 55C and 188C; 155C and 290C; 103C and 148C; and 142C and 371C, such as S55C and L188C, S155C and S290C, T103C and I148C, or L142C and N371C. Examples of pairs of such mutations include: 508C and 509C; 515C and 516C; 522C and 523C, such as K508C and S509C, N515C and V516C, or T522C and T523C.

[0161] In some embodiments, a RSV F protein mutant comprises one or more cavity filling mutations selected from the groups shown in Table 4C. Table 4C. Disulfide mutations Amino acid position Substituted with

[0162] In some embodiments, a RSV F protein mutant comprises at least one cavity filling mutation selected from the group consisting of: T54H, S190I, and V296I.

[0163] In some embodiments, a RSV F protein mutant comprises at least one electrostatic mutation selected from the groups shown in Table 4D. Table 4D. Electrostatic mutations Amino acid position Substituted with E 82, 92, 487 D, F, Q, T, S, L, H, p p .

[0165] Combinations of substitutions are shown in Table 4E. Table 4E. T103C + I148C + S190I + D486S T54H + S55C + L188C + D486S

[0166] In some embodiments, the ectodomain comprises the amino acid substitutions at T103C, I148C, S190I and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, S55C, L188C and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, T103C,I148C, S190I, V296I and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, S55C, L142C, L188C, V296I and N371C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, S55C, L188C and S190I relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at S55C, L188C, S190I and D486S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, S55C, L142C, L188C, V296I, N371C, D486S, E487Q and D489S relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T54H, S155C, S190I, S290C and V296I relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at N67I and S215P relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at N67I, S215P and E487Q relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at V56C and V164C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at I57C and S190C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at T58C and V164C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at N165C and V296C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at K168C and V296C relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises the amino acid substitutions at M396C and F483C relative to SEQ ID NO: 1. Combination of C-terminal helix-forming segment and stabilizing substitutions

[0167] In some embodiments, the disclosure provides recombinant polypeptides comprising amino acid substitutions having an engineered C-terminal alpha-helical segment that stabilize the RSV F protein in a prefusion conformation.

[0168] The native sequence of RSV / B F protein (GenBank: WDV37446.1) is shown below with the (predicted) transmembrane region with italic and the C-terminal helix of the native sequence (residues 492-501) is also bold / underlined. The signal peptide is underlined with italic / underlined. 1 MELLIHRSSA IFLTLAINAL YLTSSQNITE EFYQSTCSAV SRGYLSALRT 51 GWYTSVITIE LSNIKETKCN GTDTKVKLIK QELDKYKNAV TELQLLMQNT 101 PAVNNRARRE APQYMNYTIN TTKNLNVSIS KKRKRRFLGF LLGVGSAIAS151 GIAVSKVLHL EGEVNKIKNA LQLTNKAVVS LSNGVSVLTS RVLDLKNYIN 201 NQLLPMVNRQ SCRISNIETV IEFQQKNSRL LEITREFSVN AGVTTPLSTY 251 MLTNSELLSL INDMPITNDQ KKLMSSNVQI VRQQSYSIMS IIKEEVLAYV 301 VQLPIYGVID TPCWKLHTSP LCTTNIKEGS NICLTRTDRG WYCDNAGSVS 351 FFPQADTCKV QSNRVFCDTM NSLTLPSEVS LCNTDIFNSK YDCKIMTSKT 401 DISSSVITSL GAIVSCYGKT KCTASNKNRG IIKTFSNGCD YVSNKGVDTV 451 SVGNTLYYVN KLEGKNLYVK GEPIINYYDP LVFPSDEFDA SISQVNEKIN 501 QSLAFIRRSD ELLHNVNTGK STTNIMITAI TIVIIVVLLS LIAIGLLLYC 551 KAKNTPVTLS KDQLSGINNI AFSK (SEQ ID NO: 1242)

[0169] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 6, below, optionally lacking a p27 peptide shown in bold, and in which “X” refers to sites involving an added C-terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLI KQELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKR KRRFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSV LTFRVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNA GVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEE VLAYVVQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSV SFFPQADTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDIS SSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLY YVNKLEGKNLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXX XXXXXXXX (SEQ ID NO: 6).

[0170] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 7, below, optionally lacking a p27 peptide shown in bold, in which “X” refers to sites involving an added C- terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLI KQELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKR KRRFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSV LTFKVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNA GVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEE VLAYVVQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSV SFFPQAETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVS SSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLY YVNKQEGKSLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXX XXXXXXXX (SEQ ID NO: 7).

[0171] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 8, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLI KQELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKR KRRFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSV LTFRVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNA GVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEE VLAYVVQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSV SFFPQADTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDIS SSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLY YVNKLEGKNLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSREIIRAINI VRKIASEK (SEQ ID NO: 8).

[0172] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 9, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLI KQELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKR KRRFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSV LTFKVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNA GVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEE VLAYVVQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSV SFFPQAETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVS SSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLY YVNKQEGKSLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSREIIRAINI VRKIASEK (SEQ ID NO: 9).

[0173] In some embodiments, the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1-9.

[0174] Illustrative sequences comprising various RSV F protein ectodomains and a C-terminal alpha-helical sugement are shown in Table 4F. The signal peptide is underlined. The approximate region surrounding the p27 peptide is bold

[0175] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to sequences shown in Table 4F. Table 4F. Sequence Mutations SEQ ID NO:CDNAGSVSFFPQAETCKVQSNRVFCDTMNSLTLPSEVN LCNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKYVNKQEGKSLYVKGEPIINFYDPLVFPSDEFDASISQVN EKINQSREIIRAINIVRKIASEKIKQELDKYKNAVTELQLLMQSTPATNNRARRELPRFM T54H, S55C, NYTLNNAKKTNVTLSKKRKRRFLGFLCGVGSAIASG L142C, L188C,TREFSVAGVTTPVSTYMLTNSELLSLINDMPITNDQKK LMSNNVQIVRQQSYSIMSIIKEEVLAYWQLPLYGVIDTPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNK NRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVKQEGKMELLILKANAITTILTAVTFCFASGQNITEEFYQSTCSAV SKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIAVVSLSNGVSVLTSKVLDLKNYIDKQLLPIVNKQSCSIS NIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSN KGVDTVSVGNTLYYVNKQEGKSLYVKGEPIINFYDPL

[0176] In some embodiments, the ectodomain comprises any of the stabilizing mutations of RSV F protein disclosed in U.S. Patent No. 9,950,058, U.S. Patent No. 8,563,002, U.S. Patent No. 11,261,239, U.S. Patent No.11,629,181, and U.S. Patent No.11,655,284, each of which is hereby incorporated by reference in its entirety. Furin cleavage site

[0177] RSV F proteins are cleaved during expression by the protease furin. Constructs that replace the cleavage site for furin with a glycine-serine linker are provided herein. Sequences are provided in Table 5A. In some embodiments, RSV F protein ectodomain comprises an uncleaved furin cleavage site. Table 5A. Furin cleavage linkers Sequence Length SEQ ID NO: NNQARGSGSGRSLGF15 639

[0178] In some embodiments, the recombinant polypeptide and a protein nanostructure may be genetically fused such that they are both present in a single polypeptide, termed a “fusion protein.” The linkage between the polypeptide and the protein nanostructure allows the recombinant polypeptide to be displayed on the exterior of the self-assembling protein nanostructure.

[0179] A wide variety of polypeptide sequences can be used to link the proteins, or antigenic fragments thereof and the protein nanostructure. In some cases the linker comprises a polypeptide sequence that can be included in the encoding polynucleotide sequence. Any suitable linker polypeptide can be used. In some embodiments, the linker imposes a rigid relative orientation of the antigenic protein (e.g., ectodomain from the RSV Fusion protein) or antigenic fragment thereof to the protein nanostructure. In some embodiments, the linker flexibly links the antigenic protein (e.g., ectodomain from the RSV Fusion protein) or antigenic fragment thereof to the protein nanostructure. In some embodiments, the encoded polypeptides can include a linker betweenregions. In some embodiments, the polypeptide is a fusion protein which includes the recombinant RSV polypeptide, a linker, and the protein nanostructure component polypeptide. In some embodiments, the polypeptide is a fusion protein, which includes, in N- to C-terminal order, the recombinant RSV polypeptide, a linker, and the protein nanostructure component polypeptide. The linker can be a polypeptide. A wide variety of polypeptide sequences can be used and are well known in the art. In some embodiments, the linker may comprise a Gly-Ser linker (i.e., a linker consisting of glycine and serine residues) of any suitable length. In some embodiments, the Gly- Ser linker may be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more amino acid residues in length. Non-limiting examples of Glys-Ser linkers are presented in Table 5B. Table 5B. Sequence Length SEQ ID NO: GSS 3 646 [] n some em o mens, e n er comprses e ween an am no ac res ues. n some embodiments, the linker comprises between 4 and 24 amino acid residues. In some embodiments, the linker comprises between 8 and 24 amino acid residues. In some embodiments, the linker comprises between 10 and 24 amino acid residues. In some embodiments, the linker comprises between 12 and 24 amino acid residues. In some embodiments, the linker comprises between 16 and 24 amino acid residues. In some embodiments, the linker comprises between 18 and 24 aminoacid residues. In some embodiments, the linker comprises between 20 and 24 amino acid residues. In some embodiments, the linker comprises between 4 and 20 amino acid residues. In some embodiments, the linker comprises between 8 and 20 amino acid residues. In some embodiments, the linker comprises between 10 and 20 amino acid residues. In some embodiments, the linker comprises between 12 and 20 amino acid residues. In some embodiments, the linker comprises between 16 and 20 amino acid residues. In some embodiments, the linker comprises between 8 and 18 amino acid residues. In some embodiments, the linker comprises between 12 and 16 amino acid residues. In some embodiments, the linker comprises 3 amino acid residues. In some embodiments, the linker comprises 4 amino acid residues. In some embodiments, the linker comprises 5 amino acid residues. In some embodiments, the linker comprises 6 amino acid residues. In some embodiments, the linker comprises 7 amino acid residues. In some embodiments, the linker comprises 8 amino acid residues. In some embodiments, the linker comprises 8 amino acid residues. In some embodiments, the linker comprises 10 amino acid residues. In some embodiments, the linker comprises 11 amino acid residues. In some embodiments, the linker comprises 12 amino acid residues. In some embodiments, the linker comprises 13 amino acid residues. In some embodiments, the linker comprises 14 amino acid residues. In some embodiments, the linker comprises 15 amino acid residues. In some embodiments, the linker comprises 16 amino acid residues. In some embodiments, the linker comprises 17 amino acid residues. In some embodiments, the linker comprises 18 amino acid residues. In some embodiments, the linker comprises 19 amino acid residues. In some embodiments, the linker comprises 20 amino acid residues. In some embodiments, the linker comprises 21 amino acid residues. In some embodiments, the linker comprises 22 amino acid residues. In some embodiments, the linker comprises 23 amino acid residues. In some embodiments, the linker comprises 24 amino acid residues. In some embodiments, the linker comprises 25 amino acid residues. In some embodiments, the linker comprises 26 amino acid residues. In some embodiments, the linker comprises 27 amino acid residues. In some embodiments, the linker comprises 28 amino acid residues. In some embodiments, the linker comprises 29 amino acid residues. In some embodiments, the linker comprises 30 amino acid residues.

[0181] In some embodiments, the encoded polypeptides can include a linker between regions. In some embodiments, the polypeptide is a fusion protein which includes the recombinant RSV polypeptide, a linker, a N-terminal extension linker, and the protein nanostructure componentpolypeptide. In some embodiments, the polypeptide is a fusion protein, which includes, in N- to C-terminal order, the recombinant RSV polypeptide, a linker, a N-terminal extension linker, and the protein nanostructure component polypeptide. In some embodiments, the N-terminal extension linker is I53-50A helical extension. In some embodiments, polypeptide sequence of N-terminal extension linker is EKAAKAEEAARK (SEQ ID NO: 665).

[0182] Trimerization Domains

[0183] In some embodiments, the polypeptide may comprise a trimerization domain, such as FoldOn or a GCN4 trimerization. In some embodiments, the linker sequence comprises a FoldOn, wherein the FoldOn sequence is GYIPEAPRDGQAYVRKDGEWVLLSTFL (SEQ ID NO: 1235).

[0184] In some embodiments, the polypeptide may comprise a trimerization domain, wherein the trimerization domain sequence is DKIEEILSKIYHIENEIARIKKLIGE (SEQ ID NO: 666) (GEN). In some embodiments, the polypeptide may comprise a trimerization domain, wherein the trimerization domain sequence is EKFHQIEKEFSEVEGRIQDLEK (SEQ ID NO: 667) (HA).

[0185] In some embodiments, the polypeptide may comprise a trimerization domain, wherein the trimerization domain sequence is EDKIEEILSKIYHIENEIARIKKLIGEA (Seq ID NO: 668) (coiled-coil isoleucine zipper).

[0186] In some embodiments, the polypeptide may comprise a trimerization domain, wherein the trimerization domain sequence is GSGYIPEAPRDGQAYVRKDGEWVLLSTFL (SEQ ID NO: 669) (bacteriophage T4 fibritin).

[0187] In some embodiments, a trimerization sequence is RMKQIEDKIEEILSKIYHIENEIARIKKLIGEA (SEQ ID NO: 670) (GCN4). In some embodiments, a trimerization domain is a GCN4 variant. In some embodiments, the GCN4 variant sequence is RMKQIEDKIEEILSKIYHIENEIARIKKLIGERGGR (SEQ ID NO: 671), RMKQIEDKIEEILSKIYHIENEIARIKKLIGNRTGGR (SEQ ID NO: 672), RMKQIEDKIENITSKIYHIENEIARIKKLIGNRTGGR (SEQ ID NO: 673), RMKQIEDKIEEILSKIYNITNEIARIKKLIGNRTGGR (SEQ ID NO: 674), or RMKQIEDKIENITSKIYNITNEIARIKKLIGNRTGGR (SEQ ID NO: 675).

[0188] Illustrative sequences comprising various RSV F protein ectodomains, a C-terminal alpha- helical segment, and FoldOn are shown in Table 5C. The signal peptide is underlined with italic.The underlined FoldOn sequence may be substituted with any one of the trimerization domains described herein or any one of the multimerization domains described in Table 11 to generate embodiments that comprise such other trimerization domains.

[0189] In some embodiments, the trimeric protein complex comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to sequences shown in Table 5C. In some embodiments, the trimeric protein complex can be used as a trimeric component of a protein nanostructure. The approximate region surrounding the p27 peptide is bold. In some embodiments, the p27 peptide may be removed from the RSV F protein ectodomain through furin-based cleavage during production of antigens in cell culture. The FoldOn sequence is bold / underlined. Table 5C. Sequence Mutations SEQ ID NO:KEGSNICLTRTDRGWYCDNAGSVSFFPQAETCKVQ SNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSI MSIIKEEVLAYWQLPLYGVIDTPCWKLHTSPLCTTNVVSLSNGVSVCTIKVLDLKNYIDKQLLPIVNKQSCSI P102A, I379V, SNIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYM M447VLPRFMNYTLNNAKKTNVTLSKKRKRRFLGFLLG Naturally occurring VGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAV substitutions:KLIKQELDKYKNAVTELQLLMQSTPATNNRARREL PRFMYTLNNAKKTVTLSKKRKRRFLGFLLGVGSPRFMYTLAKKTVTLSKKRKRRFLGFLLGVGSAIA SGVAVSBVLHLEGEVKICSALLSTNKAWSLSNGVSRRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGF LLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNMELLILKANAITTILTAVTFCFASGQNITEEFYQSTC Deletion of p27 SAVSKGYLSALRTGWYTSVITIELSNIKKNKCNGTD sequenceMELLILKANAITTILTAVTFCFASQNITEEFYQSTCS AVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAin of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises (a) a C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer, (b) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1, (c) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1, (d) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1, (e) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1, (f) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1, (g) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1 or (h) any combination of (a)-(g).

[0191] In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 30 residues.

[0192] In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 1 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

[0193] In some embodiments, the C-terminal helix-forming segment comprises (1) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with A, I, L, M, V, G, T; (2) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (3) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (4) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with K, Q, R, preferably A, V, T, I; (5) an amino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with A, I, L, M, V, F, W, Y, G, T, preferably A, I, L, M, V; (6) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with any amino acids, preferably D, E, K, N, Q, R, S, T, Y; (7) an amino acid substitution at position E511 relative to SEQ ID NO: 1, wherein E is substituted with any amino acids; (8) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with D, E, K, N, Q, R, S, T, Y, preferably A, I, L, M, V, F, W, Y, G, T; (9) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with any amino acids, preferably A, I, L, M, V, F, W, Y, G , more preferably D, E, K, N, Q, R, S, T, Y; (10) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y; (11) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with any amino acids except P, preferably A, I, L, M, V, F, W, Y, G; (12) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with A, I, L, M, V, F, W, Y, G, or T, S, K; (13) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; (14) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; (15) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; and / or (16) any combination of (1)-(15).

[0194] In some embodiments, the segment comprises (1) an amino acid substitution at position L503 relative to SEQ ID NO: 1, wherein F is substituted with Q, V, K, R, N, L, (2) an amino acid substitution at position A504 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably S, T, L, A, Q, K, E, Y, (3) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with I, V, N, T, L, (4) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably Q, N, K, R, V, S, (5) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably A, N, K, E, D, Q, (6) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with T, M, V, R, (7) an amino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with T, I, K, Q, M, E, V, S, (8) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with S, K, N, D, E, (9) an amino acid substitution at position E511 relative to SEQ ID NO: 1, wherein E is substituted with R, S, E, K, A, T, L, (10) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with V, N, T, L, (11) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with D, T, H, K, E, N, R, (12) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with A, N, E, S, V, K, T, D, (13) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with I, E, L, T, Q, (14) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with E, I, K, N, R, Q, (15) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with A, S, K, E, R, (16) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with K, S, Q, R, D, E, (17) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with V, L, I, (18) an amino acid substitution at position I520 relative to SEQ ID NO: 1, wherein G is substituted with K, Q, E, N, T, (19) an amino acid substitution at position P521 relative to SEQ ID NO: 1, wherein G is substituted with H, D, E, K, R, N, Q, (20) an amino acid substitution at position E522 relative to SEQ ID NO: 1, wherein G is substituted with L, R, I, V, (21) an amino acid substitution at position A523 relative to SEQ ID NO: 1, wherein G is substituted with E, V, L, K, R I, (22) an amino acid substitution at position P524 relative to SEQ ID NO: 1, wherein G is substituted with A, K, T, E, R, (23) an amino acid substitution at position R525 relative to SEQ ID NO: 1, wherein G is substituted with H, R, S, L, N, E, D, (24) an amino acid substitution at position D526 relativeto SEQ ID NO: 1, wherein G is substituted with I, L, V, R, (25) an amino acid substitution at position G527 relative to SEQ ID NO: 1, wherein G is substituted with E, K, Q, D, (26) an amino acid substitution at position Q528 relative to SEQ ID NO: 1, wherein G is substituted with D, K, S, R, A, (27) an amino acid substitution at position A529 relative to SEQ ID NO: 1, wherein G is substituted with T, L, (28) an amino acid substitution at position Y530 relative to SEQ ID NO: 1, wherein G is substituted with L, E, T, (29) an amino acid substitution at position V531 relative to SEQ ID NO: 1, wherein G is substituted with A, R, K, (30) an amino acid substitution at position R532 relative to SEQ ID NO: 1, wherein G is substituted with V, A, and / or (31) any combination of (1)-(30).

[0195] In some embodiments, the segment comprises a polypeptide sequence listed in Table 2B or Table 2C, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10) or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10).

[0196] In some embodiments, the ectodomain comprises one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1. In some embodiments, the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: E487R + K498A, E487R + K498E, E487K + K498E, D486A + E487R + K498A, D486Q + E487R + K498A, D486E + E487A + D489A + T400D, D486A + E487M + K498A, E487Q, D486S, F488W + D489A + T400D + E487R + K498A, F140W + D489A + T400D + E487R + K498A, Q494I + S485I + K399A + 487R + 498A, Q494M + S485I + K399A; D486A + 487M + 498A, Q494L + S485A + K399V + D486A + 487M + 498A, Q494M + S485A + K399V + D486A + 487M + 498A, Q494A + S485F + K399V + D486A + 487M + 498Y, D489A + T400D + E487R + K498A, or D489A + T400D. In some embodiments, the ectodomain comprises the amino acid substitutions D489A, T400D, E487R, and K498A.

[0197] In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A. In some embodiments, the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P.

[0198] In some embodiments, the polypeptide comprises, C-terminal to the ectodomain, a heterologous multimerization domain. In some embodiments, the multimerization domain is a trimerization domain. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53- 50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64). In some embodiments, the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L.

[0199] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 6, below, optionally lacking a p27 peptide shown in bold, and in which “X” refers to sites involving an added C-terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQELDKYKNA VTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFLLGVGSAIASGIA VCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYINNQLLPMLNRQSCRISNI ETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIV RQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDN AGSVSFFPQADTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITS LGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEP IINYYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 6).

[0200] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 7, below, optionally lacking a p27 peptide shown in bold, in which “X” refers to sites involving an added C- terminal helical segment and can be any amino acid: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQELDKYKNA VTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFLLGVGSAIASGVA VCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYIDKQLLPILNKQSCSISNI ETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIV RQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDN AGSVSFFPQAETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEP IINFYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 7).

[0201] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 8, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIKQELDKYKNA VTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKRRFLGFLLGVGSAIASGIA VCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTFRVLDLKNYINNQLLPMLNRQSCRISNI ETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIV RQQSYSIMCIIKEEVLAYVVQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDN AGSVSFFPQADTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITS LGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEP IINYYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 8).

[0202] In some embodiments, the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 9, below, optionally lacking a p27 peptide shown in bold: QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQELDKYKNA VTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKRRFLGFLLGVGSAIASGVA VCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYIDKQLLPILNKQSCSISNI ETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIV RQQSYSIMCIIKEEVLAYVVQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDN AGSVSFFPQAETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITS LGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGKSLYVKGEP IINFYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 9).

[0203] In some embodiments, the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1- 9.

[0204] In another aspect, the disclosure provides a trimeric protein complex comprising a polypeptide disclosed herein. In some embodiments, the thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (a)-(h). In some embodiments, the stability, assayed by storage at about 40°C, is increased compared to a trimeric protein complex lacking modifications (a)-(h). In someembodiments, the thermal stability is increased compared to a reference RSV F protein comprising amino acid substitutions consisting essentially of S155C, S290C, S190F, and V207L (DS-Cav1).

[0205] In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide disclosed herein. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments, the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the RSV fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1 and a C- terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0206] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, D489A, T400D, E487R, and K498A relative to SEQ ID NO: 1 and a C- terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the secondpentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0207] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and T249P relative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0208] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and D486A relative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64), and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

[0209] In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19. In some embodiments, the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

[0210] In another aspect, the disclosure provides a recombinant polypeptide, comprising an alpha- helical segment and a multimerization domain, wherein the segment comprises a polypeptide sequence listed in Table 25A or Table 25B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the polypeptide comprises a trimeric pathogen protein, N-terminally or C-terminally linked to the alpha-helical segment. In some embodiments, the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, polypeptide comprises, N-terminal to the segment, an antigen. Human metapneumovirus (hMPV)

[0211] hMPV is a negative-sense, single-stranded RNA virus causing upper and lower respiratory disease. hMPV shares substantial homology with respiratory syncytial virus (RSV) in its surface glycoproteins. F protein, existing as trimers, is a type I glycoprotein.

[0212] Illustrative sequences are shown in Table 6A. A native hMPV F protein sequence was used for design. The signal peptide is underlined with italic Table 6A SEQ :LKKTNEAVSTLGNGVRVLATAVRELKDFVSKNLTR AINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNL219K, AINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDN V231I, AGITPAISKDLMTDAELARAISNMPTSAGQIKLMLast 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 179. In some embodiments, the hMPV F protein ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 180. In some embodiments, the hMPV F protein ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 181. C-terminal helix-forming segment

[0214] Illustrative sequences of C-terminal alpha-helical segments are shown in Table 6B (Rosetta remodel). Residues 468-470 of the native hMPV F protein are included as ENS (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 6B. C-terminal Alpha-helical segments for hMPV (Rosetta remodel) Name SequenceRemodeledSEQ ID LhC-Term 11ENSEIIKQAIS8 192C-Term 12ENSDRSERAQK8193C-Term 50ENSEKIERAIKKASS12 231C-Term 51ENSEKIERAIKYALS12232C-Term 89ENSQILLDAMTNTERALRS16 270C-Term 90ENSDRLKKMLEKTSKMLKS16 271C-Term 128ENSERIKQAIKKVEETLKRLKS19 310C-Term 129ENSERLEKALQTLTKAMKKTLS19 311between 5 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 10 residues. In some embodiments, the C-terminal helix- forming segment comprises between 10 and about 25 residues. In some embodiments, the C- terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 15 residues.

[0216] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 6C. Table 6C. Possible substitutions at Positions 471-489 (Rosetta remodel) Position Preferred Illustrative substitutionsD475 Polar A, D, E, H, K, N, Q, R, S, T Q476 Hydrophobic A, D, E, H, I, K, L, M, N, Q, T, Vle 6D (RFdiffusion). Residues 469-471 of the native hMPV F protein are included as NSQ (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 6D. C-terminal Alpha-helical segments for hMPV (RFdiffusion) Name SequenceRemodeledSEQ ID Len thNO

[0218] In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 35 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 30 residues. In some embodiments, the C-terminal helix-forming segmentcomprises between 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 20 residues. In some embodiments, the C-terminal helix- forming segment comprises between 20 and about 25 residues. In some embodiments, the C- terminal helix-forming segment comprises between 25 and about 30 residues.

[0219] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 6E. Table 6E. Possible substitutions at Positions 472-498 (RFdiffusion) Position Preferred Illustrative substitutions A472 Polar T N K R E SQ498 Hydrophobic V, L prises an engineered ectodomainof an hMPV fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 104 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix- forming segment, between about residue 470 and about residue 490 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 7 and about 21 residues.

[0221] In some embodiments, the segment comprises (1) an amino acid substitution at position Q471 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, I, Q, R, S, T, (2) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, R, S, T, Y, (3) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of A, I, L, M, Q, S, T, W, (4) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of A, D, E, I, K, L, N, Q, S, T, (5) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of A, D, E, H, K, N, Q, R, S, T, (6) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, H, I, K, L, M, N, Q, T, V, (7) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, E, I, K, L, M, N, Q, R, S, T, V, (8) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of A, D, E, K, N, Q, R, S, T, (9) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substituted with any one of A, D, E, F, I, K, L, M, N, Q, R, S, T, W, Y, (10) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of A, I, L, M, R, S, T, V, (11) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any oneof D, E, I, K, L, M, N, Q, R, S, T, (12) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, K, Q, R, S, T, (13) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, F, H, I, K, L, M, N, Q, R, S, T, V, W, Y, (14) an amino acid substitution at position A484 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, L, M, R, S, T, V, Y, (15) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of D, E, G, K, L, Q, R, S, T, (16) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of A, E, I, K, L, Q, R, S, T, (17) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of A, E, I, K, L, R, S, T, V, (18) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, I, K, L, N, Q, R, S, (19) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of A, D, E, K, S, and / or (20) any combination of (1)-(19). In some embodiments, the segment comprises a polypeptide sequence listed in Table 6B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0222] In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 16 and about 30 residues.

[0223] In some embodiments, the segment comprises (1) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of T, N, K, R, E, S, (2) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, I, V, (3) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of E, Q, L, D, (4) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of E, D, K, (5) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of Q, R, D, A, T, S, K, (6) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of I, V, L, (7) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of K, E, N, S, (8) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substitutedwith any one of T, D, E, S, Y, A, (9) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of L, N, (10) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, D, E, K, Q, S, N, (11) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein S is substituted with any one of E, D, S, T, Q, K, (12) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of R, K, L, E, A, (13) an amino acid substitution at position A484 relative to SEQ ID NO: 104, wherein A is substituted with any one of V, I, M, (14) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of E, A, K, H, Q, S, N, (15) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of S, E, K, R, V, D, H, (16) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of I, L, (17) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, K, R, (18) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of K, E, S, R, Q, (19) an amino acid substitution at position S490 relative to SEQ ID NO: 104, wherein S is substituted with any one of E, V, T, R, L, (20) an amino acid substitution at position G491 relative to SEQ ID NO: 104, wherein G is substituted with any one of GL, I, V, (21) an amino acid substitution at position R492 relative to SEQ ID NO: 104, wherein R is substituted with any one of E, Q, S, A, D, (22) an amino acid substitution at position E493 relative to SEQ ID NO: 104, wherein E is substituted with any one of A, E, N, L, K, Q, S, (23) an amino acid substitution at position N494 relative to SEQ ID NO: 104, wherein N is substituted with any one of I, L, (24) an amino acid substitution at position L495 relative to SEQ ID NO: 104, wherein L is substituted with any one of K, L, T, V, I, (25) an amino acid substitution at position Y496 relative to SEQ ID NO: 104, wherein Y is substituted with any one of K, E, R, Q, (26) an amino acid substitution at position F497 relative to SEQ ID NO: 104, wherein F is substituted with any one of D, R, E, Q, (27) an amino acid substitution at position Q498 relative to SEQ ID NO: 104, wherein Q is substituted with any one of V, L, and / or (28) any combination of (1)-(27). In some embodiments, the segment comprises a polypeptide sequence listed in Table 6D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0224] In some embodiments, the ectodomain further comprises one, two, three or more amino acid substitutions at positions 63, 97, 98, 99, 100, 101, 102, 140, 147, 153, 185, 188, 219, 231, 294, 365, 368, 450, 463, or 470 relative to SEQ ID NO: 104. Human Parainfluenza virus type 3 (PIV3) and type 5 (PIV5)

[0225] PIV is a negative-sense, single-stranded RNA virus which causes a variety of respiratory illnesses. It is a major cause of ubiquitous acute respiratory infections of infancy and early childhood. PIV F protein facilitates viral fusion and cell entry.

[0226] Illustrative sequences of a native PIV3 F protein are shown in Table 7A. Table 7A SEQ Description Sequence ID[ ] n some em o ments, t e ecto oma n s at east , at east , at east , at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 327.C-terminal helix-forming segment

[0228] Illustrative sequences of C-terminal alpha-helical segments are shown in Table 7B (Rosetta remodel). Residues 456-459 of the native PIV3 F protein are included as ISIE (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 7B. C-terminal Alpha-helical segments for PIV3 (Rosetta remodel) Name SequenceRemodeledSEQ ID LengthNO:20 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 25 and about 30 residues.

[0230] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 7C. Table 7C. Possible substitutions at Positions 460-477 (Rosetta remodel) Position Preferred Illustrative substitutionsS465 Polar K, S D466 Polar E, Kre shown in Table 7D (RFdiffusion). Residues 456-464 of the native MPV F protein are included as ISIELNKAK (bold underline) (alternatively, ISIELNKVK) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 7D. C-terminal Alpha-helical segments for PIV3 (RFdiffusion) NameSequenceRemodeledSEQ LengthID NO:C-Term 17ISIELNKVKERLHKLEESVKQLKKA16 354C-Term 18ISIELNKVKSDVENLKEKINKII14 355, g g p 10 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 15 residues. In some embodiments, the C-terminal helix- forming segment comprises between 15 and about 30 residues. In some embodiments, the C- terminal helix-forming segment comprises between 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 30residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 25 and about 30 residues.

[0233] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 7E. Table 7E. Possible substitutions at Positions 465-486 (RFdiffusion) Position Preferred Illustrative substitutions S465 Polar E, K, D, S, N, Q, T, R, A

[0003] n some embod ments, t e recomb nant po ypept de compr ses an engineered ectodomain of a PIV3 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 327, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprisessubstitutions relative to the reference sequence SEQ ID NO: 327 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 20 and about 28 residues.

[0235] In some embodiments, the segment comprises (1) an amino acid substitution at position L460 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, M, V, (2) an amino acid substitution at position N461 relative to SEQ ID NO: 327, wherein N is substituted with N, (3) an amino acid substitution at position K462 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R, (4) an amino acid substitution at position V463 relative to SEQ ID NO: 327, wherein V is substituted with any one of L, V, T, (5) an amino acid substitution at position K464 relative to SEQ ID NO: 327, wherein K is substituted with any one of A, K, Q, (6) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, S, (7) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of E, K, (8) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of V, L, T, (9) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, D, E, (10) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of T, Q, K, E, (11) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L, M, Y, F, W, (12) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of L, W, A, I, (13) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, E, (14) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of E, I, K, Q, (15) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of L, M, T, V, E, (16) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of S, K, R, A, (17) an amino acid substitution at position R476 relative to SEQ ID NO: 327, wherein R is substituted with any one of K, E, S, N, (18) anamino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, D, E, and / or (19) any combination of (1)-(18).

[0236] In some embodiments, the segment comprises a polypeptide sequence listed in Table 7B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 465 and about residue 490 relative to SEQ ID NO: 327, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 14 and about 30 residues.

[0237] In some embodiments, the segment comprises (1) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of E, K, D, S, N, Q, T, R, A, (2) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of D, R, K, E, M, Q, A, S, N, (3) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of I, V, L, (4) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, S, D, R, H, T, N, A, (5) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, S, E, N, T, Q, H, D, Y, (6) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of L, D, V, I, A, N, T, (7) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of E, T, L, K, N, I, R, Q, S, (8) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, Q, S, H, R, T, (9) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of R, Q, K, E, T, S, I, N, (10) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of V, L, I, Q, T, (11) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of H, K, D, T, E, S, R, N, Q, A, (12) an amino acid substitution at position R476 relative to SEQ ID NO: 327, wherein R is substituted with any one of A, T, H, E, D, K, R, Q, S, (13) an amino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L,V, (14) an amino acid substitution at position N478 relative to SEQ ID NO: 327, wherein N is substituted with any one of E, L, K, I, R, S, Q, (15) an amino acid substitution at position Q479 relative to SEQ ID NO: 327, wherein Q is substituted with any oneof K, H, E, N, Q, R, T, A, S, (16) an amino acid substitution at position K480 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R, E, T, S, L, A, I, V, (17) an amino acid substitution at position L481 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, V, I, (18) an amino acid substitution at position D482 relative to SEQ ID NO: 327, wherein D is substituted with any one of K, A, E, S, H, T, N, D, R, (19) an amino acid substitution at position S483 relative to SEQ ID NO: 327, wherein S is substituted with any one of Q, T, E, A, S, N, D, K, L, (20) an amino acid substitution at position I484 relative to SEQ ID NO: 327, wherein I is substituted with any one of I, L, A, V, (21) an amino acid substitution at position G485 relative to SEQ ID NO: 327, wherein G is substituted with any one of L, K, R, E, I, (22) an amino acid substitution at position S486 relative to SEQ ID NO: 327, wherein S is substituted with any one of T, A, E, R, H, D, S, and / or (23) any combination of (1)-(22). In some embodiments, the segment comprises a polypeptide sequence listed in Table 7D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0238] Illustrative sequences of a native PIV5 F protein are shown in Table 8A. Table 8A SEQ Descri tion Se uence ID

[0239] In some embodiments, the PIV5 protein ectodomain is at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 382. C-terminal helix-forming segment

[0240] Illustrative sequences of C-terminal alpha-helical segments are shown in Table 8B (Rosetta remodel). Residues 459-462 of the native PIV5 F protein are included as SLSD (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 8B. C-terminal Alpha-helical segments for PIV5 (Rosetta remodel) Name SequenceRemodeledSEQ ID LengthNO:C-Term 26SLSDVIKEIRKLKS10 408C-Term 27SLSDLAKIVKSLIS10 409C-Term 65SLSDLQTDIKRLKS10 447C-Term 66SLSDLAKKTKELEKKIKS14 4485 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 5 and about 15 residues. In some embodiments, the C-terminal helix- forming segment comprises between 5 and about 10 residues. In some embodiments, the C- terminal helix-forming segment comprises between 10 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 20 residues.

[0242] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 8C. Table 8C. Possible substitutions at Positions 463-488 (Rosetta remodel) Position Preferred Illustrative substitutionsL464 Polar K, I, Q, A, W, E Q465 Polar K, Q, T, E, S, R, p yp p p g omain of a PIV5 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 382, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 382 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stablealpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 6 and about 26 residues.

[0244] In some embodiments, the segment comprises (1) an amino acid substitution at position A463 relative to SEQ ID NO: 382, wherein A is substituted with any one of L, T, V, A, (2) an amino acid substitution at position L464 relative to SEQ ID NO: 382, wherein L is substituted with any one of K, I, Q, A, W, E, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 382, wherein Q is substituted with any one of K, Q, T, E, S, R, (4) an amino acid substitution at position H466 relative to SEQ ID NO: 382, wherein H is substituted with any one of K, A, E, L, I, W, R, Q, T, D, Y, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 382, wherein L is substituted with any one of V, I, L, M, FA, T, C, H, (6) an amino acid substitution at position A468 relative to SEQ ID NO: 382, wherein A is substituted with any one of D, T, K, L, E, R, I, N, S, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 382, wherein Q is substituted with any one of E, K, S, T, A, R, Q, D, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 382, wherein S is substituted with any one of A, K, L, I, T, S, V, H, Y, E, W, FR, Q, M, (9) an amino acid substitution at position D471 relative to SEQ ID NO: 382, wherein D is substituted with any one of T, E, V, L, S, I, A, K, Y, W, (10) an amino acid substitution at position T472 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, E, R, S, T, A, D, L, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 382, wherein Y is substituted with any one of T, K, S, R, Q, D, E, I, H, M, (12) an amino acid substitution at position L474 relative to SEQ ID NO: 382, wherein L is substituted with any one of T, S, L, A, D, W, Q, I, Y, V, K, E, (13) an amino acid substitution at position S475 relative to SEQ ID NO: 382, wherein S is substituted with any one of T, E, I, K, S, Q, A, L, R, D, (14) an amino acid substitution at position A476 relative to SEQ ID NO: 382, wherein A is substituted with any one of R, K, A, S, E, I, T, D, Q, (15) an amino acid substitution at position I477 relative to SEQ ID NO: 382, wherein I is substituted with any one of K, Q, R, D, T, E, I, Y, S, L, (16) an amino acid substitution at position T478 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, K, S, D, W, L, Q, I, T, (17) an amino acid substitution at position S479 relative to SEQ ID NO: 382, wherein S is substituted with any one of R, K, Q, S, A, D, E, (18) an amino acid substitution at position A480 relative to SEQ ID NO: 382, wherein A is substituted with any one of S, K, (19) an amino acid substitution at position T481 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, D, S, K, M, N, A, T, (20) an amino acid substitution at positionT482 relative to SEQ ID NO: 382, wherein T is substituted with any one of R, S, Q, L, K, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, A, S, (22) an amino acid substitution at position S484 relative to SEQ ID NO: 382, wherein S is substituted with any one of S, E, D, Y, (23) an amino acid substitution at position V485 relative to SEQ ID NO: 382, wherein V is substituted with any one of Q, T, (24) an amino acid substitution at position L486 relative to SEQ ID NO: 382, wherein L is substituted with K, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 382, wherein S is substituted with any one of S, K, (26) an amino acid substitution at position I488 relative to SEQ ID NO: 382, wherein I is substituted with any one of S, K, and / or (27) any combination of (1)- (26).

[0245] In some embodiments, the segment comprises a polypeptide sequence listed in Table 8B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. SARS-CoV-2

[0246] SARS-CoV-2 is a single, positive-strand RNA virus which can cause severe respiratory disease in humans. The SARS CoV-2 viral spike (S) protein, which is a homotrimeric class I fusion glycoprotein, binds to angiotensin-converting enzyme 2 (ACE2), which is the entry receptor utilized by SARS-CoV-2. The spike (S) protein of coronaviruses is a major surface protein and is a target for neutralizing antibodies in infected subjects or patients. Therefore, it is considered a potential protective antigen for vaccine design. Table 9A SEQ :KLNDLCFTNVYADSFVIRGDEVRQIAPGQTGKIAD YNYKLPDDFTGCVIAWNSNNLDSKVGGNYNYLYRL, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 459. C-terminal helix-forming segment

[0248] Illustrative sequences of C-terminal alpha-helical segments are shown in Table 9B (Rosetta remodel). Residues 1147-1170 of the native SARS-CoV-2 S protein are included as LQPEL (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 9B. C-terminal Alpha-helical segments for SARS (Rosetta remodel) Name SequenceRemodeledSEQ IDC-Term 3LQPELETAASIAEKIARKLLKES18 462C-Term 4LQPELESAIKKTLKIISKRNKDS18 46310 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 25 residues. In some embodiments, the C-terminal helix- forming segment comprises between 15 and about 20 residues.

[0250] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 9C. Numbering in this table reflects a single amino acid substitution relative to the reference sequence above. Table 9C. Possible substitutions at Positions 1147-1170 (Rosetta remodel) Position Preferred Illustrative substitutionsK1150 Hydrophobic I, A, L, M E1151 Polar K, S, D, R, En in Table 9D (RFdiffusion). Residues 1147-1165 of the native SARS-CoV-2 Spike (S) protein are included as LQPEL (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 9D. C-terminal Alpha-helical segments for SARS (RFdiffusion) NameSequence RemodeledSEQ IDC-Term 9LQPELNTTKTETISSIKKEIETM18 489C-Term 10LQPELEATHTRTLTTVTAA14 49010 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 25 residues.

[0253] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 9E. Table 9E. Possible substitutions at Positions 1147-1165 (RFdiffusion) Position Preferred Illustrative substitutionsP1163 Polar E, S, T, R D1164 Hydrophobic T, M, A protein,wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprises one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 459 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 10 and about 25 residues.

[0255] In some embodiments, the segment comprises (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of E, D, K, (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S is substituted with any one of T, S, K, (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with A, (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any one of I, A, L, M, (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of K, S, D, R, E, (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of I, Y, K, T, R, E, (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of T, A, (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of L, I, E, T, M, V, (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of E, K, T, R, (10) an amino acid substitution at position Y1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of I, V, K, R, (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of V, A, I, Y, T, S, (12) an amino acid substitution at position K1158 relative to SEQID NO: 459, wherein K is substituted with any one of L, R, S, K, D, W, N, I, (13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of K, T, Q, I, R, E, (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of I, L, R, E, K, S, (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, N, I, A, S, W, Y, (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of K, S, T, R, (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, D, R, K, I, A, (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of W, S, M, D, T, I, N, (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of E, A, K, L, (20) an amino acid substitution at position D1166 relative to SEQ ID NO: 459, wherein D is substituted with any one of K, S, (21) an amino acid substitution at position L1167 relative to SEQ ID NO: 459, wherein L is substituted with any one of R, K, (22) an amino acid substitution at position G1168 relative to SEQ ID NO: 459, wherein G is substituted with any one of K, S, (23) an amino acid substitution at position D1169 relative to SEQ ID NO: 459, wherein D is substituted with S, (24) an amino acid substitution at position I1170 relative to SEQ ID NO: 459, wherein I is substituted with S, and / or (25) any combination of (1)-(24).

[0256] In some embodiments, the segment comprises a polypeptide sequence listed in Table 9B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 1145 and about residue 1175 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 12 and about 22 residues.

[0257] In some embodiments, the segment comprises (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of Q, T, E, S, N, D, K, (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S is substituted with any one of T, K, N, R, S, A, E, (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with any one of L, T, I, V, (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any oneof K, Q, R, H, S, E, (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, N, A, S, K, T, D, (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, T, V, R, K, N, (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of S, V, T, A, (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, L, E, I, V, (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of H, E, S, T, Q, (10) an amino acid substitution at position Y1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of L, E, I, T, A, S, R, (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of T, V, I, A, S, M, (12) an amino acid substitution at position K1158 relative to SEQ ID NO: 459, wherein K is substituted with any one of K, E, N, R, T, A, Q, I, (13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of T, E, A, K, Q, (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of L, M, A, E, T, Y, I, S, (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, I, (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of S, R, K, N, E, Q, (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, S, T, R, (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, M, A, (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of A, L, and / or (20) any combination of (1)-(19).

[0258] In some embodiments, the segment comprises a polypeptide sequence listed in Table 9D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. Nipah virus

[0259] Nipah virus is a highly pathogenic virus, which has caused sporadic outbreaks of severe neurological and respiratory disease.Table 10A SEQ Description Sequence IDat least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 499. C-terminal helix-forming segment

[0261] Illustrative sequences of C-terminal alpha-helical segments are shown in Table 10B (Rosetta remodel). Residues 460-462 of the native Nipah F protein are included as ISS (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 10B. C-terminal Alpha-helical segments for Nipah (Rosetta remodel) Name SequenceRemodeledSEQ IDC-Term 5ISSTNKRTATIEKIVRSLLKEIKSERTR25 504C-Term 6ISSINETVTRLKKIVEKLIRELQKIK23 505nd about 35 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 20 residues. In some embodiments, the C-terminal helix- forming segment comprises between 20 and about 35 residues. In some embodiments, the C- terminal helix-forming segment comprises between 20 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 25 and about 35 residues. In some embodiments, the C-terminal helix-forming segment comprises between 25 and about 30 residues.

[0263] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 10C.Table 10C. Possible substitutions at Positions 463-489 (Rosetta remodel) Position Preferred Illustrative substitutions M463 Hydrophobic I, A, T, L, Mus a ve seque ces o -e a ap a-e ca seg e s ae s ow a e 10D (RFdiffusion). Residues 460-462 of the native Nipah F protein are included as ISS (bold underline) and are, in these embodiments, conserved with the native sequence whereas many of the other amino acid residues are modified. Table 10D. C-terminal Alpha-helical segments for Nipah (RFdiffusion) Name Se uenceRemodeledSEQ IDC-Term 1ISSLRQKISSLEKALKKAEKDLEEVRRQL26 522C-Term 2ISSLTTEVKQLQTSL12 523

[0065] n some embod ments, t e C-term na e x- orm ng segment compr ses between 10 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 10 and about 15 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 30 residues. In some embodiments, the C- terminal helix-forming segment comprises between 15 and about 25 residues. In some embodiments, the C-terminal helix-forming segment comprises between 15 and about 20 residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 30 residues. In some embodiments, the C-terminal helix-forming segment comprises between 20 and about 25 residues.

[0266] In some embodiments, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Illustrative sequences of possible substitutions are shown in Table 10E. Table 10E. Possible substitutions at Positions 463-489 (RFdiffusion) Position Preferred Illustrative substitutions M463 H dro hobic L I VL488 Hydrophobic L gineered ectodomainof a Nipah fusion (F) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 499 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 16 and about 33 residues.

[0268] In some embodiments, the segment comprises (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of I, A, T, L, M, (2) an amino acid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, K, T, S, I, D, (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of M, L, I, V, T, (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, K, A, T, S, D, R, I, Q, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of R, S, K, T, E, Q, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of T, L, I, V, A, (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, A, W, E, L, I, (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, R, Q, E, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of W, D, K, Y, I, M, E, T, (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of I, V, M, L, A, (13) an aminoacid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of T, K, M, R, E, L, A, S, (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, S, A, E, T, D, (15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of L, I, V, FT, A, M, W, K, Y, (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of I, K, A, L, E, D, S, Y, (17) an amino acid substitution at position R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of A, S, K, R, T, L, E, (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of K, E, R, Y, T, Q, (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of W, I, V, L, E, S, Q, A, T, (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, Q, E, W, T, S, A, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of S, T, K, R, Q, (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of R, E, I, S, Y, L, K, D, (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of I, R, K, W, E, T, (24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of A, T, R, K, Q, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of K, R, T, S, (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of E, V, L, K, (27) an amino acid substitution at position I489 relative to SEQ ID NO: 499, wherein I is substituted with any one of E, Q, L, K, R, and / or (28) any combination of (1)-(27).

[0269] In some embodiments, the segment comprises a polypeptide sequence listed in Table 10B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the C-terminal helix-forming segment comprises between about 12 and about 26 residues.

[0270] In some embodiments, the segment comprises (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of L, I, V, (2) an aminoacid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N, (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of Q, T, N, D, E, S, K, R, A, (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S, (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of I, V, L, A, (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q is substituted with any one of S, K, T, D, E, R, Q, (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of S, Q, N, E, A, D, K, T, R, (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of L, A, I, V, N, (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of E, Q, S, R, K, A, T, D, L, N, (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, E, Q, D, N, S, A, (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of A, S, R, T, V, E, I, K, L, D, Q, (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of L, I, V, (13) an amino acid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, H, D, T, A, S, R, Q, E, N, (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, R, S, E, A, T, H, D, (15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of A, L, I, V, (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, T, R, K, Q, S, I, (17) an amino acid substitution at position R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of K, N, E, Q, S, T, H, R, A, (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of D, L, E, K, T, R, V, I, Q, (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of L, V, I, (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of E, K, N, D, L, Q, H, (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of E, K, S, Q, A, T, (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of V, L, I, (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of R, K, L, V, E, Q, I,(24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of R, E, A, S, L, (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of Q, R, T, S, L, (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of L, and / or (27) any combination of (1)-(26).

[0271] In some embodiments, the segment comprises a polypeptide sequence listed in Table 10D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. III. Protein nanostructures

[0272] The disclosure further provides protein nanostructures comprising any of the engineered ectodomains described herein. For example, the disclosure provides protein nanostructures comprising a trimeric component comprising a recombinant polypeptide comprising an ectodomain of a viral membrane fusion (F) protein of Respiratory Syncytial Virus (RSV) having an engineered C-terminal alpha-helical segment that stabilizes the F protein in a prefusion conformation and pentameric component.

[0273] Further provided are compositions in which any of the alpha-helical segments described herein are used as a fusion to a trimeric protein complex or to a trimeric component of a nanostructure to stabilize the complex orcomponent. For example, the alpha-helical segments described herein may be used without any antigen (e.g., ectodomain) or with an antigen or other molecule attached to the complex or nanostructure by other means, such as bioconjugate chemistry. In some embodiments, the alpha-helical segments described herein are used as fusion proteins to monomeric antigens, including but not limited to the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein.

[0274] The protein nanostructures of the present invention may comprise multimeric protein assemblies adapted for display of molecules such as antigens (e.g., engineered ectodomains). The protein nanostructures, in some embodiments described herein, comprise at least a first component displaying an engineered ectodomain and, optionally, a second component. The engineered ectodomain may include one or more amino acid substitutions, a C-terminal helix-forming segment, or a combination thereof. The first component may comprise or consist of three copies of a fusion protein. In some embodiments, the fusion protein comprises an assembly domain having a protein sequence designed by computational methods to assemble to form ananostructure. In some embodiments, the first component is a trimeric component in which the assembly domains form trimers related by 3-fold rotational symmetry, and / or the second component is a pentameric component, in which the assembly domains form pentamers related by 5-fold rotational symmetry. In some embodiments, the combination of the two components form an “icosahedral particle” having I53 symmetry. Together these components may be arranged such that the members of each component are related to one another by symmetry operators. A general computational method for designing self-assembling protein materials, involving symmetrical docking of protein building blocks in a target symmetric architecture, is disclosed in Patent Pub. No. US 2015 / 0356240 A1.

[0275] The “core” of the protein nanostructure is used herein to describe the central portion of the protein nanostructure. For clarity, the term “core” as used herein excludes molecules displayed by the nanostructure. The core may serve to assemble multiple copies of the displayed molecule, such as an antigen (e.g., an engineered ectodomain). Without being bound by theory, this may increase the immunogenicity of an antigen. The disclosure envisions nanostructures in which the core is either non-covalently associated with the displayed antigen; covalently linked to the display antigen (such as by chemical conjugation); or, in preferred embodiments, linked to the displayed antigen through a polypeptide linker in a fusion protein. In some embodiments, the fusion protein comprises a first polypeptide comprising an antigen (e.g., an ectodomain), and a first assembly domain. In some embodiments, an antigen (e.g., an ectodomain) is non-covalently or covalently linked to the assembly domain. For example, an antigen (e.g., an ectodomain) may be fused to the first component and configured to bind a portion of the first component, or a chemical tag on the first component. For example, a streptavidin-biotin (or neutravidin-biotin) linker can be employed. Alternatively, various bioconjugate linkers may be used. In some embodiments of the present disclosure, the antigen comprises further polypeptide sequences in addition to RSV F protein.

[0276] In some embodiments, three copies of an antigen (e.g., an ectodomain) polypeptide are displayed on a 3-fold axis. Thus, the protein nanostructure is capable of displaying 60 monomeric antigen (e.g., an ectodomain) polypeptides. In some embodiments, the protein nanostructure is adapted for display of up to 12, 24, or 60 monomers. In some embodiments, a component may comprise a polypeptide linked to diverse engineered ectodomains, such that the protein nanostructure displays different ectodomains on the same nanostructure. In some embodiments, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,or more different ectodomains are displayed. Non-limiting illustrative protein nanostructure are provided in Bale et al. Science 353:389-94 (2016); Heinze et al. J. Phys. Chem B.120:5945-5952 (2016); King et al. Nature 510:103-108 (2014); and King et al. Science 336:1171-71 (2012). Attachment Modalities

[0277] The protein nanostructures of the present disclosure display antigenic proteins in various ways including as gene fusion or by other means disclosed herein. As used herein, “linked to” or “attached to” denotes any means known in the art for causing two polypeptides to associate. The association may be direct or indirect, reversible or irreversible, weak or strong, covalent or non- covalent, and selective or nonselective.

[0278] In some embodiments, attachment is achieved by genetic engineering to create an N- or C- terminal fusion of potentially antigenic polypeptides of the protein nanostructure.

[0279] In some embodiments, attachment is achieved by post-translational covalent attachment of one or more pluralities of antigenic protein. In some embodiments, chemical cross-linking is used to non-specifically attach the antigen to a protein nanostructure. In some embodiments, chemical cross-linking is used to specifically attach the antigenic protein to a protein nanostructure (e.g., to the first polypeptide or the second polypeptide). Various specific and non-specific cross-linking chemistries are known in the art, such as Click chemistry and other methods. In general, any cross- linking chemistry / bioconjugate used to link two proteins may be adapted for use in the presently disclosed protein nanostructures. In particular, chemistries used in creation of immunoconjugates or antibody drug conjugates may be used. In some embodiments, a protein nanostructure is created using a cleavable or non-cleavable linker. Processes and methods for conjugation of antigens to carriers are provided by, e.g., Patent Pub. No. US 2008 / 0145373 A1.

[0280] The protein nanostructures may employ a variety of coupling techniques to attach an antigen to the core, including but not limited to the SpyCatcher system described in, e.g., Escolano et al. Nature 570:468-473 (2019), He et al. Sci Adv. 7(12):eabf1591 (2021), and Tan et al. Nat. Commun.12(1):542 (2021).

[0281] In some embodiments, attachment is achieved by non-covalent attachment between a component and the ectodomain. In some embodiments the ectodomain is engineered to be negatively charged on at least one surface and the core polypeptide is engineered to be positivelycharged on at least one surface, or positively and negatively charged, respectively. This can promote intermolecular association between the ectodomain and the component core polypeptide by electrostatic force. In some embodiments, shape complementarity is employed to cause linkage of ectodomain to component core. Shape complementarity can be pre-existing or rationally designed. In some embodiments, computational design of protein-protein interfaces is used to achieve attachment.

[0282] In another aspect, the disclosure provides a trimeric protein complex comprising a polypeptide disclosed herein. In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide disclosed herein.

[0283] In some embodiments, the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component. In some embodiments, the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a human Metapneumovirus (hMPV) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a human Parainfluenza virus type 3 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a human Parainfluenza virus type 5 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a SARS-CoV-2 spike (S) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises an engineered ectodomain of a Nipaha virus fusion (F) polypeptide and an I53-50A polypeptide. In some embodiments the first, trimeric component comprises a fusion protein comprising, in N- to C- terminal order, the engineered fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the engineered spike (S) polypeptide, an amino acid linker, and the I53-50A polypeptide. In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences. In someembodiments the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74. Polypeptide Sequences

[0284] Patent Pub No. US 2015 / 0356240 A1 describes various methods for designing protein assemblies. As described in US Patent Pub No. US 2016 / 0122392 A1 and in International Patent Pub. No. WO 2014 / 124301 A1, the isolated polypeptides of SEQ ID NOs:13-63 were designed for their ability to self-assemble in pairs to form protein nanostructures, such as icosahedral particles. The design involved design of suitable interface residues for each member of the polypeptide pair that can be assembled to form the protein nanostructures. The protein nanostructures so formed include symmetrically repeated, non-natural, non-covalent polypeptide- polypeptide interfaces that orient a first assembly domain and a second assembly domain into protein nanostructures, such as one with an icosahedral symmetry. Thus, in one embodiment a first assembly domain and second assembly domain of the component are selected from the group consisting of SEQ ID NOs:13-63. In each case, an N-terminal methionine residue present in the full length protein is included, but may be removed to make a fusion that is not included in the sequence. The identified residues in Table 11 are numbered beginning with an N-terminal methionine (not shown). In various embodiments, one or more additional residues are deleted from the N-terminus and / or additional residues are added to the N-terminus (e.g., to form a helical extension). Table 11. ComponentIdentified A i A i i f e , 8 2 , 9 ,Identified NameCMomulptiomneerntAmino Acid Sequenceinterface id 1 , 9 , 1 , 2 , , , 1 , , , 1 2 1 3 , , 6 , 2 1 ,CMomulptiomneerntIdentified Name Amino Acid Sequenceinterface id 1 , 1 3 , , , , , , 7 , 1 3 , , , 1 2Identified NameCMomulptiomneerntAmino Acid Sequenceinterface id 2 9 , 1 5 , , 1 2 , 1 4 8 , 9 , 1 , 2 , , , , , 1 ,Identified NameCMomulptiomneerntAmino Acid Sequenceinterface id , 1 , , , , , 1 2 2 , , 1 2 2 , , 1 2 2 ,CMomulptioIdentified Name mneerntAmino Acid Sequenceinterface idCMomulptioIdentified Name mneerntAmino Acid Sequenceinterface idCMomulptioIdentified Name mneerntAmino Acid Sequenceinterface idCMomulptioIdentified Name mneerntAmino Acid Sequenceinterface idCMomulptioIdentified Name mneerntAmino Acid Sequenceinterface idCMomulpIdentified Name tiomneerntAmino Acid Sequenceinterface idbly domain of embodiments of the present disclosure. In each case, the pairs of sequences together form an I53 multimer with icosahedral symmetry. The right hand column in Table 11 identifies the residue numbers in each illustrative polypeptide that were identified as present at the interface of resulting assembled protein nanostructures (i.e., “identified interface residues”). As can be seen, the number of interface residues for the illustrative polypeptides of SEQ ID NO:13-46 range from 4-13. In various embodiments, a first assembly domain and second assembly domain comprise an amino acid sequence that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical over its length, and identical at least at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13 identified interface positions (depending on the number of interface residues for a given polypeptide), to the amino acid sequence of a polypeptide selected from the group consisting of SEQ ID NOs: 13-46. SEQ ID NOs: 47-63 represent other amino acid sequences of a first assembly domain and second assembly domain from embodiments of the present disclosure. In other embodiments, a first assembly domain and / or second assembly domain comprise an amino acid sequence that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical over its length, and identical at least at 20%, 25%, 33%, 40%, 50%, 60%, 70%, 75%, 80%, 90%, or 100% of the identified interface positions, to the amino acid sequence of a polypeptide selected from the group consisting of SEQ ID NOs:13-63.

[0286] As is the case with proteins in general, the polypeptides are expected to tolerate some variation in the designed sequences without disrupting subsequent assembly into protein nanostructures: particularly when such variation comprises conservative amino acid substitutions. As used herein, “conservative amino acid substitution” means that: hydrophobic amino acids (Ala,Gly, Met, Val, Ile, Leu, Thr) are substituted with other hydrophobic amino acids; hydrophobic amino acids with bulky side chains (Phe, Tyr, Trp) are substituted with other hydrophobic aminoacids with bulky side chains; polar amino acids (Asp, Glu, Lys, Arg, Ser, Thr, Asn, Gly Tyr) are substituted with other polar amino acids; amino acids with positively charged side chains (Arg, His, Lys) are substituted with other amino acids with positively charged side chains; and amino acids with negatively charged side chains (Asp, Glu) are substituted with other amino acids with negatively charged side chains.

[0287] In various embodiments of the protein nanostructures of the invention, a first assembly domain and second assembly domain, or the vice versa, comprise polypeptides with the amino acid sequence selected from the following pairs, or modified versions thereof (i.e., permissible modifications as disclosed for the polypeptides of the invention: isolated polypeptides comprising an amino acid sequence that is at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, at least 100% identical over its length, and / or identical at least at one identified interface position, to the amino acid sequence indicated by the SEQ ID NO: SEQ ID NO:13 and SEQ ID NO:14 (I53-34A and I53-34B); SEQ ID NO:15 and SEQ ID NO:16 (I53-40A and I53-40B); SEQ ID NO:15 and SEQ ID NO:36 (I53-40A and I53-40B.1); SEQ ID NO:35 and SEQ ID NO:16 (I53-40A.1 and I53-40B); SEQ ID NO:47 and SEQ ID NO:48 (I53-40A genus and I53-40B genus); SEQ ID NO:17 and SEQ ID NO:18 (I53-47A and I53-47B); SEQ ID NO:17 and SEQ ID NO:39 (I53-47A and I53-47B.1); SEQ ID NO:17 and SEQ ID NO:40 (I53-47A and I53-47B.1NegT2); SEQ ID NO:37 and SEQ ID NO:18 (I53-47A.1 and I53-47B); SEQ ID NO:37 and SEQ ID NO:39 (I53-47A.1 and I53-47B.1); SEQ ID NO:37 and SEQ ID NO:40 (I53-47A.1 and I53-47B.1NegT2); SEQ ID NO:38 and SEQ ID NO:18 (I53-47A.1NegT2 and I53-47B); SEQ ID NO:38 and SEQ ID NO:39 (I53-47A.1NegT2 and I53-47B.1); SEQ ID NO:38 and SEQ ID NO:40 (I53-47A.1NegT2 and I53-47B.1NegT2); SEQ ID NO:49 and SEQ ID NO:50 (I53-47A genus and I53-47B genus); SEQ ID NO:19 and SEQ ID NO:20 (I53-50A and I53-50B); SEQ ID NO:19 and SEQ ID NO:44 (I53-50A and I53-50B.1); SEQ ID NO:19 and SEQ ID NO:45 (I53-50A and I53-50B.1NegT2);SEQ ID NO:19 and SEQ ID NO:46 (I53-50A and I53-50B.4PosT1); SEQ ID NO:41 and SEQ ID NO:20 (I53-50A.1 and I53-50B); SEQ ID NO:41 and SEQ ID NO:44 (I53-50A.1 and I53-50B.1); SEQ ID NO:41 and SEQ ID NO:45 (I53-50A.1 and I53-50B.1NegT2); SEQ ID NO:41 and SEQ ID NO:46 (I53-50A.1 and I53-50B.4PosT1); SEQ ID NO:42 and SEQ ID NO:20 (I53-50A.1NegT2 and I53-50B); SEQ ID NO:42 and SEQ ID NO:44 (I53-50A.1NegT2 and I53-50B.1); SEQ ID NO:42 and SEQ ID NO:45 (I53-50A.1NegT2 and I53-50B.1NegT2); SEQ ID NO:42 and SEQ ID NO:46 (I53-50A.1NegT2 and I53-50B.4PosT1); SEQ ID NO:43 and SEQ ID NO:20 (I53-50A.1PosT1 and I53-50B); SEQ ID NO:43 and SEQ ID NO:44 (I53-50A.1PosT1 and I53-50B.1); SEQ ID NO:43 and SEQ ID NO:45 (I53-50A.1PosT1 and I53-50B.1NegT2); SEQ ID NO:43 and SEQ ID NO:46 (I53-50A.1PosT1 and I53-50B.4PosT1); SEQ ID NO:51 and SEQ ID NO:52 (I53-50A genus and I53-50B genus); SEQ ID NO:21 and SEQ ID NO:22 (I53-51A and I53-51B); SEQ ID NO:23 and SEQ ID NO:24 (I52-03A and I52-03B); SEQ ID NO:25 and SEQ ID NO:26 (I52-32A and I52-32B); SEQ ID NO:27 and SEQ ID NO:28 (I52-33A and I52-33B) SEQ ID NO:29 and SEQ ID NO:30 (I32-06A and I32-06B); SEQ ID NO:31 and SEQ ID NO:32 (I32-19A and I32-19B); SEQ ID NO:33 and SEQ ID NO:34 (I32-28A and I32-28B); SEQ ID NO:35 and SEQ ID NO:36 (I53-40A.1 and I53-40B.1); SEQ ID NO:53 and SEQ ID NO:54 (T32-28A and T32-28B); SEQ ID NO:55 and SEQ ID NO:56 (T33-09A and T33-09B); SEQ ID NO:57 and SEQ ID NO:58 (T33-15A and T33-15B); SEQ ID NO:59 and SEQ ID NO:60 (T33-21A and T33-21B); SEQ ID NO:61 and SEQ ID NO:62 (T33-28A and T32-28B); and SEQ ID NO:63 and SEQ ID NO:56 (T33-31A and T33-09B (also referred to as T33-31B)).

[0288] In some embodiments, the assembly domains are I53_dn5B (trimer, optionally linked to the antigen) and I53_dn5A or I53_dn5A.1 or I53_dn5A.2 (pentamer). I53_dn5 nanostructures are described in US 2022 / 0072120 A1, the contents of which are incorporated by reference. I53_dn5variants may include one or more amino acid substitutions, such as C94A, C119A, W18G, K84R, M88P, E91D, L117I, or L120D (together “I53_dn5A.1”; Ueda et al. eLife 9:e57659 (2020)) or A25E, M88A, C119T, L120E, A127E, L131T, I132K, E133A, or a deletion of positions 135-137 (“I53_dn5A.2”; Wang et al. bioRxiv 2022.08.04.502842).

[0289] In some embodiments, the ectodomains are expressed as a fusion protein with a first assembly domain. In some embodiments, the first assembly domain and the ectodomain are joined by a linker sequence.

[0290] Non-limiting examples of designed protein complexes useful in protein nanostructures of the present disclosure include those disclosed in U.S. Patent No. 9,630,994; Int’l Pat. Pub No. WO2018187325A1; U.S. Pat. Pub. No.2018 / 0137234 A1; U.S. Pat. Pub. No.2019 / 0155988 A2, each of which is incorporated herein in its entirety.

[0291] In various embodiments of the protein nanostructures of the disclosure, the assembly domains are polypeptides with the amino acid sequence selected from the following pairs, or modified versions thereof (i.e., permissible modifications as disclosed for the polypeptides of the invention: isolated polypeptides comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical over its length, and / or identical at least at one identified interface position, to the amino acid sequence indicated by the SEQ ID NO): SEQ ID NO: 65 and SEQ ID NO: 66 (T33_dn2A and T33_dn2B); SEQ ID NO: 67 and SEQ ID NO: 68 (T33_dn5A and T33_dn5B); SEQ ID NO: 69 and SEQ ID NO: 70 (T33_dn10A and T33_dn10B); or SEQ ID NO: 71 and SEQ ID NO: 72 (I53_dn5A and I53_dn5B).

[0292] Various protein nanostructures are known in the art and described, for example in U.S. Pat. Pub. Nos. US 2015 / 0356240 A1; US 2016 / 0122392 A1, US 2018 / 0030429 A1, US 2019 / 0341124 A1, and US 2022 / 0072120 A1, the contents of which are incorporated by reference herein. In some embodiments, the protein nanostructure comprises, as an assembly domain, a variant of KDPG aldolase (Protein Data Bank code 1WA3) engineered to self-assemble into a protein nanostructure. In its native form, 1WA3 non-covalently assembles to form a trimer via a first interface (the trimer interface). When 20 copies of the trimer (60 monomers) are computationally docked to form a one-component icosahedral protein nanostructure, sets of fivemonomers of 1WA3 contact one another via a second interface (the pentamer interface). By introducing amino acid substitutions, the pentamer interface may be stabilized such that the protein nanostructure will spontaneously self-assemble, e.g., within the expressing cell or when isolated trimers (or monomers) are mixed under suitable conditions.

[0293] In some embodiments, the pentamer interface comprises 1, 2, 3, 4 or more interface residues, such as residues in positions 33, 61, 187, and 190 numbered according to SEQ ID NO: 107. In some embodiments, the assembly domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 107. In some embodiments, the assembly domain comprises amino acid substitutions at 1, 2, 3, 4 of positions 33, 61, 187, and 190 compared to SEQ ID NO: 107. In some embodiments, a plurality of the amino acid substitutions are substitutions of a polar residue for a non-poplar residue (e.g., A, L, I, M, V, F, or W). In some embodiments, some or all of the amino acid substitutions are substitutions of a polar residue for a small, non-polar residue (e.g., A, L, I, M, or V). In some embodiments, the protein nanostructure comprises amino acid substitutions E33L or E33V; K61L or K61M; D187A or D187V; and / or R190A. In some embodiments, the protein nanostructure comprises amino acid substitutions E33L, K61M, D187V, and R190A. In some embodiments, the protein nanostructure comprises amino acid substitutions E33V, K61L, D187A, and R190A. In some embodiments, the assembly domain comprises an amino acid substitution to negate the enzymatic activity of the assembly domain (e.g., K129A). In embodiments, the assembly domain may comprise further amino acid substitutions (e.g., MI3; E56M or E56K; P186I; E191A; and / or K194A). In some embodiments, the assembly domain comprises amino acid substitutions that remove cysteine residues. In some embodiments, the assembly domain comprises C76A and / or C100A substitutions.

[0294] In some embodiments, the polypeptide comprises a heterologous multimerization domain. In some embodiments, the multimerization domain is a trimerization domain. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64). In some embodiments, the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L.

[0295] In some embodiments, the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences. Ferritin-based nanostructures

[0296] In some embodiments, the assembly domain is a ferritin polypeptide. In some embodiments, the assembly domain of a ferritin protein nanostructure comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the following sequences: MLSKDIIKLLNEQVNKEMNSSNLYMSMSSWCYTHSLDGAGLFLFDHAAEEYEHAKKLII FLNENNVPVQLTSISAPEHKFEGLTQIFQKAYEHEQHISESINNIVDHAIKSKDHATFNFLQ WYVAEQHEEEVLFKDILDKVELIGNENHGLYLADQYVKGIAKSRKS. (SEQ ID NO: 114) MLKPEMIEKLNEQMNLELYSSLLYQQMSAWCSYHTFEGAAAFLRRHAQEEMTHMQRL FDYLTDTGNLPRINTVESPFAEYSSLDELFQETYKHEQLITQKINELAHAAMTNQDYPTF NFLQWYVSEQHEEEKLFKSIIDKLSLAGKSGEGLYFIDKELSTLDAQN. (SEQ ID NO: 115) NFHQDCEAGLNRTVNLKFHSSYVYLSMASYFNRDDVALSNFAKFFRERSEEEKEHAEK LIEYQNQRGGRVFLQSVEKPERDDWANGLEALQTALKLQKSVNQALLDLHAVAADKS DPHMTDFLESPYLSESVETIKKLGDHITSLKKLWSSHPGMAEYLFNKHTLG. (SEQ ID NO: 116) QFSKDIEKLLNEQVNKEMQSSNLYMSMSSWCYTHSLDGAGLFLFDHAAEEYEHAKKLII FLNENNVPVQLTSISAPEHKFEGLTQIFQKAYEHEQHISESINNIVDHAIKSKDHATFNFLQ WYVAEQHEEEVLFKDILDKIELIGNENHGLYLADQYVKGIAKSRKSGS. (SEQ ID NO: 117) SGESQVRQNFKPEMEEKLNEQMNLELYSSLLYQQMSAWCSYHTFEGAAAFLRRHAQEE MTHMQRLFDYLTDTGNLPRINTVESPFAEYSSLDELFQETYKHEQLITQKINELAHAAMT NQDYPTFNFLQWYVSEQHEEEKLFKSIIDKLSLAGKSGEGLYFIDKELSTLDGS. (SEQ ID NO: 118)

[0297] In some embodiments, the C-terminal helix-forming segment links antigen with any nanoparticle known in the art – including but not limited to HPV particle (with SpyCatcher), or Ferritin.Other nanostructures or nanoparticles

[0298] In some embodiments, the ecotdomains described herein are displayed on any nanostructure or nanoparticle known in the art. Illustrative nanostructures and nanoparticles include, but are not limited to Human papillomavirus (HPV) virus-like particles (VLPs), Chikungunya VLPs, AP205 capsid protein VLPs, phage VLPs (e.g., bacteriophage). Display on these and other platforms may be performed by creating a fusion protein of the ectodomain to a relevant protein of the system, by bioconjugate chemistry (e.g., SpyCatcher), or other means known in the art. The protein nanostructure may be a lumazine synthase nanoparticle as described, e.g., in Geng et al. PLoS Pathog. 17(9):e1009897 (2021). The protein nanostructure may be a ferritin nanoparticle as described, e.g., in Joyce et al. bioRxiv 2021.05.09.443331 and in U.S. Pat. Pub. No. US 2019 / 0330279 A1.

[0299] In another aspect, the disclosure provides a recombinant polypeptide for use in displaying a molecule such as an antigen, comprising an alpha-helical segment and a multimerization domain, wherein the alpha-helical segment comprises one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer. In some embodiments, the alpha-helical segment has improved hydrophobic packing. In some embodiments, the alpha-helical segment comprises between about 7 and about 31 residues. In some embodiments, the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids.

[0300] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to any one of LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566), LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567), LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568), EKIXXAIKKAXKL (SEQ ID NO: 569), EXIXKAIKXLXXXXX (SEQ ID NO: 570), XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571), XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

[0301] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

[0302] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according to a) L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573), b) L V [A / T] T X2 K X2 L X2 D L I X2 X2 L [K / E] X2 L L X2 K L X2 X2 (SEQ ID NO: 574), or c) L N KV K K X2V X2X2L X2X2X2V X2X2L E K X2L X2(SEQ ID NO: 575), wherein X2is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0303] In some embodiments, the alpha-helical segment comprises a polypeptide sequence according a) E K I X2X2A I K K A X2K L (SEQ ID NO: 576), b) E X2I X2K A I K X2L [L / X2] X2 X2 [X1 / X2] X2 (SEQ ID NO: 577), and c) X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2 X2 X1 X2 X2 (SEQ ID NO: 578) , or d) X2 X2 L K K A A X2 I X1 K K X1 L K X2X2(SEQ ID NO: 579) , wherein X1is apolar residues selected from A, I, L, and M, wherein X2is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

[0304] In some embodiments, the alpha-helical segment comprises a polypeptide sequence listed in Table 25A or Table 25B or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In some embodiments, the alpha-helical segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

[0305] In some embodiments, the multimerization domain is I53-50A or a variant thereof. In some embodiments, the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64). In some embodiments, the polypeptide comprises an N-terminal fusion of the alpha-helical segment to the multimerization domain via a peptide bond or polypeptide linker. In some embodiments, the polypeptide comprises, N-terminal to the alpha- helical segment, an antigen polypeptide.

[0306] In another aspect, the disclosure provides a polypeptide comprising an alpha-helical segment, comprising a polypeptide sequence listed in Table 25A or Table 25B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto. In another aspect, the disclosure provides a protein nanostructure comprising a trimeric component comprising a polypeptide described herein. In some embodiments, the nanostructure is a two-component nanostructure comprising a first, trimeric component and a second, pentameric component. In someembodiments, the nanostructure is a two-component nanostructure comprising a second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71. IV. Polynucleotides

[0307] In another aspect, the present disclosure provides polynucleotides encoding any of the polypeptides, complex, components, nanostructures, or other compositiosn of the disclosure. The polynucleotides sequences may comprise RNA or DNA. As used herein, “polynucleotides ” are those that have been removed from their normal surrounding polynucleotides sequences in the genome or in cDNA sequences. Such polynucleotides sequences may comprise additional sequences useful for promoting expression and / or purification of the encoded protein, including but not limited to polyA sequences, modified Kozak sequences, and sequences encoding epitope tags, export signals, and secretory signals, nuclear localization signals, and plasma membrane localization signals. It will be apparent to those of skill in the art, based on the teachings herein, what nucleic acid sequences will encode the proteins of the disclosure. V. Delivery vehicles

[0308] In some embodiments, polynucleotides (e.g., mRNA) encoding protein nanostructures including a component comprising a viral protein monomer of a trimeric viral antigen are formulated in a delivery vehicle. In some embodiments, the delivery vehicle is a non-viral vector. In some embodiments, the delivery vehicle is a lipid nanoparticle (LNP). In some embodiments, the delivery vehicle is a liposome. In some embodiments, the delivery vehicle is a polymeric-non- viral vector, such as spermine, Polyethylenimine, chitosan, or polyurethane. In some embodiments, the delivery vehicle is a polymer delivery system, such as poly-amido-amine (PAA), poly-beta aminoesters (PBAEs) or polyethylenimine (PEI). In some embodiments, the delivery vehicle is a ferritin nanoparticle. In some embodiments, the delivery vehicle is an encapsulin.

[0309] In some embodiments, polynucleotides (e.g., mRNA) encoding protein nanostructures including a component comprising a viral protein monomer of a trimeric viral antigen are formulated in a nanoparticle. In some embodiments, the nanoparticle is a lipid nanoparticle (LNP).In some embodiments, the polynucleotides are formulated in a lipid-polycation complex, referred to as a cationic LNP. As a non-limiting example, the polycation may include a cationic peptide or a polypeptide such as, but not limited to, polylysine, polyornithine and / or polyarginine. In some embodiments, the polynucleotides are formulated in a LNP that includes a non-cationic lipid such as, but not limited to, cholesterol or dioleoyl phosphatidylethanolamine (DOPE).

[0310] In various embodiments, the lipid nanoparticles have a mean diameter from about 30 nm to about 150 nm, from about 40 nm to about 150 nm, from about 50 nm to about 150 nm, from about 60 nm to about 130 nm, from about 70 nm to about 110 nm, from about 70 nm to about 100 nm, from about 80 nm to about 100 nm, from about 90 nm to about 100 nm, from about 70 to about 90 nm, from about 80 nm to about 90 nm, from about 70 nm to about 80 nm, or about 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, or 150 nm. In some embodiments, the LNPs are substantially non-toxic. In certain embodiments, polynucleotides, when present in the LNPs, are resistant in aqueous solution to degradation with a nuclease. Lipids and LNPs comprising polynucleotides and their method of preparation are described in, e.g., U.S. Pat. Nos. 8,569,256, 5,965,542 and U.S. Patent Publication Nos. 2021 / 0323914, 2016 / 0199485, 2016 / 0009637, 2015 / 0273068, 2015 / 0265708, 2015 / 0203446, 2015 / 0005363, 2014 / 0308304, 2014 / 0200257, 2013 / 086373, 2013 / 0338210, 2013 / 0323269, 2013 / 0245107, 2013 / 0195920, 2013 / 0123338, 2013 / 0022649, 2013 / 0017223, 2012 / 0295832, 2012 / 0183581, 2012 / 0172411, 2012 / 0027803, 2012 / 0058188, 2011 / 0311583, 2011 / 0311582, 2011 / 0262527, 2011 / 0216622, 2011 / 0117125, 2011 / 0091525, 2011 / 0076335, 2011 / 0060032, 2010 / 0130588, 2007 / 0042031, 2006 / 0240093, 2006 / 0083780, 2006 / 0008910, 2005 / 0175682, 2005 / 017054, 2005 / 0118253, 2005 / 0064595, 2004 / 0142025, 2007 / 0042031, 1999 / 009076 and PCT Pub. Nos. WO 99 / 39741, WO 2017 / 004143, WO 2017 / 075531, WO 2015 / 199952, WO 2014 / 008334, WO 2013 / 086373, WO 2013 / 086322, WO 2013 / 016058, WO 2013 / 086373, WO2011 / 141705, WO 2017 / 049245, WO 2010 / 144740, WO / 2017 / 075531, and WO 2001 / 07548, the contents of which are incorporated by reference herein.

[0311] Further exemplary lipids and LNPs and their manufacture are known in the art – for example in U.S. Pat. Pub. No. U.S. 2012 / 0276209, Semple et al., 2010, Nat Biotechnol., 28(2):172-176; Akinc et al., 2010, Mol Ther., 18(7): 1357-1364; Basha et al., 2011, Mol Ther, 19(12): 2186-2200;Leung et al., 2012, J Phys Chem C Nanomater Interfaces, 116(34): 18440-18450; Lee et al., 2012, Int J Cancer., 131(5): E781-90; Belliveau et al., 2012, Mol Ther nucleic Acids, 1: e37; Jayaraman et al., 2012, Angew Chem Int Ed Engl., 51(34): 8529-8533; Mui et al., 2013, Mol Ther Nucleic Acids. 2, e139; Maier et al., 2013, Mol Ther., 21(8): 1570-1578; and Tam et al., 2013, Nanomedicine, 9(5): 665-74, each of which are incorporated by reference herein. Lipids and their manufacture can be found, for example, in U.S. Pat. Pub. Nos.2015 / 0376115 and 2016 / 0376224, the contents of which are incorporated by reference herein. VI. Pharmaceutical Compositions

[0312] The disclosure also provides pharmaceutical compositions. Such pharmaceutical compositions can be used for generating an immune response against an infectious disease in a subject. The pharmaceutical compositions of the disclosure may include a pharmaceutically acceptable carrier. A thorough discussion of such carriers is available in Chapter 30 of Remington: The Science and Practice of Pharmacy (23rded., 2021).

[0313] In some embodiments, the pharmaceutical composition can also include excipients and / or additives. Examples of these are surfactants, stabilizers, complexing agents, antioxidants, or preservatives which prolong the duration of use of the finished pharmaceutical formulation, flavorings, vitamins, or other additives known in the art. Complexing agents include, but are not limited to, ethylenediaminetetraacetic acid (EDTA) or a salt thereof, such as the disodium salt, citric acid, nitrilotriacetic acid and the salts thereof. In some embodiments, preservatives include, but are not limited to, those that protect the solution from contamination with pathogenic particles, including benzalkonium chloride or benzoic acid, or benzoates such as sodium benzoate. Antioxidants include, but are not limited to, vitamins, provitamins, ascorbic acid, vitamin E, salts or esters thereof.

[0314] Non-limiting examples of pharmaceutically acceptable excipients include water, NaCl, normal saline solutions, lactated Ringer’s, normal sucrose, normal glucose, binders, fillers, disintegrants, lubricants, coatings, sweeteners, flavors, salt solutions (such as Ringer's solution), alcohols, oils, gelatins, carbohydrates such as lactose, amylose or starch, fatty acid esters, hydroxymethyl cellulose, polyvinyl pyrrolidine, and colors, and the like. Such preparations can be sterilized and, if desired, mixed with auxiliary agents such as lubricants, preservatives, stabilizers, wetting agents, emulsifiers, salts for influencing osmotic pressure, buffers, coloring, and / oraromatic substances and the like that do not deleteriously react with the compounds of the disclosure. One of skill in the art will recognize that other pharmaceutical excipients are useful in the present disclosure.

[0315] In some embodiments, one or more tonicity agents may be added to provide the desired ionic strength. Tonicity agents for use herein include those which display no or only negligible pharmacological activity after administration. Both inorganic and organic tonicity adjusting agents may be used.

[0316] In another aspect, the disclosure provides a pharmaceutical composition comprising a polypeptide, a protein complex, or a nanostructure disclosed herein. VII. Vaccines

[0317] In another aspect, the disclosure provides a vaccine comprising a polypeptide, a protein complex, or a nanostructure as disclosed herein. In another aspect, the disclosure provides a vaccine comprising a polypeptide or a nanostructure described herein.

[0318] In some embodiments, the vaccine comprises an adjuvant.

[0319] In some embodiments, the pharmaceutical composition provided herein is administered as a RSV vaccine, for example, an RSV / A vaccine, and RSV / B vaccine, or a bivalent RSV A / B vaccine. In some embodiments, the pharmaceutical composition provided herein is administered as a hMPV vacccine. In some embodiments, the pharmaceutical composition provided herein is administered as a hMPV / A vacccine. In some embodiments, the pharmaceutical composition provided herein is administered as a hMPV / B vacccine. In some embodiments, the pharmaceutical composition provided herein is administered as a hMPV / A and hMPV / B bivalent vacccine. In some embodiments, the pharmaceutical composition provided herein is administered as a hMPV / A and RSV bivalent vacccine In some embodiments, the pharmaceutical composition provided herein is administered as a PIV3 vaccine. In some embodiments, the pharmaceutical composition provided herein is administered as a PIV5 vaccine. In some embodiments, the pharmaceutical composition provided herein is administered as a SARS-CoV-2 vaccine. In some embodiments, the pharmaceutical composition provided herein is administered as a Nipah vaccine. In some embodiments, the pharmaceutical composition provided herein is administered as a bivalent RSV / hMPV vaccine.Adjuvants

[0320] Adjuvants or immune potentiators may also be administered with or in combination with a lipid nanoparticle composition. Advantages of adjuvants include, but are not limited to, the enhancement of the immunogenicity of antigens, modification of the nature of the immune response, the reduction of the antigen amount needed for a successful immunization, the reduction of the frequency of booster immunizations needed and an improved immune response in elderly and immunocompromised vaccinees. These may be co-administered by any route, e.g., intramuscular, subcutaneous, intravenous, or intradermal injections.

[0321] Adjuvants may include, but are not limited to, a natural or a synthetic adjuvant. Adjuvants may be organic or inorganic.

[0322] Adjuvants may be selected from any of the classes (1) mineral salts, e.g., aluminum hydroxide and aluminum or calcium phosphate gels; (2) emulsions including: oil emulsions and surfactant based formulations, e.g., microfluidised detergent stabilized oil-in-water emulsion, purified saponin, oil-in-water emulsion, stabilized water-in-oil emulsion; (3) particulate adjuvants, e.g., virosomes (unilamellar liposomal vehicles incorporating influenza hemagglutinin), structured complex of saponins and lipids, polylactide co-glycolide (PLG); (4) microbial derivatives; (5) endogenous human immunomodulators; (6) inert vehicles, such as gold particles; (7) microorganism derived adjuvants; (8) tensoactive compounds; (9) carbohydrates; or combinations thereof.

[0323] Adjuvants for nucleic acid vaccines (DNA) have been disclosed in, for example, Kobiyama, et al., Vaccines, 2013, 1(3), 278-292, the contents of which are incorporated herein by reference in their entirety. Any of the adjuvants disclosed by Kobiyama et al., may be used in the vaccines as described herein.

[0324] Other adjuvants which may be utilized include any of those listed on the web-based vaccine adjuvant database, on the World Wide Web at violinet.org / vaxjo / and described in for example Sayers, et al., J. Biomedicine and Biotechnology, volume 2012 (2012), Article ID 831486, 13 pages, the contents of which are incorporated herein by reference in their entirety.

[0325] Specific adjuvants may include cationic liposome-DNA complex JVRS-100, aluminum hydroxide vaccine adjuvant, aluminum phosphate vaccine adjuvant, aluminum potassium sulfateadjuvant, alhydrogel, ISCOM(s)™, Freund's Complete Adjuvant, Freund's Incomplete Adjuvant, CpG DNA Vaccine Adjuvant, Cholera toxin, Cholera toxin B subunit, Liposomes, Saponin Vaccine Adjuvant, DDA Adjuvant, Squalene-based Adjuvants, Etx B subunit Adjuvant, IL-12 Vaccine Adjuvant, LTK63 Vaccine Mutant Adjuvant, TiterMax Gold Adjuvant, Ribi Vaccine Adjuvant, Montanide ISA 720 Adjuvant, Corynebacterium-derived P40 Vaccine Adjuvant, MPL™ Adjuvant, AS04, AS02, AS01E, Lipopolysaccharide Vaccine Adjuvant, Muramyl Dipeptide Adjuvant, CRL1005, Killed Corynebacterium parvum Vaccine Adjuvant, Montanide ISA 51, Bordetella pertussis component Vaccine Adjuvant, Cationic Liposomal Vaccine Adjuvant, Adamantylamide Dipeptide Vaccine Adjuvant, Arlacel A, VSA-3 Adjuvant, Aluminum vaccine adjuvant, Polygen Vaccine Adjuvant, ADJUMER™, Algal Glucan, Bay R1005, Theramide®, Stearyl Tyrosine, Specol, Algammulin, AVRIDINE®, Calcium Phosphate Gel, CTA1-DD gene fusion protein, DOC / Alum Complex, Gamma Inulin, Gerbu Adjuvant, GM-CSF, GMDP, Recombinant hIFN-gamma / Interferon-g, Interleukin-1β, Interleukin-2, Interleukin-7, Sclavo peptide, Rehydragel LV, Rehydragel HPA, Loxoribine, MF59, MTP-PE Liposomes, Murametide, Murapalmitine, D-Murapalmitine, NAGO, Non-Ionic Surfactant Vesicles, PMMA, Protein Cochleates, QS-21, SPT (Antigen Formulation), nanoemulsion vaccine adjuvant, AS03, Quil-A vaccine adjuvant, RC529 vaccine adjuvant, LTR192G Vaccine Adjuvant, E. coli heat- labile toxin, LT, amorphous aluminum hydroxyphosphate sulfate adjuvant, Calcium phosphate vaccine adjuvant, Montanide Incomplete Seppic Adjuvant, Imiquimod, Resiquimod, AF03, Flagellin, Poly(I:C), ISCOMATRIX®, Abisco-100 vaccine adjuvant, Albumin-heparin microparticles vaccine adjuvant, AS-2 vaccine adjuvant, B7-2 vaccine adjuvant, DHEA vaccine adjuvant, Immunoliposomes Containing Antibodies to Costimulatory Molecules, SAF-1, Sendai Proteoliposomes, Sendai-containing Lipid Matrices, Threonyl muramyl dipeptide (TMDP), Ty Particles vaccine adjuvant, Bupivacaine vaccine adjuvant, DL-PGL (Polyester poly (DL-lactide- co-glycolide)) vaccine adjuvant, IL-15 vaccine adjuvant, LTK72 vaccine adjuvant, MPL-SE vaccine adjuvant, non-toxic mutant E112K of Cholera Toxin mCT-E112K, and / or Matrix-S.

[0326] In some embodiments, the adjuvant comprises squalene. In some embodiments, the adjuvant comprises aluminum hydroxide. In some embodiments, the adjuvant comprises AS01E.VIII. Methods of Use

[0327] In another aspect, the disclosure provides methods of administration for the composition, the pharmaceutical composition, or the vaccine described herein.

[0328] In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of generating an immune response in a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of treating or preventing disease including, but not limited to, RSV, hMPV, PIV3, PIV5, Nipah and / or SARS-CoV-2 in a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a composition of the disclosure for use in vaccinating, generating an immune response, or treating or preventing disease including, but not limited to, RSV, hMPV, PIV3, PIV5, Nipah and / or SARS-CoV-2. In another aspect, the disclosure provides a composition, method, or use as described herein. In another aspect, the disclosure provides a method of treating or preventing coronavirus disease in a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a composition of the disclosure for use in vaccinating, generating an immune response, or treating or preventing coronavirus disease. In another aspect, the disclosure provides a composition, method, or use as described herein.

[0329] In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition described herein. In another aspect, the disclosure provides a method of generating an immune response or treating or preventing a viral infection in a subject, comprising administering to the subject a polypeptide or a nanostructure described herein. In another aspect, the disclosure provides a method of making a polypeptide or a nanostructure described herein, comprising culturing host cells modified to express one or more polypeptides as described herein.

[0330] In some embodiments, the method comprises administering the vaccine described herein. In some embodiments, the subject is immunized against infection to RSV, hMPV, PIV3, PIV5, Nipah and / or SARS-CoV-2 S. In some embodiments, the subject is immunized against infection by coronavirus. In some embodiments, the vaccine is administered by subcutaneous injection. In some embodiments, the vaccine is administered by intramuscular injection. In some embodiments,the vaccine is administered by intradermal injection. In some embodiments, the vaccine is administered intranasally. In one aspect, the disclosure provides a pre-filled syringe comprising the vaccine described herein. In one aspect, the disclosure provides a kit comprising the vaccine described herein or the pre-filled syringe described herein. In one aspect, the disclosure provides a kit comprising the vaccine described herein or the lysophilized vaccine described herein

[0331] In some embodiments, the unit dose of the pharmaceutical composition comprises about 0.5 µg to about 1 µg, about 20 µg to about 25 µg, about 25 µg to about 50 µg, about 50 µg to about 70 µg, about 70 µg to about 75 µg, about 75 µg to about 100 µg, about 100 µg to about 125 µg, about 100 µg to about 150 µg, about 125 µg to about 150 µg, about 125 µg to about 175 µg, about 150 µg to about 175 µg, about 175 µg to about 200 µg, about 200 µg to about 250 µg, about 225 µg to about 300 µg, about 250 µg to about 300 µg, or about 250 µg to about 350 µg of the protein nanostructures.

[0332] In some embodiments, the subject is at risk of disease including, but not limited to, RSV, hMPV, PIV3, PIV5, Nipah and / or SARS-CoV-2. In some embodiments, the subject is at risk of hMPV disease. In some embodiments, the subject is at risk of PIV3 disease. In some embodiments, the subject is at risk of PIV5 disease. In some embodiments, the subject is at risk of coronavirus disease. In some embodiments, the subject is an adult of over 60 years of age. In some embodiments, the subject is a healthy adult of 18-45 years of age. In some embodiments, the subject is a pregnant women between week 32 and week 36 of pregnancy. In some embodiments, the subject is a pregnant women between week 30 and week 38 of pregnancy. In some embodiments, the subject is a pregnant women between week 28 and week 38 of pregnancy.

[0333] In another aspect, the disclosure provides a method of vaccinating a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a method of generating an immune response in a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a method of treating or preventing a viral infection in a subject, comprising administering to the subject a composition disclosed herein. In another aspect, the disclosure provides a composition for use in vaccinating, generating an immune response, or treating or preventing any viral infectious disease disclosed herein. In another aspect, the disclosure provides a composition, method, or use asdescribed herein. In another aspect, the disclosure provides a method of making a composition, comprising culturing host cells modified to express one or more polypeptides as described herein.

[0334] Any aspect or embodiment described herein can be combined with any other aspect or embodiment as disclosed herein EXAMPLES

[0335] The following Examples are merely illustrative and are not intended to limit the scope or content of the invention in any way. Example 1. Remodeling the C-terminus of RSV F protein

[0336] This Example describes remodeling the C terminus of the RSV F protein to create a stable helix-forming segment.

[0337] RSV F protein, like other class I viral membrane fusion protein, forms a trimer with two primary conformations (prefusion and postfusion). The C terminus of the ectodomain, adjacent to the transmembrane domain, is believed to form a helical bundle in the context of the native protein. Structures of the prefusion F protein generally model the C terminus as alpha-helical, with structured density ending at about residue 510 or 512 (e.g., PDB 5C6B and 5UDD, respectively). The native sequence after residue 513 is often replaced with a four-residue linker (SAIG) and the trimeric FoldOn domain. The predicted transmembrane domain begins at residue 527. The sequence of a native RSV / B F protein sequence (GenBank: WDV37446.1) is shown here with the transmembrane domain bold / underlined: 1 MELLIHRSSA IFLTLAINAL YLTSSQNITE EFYQSTCSAV 41 SRGYLSALRT GWYTSVITIE LSNIKETKCN GTDTKVKLIK 81 QELDKYKNAV TELQLLMQNT PAVNNRARRE APQYMNYTIN 121 TTKNLNVSIS KKRKRRFLGF LLGVGSAIAS GIAVSKVLHL 161 EGEVNKIKNA LQLTNKAVVS LSNGVSVLTS RVLDLKNYIN 201 NQLLPMVNRQ SCRISNIETV IEFQQKNSRL LEITREFSVN 241 AGVTTPLSTY MLTNSELLSL INDMPITNDQ KKLMSSNVQI 281 VRQQSYSIMS IIKEEVLAYV VQLPIYGVID TPCWKLHTSP 321 LCTTNIKEGS NICLTRTDRG WYCDNAGSVS FFPQADTCKV 361 QSNRVFCDTM NSLTLPSEVS LCNTDIFNSK YDCKIMTSKT 401 DISSSVITSL GAIVSCYGKT KCTASNKNRG IIKTFSNGCD 441 YVSNKGVDTV SVGNTLYYVN KLEGKNLYVK GEPIINYYDP 481 LVFPSDEFDA SISQVNEKIN QSLAFIRRSD ELLHNVNTGK 521 STTNIMITAI TIVIIVVLLS LIAIGLLLYC KAKNTPVTLS 561 KDQLSGINNI AFSK (SEQ ID NO: 1)

[0338] We hypothesized that the poor structural resolution of the C terminus of the ectodomain reflects imperfect hydrophobic packing of the helical bundle in the native protein when it is expressed recombinantly. We developed a pipeline to remodel the C terminus of the ectodomain to generate improved antigens for use in vaccines. Our method structurally remodels the segment (corresponding to about residue 500 and about residue 530 relative to native sequence) into a more structurally stable helical bundle by substituting residues (e.g., to generate new non-covalent interactions, prevent clashing of residues, or adjust the polypeptide backbone), as well as preserve or enhance polar exposed surfaces, and thereby decrease the free energy of self-association of the protomers (as predicted ddG and measuring thermal denaturation temperature). The remodeling pipeline included manual selection of sequences predicted to form structures capable of serving as adaptors to connect the C terminus of the ectodomain to a trimerization domain, such as an I53- 50A multimerization domain. Manual selection was performed based on a combination of polypeptide sequence diversity and computational metrics, which included geometry design space, hydrophobic core packages, termini availability, and lack of obvious errors in conformation (i.e., solvent exposed tryptophans).

[0339] Structural models from the Protein Data Bank (PDB) were prepared for design by symmetrization, removal of hetero-atoms, renumbering, relaxing, and marking of glycosylation sites. Rosetta blueprint files were written to generate a gradient between the native structure and the remodeled domain. Generally, the blueprint included a two-residue native sequence that is remodeled using helical constraints; followed by one or more existing helical residues that contribute to hydrophobic contacts at the C-terminus, which are allowed to design; followed by an alpha-helical segment ranging in length from approximately 1-10 amino acids in length, determined empirically by the designer. For example, to remodel this sequence: 481 LVFPSDEFDA SISQVNEKIN QSLAFIRRSD ELLHNVNTG (SEQ ID NO: 710) a blueprint may be generated were the amino acid residue is set to match the native sequence (A), to start with native sequence but allow substitutions (A), to newly modeled as any amino acid (X) (top line), while the three-dimensional structure of the polypeptide is set to either match the native structure (.) or to be constrained to be helical (H):481 LVFPSDEFDA SISQVNEKIN QSLAFIRRSX XXXXXXXXX (SEQ ID NO: 711) .......... .......... HHHHHHHHHH HHHHHHHHH (SEQ ID NO:712)

[0340] Using this or similar blueprints, designs were generated with Rosetta Remodel. Remodel results were filtered by ddG calculations from the design model directly, and manually reverted to remove any introduced glycosylation sites, exposed hydrophobic residues, or buried polar residues. The resulting models were relaxed and then ddG’s were again calculated.

[0341] Alternatively, remodeling was performed using RFdiffusion. Relaxed structures used as input for remodel were also used as input for RFdiffusion, except that only the C-terminal helices were used as input for diffusion. This protocol significantly reduces computational time. Positions with WT sequence identities from Rosetta Remodel were likewise preserved for diffusion. Unlike with Rosetta Remodel all structural data of subsequent residues was ignored. This is a limitation of RFdiffusion, not a scientific constraint placed on the design problem. Intra-protomer and inter- protomer weights were both set to 1, with a quadratic guide decay and a guide scale of 2. C- terminal lengths varied between 12 and 31 residues depending on the source virus, and 15 remodeled helices were generated for each length. Sequences were generated using Protein MPNN, with a sampling temperature of 0.4 and a negative bias against C (-10), and F, G, P, W, and Y (-1). 100 sequences were generated for each remodeled domain. The top 2% based on MPNN sample score were selected for computational characterization.

[0342] Designs were analyzed based on the following criteria: 1) ColabFold validates the design performed with Rossetta by predicting ordered terminal helix consistent with design model (assuming ColabFold method can provide reliable results for a particular fusion protein); 2) decrease in ddG by 5-15 Rosetta Energy Unit (REU) and 3); design has a well-packed hydrophobic core without extraneous elements (i.e., helical segments with no interprotomer hydrophobic packing). To calculate ddG, two models are generated, one in which all protomers are correctly in contact as trimers and one in which the protomers are moved distant from each other. Sidechains in both models are repacked and minimized, and then both models are scored. The ddG is the difference in the scores, as in (Distant state) – (Trimeric state).

[0343] FIG. 2 shows a structural model of a representative experimental model of the RSV F protein (left) compared to the predicted structure of a representative design (right), provided from PDB 4MMU. The optimal length for the remodeled C terminus was determined by plotting average ddG against the length of the C-terminal helix, as shown in FIG.3. When using Rosetta Remodel,the average ddG will decrease until an optimum length is achieved, at which point the ddG will tend to stay the same or increase again. This may be because Remodel can struggle when building larger segments due to increasing degrees of freedom. Ideal linker lengths are those near the minimum ddG. In this case, it was determined that an optimal C-terminal helix would terminate at about position 519. It was observed empirically that a ddG was minimized when the helical segment extended about 6 residues past the native position 513 (i.e., to position 519).

[0344] Computational modeling (with Rosetta Remodel) of the RSV / B protein was used to generate artificial polypeptide sequences, each predicted to form a stable alpha helix, shown in Table 12. Residues 500-502 of the native RSV F protein are included as NQS. Residues Q501 and S502 were remodeled with helical constraints while preserving the native sequence identities. This optimizes the helical backbone of these residues with side chains represented as centroids and then repacks the side chains in all-atom mode. Residues 503-509 were remodeled with helical constraints and without sequence constraints. The helical backbone is first optimized with side chains represented as centroids, and the side chains are designed in all-atom mode. As a result there is some bias towards the native sequence. Six to 14 additional amino acids were added with helical constraints. Side chains are represented as valine centroids during backbone sampling, then the sequence is sampled in all-atom mode. All backbone sampling of these elements in centroid mode is performed simultaneously and sequence design in all-atom mode is likewise performed simultaneously. Designs were manually refined to remove exposed hydrophobic residues or buried polar residues with identities preferentially selected from the nearest residue in the WT sequence or rationally where the WT residue was suboptimal.

[0345] The I53-50A molecule is well-suited for genetic fusion to many trimeric antigens, and features symmetric N-termini that are approximately 5 nm apart. Due to the remodeled C-terminus of the C-Term 1 design being more distanced laterally from the symmetric axis of the antigen (FIG. 2), it appeared possible that this modification could minimize strain in genetic fusions to I53-50A relative to commonly-studied antigen fragments that end at residue 513. Four sequences were selected for experimental testing (Table 12) as genetic fusions to a version of I53-50A (I53- 50Aδcys), with antigens also containing DS-Cav1 mutations.Table 12. Illustrative C-terminal helix-forming segments Name SequenceRemodeledSEQ ID NO: LengthC-Term 36 NQSRKLEEIARIVEQKKRTEEKRS 21 150 C-Term 37NQSAETKKAIERAREL13 151ent ISQVNEKINQSLAFIRRSDE (SEQ ID NO: 713).

[0347] In context the C-terminal alpha-helix of the modified construct is ISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 714) and is only nine residues longer than the portion of the native structure known to be helical, and two residues lower than the predicted helical segment. Contact residues are bold and underlined. Native ISQVNEKINQSLAFIRRSDELLHNVN (SEQ ID NO: 715) Remodel ISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 714)

[0348] Whereas the WT sequence has a three-residue hydrophobic segment leading into the designed helix, and a five-residue polar segment in the middle, which contributes to sub-optimal packing, the remodeled sequences are characterized by a pattern of alternating hydrophobic and polar segments with no hydrophobic segment longer than two consecutive residues and no polar segment longer than three consecutive residues (FIG. 4). The remodeled helix has at minimum two hydrophobic segments at positions 508 and / or 509 and 511 and / or 512 and optimally four hydrophobic segments at positions 505 and / or 506, 508 and / or 509, 511 and / or 512, and 515 and / or 516.

[0349] Published structure of the RSV protein generally does not include the residues C-terminal to about residue 500. Either the residues are not included in the recombinant protein studied, orthey are not visible in the electronic density observed. Nonetheless, modeling suggests that the following substitutions will stabilize the portion of the F protein in a helical conformation. Table 13. Possible substitutions at Position 505-516 Position Preferences suggested by modeling Illustrative Substitutions F505 Hydrophobic or threonine, not WFY A, I, L, M, V, G, T; not F, Y, W P; Y P; Y T; E, he G, S, E, A, P; Y P; G K P; Y P; Y P; Ysome embodiments, polar amino acids include charged amino acid residues. In some embodiments, charged amino acids refer to E, D, R, K, and H. In some embodiments, hydrophobic amino acids refer to A, I, L, M, V, F, Y, and W.

[0351] A small-scale screen showed that three of the four selected designs expressed. Table 14 shows binding of antibodies D25, AM14, and 4D7 to RSV / B F proteins fused to I53-50A to form trimeric protein complexes (but not assembled with I53-50B). Both D25 and AM14 are specific to the prefusion state, however D25 can bind both prefusion monomers and trimers while AM14 can only bind closed trimeric prefusion trimers.4D7 is specific to the postfusion state. C-Term1 was well expressed and showed the highest binding to AM14. Table 14. Summary of antibody binding screening data for designed RSV / B F proteins Name Expression D25 AM14 4D7 C-Term1 ++ +++ +++ + Example 2. D

[0352] This Example describes sets of stabilizing mutations for stabilization of the prefusion state of RSV F protein. Based on a structure of RSV F in the prefusion conformation (FIG.1) compared to its postfusion conformation (not shown), stabilizing mutations at the interfaces between protomers were designed to either lower the energy of the prefusion state or raise the energy of the postfusion state.

[0353] Computational modeling was used to identify amino acid substitutions to stabilize RSV / B F protein in the prefusion conformation. These mutations are listed in Table 15. Table 15. stabilizing substitutions Space Substitutions S, , ,Space 3 K75Q, N216S, N216D, E218P, T219S ,

[0354] Based o rgize include:E487R + K498A E487R + K498E

[0355] RSV F. s that replace the cleavage site for furin (residues 104-140) with a native linker were also tested. Linker sequences are provided in Table 16, which were tested in between residues 103 and 141. Table 16. Furin cleavage linkers Sequence Length SEQ ID NO:Example 3. Experimental Evaluation of RSV F proteins

[0356] This Example shows that the C-terminal helix-forming segments described in Example 1 increase thermal stability of the recombinant polypeptides by as much as about 20-25°C or more and increase storage stability under accelerated degradation conditions (storage at 40°C). Further improvement is observed when the C-terminal helix-forming segment is combined with stabilizing mutations described in Example 2. The recombinant polypeptides retain the ability to self- assemble to form a two-component I53-50-type nanostructure.

[0357] Recombinant polypeptides that include RSV / B F protein ectodomains (B18537 strain with DS-Cav1 mutations) fused to I53-50AΔcys were tested using small-scale HEK293 expression. Supernatants were screened for relative expression by bio-layer interferometry (BLI) with a monoclonal antibody (16A8) that binds specifically to I53-50A. BLI was used to measure binding to known RSV F protein antibodies D25 (specific to prefusion state), AM14 (specific to closed trimeric prefusion state), and 4D7 (specific to postfusion state). Measurements were normalized to binding by palivizumab (conformation independent). Increased AM14 was observed to several designs featuring mutations in Space 1, C-terminal remodeling, or both.

[0358] Scaled-up protein preparation for select designs were incubated for six days at either 4 ^C or 40 ^C. Designs were identified which showed less loss in D25 or AM14 binding at 40 ^C compared to DS-Cav1 mutations alone, as well as smaller increases in binding to 4D7 at 40 ^C. The C-term1 design (Example 1) that includes a remodeled C terminus showed nearly no decrease in AM14 binding and no increase in 4D7 binding.

[0359] Sets of mutations were selected for analysis in combination with each other. In these experiments, an ectodomain sequence from a contemporary RSV / B strain was used (hRSV / B / Australia / VIC-RCH056 / 2019). Antibody binding was normalized to 16A8 mAb, which is specific to the I53-50A fusion partner. Multiple designs were characterized that increased ratios of binding to AM14 (prefusion) or decreased the binding to 4D7 (postfusion) (FIG. 1). Six-day thermal stress tests were performed for select scaled-up proteins.

[0360] Fourteen designs were selected for further analysis after scale-up and purification. Antigenic measurements confirmed increases in AM14 binding for all tested designs relative to DS-Cav1 mutations alone. Constructs incorporating C-terminal remodeling generally showed greater thermal stability under storage (i.e., reduced rate of decrease in 4D7 binding).

[0361] Constructs selected for thermal denaturation and storage testing are shown in Table 17. All tested RSV / B constructs were based on the sequence of strain hRSV / B / Australia / VIC- RCH056 / 2019, including the DS-Cav1 mutations, fused to I53-50AΔcys. All proteins were tested as soluble, trimeric fusions (prior to assembly with I53-50B to form a nanostructure). RSV / A.03 (based on the A2 strain) and RSV / B.002 were controls containing the DS-Cav1 substitutions. The data in Table 17 show that the C-terminal alpha-helical segment by itself can increase thermal stability by up to about 25oC (compare construct RSV / B.002 to RSV / B.195, construct RSV / B.093 to RSV / B.189). Furthermore, all constructs having the C-terminal alpha-helical segment maintain the prefusion conformation when stored at 40oC for seven days. One construct without the C- terminal alpha-helical segment, RSV / B.093, was also stable prefusion at 40oC, but its melting temperature was lower than constructs containing C-terminal remodeling. Table 17 NanoDSF Storage Construct Serotype Substitutio Alpha- Tonset Tm (oC) Stable at 40oCK498A RSV / B.148 B1E487R Yes264.4 77.3 ++ K498ANQS N V S (S Q NO: 0)3Based on A2 strain4In addition to DS-Cav1 (S155C, S290C, S190F, and V207L)

[0362] Selected constructs were incubated with a second component, I53-50B, to form nanostructures. Dynamic Light Scattering (DLS) and negative-stain electron microscopy (nsEM) confirm assembly as nanostructure. Results are shown in Table 18. A representative electron micrograph is shown in FIG.5 (RSV / B.195, having the DS-Cav).Table 18 Nanostructure Construct Serot Substitutions2Alpha- Self- Compact In vivoT249P RSV / B.178 B1D489A Yes Yes Not tested No T400D2In addition to DS-Cav1 (S155C, S290C, S190F, and V207L)3NQSREIIRAINIVRKIASEK (SEQ ID NO: 10)4Based on A2 strain

[0363] Sequences for designed constructs used in Table 18 are shown in Table 19. SEQ ID NO: 1 is used for the reference sequence. In each case, the signal peptide at the N terminus or the tag at the C terminus, shown in underlined may be inserted with known alternatives or deleted. RSV F protein is known to be cleaved at two furin cleavage sites leading to loss of a peptide sequence known as “p27.” (Rezende et al. Front. Microbiol., Vol. 14 (2023).) As used herein, the term “polypeptide” includes polypeptides lacking the p27 peptide due to this cleavage reaction. The approximate region surrounding the p27 peptide is italicized, and may be removed through furin- based cleavage during production of antigens in cell culture. Table 19 SEQ :DQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQLPLYGVIDTP CWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAERSV / A.01 MELLILKANAITTILTAVTFCFASGQNITEEFYQSTCSAVSKGYLS 79 6 ALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQELDKYKNAVARKMEELFKKHKIVAVLRANSVEEAIEKAVAVFAGGVHLIEITF TVPDADTVIKALSVLKEKGAIIGAGTVTSVEQARKAVESGAEFIVDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQLPLYGVIDTP CWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAEAGVLAVGVGSALVKGTPDEVREKAKAFVEKIRGATELEHHHHH HTCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKD DVSSSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRK RRFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVLDEEISQFAKEKGVFYMPGVMTPTELVKAMKLGHTILKLFPGEV VGPQFVKAMKGPFPNVKFVPTGGVNLDNVAEWFKAGVLAVGVSSSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGV DTVSVGNTLYYVNKLEGKNLYVKGEPIINYYDPLVFPSDRFDAS 0RRFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAV VSLSNGVSVLTFRVLDLKNYINNQLLPMLNRQSCRISNIETVIEF 1 2LKLFPGEVVGPQFVKAMKGPFPNVKFVPTGGVNLDNVAEWFK AGVLAVGVGSALVKGTPDEVREKAKAFVEKIRGATELEHHHHH 3

[0364] Relative expression and antibody binding of each design are shown in Table 20. Table 20. Relative expression and antibody binding by BLI Construct # Expression D25 AM14 4D7 Palivizumab RSV / A03 +++ +++ ++ ++ +++RSV / B.063 +++ +++ +++ + +++ RSV / B.064 +++ +++ +++ ++ ++++RSV / B.109 ++ +++ +++ + ++ RSV / B.110 + +++ +++ +++ ++RSV / B.155 + +++ ++ + N / A RSV / B.156 ++ +++ +++ + N / A

[0365] Mutations of designed constructs used in the experiments are shown in Table 21. All sequences featured the ectodomain of RSV F (with DS-Cav1 mutations) genetically fused to I53- 50AΔcys (SEQ ID NO: 64) with a flexible glycine- and serine-based linker. Designs that contain a C-terminal alpha-helical segment place this segment at the C-terminus of the ectodomain asdescribed earlier, and prior to the flexible linker. SEQ ID NO: 1 is used for the reference sequence. In each case, the signal peptide at the N terminus or the tag at the C terminus may be replaced with known alternatives or deleted. “o” indicated that an amino acid substitution was used. Table 21. Mutations of constructs used in the experiments Construct T58M V154I R Alpha- #Space 1V296A A298LT249P235Y E232AT67Vhelical t1RSV / B.069 Q494M, S485I, K399A, D486A + 487M + 498ARSV / B.092 F140W + D489A + T400D + E487R + K498A + D486Ao o oRSV / B.120 Q494M, S485I, K399A, D486A + 487M + 498Ao oRSV / B.143 F488W + D489A + T400D + E487R + K498A + D486Ao o oRSV / B.171 D489A + T400D + E487R + K498AORSV / B.194 F488W + D489A + T400D + E487R + K498A + D486Ao o o O

[0366] To test whether these stabilizing modifications are generalizable outside of RSV / B-based antigens, two novel designs were also evaluated in the context of an RSV / A antigen sequence (RSV / A.013 and RSV / A.023). Both designs contained DS-Cav1 mutations and were genetically fused to I53-50AΔcys, with RSV / A.013 adding a C-terminal alpha-helical segment (equivalent to the RSV / B.195 design) and RSV / A.023 adding both a C-terminal alpha-helical segment and D489A, T400D, E487R and K498A mutations (equivalent to the RSV / B.171 design). Sequences and mutations for these designs are further detailed in Table 19 and Table 21 respectively. Both thermal stability and storage stability at 40°C were strongly increased relative to the RSV / A.03 design, which did not include a C-terminal alpha-helical segment or D489A, T400D, E487R and K498A mutations (Table 17). RSV / A.013 and RSV / A.023 showed increases in melting temperature of 4.5oC and 19.0oC relative to RSV / A.03, which demonstrates that the C-terminal alpha-helical segment can be alone used to improve the thermal stability of both RSV / A and RSV / B antigens, and that the combination of the C-terminal alpha-helical segment with further stabilizing mutations can more rigorously improve the thermal stability of both RSV / A and RSV / B antigens. Further, both RSV / A.013 and RSV / A.023 were capable of in vitro assembly into nanostructures with addition of I53-50B as evaluated by DLS (Table 18).

[0367] In order to evaluate the immunogenicity of different designs based on either RSV / B or RSV / A, two in vivo studies were performed in BALB / c mice. In one study, RSV / B neutralizing titers elicited by immunization with either a 0.02 mg or 0.1 mg dose of assembled nanostructures based on RSV / B.002, RSV / B.093, RSV / B.195, RSV / B.160, or RSV / B.171 were evaluated, all of which were adjuvanted with AddaVax (FIG.17). No statistically significant differences between any of the designs were observed at either dose. Similarly, no statistically significant differences were observed between mice immunized with either a 5 mg unadjuvanted or 0.01 mg AddaVax- adjuvanted dose of assembled nanostructures based on RSV / A.03, RSV / A.013, or RSV / A.023 (FIG.18). However, mice immunized with 1 mg of unadjuvanted RSV / A.023 nanostructure didhave significantly higher RSV / A neutralizing titers than mice immunized with the same dose of unadjuvanted RSV / A.03. Example 4. Diffusion Methods to Generate a C terminus

[0368] Relaxed structures used as input for Rosetta Remodel were also used as input for RFdiffusion, except that only the C-terminal helices and neighboring residues were used as input for diffusion. This significantly reduces computational time. Positions with WT sequence identities from Rosetta Remodel were likewise preserved for diffusion. Unlike with Rosetta Remodel all structural data of subsequent residues was ignored. This is a limitation of RFdiffusion, not a scientific constraint placed on the design problem. Intra-protomer and inter-protomer weights were both set to 1, with a quadratic guide decay and a guide scale of 2. The non-standard weights Base_epoch8_ckpt.pt were applied and C3 symmetry was enforced. C-terminal lengths varied between 12 and 31 residues depending on the source virus, and 15 remodeled helices were generated for each length. Sequences were generated using Protein MPNN, with a sampling temperature of 0.4 and a negative bias against C (-10), and F, G, P, W, and Y (-1).100 sequences were generated for each remodeled domain. The top 2% based on MPNN sample score were selected for computational characterization.

[0369] A set of unique all alpha-helical bundles were generated for each input structure. For most inputs, Rosetta Remodel (Remodel) and RFDiffusion (Diffusion) were both used, except for PIV5 where Remodel generated ample unique results. The number and quality of the output structures was highly variable, depending on the input structure. For example, the C-terminal residues in most structures suffer from low data quality, likely due to local flexibility. This, combined with consistent evidence for a lack of effort in refining this region, may have resulted in sub-optimal bond angles and lengths. Furthermore, many fusion proteins are slightly asymmetric. Symmetrization could have introduced strain. Collectively these effects can influence the quality and number of outputs passing the ddG filter, and also the results generated by diffusion. For that reason, both remodel and diffusion were used where remodel alone was not sufficient to generate enough quality outputs.

[0370] Remodeled C-terminal domains generally fell into two categories based on the geometry of the input structure. Where the input domain already consists of a relatively tight helical structure (for example FIGs. 6A-6D) the remodeled domain continues the helical bundle with straight orslightly twisted helical bundles with remodel lengths between 10 and 24 residues being optimal (FIG.7). The input domain consists of converging alpha-helices, helices in the remodeled domain cross, with a well-packed hydrophobic core (FIG. 6E) RFdiffusion was also able to generate outputs where the helices converge into a tight helical bundle (FIG.8). Optimal remodel lengths for these constructs were greater than 10 residues (FIG.9). In some cases all remodeled lengths resulted in significantly better scores than the WT sequence (FIG.9), in which case designs were selected based on their score relative to the average for that remodel length.

[0371] Selected remodeled sequences all result in helical bundles with repeating patterns of hydrophobic and hydrophilic residues. In most cases the WT sequence has a similar pattern, except that one of the repeats is much less hydrophobic than the remodeled sequences. For example, remodel position 8 is a serine in PIV5 and in remodeled designs is typically a leucine, isoleucine, valine, or alanine (FIG. 10). Designs with more distant C-terminal helices tended to result in designs where the pattern of polar and hydrophobic residues shifted relative to WT (FIG.11).

[0372] PIV5: The input structure for PIV5 was 4GIP (Ref.4). PIV5 has a glycan at position 457 which was preserved. The B-factors increase significantly from residue 460 to 464, so for that reason 459 and 460 were allowed to repack, and de novo sequences were generated for subsequent residues. 76 remodeled sequences were generated, ranging from six (6) to 26 residues in length. The designs generally improve hydrophobic packing, particularly at position 470 and 471. Some short remodeled sequences had excellent predicted ddG’s but from the ddG plot the optimal length is ~12-14 residues (FIG.7).

[0373] PIV3: The input for PIV3 was 8DG8 (Ref.5). There are no glycans in the PIV3 C-terminal helical bundle. The cryo-EM map quality deteriorates progressively along the length of the C- terminal helices and there is no side-chain resolved after residue 469. There is some sub-optimal packing at position 460, and so this position was allowed to design when using Rosetta remodel. Because residue 461 makes native contacts with the rest of the ectodomain its identity was preserved, and subsequent positions were allowed to design de novo. The residues after position 468 were removed. RFdiffusion does not allow for extension and partial diffusion simultaneously, so diffusion models start at residue 465. Ten (10) sequences were generated by Rosetta Remodel and 44 sequences by diffusion. The optimal length was 14-16 residues (FIG. 7). Therefore, remodeled lengths of 14 or more were selected for RFdiffusion.

[0374] Nipah: The input for Nipah was 7UP9 (Ref. 6). Nipah contains a glycan at residue 464 which was preserved in all designs. Because Nipah has a low-entropy methionine at residue 463, and no significant contacts with the rest of the ectodomain, remodel and diffusion both were allowed to design de...

Claims

CLAIMS What is claimed is:

1. A recombinant polypeptide, comprising an engineered ectodomain of a trimeric viral protein, wherein the ectodomain comprises: a C-terminal helix forming segment comprising one or more amino acid substitutions, relative to a native reference sequence of the viral protein, selected such that the segment forms a stable alpha-helical homotrimer.

2. The recombinant polypeptide of claim 1, wherein the C-terminal helix forming segment has improved hydrophobic packing compared to the native reference sequence.

3. The recombinant polypeptide of claim 1 or claim 2, wherein the C-terminal helix forming segment comprises between about 7 and about 31 residues.

4. The recombinant polypeptide of any one of claims 1 to 3, wherein the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids.

5. The recombinant polypeptide of any one of claims 1 to 4, wherein the C-terminal helix forming segment comprises a polypeptide sequence according to any one of: LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566) LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567) LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568) EKIXXAIKKAXKL (SEQ ID NO: 569) EXIXKAIKXLXXXXX (SEQ ID NO: 570) XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571) XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

6. The recombinant polypeptide of any one of claims 1 to 5, wherein the C-terminal helix forming segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

7. The recombinant polypeptide of any one of claims 1 to 6, wherein the segment comprises a polypeptide sequence according: a) L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573); b) L V [A / T] T X2K X2L X2D L I X2X2L [K / E] X2L L X2K L X2X2(SEQ ID NO: 574); orc) L N K V K K X2V X2X2L X2X2X2V X2X2L E K X2L X2(SEQ ID NO: 575), wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

8. The recombinant polypeptide of any one of claims 1 to 6, wherein segment comprises a polypeptide sequence according: a) E K I X2 X2 A I K K A X2 K L (SEQ ID NO: 576); b) E X2I X2K A I K X2L [L / X2] X2X2[X1 / X2] X2(SEQ ID NO: 577); and c) X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2 X2 X1 X2 X2 (SEQ ID NO: 578) ; or d) X2X2L K K A A X2I X1K K X1L K X2X2(SEQ ID NO: 579) , wherein X1is apolar residues selected from A, I, L, and M, wherein X2is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

9. The recombinant polypeptide of any one of claims 1 to 8, wherein the segment comprises a polypeptide sequence listed in Table 25A or Table 25B.

10. The recombinant polypeptide of any one of claims 1 to 9, wherein the native reference sequence of the viral protein is any one of SEQ ID NOs: 1, 104, 327, 382, 459, 499.

11. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a hMPV fusion (F) protein, wherein the ectodomain comprises a C-terminal helix- forming segment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

12. The polypeptide of claim 11, wherein the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 104 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

13. The polypeptide of claim 11 or claim 12, wherein the ectodomain comprises the C- terminal helix-forming segment, between about residue 470 and about residue 490 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

14. The polypeptide of any one of claims 11 to 13, wherein the C-terminal helix-forming segment comprises between about 7 and about 21 residues.

15. The polypeptide of any one of claims 11 to 14, wherein the segment comprises: (1) an amino acid substitution at position Q471 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, I, Q, R, S, T; (2) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, R, S, T, Y; (3) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of A, I, L, M, Q, S, T, W; (4) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of A, D, E, I, K, L, N, Q, S, T; (5) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of A, D, E, H, K, N, Q, R, S, T; (6) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of A, D, E, H, I, K, L, M, N, Q, T, V; (7) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, E, I, K, L, M, N, Q, R, S, T, V; (8) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of A, D, E, K, N, Q, R, S, T; (9) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substituted with any one of A, D, E, F, I, K, L, M, N, Q, R, S, T, W, Y; (10) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of A, I, L, M, R, S, T, V; (11) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any one of D, E, I, K, L, M, N, Q, R, S, T; (12) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, K, Q, R, S, T; (13) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of A, D, E, F, H, I, K, L, M, N, Q, R, S, T, V, W, Y; (14) an amino acid substitution at position A484 relative to SEQ ID NO: 104, wherein A is substituted with any one of A, D, E, I, K, L, M, R, S, T, V, Y;(15) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of D, E, G, K, L, Q, R, S, T; (16) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of A, E, I, K, L, Q, R, S, T; (17) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of A, E, I, K, L, R, S, T, V; (18) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, I, K, L, N, Q, R, S; (19) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of A, D, E, K, S; and / or (20) any combination of (1)-(19).

16. The polypeptide of any one of claims 11 to 15, wherein the segment comprises a polypeptide sequence listed in Table 6B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

17. The polypeptide of any one of claims 11 to 16, wherein the ectodomain comprises the C-terminal helix-forming segment, between about residue 470 and about residue 500 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

18. The polypeptide of any one of claims 11 to 17, wherein the C-terminal helix-forming segment comprises between about 16 and about 30 residues.

19. The polypeptide of any one of claims 11 to 18, wherein the segment comprises: (1) an amino acid substitution at position A472 relative to SEQ ID NO: 104, wherein A is substituted with any one of T, N, K, R, E, S; (2) an amino acid substitution at position L473 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, I, V; (3) an amino acid substitution at position V474 relative to SEQ ID NO: 104, wherein V is substituted with any one of E, Q, L, D; (4) an amino acid substitution at position D475 relative to SEQ ID NO: 104, wherein D is substituted with any one of E, D, K; (5) an amino acid substitution at position Q476 relative to SEQ ID NO: 104, wherein Q is substituted with any one of Q, R, D, A, T, S, K;(6) an amino acid substitution at position S477 relative to SEQ ID NO: 104, wherein S is substituted with any one of I, V, L; (7) an amino acid substitution at position N478 relative to SEQ ID NO: 104, wherein N is substituted with any one of K, E, N, S; (8) an amino acid substitution at position R479 relative to SEQ ID NO: 104, wherein R is substituted with any one of T, D, E, S, Y, A; (9) an amino acid substitution at position I480 relative to SEQ ID NO: 104, wherein I is substituted with any one of L, N; (10) an amino acid substitution at position L481 relative to SEQ ID NO: 104, wherein L is substituted with any one of T, D, E, K, Q, S, N; (11) an amino acid substitution at position S482 relative to SEQ ID NO: 104, wherein S is substituted with any one of E, D, S, T, Q, K; (12) an amino acid substitution at position S483 relative to SEQ ID NO: 104, wherein S is substituted with any one of R, K, L, E, A; (13) an amino acid substitution at position A484 relative to SEQ ID NO: 104, wherein A is substituted with any one of V, I, M; (14) an amino acid substitution at position E485 relative to SEQ ID NO: 104, wherein E is substituted with any one of E, A, K, H, Q, S, N; (15) an amino acid substitution at position K486 relative to SEQ ID NO: 104, wherein K is substituted with any one of S, E, K, R, V, D, H; (16) an amino acid substitution at position G487 relative to SEQ ID NO: 104, wherein G is substituted with any one of I, L; (17) an amino acid substitution at position N488 relative to SEQ ID NO: 104, wherein N is substituted with any one of E, K, R; (18) an amino acid substitution at position T489 relative to SEQ ID NO: 104, wherein T is substituted with any one of K, E, S, R, Q; (19) an amino acid substitution at position S490 relative to SEQ ID NO: 104, wherein S is substituted with any one of E, V, T, R, L; (20) an amino acid substitution at position G491 relative to SEQ ID NO: 104, wherein G is substituted with any one of GL, I, V;(21) an amino acid substitution at position R492 relative to SEQ ID NO: 104, wherein R is substituted with any one of E, Q, S, A, D; (22) an amino acid substitution at position E493 relative to SEQ ID NO: 104, wherein E is substituted with any one of A, E, N, L, K, Q, S; (23) an amino acid substitution at position N494 relative to SEQ ID NO: 104, wherein N is substituted with any one of I, L; (24) an amino acid substitution at position L495 relative to SEQ ID NO: 104, wherein L is substituted with any one of K, L, T, V, I; (25) an amino acid substitution at position Y496 relative to SEQ ID NO: 104, wherein Y is substituted with any one of K, E, R, Q; (26) an amino acid substitution at position F497 relative to SEQ ID NO: 104, wherein F is substituted with any one of D, R, E, Q; (27) an amino acid substitution at position Q498 relative to SEQ ID NO: 104, wherein Q is substituted with any one of V, L; and / or (28) any combination of (1)-(27).

20. The polypeptide of any one of claims 11 to 19, wherein the segment comprises a polypeptide sequence listed in Table 6D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

21. The polypeptide of any one of claims 11 to 20, wherein the ectodomain further comprises: one, two, three or more amino acid substitutions at positions 63, 97, 98, 99, 100, 101, 102, 140, 147, 153, 185, 188, 219, 231, 294, 365, 368, 450, 463, or 470 relative to SEQ ID NO:

104.

22. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a PIV3 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix- forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 327, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

23. The polypeptide of claim 22, wherein the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 327 at two or more, threeor more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

24. The polypeptide of claim 22 or claim 23, wherein the ectodomain comprises the C- terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 104, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

25. The polypeptide of any one of claims 22 to 24, wherein the C-terminal helix-forming segment comprises between about 20 and about 28 residues.

26. The polypeptide of any one of claims 22 to 25, wherein the segment comprises: (1) an amino acid substitution at position L460 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, M, V; (2) an amino acid substitution at position N461 relative to SEQ ID NO: 327, wherein N is substituted with N; (3) an amino acid substitution at position K462 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R; (4) an amino acid substitution at position V463 relative to SEQ ID NO: 327, wherein V is substituted with any one of L, V, T; (5) an amino acid substitution at position K464 relative to SEQ ID NO: 327, wherein K is substituted with any one of A, K, Q; (6) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, S; (7) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of E, K; (8) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of V, L, T; (9) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, D, E; (10) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of T, Q, K, E; (11) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L, M, Y, F, W;(12) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of L, W, A, I; (13) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, E; (14) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of E, I, K, Q; (15) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of L, M, T, V, E; (16) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of S, K, R, A; (17) an amino acid substitution at position R476 relative to SEQ ID NO: 327, wherein R is substituted with any one of K, E, S, N; (18) an amino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of K, D, E; and / or (19) any combination of (1)-(18).

27. The polypeptide of any one of claims 22 to 26, wherein the segment comprises a polypeptide sequence listed in Table 7B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

28. The polypeptide of any one of claims 22 to 27, wherein the ectodomain comprises the C-terminal helix-forming segment, between about residue 465 and about residue 490 relative to SEQ ID NO: 327, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

29. The polypeptide of any one of claims 22 to 28, wherein the C-terminal helix-forming segment comprises between about 14 and about 30 residues.

30. The polypeptide of any one of claims 22 to 29, wherein the segment comprises: (1) an amino acid substitution at position S465 relative to SEQ ID NO: 327, wherein S is substituted with any one of E, K, D, S, N, Q, T, R, A; (2) an amino acid substitution at position D466 relative to SEQ ID NO: 327, wherein D is substituted with any one of D, R, K, E, M, Q, A, S, N; (3) an amino acid substitution at position L467 relative to SEQ ID NO: 327, wherein L is substituted with any one of I, V, L;(4) an amino acid substitution at position E468 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, S, D, R, H, T, N, A; (5) an amino acid substitution at position E469 relative to SEQ ID NO: 327, wherein E is substituted with any one of K, S, E, N, T, Q, H, D, Y; (6) an amino acid substitution at position S470 relative to SEQ ID NO: 327, wherein S is substituted with any one of L, D, V, I, A, N, T; (7) an amino acid substitution at position K471 relative to SEQ ID NO: 327, wherein K is substituted with any one of E, T, L, K, N, I, R, Q, S; (8) an amino acid substitution at position E472 relative to SEQ ID NO: 327, wherein E is substituted with any one of E, K, Q, S, H, R, T; (9) an amino acid substitution at position W473 relative to SEQ ID NO: 327, wherein W is substituted with any one of R, Q, K, E, T, S, I, N; (10) an amino acid substitution at position Y474 relative to SEQ ID NO: 327, wherein Y is substituted with any one of V, L, I, Q, T; (11) an amino acid substitution at position R475 relative to SEQ ID NO: 327, wherein R is substituted with any one of H, K, D, T, E, S, R, N, Q, A; (12) an amino acid substitution at position R476 relative to SEQ ID NO: 327, wherein R is substituted with any one of A, T, H, E, D, K, R, Q, S; (13) an amino acid substitution at position S477 relative to SEQ ID NO: 327, wherein S is substituted with any one of I, L,V; (14) an amino acid substitution at position N478 relative to SEQ ID NO: 327, wherein N is substituted with any one of E, L, K, I, R, S, Q; (15) an amino acid substitution at position Q479 relative to SEQ ID NO: 327, wherein Q is substituted with any one of K, H, E, N, Q, R, T, A, S; (16) an amino acid substitution at position K480 relative to SEQ ID NO: 327, wherein K is substituted with any one of K, R, E, T, S, L, A, I, V; (17) an amino acid substitution at position L481 relative to SEQ ID NO: 327, wherein L is substituted with any one of L, V, I; (18) an amino acid substitution at position D482 relative to SEQ ID NO: 327, wherein D is substituted with any one of K, A, E, S, H, T, N, D, R;(19) an amino acid substitution at position S483 relative to SEQ ID NO: 327, wherein S is substituted with any one of Q, T, E, A, S, N, D, K, L; (20) an amino acid substitution at position I484 relative to SEQ ID NO: 327, wherein I is substituted with any one of I, L, A, V; (21) an amino acid substitution at position G485 relative to SEQ ID NO: 327, wherein G is substituted with any one of L, K, R, E, I; (22) an amino acid substitution at position S486 relative to SEQ ID NO: 327, wherein S is substituted with any one of T, A, E, R, H, D, S; and / or (23) any combination of (1)-(22).

31. The polypeptide of any one of claims 22 to 30, wherein the segment comprises a polypeptide sequence listed in Table 7D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

32. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a PIV5 fusion (F) protein, wherein the ectodomain comprises a C-terminal helix- forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 382, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

33. The polypeptide of claim 32, wherein the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 382 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

34. The polypeptide of claim 32 to 33, wherein the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 480 relative to SEQ ID NO: 382, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

35. The polypeptide of any one of claims 32 to 34, wherein the C-terminal helix-forming segment comprises between about 6 and about 26 residues.

36. The polypeptide of any one of claims 32 to 35, wherein the segment comprises: (1) an amino acid substitution at position A463 relative to SEQ ID NO: 382, wherein A is substituted with any one of L, T, V, A;(2) an amino acid substitution at position L464 relative to SEQ ID NO: 382, wherein L is substituted with any one of K, I, Q, A, W, E; (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 382, wherein Q is substituted with any one of K, Q, T, E, S, R; (4) an amino acid substitution at position H466 relative to SEQ ID NO: 382, wherein H is substituted with any one of K, A, E, L, I, W, R, Q, T, D, Y; (5) an amino acid substitution at position L467 relative to SEQ ID NO: 382, wherein L is substituted with any one of V, I, L, M, FA, T, C, H; (6) an amino acid substitution at position A468 relative to SEQ ID NO: 382, wherein A is substituted with any one of D, T, K, L, E, R, I, N, S; (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 382, wherein Q is substituted with any one of E, K, S, T, A, R, Q, D; (8) an amino acid substitution at position S470 relative to SEQ ID NO: 382, wherein S is substituted with any one of A, K, L, I, T, S, V, H, Y, E, W, FR, Q, M; (9) an amino acid substitution at position D471 relative to SEQ ID NO: 382, wherein D is substituted with any one of T, E, V, L, S, I, A, K, Y, W; (10) an amino acid substitution at position T472 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, E, R, S, T, A, D, L; (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 382, wherein Y is substituted with any one of T, K, S, R, Q, D, E, I, H, M; (12) an amino acid substitution at position L474 relative to SEQ ID NO: 382, wherein L is substituted with any one of T, S, L, A, D, W, Q, I, Y, V, K, E; (13) an amino acid substitution at position S475 relative to SEQ ID NO: 382, wherein S is substituted with any one of T, E, I, K, S, Q, A, L, R, D; (14) an amino acid substitution at position A476 relative to SEQ ID NO: 382, wherein A is substituted with any one of R, K, A, S, E, I, T, D, Q; (15) an amino acid substitution at position I477 relative to SEQ ID NO: 382, wherein I is substituted with any one of K, Q, R, D, T, E, I, Y, S, L; (16) an amino acid substitution at position T478 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, K, S, D, W, L, Q, I, T;(17) an amino acid substitution at position S479 relative to SEQ ID NO: 382, wherein S is substituted with any one of R, K, Q, S, A, D, E; (18) an amino acid substitution at position A480 relative to SEQ ID NO: 382, wherein A is substituted with any one of S, K; (19) an amino acid substitution at position T481 relative to SEQ ID NO: 382, wherein T is substituted with any one of E, D, S, K, M, N, A, T; (20) an amino acid substitution at position T482 relative to SEQ ID NO: 382, wherein T is substituted with any one of R, S, Q, L, K; (21) an amino acid substitution at position T483 relative to SEQ ID NO: 382, wherein T is substituted with any one of K, A, S; (22) an amino acid substitution at position S484 relative to SEQ ID NO: 382, wherein S is substituted with any one of S, E, D, Y; (23) an amino acid substitution at position V485 relative to SEQ ID NO: 382, wherein V is substituted with any one of Q, T; (24) an amino acid substitution at position L486 relative to SEQ ID NO: 382, wherein L is substituted with K; (25) an amino acid substitution at position S487 relative to SEQ ID NO: 382, wherein S is substituted with any one of S, K; (26) an amino acid substitution at position I488 relative to SEQ ID NO: 382, wherein I is substituted with any one of S, K; and / or (27) any combination of (1)-(26).

37. The polypeptide of any one of claims 32 to 36, wherein the segment comprises a polypeptide sequence listed in Table 8B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

38. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a SARS-CoV2 spike (S) protein, wherein the ectodomain comprises a C-terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

39. The polypeptide of claim 38, wherein the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 459 at two or more, threeor more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

40. The polypeptide of claim 38 or claim 39, wherein the ectodomain comprises the C- terminal helix-forming segment, between about residue 1140 and about residue 1170 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

41. The polypeptide of any one of claims 38 to 40, wherein the C-terminal helix-forming segment comprises between about 10 and about 25 residues.

42. The polypeptide of any one of claims 38 to 41, wherein the segment comprises: (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of E, D, K; (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S is substituted with any one of T, S, K; (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with A; (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any one of I, A, L, M; (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of K, S, D, R, E; (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of I, Y, K, T, R, E; (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of T, A; (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of L, I, E, T, M, V; (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of E, K, T, R; (10) an amino acid substitution at position Y1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of I, V, K, R; (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of V, A, I, Y, T, S;(12) an amino acid substitution at position K1158 relative to SEQ ID NO: 459, wherein K is substituted with any one of L, R, S, K, D, W, N, I; (13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of K, T, Q, I, R, E; (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of I, L, R, E, K, S; (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, N, I, A, S, W, Y; (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of K, S, T, R; (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, D, R, K, I, A; (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of W, S, M, D, T, I, N; (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of E, A, K, L; (20) an amino acid substitution at position D1166 relative to SEQ ID NO: 459, wherein D is substituted with any one of K, S; (21) an amino acid substitution at position L1167 relative to SEQ ID NO: 459, wherein L is substituted with any one of R, K; (22) an amino acid substitution at position G1168 relative to SEQ ID NO: 459, wherein G is substituted with any one of K, S; (23) an amino acid substitution at position D1169 relative to SEQ ID NO: 459, wherein D is substituted with S; (24) an amino acid substitution at position I1170 relative to SEQ ID NO: 459, wherein I is substituted with S; and / or (25) any combination of (1)-(24).

43. The polypeptide of any one of claims 38 to 42, wherein the segment comprises a polypeptide sequence listed in Table 9B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

44. The polypeptide of any one of claims 38 to 43, wherein the ectodomain comprises the C-terminal helix-forming segment, between about residue 1145 and about residue 1175 relative to SEQ ID NO: 459, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

45. The polypeptide of any one of claims 38 to 44, wherein the C-terminal helix-forming segment comprises between about 12 and about 22 residues.

46. The polypeptide of any one of claims 38 to 45, wherein the segment comprises: (1) an amino acid substitution at position D1147 relative to SEQ ID NO: 459, wherein D is substituted with any one of Q, T, E, S, N, D, K; (2) an amino acid substitution at position S1148 relative to SEQ ID NO: 459, wherein S is substituted with any one of T, K, N, R, S, A, E; (3) an amino acid substitution at position F1149 relative to SEQ ID NO: 459, wherein F is substituted with any one of L, T, I, V; (4) an amino acid substitution at position K1150 relative to SEQ ID NO: 459, wherein K is substituted with any one of K, Q, R, H, S, E; (5) an amino acid substitution at position E1151 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, N, A, S, K, T, D; (6) an amino acid substitution at position E1152 relative to SEQ ID NO: 459, wherein E is substituted with any one of E, T, V, R, K, N; (7) an amino acid substitution at position L1153 relative to SEQ ID NO: 459, wherein L is substituted with any one of S, V, T, A; (8) an amino acid substitution at position D1154 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, L, E, I, V; (9) an amino acid substitution at position K1155 relative to SEQ ID NO: 459, wherein K is substituted with any one of H, E, S, T, Q; (10) an amino acid substitution at position Y1156 relative to SEQ ID NO: 459, wherein Y is substituted with any one of L, E, I, T, A, S, R; (11) an amino acid substitution at position F1157 relative to SEQ ID NO: 459, wherein F is substituted with any one of T, V, I, A, S, M; (12) an amino acid substitution at position K1158 relative to SEQ ID NO: 459, wherein K is substituted with any one of K, E, N, R, T, A, Q, I;(13) an amino acid substitution at position N1159 relative to SEQ ID NO: 459, wherein N is substituted with any one of T, E, A, K, Q; (14) an amino acid substitution at position H1160 relative to SEQ ID NO: 459, wherein H is substituted with any one of L, M, A, E, T, Y, I, S; (15) an amino acid substitution at position T1161 relative to SEQ ID NO: 459, wherein T is substituted with any one of L, I; (16) an amino acid substitution at position S1162 relative to SEQ ID NO: 459, wherein S is substituted with any one of S, R, K, N, E, Q; (17) an amino acid substitution at position P1163 relative to SEQ ID NO: 459, wherein P is substituted with any one of E, S, T, R; (18) an amino acid substitution at position D1164 relative to SEQ ID NO: 459, wherein D is substituted with any one of T, M, A; (19) an amino acid substitution at position V1165 relative to SEQ ID NO: 459, wherein V is substituted with any one of A, L; and / or (20) any combination of (1)-(19).

47. The polypeptide of any one of claims 38 to 46, wherein the segment comprises a polypeptide sequence listed in Table 9D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

48. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a Nipah fusion (F) protein, wherein the ectodomain comprises a C-terminal helix- forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

49. The polypeptide of claim 48, wherein the C-terminal helix-forming segment comprises substitutions relative to the reference sequence SEQ ID NO: 499 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

50. The polypeptide of claim 48 or claim 49, wherein the ectodomain comprises the C- terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: NopahX, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

51. The polypeptide of any one of claims 48 to 50, wherein the C-terminal helix-forming segment comprises between about 16 and about 33 residues.

52. The polypeptide of any one of claims 48 to 51, wherein the segment comprises: (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of I, A, T, L, M; (2) an amino acid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N; (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, K, T, S, I, D; (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S; (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of M, L, I, V, T; (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, K, A, T, S, D, R, I, Q; (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of R, S, K, T, E, Q; (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of T, L, I, V, A; (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, A, W, E, L, I; (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, R, Q, E; (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of W, D, K, Y, I, M, E, T; (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of I, V, M, L, A; (13) an amino acid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of T, K, M, R, E, L, A, S; (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, S, A, E, T, D;(15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of L, I, V, FT, A, M, W, K, Y; (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of I, K, A, L, E, D, S, Y; (17) an amino acid substitution at position R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of A, S, K, R, T, L, E; (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of K, E, R, Y, T, Q; (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of W, I, V, L, E, S, Q, A, T ; (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, Q, E, W, T, S, A; (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of S, T, K, R, Q; (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of R, E, I, S, Y, L, K, D; (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of I, R, K, W, E, T; (24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of A, T, R, K, Q; (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of K, R, T, S; (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of E, V, L, K; (27) an amino acid substitution at position I489 relative to SEQ ID NO: 499, wherein I is substituted with any one of E, Q, L, K, R; and / or (28) any combination of (1)-(27).

53. The polypeptide of any one of claims 48 to 52, wherein the segment comprises a polypeptide sequence listed in Table 10B, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

54. The polypeptide of any one of claims 48 to 53, wherein the ectodomain comprises the C-terminal helix-forming segment, between about residue 460 and about residue 490 relative to SEQ ID NO: 499, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

55. The polypeptide of any one of claims 48 to 54, wherein the C-terminal helix-forming segment comprises between about 12 and about 26 residues.

56. The polypeptide of any one of claims 48 to 55, wherein the segment comprises: (1) an amino acid substitution at position M463 relative to SEQ ID NO: 499, wherein M is substituted with any one of L, I, V; (2) an amino acid substitution at position N464 relative to SEQ ID NO: 499, wherein N is substituted with N; (3) an amino acid substitution at position Q465 relative to SEQ ID NO: 499, wherein Q is substituted with any one of Q, T, N, D, E, S, K, R, A; (4) an amino acid substitution at position S466 relative to SEQ ID NO: 499, wherein S is substituted with S; (5) an amino acid substitution at position L467 relative to SEQ ID NO: 499, wherein L is substituted with any one of I, V, L, A; (6) an amino acid substitution at position Q468 relative to SEQ ID NO: 499, wherein Q is substituted with any one of S, K, T, D, E, R, Q; (7) an amino acid substitution at position Q469 relative to SEQ ID NO: 499, wherein Q is substituted with any one of S, Q, N, E, A, D, K, T, R; (8) an amino acid substitution at position S470 relative to SEQ ID NO: 499, wherein S is substituted with any one of L, A, I, V, N; (9) an amino acid substitution at position K471 relative to SEQ ID NO: 499, wherein K is substituted with any one of E, Q, S, R, K, A, T, D, L, N; (10) an amino acid substitution at position D472 relative to SEQ ID NO: 499, wherein D is substituted with any one of K, T, E, Q, D, N, S, A; (11) an amino acid substitution at position Y473 relative to SEQ ID NO: 499, wherein Y is substituted with any one of A, S, R, T, V, E, I, K, L, D, Q; (12) an amino acid substitution at position I474 relative to SEQ ID NO: 499, wherein I is substituted with any one of L, I, V;(13) an amino acid substitution at position K475 relative to SEQ ID NO: 499, wherein K is substituted with any one of K, H, D, T, A, S, R, Q, E, N; (14) an amino acid substitution at position E476 relative to SEQ ID NO: 499, wherein E is substituted with any one of K, R, S, E, A, T, H, D; (15) an amino acid substitution at position A477 relative to SEQ ID NO: 499, wherein A is substituted with any one of A, L, I, V; (16) an amino acid substitution at position Q478 relative to SEQ ID NO: 499, wherein Q is substituted with any one of E, L, T, R, K, Q, S, I; (17) an amino acid substitution at position R479 relative to SEQ ID NO: 499, wherein R is substituted with any one of K, N, E, Q, S, T, H, R, A; (18) an amino acid substitution at position L480 relative to SEQ ID NO: 499, wherein L is substituted with any one of D, L, E, K, T, R, V, I, Q; (19) an amino acid substitution at position L481 relative to SEQ ID NO: 499, wherein L is substituted with any one of L, V, I; (20) an amino acid substitution at position D482 relative to SEQ ID NO: 499, wherein D is substituted with any one of E, K, N, D, L, Q, H; (21) an amino acid substitution at position T483 relative to SEQ ID NO: 499, wherein T is substituted with any one of E, K, S, Q, A, T; (22) an amino acid substitution at position V484 relative to SEQ ID NO: 499, wherein V is substituted with any one of V, L, I; (23) an amino acid substitution at position N485 relative to SEQ ID NO: 499, wherein N is substituted with any one of R, K, L, V, E, Q, I; (24) an amino acid substitution at position P486 relative to SEQ ID NO: 499, wherein P is substituted with any one of R, E, A, S, L; (25) an amino acid substitution at position S487 relative to SEQ ID NO: 499, wherein S is substituted with any one of Q, R, T, S, L; (26) an amino acid substitution at position L488 relative to SEQ ID NO: 499, wherein L is substituted with any one of L; and / or (27) any combination of (1)-(26).

57. The polypeptide of any one of claims 48 to 56, wherein the segment comprises a polypeptide sequence listed in Table 10D, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

58. The recombinant polypeptide of any one of claims 1 to 10, comprising an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises: (a) a C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer; (b) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1; (c) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1; (d) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1; (e) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1; (f) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1; (g) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1; or (h) any combination of (a)-(g).

59. The polypeptide of claim 58, wherein the ectodomain comprises (a) the C-terminal helix-forming segment, between about residue 500 and about residue 530 relative to SEQ ID NO: 1, comprising one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

60. The polypeptide of claim 59, wherein the C-terminal helix-forming segment comprises between about 10 and about 30 residues.

61. The polypeptide of claim 59, wherein the segment comprises substitutions relative to the reference sequence SEQ ID NO: 1 at two or more, three or more, or four or more residues that generate hydrophobic contacts between the segments in the alpha-helical homotrimer.

62. The polypeptide of claim 59, wherein the segment comprises: (1) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with A, I, L, M, V, G, T; (2) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (3) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y or A, I, L, V; (4) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with K, Q, R, preferably A, V, T, I; (5) an amino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with A, I, L, M, V, F, W, Y, G, T, preferably A, I, L, M, V; (6) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with any amino acids, preferably D, E, K, N, Q, R, S, T, Y; (7) an amino acid substitution at position E511 relative to SEQ ID NO: 1, wherein E is substituted with any amino acids; (8) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with D, E, K, N, Q, R, S, T, Y , preferably A, I, L, M, V, F, W, Y, G, T; (9) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with any amino acids, preferably A, I, L, M, V, F, W, Y, G , more preferably D, E, K, N, Q, R, S, T, Y; (10) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with any amino acids except P, preferably D, E, K, N, Q, R, S, T, Y; (11) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with any amino acids except P, preferably A, I, L, M, V, F, W, Y, G; (12) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with A, I, L, M, V, F, W, Y, G, or T, S, K;(13) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; (14) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with Any except P, preferably D, E, K, N, Q, R, S, T, Y; (15) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with any amino acid except P, preferably D, E, K, N, Q, R, S, T, Y; and / or (16) any combination of (1)-(15).

63. The polypeptide of claim 59, wherein the segment comprises: (1) an amino acid substitution at position L503 relative to SEQ ID NO: 1, wherein F is substituted with Q, V, K, R, N, L; (2) an amino acid substitution at position A504 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably S, T, L, A, Q, K, E, Y; (3) an amino acid substitution at position F505 relative to SEQ ID NO: 1, wherein F is substituted with I, V, N, T, L; (4) an amino acid substitution at position I506 relative to SEQ ID NO: 1, wherein I is substituted with any amino acids except P, preferably Q, N, K, R, V, S; (5) an amino acid substitution at position R507 relative to SEQ ID NO: 1, wherein R is substituted with any amino acids except P, preferably A, N, K, E, D, Q; (6) an amino acid substitution at position K508 relative to SEQ ID NO: 1, wherein R is substituted with T, M, V, R; (7) an amino acid substitution at position S509 relative to SEQ ID NO: 1, wherein S is substituted with T, I, K, Q, M, E, V, S; (8) an amino acid substitution at position D510 relative to SEQ ID NO: 1, wherein D is substituted with S, K, N, D, E; (9) an amino acid substitution at position E511 relative to SEQ ID NO: 1, wherein E is substituted with R, S, E, K, A, T, L; (10) an amino acid substitution at position L512 relative to SEQ ID NO: 1, wherein L is substituted with V, N, T, L;(11) an amino acid substitution at position L513 relative to SEQ ID NO: 1, wherein L is substituted with D, T, H, K, E, N, R; (12) an amino acid substitution at position H514 relative to SEQ ID NO: 1, wherein H is substituted with A, N, E, S, V, K, T, D; (13) an amino acid substitution at position N515 relative to SEQ ID NO: 1, wherein N is substituted with I, E, L, T, Q; (14) an amino acid substitution at position V516 relative to SEQ ID NO: 1, wherein V is substituted with E, I, K, N, R, Q; (15) an amino acid substitution at position N517 relative to SEQ ID NO: 1, wherein N is substituted with A, S, K, E, R; (16) an amino acid substitution at position T518 relative to SEQ ID NO: 1, wherein T is substituted with K, S, Q, R, D, E; (17) an amino acid substitution at position G519 relative to SEQ ID NO: 1, wherein G is substituted with V, L, I; (18) an amino acid substitution at position I520 relative to SEQ ID NO: 1, wherein G is substituted with K, Q, E, N, T ; (19) an amino acid substitution at position P521 relative to SEQ ID NO: 1, wherein G is substituted with H, D, E, K, R, N, Q; (20) an amino acid substitution at position E522 relative to SEQ ID NO: 1, wherein G is substituted with L, R, I, V; (21) an amino acid substitution at position A523 relative to SEQ ID NO: 1, wherein G is substituted with E, V, L, K, R I; (22) an amino acid substitution at position P524 relative to SEQ ID NO: 1, wherein G is substituted with A, K, T, E, R; (23) an amino acid substitution at position R525 relative to SEQ ID NO: 1, wherein G is substituted with H, R, S, L, N, E, D; (24) an amino acid substitution at position D526 relative to SEQ ID NO: 1, wherein G is substituted with I, L, V, R; (25) an amino acid substitution at position G527 relative to SEQ ID NO: 1, wherein G is substituted with E, K, Q, D;(26) an amino acid substitution at position Q528 relative to SEQ ID NO: 1, wherein G is substituted with D, K, S, R, A; (27) an amino acid substitution at position A529 relative to SEQ ID NO: 1, wherein G is substituted with T, L; (28) an amino acid substitution at position Y530 relative to SEQ ID NO: 1, wherein G is substituted with L, E, T; (29) an amino acid substitution at position V531 relative to SEQ ID NO: 1, wherein G is substituted with A, R, K; (30) an amino acid substitution at position R532 relative to SEQ ID NO: 1, wherein G is substituted with V, A; and / or (31) any combination of (1)-(30).

64. The polypeptide of claim 59, wherein the segment comprises a polypeptide sequence listed in Table 2B or Table 2C, or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

65. The polypeptide of claim 59, wherein the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

66. The polypeptide of claim 59, wherein the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10) or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

67. The polypeptide of claim 59, wherein the segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10).

68. The polypeptide of any one of claims 58 to 67, wherein the ectodomain comprises (b) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO:

1.

69. The polypeptide of any one of claims 58 to 67, wherein the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: E487R + K498A; E487R + K498E;E487K + K498E; D486A + E487R + K498A; D486Q + E487R + K498A; D486E + E487A + D489A + T400D; D486A + E487M + K498A; E487Q; D486S; F488W + D489A + T400D + E487R + K498A; F140W + D489A + T400D + E487R + K498A; Q494I + S485I + K399A + 487R + 498A; Q494M + S485I + K399A; D486A + 487M + 498A; Q494L + S485A + K399V + D486A + 487M + 498A; Q494M + S485A + K399V + D486A + 487M + 498A; Q494A + S485F + K399V + D486A + 487M + 498Y; D489A + T400D + E487R + K498A; or D489A + T400D.

70. The polypeptide of claim 68, wherein the ectodomain comprises the amino acid substitutions D489A, T400D, E487R, and K498A.

71. The polypeptide of claim 68, wherein the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A.

72. The polypeptide of claim 68, wherein the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P.

73. The polypeptide of any one of claims 58 to 72, wherein the polypetide comprises, C- terminal to the ectodomain, a heterologous multimerization domain.

74. The polypeptide of claim 73, wherein the multimerization domain is a trimerization domain.

75. The polypeptide of claim 73 or claim 74, wherein the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64).

76. The polypeptide of any one of claims 58 to 75, wherein the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L .

77. The polypeptide of any one of claims 58 to 76, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIK QELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKR RFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTF RVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTT PLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYV VQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQA DTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGK NLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 6), optionally lacking a p27 peptide shown in bold.

78. The polypeptide of any one of claims 58 to 77, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIK QELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKR RFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTF KVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTT PVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYV VQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQA ETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGK SLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 7), optionally lacking a p27 peptide shown in bold.

79. The polypeptide of any one of claims 58 to 77, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIK QELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKR RFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTF RVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTTPLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYV VQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQA DTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGK NLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 8), optionally lacking a p27 peptide shown in bold.

80. The polypeptide of any one of claims 58 to 77, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIK QELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKR RFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTF KVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTT PVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYV VQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQA ETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGK SLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 9), optionally lacking a p27 peptide shown in bold.

81. The polypeptide of any one of claims 58 to 77, wherein the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1-9.

82. A trimeric protein complex comprising a polypeptide according to any one of claims 1 to 81.

83. The trimeric protein complex of claim 81, wherein thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (a)-(h).

84. The trimeric protein complex of claim 82 wherein stability, assayed by storage at about 40°C, is increased compared to a trimeric protein complex lacking modifications (a)-(h).

85. The trimeric protein complex of claim 83, wherein the thermal stability is increased compared to a reference RSV F protein comprising amino acid substitutions consisting essentially of S155C, S290C, S190F, and V207L (DS-Cav1).

86. A protein nanostructure comprising a trimeric component comprising a polypeptide according to any one of claims 1 to 81.

87. The nanostructure of claim 86, wherein the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component.

88. The nanostructure of claim 87, wherein the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide.

89. The nanostructure of any one of claims 86 to 88, wherein the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the RSV fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide.

90. The nanostructure of any one of claims 86 to 89, wherein the nanostructure is a two- component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1 and a C-terminal helix- forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10); and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

91. The nanostructure of any one of claims 86 to 89, wherein the nanostructure is a two- component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, D489A, T400D, E487R, and K498A relative to SEQ IDNO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10); and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

92. The nanostructure of any one of claims 86 to 89, wherein the nanostructure is a two- component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and T249P relative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10); and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

93. The nanostructure of any one of claims 86 to 89, wherein the nanostructure is a two- component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and D486Arelative to SEQ ID NO: 1 and a C-terminal helix-forming segment comprising the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10); and a multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53- 50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

94. The nanostructure of any one of claims 86 to 93, wherein the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences.

95. The nanostructure of any one of claims 86 to 94, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

96. A pharmaceutical composition comprising a polypeptide according to any one of claims 58 to 81, a protein complex according to any one of claims 82 to 85, or a nanostructure according to any one of claims 86 to 95.

97. The nanostructure of claim 86, wherein the nanostructure is a two-component nanostructure.

98. The nanostructure of claim 86, wherein the nanostructure is a one-component nanostructure.

99. The nanostructure of claim 97 or claim 98, wherein the nanostructure is a spherical nanostructure and / or a naturally occurring nanostructure, optionally a virus-like particle (VLP), optionally a Human papillomavirus VLP.

100. The nanostructure of claim 97 or claim 99, wherein the nanostructure is a two- component nanostructure comprising the first, trimeric component and a second, pentameric component.

101. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide.

102. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a human Metapneumovirus (hMPV) fusion (F) polypeptide and an I53-50A polypeptide.

103. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a human Parainfluenza virus type 3 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide.

104. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a human Parainfluenza virus type 5 (PIV3) fusion (F) polypeptide and an I53-50A polypeptide.

105. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a SARS-CoV-2 spike (S) polypeptide and an I53-50A polypeptide.

106. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises an engineered ectodomain of a Nipaha virus fusion (F) polypeptide and an I53-50A polypeptide.

107. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the engineered fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide.

108. The nanostructure of any proceeding claim, wherein the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the engineered spike (S) polypeptide, an amino acid linker, and the I53-50A polypeptide.

109. The nanostructure of any proceeding claim, wherein the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of the sequences listed in Table 19 or to any one of thesequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences.

110. The nanostructure of any proceeding claim, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

111. A polynucleotide encoding the polypeptide, protein complex, or nanostructure of any preceding claim.

112. A delivery vehicle comprising a polynucleotide encoding the polypeptide, protein complex, or nanostructure of any preceding claim, wherein optionally the delivery vehicle is a lipid nanoparticle (LNP).

113. A pharmaceutical composition or vaccine comprising a polypeptide, protein complex, or nanostructure, polynucleotide, or delivery vehicle according to any preceding claim.

114. A method of vaccinating a subject and / or generating an immune response in subject, comprising administering to the subject a composition according to any preceding claim.

115. A method of treating or preventing a viral infection in a subject, comprising administering to the subject a composition according to any preceding claim.

116. A composition according to any preceding claim for use in vaccinating, generating an immune response, or treating or preventing any viral infection disease.

117. A method of making a compositions according to any preceding claim, comprising culturing host cells modified to express one or more polypeptides as described herein.

118. A composition, method, or use as described herein.

119. A recombinant polypeptide for use in displaying a molecule such as an antigen, comprising an alpha-helical segment and a multimerization domain, wherein the alpha-helical segment comprises one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

120. The polypeptide of claim 119, wherein the alpha-helical segment has improved hydrophobic packing compared to a reference alpha-helical segment without the amino acid substitutions.

121. The polypeptide of claim 119 or claim 120, wherein the alpha-helical segment comprises between about 7 and about 31 residues.

122. The polypeptide of any one of claims 119 to 121, wherein the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids.

123. The polypeptide of any one of claims 119 to 122, wherein the alpha-helical segment comprises a polypeptide sequence according to any one of: LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566) LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567) LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568) EKIXXAIKKAXKL (SEQ ID NO: 569) EXIXKAIKXLXXXXX (SEQ ID NO: 570) XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571) XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

124. The polypeptide of any one of claims 119 to 123, wherein the alpha-helical segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

125. The polypeptide of any one of claims 119 to 124, wherein the alpha-helical segment comprises a polypeptide sequence according: a) L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573); b) L V [A / T] T X2K X2L X2D L I X2X2L [K / E] X2L L X2K L X2X2(SEQ ID NO: 574); or c) L N K V K K X2 V X2 X2 L X2 X2 X2 V X2 X2 L E K X2 L X2 (SEQ ID NO: 575), wherein X2is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

126. The polypeptide of any one of claims 119 to 124, wherein the alpha-helical segment comprises a polypeptide sequence according: a) E K I X2X2A I K K A X2K L (SEQ ID NO: 576); b) E X2 I X2 K A I K X2 L [L / X2] X2 X2 [X1 / X2] X2 (SEQ ID NO: 577); and c) X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2 X2 X1 X2 X2 (SEQ ID NO: 578) ; or d) X2X2L K K A A X2I X1K K X1L K X2X2(SEQ ID NO: 579) , wherein X1is apolar residues selected from A, I, L, and M, wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

127. The polypeptide of any one of claims 119 to 126, wherein the alpha-helical segment comprises a polypeptide sequence listed in Table 25A or Table 25B or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

128. The polypeptide of any one of claims 119 to 127, wherein the alpha-helical segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

129. The polypeptide of any one of claims 119 to 128, wherein the multimerization domain is I53-50A or a variant thereof.

130. The polypeptide of any one of claims 119 to 129, wherein the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64).

131. The polypeptide of any one of claims 119 to 130, wherein the polypeptide comprises an N-terminal fusion of the alpha-helical segment to the multimerization domain via a peptide bond or polypeptide linker.

132. The polypeptide of any one of claims 119 to 131, wherein the polypeptide comprises, N-terminal to the alpha-helical segment, an antigen polypeptide.

133. A polypeptide comprising an alpha-helical segment, wherein the alpha-helical segment comprises one or more amino acid substitutions selected such that the segment forms a stable alpha-helical homotrimer.

134. The polypeptide of claim 133, wherein the alpha-helical segment has improved hydrophobic packing compared to a reference alpha-helical segment without the amino acid substitutions.

135. The polypeptide of claim 133 or claim 134, wherein the alpha-helical segment comprises between about 7 and about 31 residues.

136. The polypeptide of any one of claims 133 to 135, wherein the amino acid substitutions comprise polar, charged and / or hydrophobic amino acids.

137. The polypeptide of any one of claims 133 to 136, wherein the alpha-helical segment comprises a polypeptide sequence according to any one of:LXXTIXXLLXIXXXLXXXL (SEQ ID NO: 566) LVXTXKXLXDLIXXLXXLLXKLXX (SEQ ID NO: 567) LNKVKKXVXXLXXXVXXLEKXLX (SEQ ID NO: 568) EKIXXAIKKAXKL (SEQ ID NO: 569) EXIXKAIKXLXXXXX (SEQ ID NO: 570) XKXXEXXXXVXXXXXXXXX (SEQ ID NO: 571) XXLKKAAXIXKKXLKXX (SEQ ID NO: 572).

138. The polypeptide of any one of claims 133 to 137, wherein the alpha-helical segment comprises a polypeptide sequence according to any one of the consensus sequences in Table 24.

138. The polypeptide of any one of claims 133 to 138, wherein the alpha-helical segment comprises a polypeptide sequence according: a) L X2 X2 T I X2 X2 L L X2 I [V / I] X2 X2 L [I / L] X2 X2 L (SEQ ID NO: 573); b) L V [A / T] T X2 K X2 L X2 D L I X2 X2 L [K / E] X2 L L X2 K L X2 X2 (SEQ ID NO: 574); or c) L N K V K K X2V X2X2L X2X2X2V X2X2L E K X2L X2(SEQ ID NO: 575), wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

140. The polypeptide of any one of claims 133 to 139, wherein the alpha-helical segment comprises a polypeptide sequence according: a) E K I X2X2A I K K A X2K L (SEQ ID NO: 576); b) E X2I X2K A I K X2L [L / X2] X2X2[X1 / X2] X2(SEQ ID NO: 577); and c) X2 K [X1 / T] [L / E] E [T / A] X1 X2 [I / X2] V X2 X2 [X1 / X2] [X1 / X2] X2 X2 X1 X2 X2 (SEQ ID NO: 578) ; or d) X2X2L K K A A X2I X1K K X1L K X2X2(SEQ ID NO: 579), wherein X1is apolar residues selected from A, I, L, and M, wherein X2 is polar and charged residues selected from S, T, N, Q, E, D, R, K, and H, preferably wild type amino acid.

141. The polypeptide of any one of claims 133 to 140, wherein the alpha-helical segment comprises a polypeptide sequence listed in Table 25A or Table 25B or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

142. The polypeptide of any one of claims 133 to 141, wherein the alpha-helical segment comprises the polypeptide sequence NQSREIIRAINIVRKIASEK (SEQ ID NO: 10), NQSALWLEAAKYVKQAREKS (SEQ ID NO: 11), NQSAKNAEAAKIAEETKRKD (SEQ ID NO: 12), or NQSRETAKAVSAVK (SEQ ID NO: 75), or a polypeptide sequence having between 1 and 5 amino acid substitutions thereto.

143. A protein nanostructure comprising a trimeric component comprising a polypeptide according to any one of claims 119 to 142.

144. The nanostructure of claim 143, wherein the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component.

145. The nanostructure of claim 143, wherein the nanostructure is a two-component nanostructure comprising a second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

146. A pharmaceutical composition comprising a polypeptide according to any one of claims 119 to 142 or a nanostructure according to any one of claims 143 to 145.

147. A vaccine comprising a polypeptide according to any one of claims 119 to 142 or a nanostructure according to any one of claims 143 to 145.

148. A method of vaccinating a subject, comprising administering to the subject a polypeptide according to any one of claims 119 to 142 or a nanostructure according to any one of claims 143 to 145.

149. A method of generating an immune response or treating or preventing a viral infection in a subject, comprising administering to the subject a polypeptide according to any one of claims 119 to 142, a nanostructure according to any one of claims 143 to 145, a pharmaceutical composition according to claim 146, or a vaccine according to 147.

150. A method of making a polypeptide according to any one of claims 119 to 142, a nanostructure according to any one of claims 143 to 145, a pharmaceutical composition according to claim 146, or a vaccine according to 147, comprising culturing host cells modified to express one or more polypeptides as described herein.

151. A recombinant polypeptide, comprising an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) protein, wherein the ectodomain comprises:(a) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO: 1; (b) one, two, three or more amino acid substitutions at positions 56, 58, 154, 187, 296, or 298 relative to SEQ ID NO: 1; (c) one, two, three or more amino acid substitutions at positions 75, 216, 218, or 219 relative to SEQ ID NO: 1; (d) one, two, three or more amino acid substitutions at positions 92, 232, 235, 238, 249, 250, or 254 relative to SEQ ID NO: 1; (e) one, two, three or more amino acid substitutions at positions 67, 137, or 339 relative to SEQ ID NO: 1; (f) a substitution of a non-cleavable linker in place of a furin cleavage site at about residue 100 to about residue 140 relative to SEQ ID NO: 1; or (g) any combination of (a)-(f).

152. The polypeptide of claim 152, wherein the ectodomain comprises (a) one, two, three or more amino acid substitutions at positions 140, 399, 400, 485, 486, 487, 488, 489, 494, or 498 relative to SEQ ID NO:

1.

153. The polypeptide of claim 151 or claim 152, wherein the ectodomain comprises one or more of the following sets of amino acid substitutions relative to SEQ ID NO: 1: E487R + K498A; E487R + K498E; E487K + K498E; D486A + E487R + K498A; D486Q + E487R + K498A; D486E + E487A + D489A + T400D; D486A + E487M + K498A; E487Q; D486S; F488W + D489A + T400D + E487R + K498A; F140W + D489A + T400D + E487R + K498A; Q494I + S485I + K399A + 487R + 498A; Q494M + S485I + K399A; D486A + 487M + 498A;Q494L + S485A + K399V + D486A + 487M + 498A; Q494M + S485A + K399V + D486A + 487M + 498A; Q494A + S485F + K399V + D486A + 487M + 498Y; D489A + T400D + E487R + K498A; or D489A + T400D.

154. The polypeptide of claim 152, wherein the ectodomain comprises the amino acid substitutions D489A, T400D, E487R, and K498A.

155. The polypeptide of claim 152, wherein the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and D486A.

156. The polypeptide of claim 152, wherein the ectodomain comprises the amino acid substitutions F488W, D489A, T400D, E487R, K498A, and T249P.

157. The polypeptide of any one of claims 151 to 156, wherein the polypeptide comprises a heterologous multimerization domain.

158. The polypeptide of claim 157, wherein the multimerization domain is a trimerization domain.

159. The polypeptide of claim 157 or claim 158, wherein the multimerization domain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64).

160. The polypeptide of any one of claims 151 to 159, wherein the ectodomain comprises the amino acid substitutions S155C, S290C, S190F, and V207L.

161. The polypeptide of any one of claims 151 to 160, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIK QELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKR RFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTF RVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTT PLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYV VQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQA DTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGKNLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 6), optionally lacking a p27 peptide shown in bold.

162. The polypeptide of any one of claims 151 to 161, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIK QELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKR RFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTF KVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTT PVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYV VQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQA ETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGK SLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSXXXXXXXXXXXXXXXXX (SEQ ID NO: 7), optionally lacking a p27 peptide shown in bold.

163. The polypeptide of any one of claims 151 to 161, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSRGYLSALRTGWYTSVITIELSNIKETKCNGTDTKVKLIK QELDKYKNAVTELQLLMQNTPAVNNRARREAPQYMNYTINTTKNLNVSISKKRKR RFLGFLLGVGSAIASGIAVCKVLHLEGEVNKIKNALQLTNKAVVSLSNGVSVLTF RVLDLKNYINNQLLPMLNRQSCRISNIETVIEFQQKNSRLLEITREFSVNAGVTT PLSTYMLTNSELLSLINDMPITNDQKKLMSSNVQIVRQQSYSIMCIIKEEVLAYV VQLPIYGVIDTPCWKLHTSPLCTTNIKEGSNICLTRTDRGWYCDNAGSVSFFPQA DTCKVQSNRVFCDTMNSLTLPSEVSLCNTDIFNSKYDCKIMTSKTDISSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKLEGK NLYVKGEPIINYYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 8), optionally lacking a p27 peptide shown in bold.

164. The polypeptide of any one of claims 151 to 161, wherein the ectodomain comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to QNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIK QELDKYKNAVTELQLLMQSTPATNNRARRELPRFMNYTLNNAKKTNVTLSKKRKR RFLGFLLGVGSAIASGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTF KVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQQKNNRLLEITREFSVNAGVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYV VQLPLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQA ETCKVQSNRVFCDTMNSLTLPSEVNLCNVDIFNPKYDCKIMTSKTDVSSSVITSL GAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYYVNKQEGK SLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSREIIRAINIVRKIASEK (SEQ ID NO: 9), optionally lacking a p27 peptide shown in bold.

165. The polypeptide of any one of claims 151 to 161, wherein the polypeptide comprises a sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 1-9.

166. A trimeric protein complex comprising a polypeptide according to any one of claims 151 to 165.

167. The trimeric protein complex of claim 166, wherein thermal stability, assayed by nanoDSF, is increased by at least 10°C, at least 15°C, at least 20°C, about 10°C to about 30°C, about 10°C to about 20°C, or about 20°C to about 30°C compared to a trimeric protein complex lacking modifications (a)-(g).

168. The trimeric protein complex of claim 166, wherein stability, assayed by storage at about 40°C, is increased compared to a trimeric protein complex lacking modifications (a)-(g).

169. The trimeric protein complex of claim 167, wherein the thermal stability increased compared to a reference RSV F protein comprising amino acid substitutions consisting essentially of S155C, S290C, S190F, and V207L (DS-Cav1).

170. A protein nanostructure comprising a trimeric component comprising a polypeptide according to any one of claims 151 to 165.

171. The nanostructure of claim 170, wherein the nanostructure is a two-component nanostructure comprising the first, trimeric component and a second, pentameric component.

172. The nanostructure of claim 171, wherein the first, trimeric component comprises an engineered ectodomain of a Respiratory Syncytial Virus (RSV) fusion (F) polypeptide and an I53-50A polypeptide.

173. The nanostructure of any one of claims 170 to 172, wherein the first, trimeric component comprises a fusion protein comprising, in N- to C-terminal order, the RSV fusion (F) polypeptide, an amino acid linker, and the I53-50A polypeptide.

174. The nanostructure of any one of claims 170 to 173, wherein the nanostructure is a two-component nanostructure comprising:the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, and V207L relative to SEQ ID NO: 1; and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

175. The nanostructure of any one of claims 170 to 173, wherein the nanostructure is a two-component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an e engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, D489A, T400D, E487R, and K498A relative to SEQ ID NO: 1; and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

176. The nanostructure of any one of claims 170 to 173, wherein the nanostructure is a two-component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and T249P relative to SEQ ID NO: 1; andan multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

177. The nanostructure of any one of claims 170 to 173, wherein the nanostructure is a two-component nanostructure comprising: the first, trimeric component, wherein the first trimeric component comprises an engineered ectodomain of a RSV F polypeptide comprising an amino acid substitutions at position S155C, S290C, S190F, V207L, F488W, D489A, T400D, E487R, K498A, and D486A relative to SEQ ID NO: 1; and an multimerization domain comprising a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to I53-50A (SEQ ID NO: 19) or I53-50A ΔCys (SEQ ID NO: 64); and / or the second pentameric component, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to SEQ ID NO: 20 or 71.

178. The nanostructure of any one of claims 170 to 177, wherein the trimeric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to any one of sequence listed in Table 19 or to any one of the sequences listed in Table 19 without the underlined and / or bold / italicized polypeptide sequences.

179. The nanostructure of any one of claims 170 to 178, wherein the pentameric component comprises a polypeptide sequence at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least98%, at least 99%, or 100% identical to any one or more of SEQ ID NOs: 20, 44, 45, 52, 71, 73, 74.

180. A pharmaceutical composition comprising a polypeptide according to any one of claims 151 to 165, a protein complex according to any one of claims 166 to 169, or a nanostructure according to any one of claims 170 to 179.

181. A vaccine comprising a polypeptide according to any one of claims 151 to 165, a protein complex according to any one of claims 166 to 169, or a nanostructure according to any one of claims 170 to 179.

182. A method of vaccinating a subject, comprising administering to the subject a composition according to any preceding claim.

183. A method of generating an immune response in a subject, comprising administering to the subject a composition according to any preceding claim.

184. A method of treating or preventing RSV disease in a subject, comprising administering to the subject a composition according to any preceding claim.

185. A composition according to any preceding claim for use in vaccinating, generating an immune response, or treating or preventing RSV disease.

186. A method of making a compositions according to any preceding claim, comprising culturing a host cells modified to express one or more polypeptides as described herein.

187. A polynucleotide encoding a composition according to any preceding claim.

188. A delivery vehicle comprising the polynucleotide of claim 187, wherein optionally the delivery vehicle is a lipid nanoparticle (LNP).

189. A lipid nanoparticle (LNP) comprising the polynucleotide of claim 187.

190. A host cell comprising the polynucleotide of claim 187.

191. A composition, method, or use as described herein.

Images