Biosensor platform for the rapid detection of analytes

Abstract

Described herein are fusion proteins for detecting analytes in test samples. Also described herein are methods for detecting analytes in test samples using such fusion protein. Further provided herein are systems or kits comprising such fusion proteins for detection of analytes in test samples.

Description

[0001]Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 BIOSENSOR PLATFORM FOR THE RAPID DETECTION OF ANALYTES CLAIM OF PRIORITY This application claims the benefit of U.S. Provisional Application Serial Nos. 63 / 583,409, filed on September 18, 2023, and 63 / 583,421, filed on September 18, 2023. The entire contents of the foregoing are incorporated herein by reference. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with Government support under Grant No. HL119145- 08 awarded by the National Institutes of Health. The Government has certain rights in the invention. TECHNICAL FIELD The present disclosure generally relates to fusion proteins and methods for detection of analytes in test samples. BACKGROUND The recurrent large-scale epidemics or pandemics from emerging viruses such as influenza, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome coronaviruses (MERS-CoV), Ebola virus, and Zika virus have threatened public health worldwide (1, 2). The coronavirus disease 2019 (COVID-19) pandemic caused by SARS-CoV-2 has reminded us again of the importance of large-population screening of infectious diseases in preventing the spread between communities (3, 4). Quantitative reverse-transcription polymerase chain reaction (RT-qPCR) is employed as the gold standard for the confirmatory diagnosis of infectious diseases by amplifying and identifying the nucleic acids from pathogens with high sensitivity and specificity (5, 6), yet it is not readily adopted as the prevalent method for home diagnostics due to the requirement of sample transportation to laboratories, complicated sample preparation, skilled technicians, expensive equipment, and long turnaround time (12h - 72h). Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 SUMMARY In a first aspect, the present disclosure provides a method for detecting the presence or absence of an analyte, optionally a multimeric analyte (e.g., a multimeric protein expressed by a pathogenic organism), in a test sample, the method comprising: a) combining the test sample with a first fusion protein and a second fusion protein, wherein the first fusion protein comprises a first reporter protein fragment and one or more binding domains, and the second fusion protein comprises a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to the analyte, the first reporter protein fragment and second reporter protein fragment, when together, comprise the reporter protein in its entirety and restore its function, and wherein the test sample, the first fusion protein, and the second fusion protein are combined under conditions effective for the reporter protein to generate a detectable signal; and b) detecting the presence or absence of the analyte in the test sample based on presence or absence of the detectable signal generated by the reporter protein. In some embodiments, the reporter protein generates the detectable signal when the first and second reporter protein fragments come together to reconstitute the function of the reporter protein in presence of the analyte in the test sample. In some embodiments, the first fusion protein comprises the first reporter protein fragment at the N-terminal or at the C-terminal. In some embodiments, second fusion protein comprises the second reporter protein fragment at the N-terminal or at the C- terminal. In some embodiments, the first reporter protein fragment is fused to a binding domain via a linker. In some embodiments, the second reporter protein fragment is fused to a binding domain via a linker. In some embodiments, the first fusion protein comprises one or more linkers between two binding domains. In some embodiments, the second fusion protein comprises one or more linkers between two binding domains. In some embodiments, the one or more binding domains are binders that recognize and bind at or near the apex of the analyte. In some embodiments, the conditions effective for the reporter protein to generate a detectable signal comprises combining the test sample, the first fusion protein, and the Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 second fusion protein in a composition comprising a substrate for the reporter protein. In some embodiments, the reporter protein is luciferase, beta-lactamase, beta-galactosidase, beta-glucuronidase, green fluorescent protein (GFP), horseradish peroxidase, chloramphenicol acetyl transferase, or alkaline phosphatase. In some embodiments, the reporter protein is luciferase. In certain embodiments, the conditions effective for the reporter protein to generate a detectable signal comprises combining the test sample, the first fusion protein, and the second fusion protein in a composition comprising luciferin or a derivative of luciferin. In some embodiments, the first fusion protein comprises a small luciferase fragment and the second fusion protein comprises a large luciferase fragment. In some embodiments, the first fusion protein comprises a large luciferase fragment and the second fusion protein comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises 10-14, optionally 11, amino acids starting from the C terminus of luciferase (e.g., deep sea shrimp luciferase or a variant thereof). In some embodiments, the large luciferase fragment comprises an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises 156-160, optionally 158, amino acids, starting from the N terminus of luciferase (e.g., deep sea shrimp luciferase or a variant thereof). In some embodiments, the first fusion protein further comprises a purification tag. In some embodiments, the second fusion protein further comprises a purification tag. In certain embodiments, the purification tag is FLAG, streptavidin, cMyc tag, His-tag, or biotin tag. In some embodiments, the analyte is a mammalian protein. In certain embodiments, the analyte is TIM1. In some embodiments, the analyte is a multimeric protein expressed by a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 pathogenic virus. In some embodiments, the pathogenic virus is SARS CoV-2, influenza virus, human immunodeficiency virus (HIV), respiratory syncytial virus (RSV), measles, ebola, hepatitis C, zika, monkeypox, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, rotavirus or poliovirus. In some embodiments, the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 3; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 8; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 11; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 21. In certain embodiments, the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 3; a binding domain comprising the amino acid sequence of SEQ ID NO: 8; a binding domain comprising the amino acid sequence of SEQ ID NO: 11; a binding domain comprising the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising the amino acid sequence of SEQ ID NO: 21. In some embodiments, the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 26; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 27; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 29. In certain embodiments, the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 26; a binding domain comprising the amino acid sequence of SEQ ID NO: 27; a binding domain comprising the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 38. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and the second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34; a binding domain comprising the amino acid sequence of SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 36; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 38. In some embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36. In some embodiments, the pathogenic virus is HIV, and the one or more binding Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO: 38. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO: 38. In some embodiments, the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 45. In certain embodiments, the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising the amino acid sequence of SEQ ID NO: 45. In some embodiments, the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 50; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 60. In certain embodiments, the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 50; a binding domain comprising the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising the amino acid sequence of SEQ ID NO: 60. In some embodiments, the test sample is a biological sample. In some embodiments, the biological sample is pus, mucus, blood, plasma, serum, semen, sputum, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 nasal fluid, urine, saliva, or milk. In some embodiments, the reporter protein generates the detectable signal, and / or the method comprises detecting presence or absence of a detectable signal generated by the reporter protein, in less than 180 seconds, less than 120 seconds, less than 60 seconds, less than 30 seconds, less than 20 seconds, less than 10 seconds, less than 5 seconds, or less than 3 seconds after contacting the first fusion protein and the second fusion protein with the test sample, when the analyte is present in the test sample. In some embodiments, the first and second fusion proteins are configured to bind to the analyte to bring the first and second reporter protein fragments together within around 50 Å of each other. In certain embodiments, the first and second fusion proteins are configured to bind to the analyte to bring the first and second reporter protein fragments together within less than 45 Å, less than 40 Å, less than 35 Å, less than 30 Å, less than 25 Å, less than 20 Å, less than 15 Å, less than 10 Å, less than 5 Å, or less than 1 Å, such as, about 45 Å to about 50 Å, about 40 Å to about 45 Å, about 35 Å to about 40 Å, about 30 Å to about 35 Å, about 25 Å to about 30 Å, about 20 Å to about 25 Å, about 15 Å to about 20 Å, about 10 Å to about 15 Å, about 5 Å to about 10 Å, about 1 Å to about 5 Å (e.g., about 50 Å, about 45 Å, about 40 Å, about 35 Å, about 30 Å, about 25 Å, about 20 Å, about 15 Å, about 10 Å, about 5 Å, or about 1 Å) of each other. Also provided herein is a system or kit for detecting an analyte, optionally a multimeric analyte in a test sample, the system or kit comprising: i) a first fusion protein comprising a first reporter protein fragment and one or more binding domains; and ii) a second fusion protein comprising a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to the multimeric analyte, and the first reporter protein fragment and second reporter protein fragment, when together, comprise the reporter protein in its entirety and restore its function. In some embodiments, the reporter protein generates a detectable signal when the first and second reporter protein fragments come together to restore the function of the reporter protein in presence of the multimeric analyte in the test sample. In some embodiments, the first fusion protein and the second fusion protein are lyophilized. In some embodiments, the system or kit further comprises a substrate for the reporter protein. In some embodiments, the system or kit further comprises a negative Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 control sample. In certain embodiments, the negative control sample comprises 1-10% PBS. In some embodiments, the system or kit further comprises a positive control sample. In certain embodiments, the positive control sample comprises the analyte or a component thereof that is known to specifically bind to the first and second fusion proteins and reconstitute the function of the reporter protein. In some embodiments, the one or more binding domains are binders that recognize and bind at or near the apex of the analyte. In some embodiments, the first fusion protein comprises the first reporter protein fragment at the N-terminal or at the C- terminal. In some embodiments, the seocnd fusion protein comprises the second reporter protein fragment at the N-terminal or at the C-terminal. In some embodiments, the first reporter protein fragment is fused to a binding domain via a linker. In some embodiments, the second reporter protein fragment is fused to a binding domain via a linker. In some embodiments, the first fusion protein comprises one or more linkers between two binding domains. In some embodiments, the second fusion protein comprises one or more linkers between two binding domains. In some embodiments, the reporter protein is luciferase, beta-lactamase, beta- galactosidase, beta-glucuronidase, green fluorescent protein (GFP), horseradish peroxidase, chloramphenicol acetyl transferase, or alkaline phosphatase. In certain embodiments, the reporter protein is luciferase. In some embodiments, the system or kit further comprises luciferin or a derivative of luciferin. In some embodiments, the first fusion protein comprises a small luciferase fragment and the second fusion protein comprises a large luciferase fragment. In some embodiments, the first fusion protein comprises a large luciferase fragment and the second fusion protein comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1, or an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises 10-14, optionally 11, amino acids starting from the C terminus of luciferase (e.g., deep sea shrimp luciferase or a variant thereof). In some embodiments, the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2, or an amino acid sequence having at least 85% sequence Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises 156-160, optionally 158, amino acids, starting from the N terminus of luciferase (e.g., deep sea shrimp luciferase or a variant thereof). In some embodiments, the first fusion protein further comprises a purification tag. In some embodiments, the second fusion protein further comprises a purification tag. In certain embodiments, the purification tag is FLAG, cMyc tag, His-tag, streptavidin, or biotin tag. In some embodiments, the analyte is a mammalian protein. In certain embodiments, the analyte is TIM1. In some embodiments, the analyte is a multimeric protein expressed by a pathogenic virus. In some embodiments, the pathogenic virus is SARS CoV-2, influenza virus, human immunodeficiency virus (HIV), respiratory syncytial virus (RSV), measles, ebola, hepatitis C, zika, monkeypox, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, rotavirus or poliovirus. In some embodiments, the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 3; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 8; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 11; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 21. In certain embodiments, the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 3; a binding domain comprising the amino acid sequence of SEQ ID NO: 8; a binding domain comprising the amino acid sequence of SEQ ID NO: 11; a binding domain comprising the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising the amino acid sequence of SEQ ID NO: 21. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 26; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 27; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 29. In certain embodiments, the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 26; a binding domain comprising the amino acid sequence of SEQ ID NO: 27; a binding domain comprising the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 38. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and the second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34; a binding domain comprising the amino acid sequence of SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 36; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 38. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36. In some embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO: 38. In certain embodiments, the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO: 38. In some embodiments, the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 45. In certain embodiments, the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising the amino acid sequence of SEQ ID NO: 45. In some embodiments, the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 50; a binding domain comprising an Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 60. In certain embodiments, the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 50; a binding domain comprising the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising the amino acid sequence of SEQ ID NO: 60. In some embodiments, the test sample is a biological sample. In some embodiments, the biological sample is pus, mucus, blood, plasma, serum, semen, sputum, nasal fluid, urine, saliva, or milk. In some embodiments, the reporter protein generates the detectable signal in less than 180 seconds, less than 120 seconds, less than 60 seconds, less than 30 seconds, less than 20 seconds, less than 10 seconds, less than 5 seconds, or less than 3 seconds after contacting the first fusion protein and the second fusion protein with the test sample, when the analyte is present in the test sample. In some embodiments, the first and second fusion proteins are configured to bind to the analyte to bring the first and second reporter protein fragments together within around 50 Å of each other. In certain embodiments, the first and second fusion proteins are configured to bind to the analyte to bring the first and second reporter protein fragments together within less than 45 Å, less than 40 Å, less than 35 Å, less than 30 Å, less than 25 Å, less than 20 Å, less than 15 Å, less than 10 Å, less than 5 Å, or less than 1 Å, such as, about 45 Å to about 50 Å, about 40 Å to about 45 Å, about 35 Å to about 40 Å, about 30 Å to about 35 Å, about 25 Å to about 30 Å, about 20 Å to about 25 Å, about 15 Å to about 20 Å, about 10 Å to about 15 Å, about 5 Å to about 10 Å, about 1 Å to about 5 Å (e.g., about 50 Å, about 45 Å, about 40 Å, about 35 Å, about 30 Å, about 25 Å, about 20 Å, about 15 Å, about 10 Å, about 5 Å, or about 1 Å) of each other. Also provided herein is a test strip apparatus, comprising: an elongate strip body, including first and second longitudinally separated strip ends; at least one well extending laterally downward from the strip body proximate the first strip end, the well having a cup shape with an at least partially optically accessible bottom well surface spaced Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 laterally from the strip body by an at least partially opaque cup sidewall; at least one cap extending laterally upward from the strip proximate the second strip end; at least one of the well and the cap configured to maintain a test reagent therein; at least one of the well and the cap being configured to selectively receive a biofluid therein for reaction with the test reagent to produce an optically detectable test result, wherein the strip body is manipulable to superimpose the cap over the well, and wherein the strip body and at least one of the well and the cap comprise a single unitary structure. In some embodiments, the test reagent comprises: i) a first fusion protein comprising a first reporter protein fragment and one or more binding domains; and ii) a second fusion protein comprising a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to a multimeric analyte, and the first reporter protein fragment and second reporter protein fragment, when together, comprise the reporter protein in its entirety and restore its function. Also provided herein is a system for optically detecting biofluid test results, comprising: a test strip apparatus described hereinabove; an optical detector configured to detect an optical property of the biofluid through the optically accessible portion of the well bottom and responsively produce a test result signal; and a user-perceptible result indicator configured to selectively display the test result signal to a user. Unless otherwise defined, 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 invention belongs. Methods and materials are described herein for use in the present invention; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. Other features and advantages of the invention will be apparent from the following detailed description and figures, and from the claims. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 DESCRIPTION OF DRAWINGS FIGs.1A-1C, Illustration of exemplary embodiments of the disclosure as applied to enveloped viruses, specifically the luminescent fusion proteins for SARS-CoV-2 and for influenza detection. FIG.1A, The trimeric spike protein on the SARS-CoV-2 surface brings two luciferase fragments into close proximity to assemble a functional luciferase protein, which catalyzes the substrate (fluorofurimazine, FFz) to emit a bright blue bioluminescent light signal. Mini ACE2 mimic binder is genetically fused to each of the luciferase fragments and each fragment binds to the apex of the spike (inset circle). FIG. 1B, Illustration showing the universal diagnostics of influenza A and B using the split luciferase probes containing the universal binder that recognizes the trimeric hemagglutinin (HA) of influenza A and B. The HA proteins on the surface of influenza viruses bring two multidomain luciferase fragments into close proximity to assemble a functional luciferase protein (intramolecular complementation) to generate the bright blue bioluminescence. The high density of closely-packed HAs on influenza virus surface also facilitates the intermolecular complementation of two luciferase fragments between neighboring HAs. complementation of two luciferase fragments between neighboring HAs. FIG.1C, Frames from a video are provided showing that the luminescence appears in less than 3 seconds from adding the spike and the luciferase probes into the substrate to detect the SARS-CoV-2 virus. FIGs.2A-2F, Binding and cryo-EM analysis of B3S / B3L in complex with SARS- CoV-2 spike proteins. FIG.2A, Coomassie blue-stained SDS-PAGE showing the purity of B3L and B3S produced using E. coli expression system and purified by size exclusion chromatography (SEC). Protein structures of B3L and B3S are predicted by AlphaFold. L: large luciferase fragment, S: small luciferase fragment. FIG.2B, Binding affinity analysis of B3L and B3S to the spike protein by BLI. The test was repeated three times and the variations of the the affinity equilibrium constants for each test originates from the limited capability of BLI technology for measuring the ultrahigh binding affinity with the value less than 1 nM. FIG.2C, SEC elution profiles of the spike-B3L-B3S complex (solid curve) and the corresponding SDS-PAGE gel analysis. Spike, B3L, and B3S are all present in the purified complex in an approximately equimolar ratio. Pure spike Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 proteins (dash curve) is included in SEC elution profiles for comparison. FIG.2D, Cryo- EM structure of B3L / B3S in complex with the spike trimer in a two-RBD-up conformation (class I RBD), showing the close proximity of two luciferase fragments. FIG.2E, Close-up view of the interaction between B3L / B3S and the RBD of the spike trimer in a two-RBD-up conformation (class I RBD). Left, the cryo-EM structure of the helix region of either B3L or B3S (in dark gray) makes contact with the receptor-binding motif of the RBD (in light gray). Right, zoom-in views of the binding interface, showing helix I and helix II wedged into a gap formed by the receptor-binding motif region of the RBD. R403 of the RBD interacts with D29 from helix I through a salt bridge. FIG.2F, Quantification of the molar ratio of spike, B3L, and B3S for the complex after SEC purification. Left: SDS-PAGE analysis of the main SEC peak (spike-B3LB3S complex). Right: SDS-PAGE analysis of serial dilutions of spike, B3L, and B3S showing the degree of interaction between the proteins and Coomassie stain. FIGs.3A-3H, Luminescence fusion proteins for the detection of SARS-CoV-2 viruses.. FIG.3A, Emission spectra of bright blue bioluminescence. The insert image was taken by iPhone 11. FIG.3B, Bioluminescence noise evaluation of split luciferase probes. FIG.3C, Stability evaluation of the bioluminescence signal in the presence of different concentrations of SARS-CoV-2 spike proteins. FIG.3D, Evaluation of detection limit with varying concentrations of SARS-CoV-2 trimeric spike proteins and RBD (monomer). FIG.3E, Specificity evaluation of B3L and B3S for SARS-CoV-2 spike proteins from the antigen proteins of other viruses. FIG.3F, Evaluation of detection limit with different concentrations of Gamma-inactivated SARS-CoV-2 viruses spiked in saliva samples. The detection limit was calculated at three times the standard deviation of the signal from a blank. FIG.3G, Testing of clinical saliva samples with the bioluminescence biosensor for COVID-19 diagnostics. Bioluminescence intensity was significantly higher in the patients with COVID-19 (+) than in healthy controls (-). FIG. 3H, The receiver operation characteristic curve was used to determine the sensitivity (90%) and specificity (100%) with the area under the curve (AUC) of 0.985. FIGs.4A-4E, Design and characterization of luminescence fusion proteins for the universal diagnostics of influenza A and influenza B. FIG.4A, Coomassie blue-stained SDS-PAGE showing the purity of L84 and S84 produced using mammalian cell Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 expression system and purified by Ni column. Protein structures of L84 and S84 are predicted by AlphaFold. L: large luciferase fragment, S: small luciferase fragment. FIG. 4B, Binding analysis of L84 and S84 with the HA of influenza A (A / Guangdong / 17SF003 / 2016 (H7N9)) and influenza B (B / Ohio / 1 / 2005) by microscale thermophoresis. FIG.4C, Specificity evaluation of L84 and S84 for influenza HA proteins from the antigen proteins of other viruses. FIG.4D, Evaluation of the detection limit of L84 and S84 for HA proteins from influenza A (left) and influenza B (right) viruses. The detection limit was calculated at three times the standard deviation of the signal from a blank. FIG.4E, Evaluation of detection limit of L84 and S84 for influenza A (left) and influenza B (right) viruses. FIGs.5A-5B, Luminescence biosensor for the detection of spike proteins from SARS-CoV-2 Omicron variants using IL and IS pair. FIG.5A, Coomassie blue-stained SDS-PAGE showing the purity of I1 and I2 produced using mammalian cell expression system and purified by SEC (IL) and Ni column (IS). Protein structures of IL and IS are predicted by AlphaFold. FIG.5B, Evaluation of detection limit with varying concentrations of SARS-CoV-2 BA.1 and XBB spike proteins. FIG.6 provides a schematic diagram showing the construct design of B3S (left panel) and the protein structure of B3S predicted by AlphaFold (right panel). FIG.7 provides a schematic diagram showing the construct design of SB3 (left panel) and the protein structure of SB3 predicted by AlphaFold (right panel). FIG.8 provides a schematic diagram showing the construct design of B3L (left panel) and the protein structure of B3L predicted by AlphaFold (right panel). FIG.9 provides a schematic diagram showing the construct design of LB3 (left panel) and the protein structure of LB3 predicted by AlphaFold (right panel). FIG.10 provides a schematic diagram showing the construct design of IL (left panel) and the protein structure of IL predicted by AlphaFold (right panel). FIG.11 provides a schematic diagram showing the construct design of IS (left panel) and the protein structure of IS predicted by AlphaFold (right panel). FIG.12 provides a schematic diagram showing the construct design of CTC-S (left panel) and the protein structure of CTC-S predicted by AlphaFold (right panel). Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 FIG.13 provides a schematic diagram showing the construct design of S-CTC (left panel) and the protein structure of S-CTC predicted by AlphaFold (right panel). FIG.14 provides a schematic diagram showing the construct design of CTC-L (left panel) and the protein structure of CTC-L predicted by AlphaFold (right panel). FIG.15 provides a schematic diagram showing the construct design of L-CTC (left panel) and the protein structure of L-CTC predicted by AlphaFold (right panel). FIG.16 provides a schematic diagram showing the construct design of CT30-S (left panel) and the protein structure of CT30-S predicted by AlphaFold (right panel). FIG.17 provides a schematic diagram showing the construct design of S-CT30 (left panel) and the protein structure of S-CT30 predicted by AlphaFold (right panel). FIG.18 provides a schematic diagram showing the construct design of CT30-L (left panel) and the protein structure of CT30-L predicted by AlphaFold (right panel). FIG.19 provides a schematic diagram showing the construct design of L-CTC- N30 (left panel) and the protein structure of L-CTC-N30 predicted by AlphaFold (right panel). FIG.20 provides a schematic diagram showing the construct design of ACE2-S (left panel) and the protein structure of ACE2-S predicted by AlphaFold (right panel). FIG.21 provides a schematic diagram showing the construct design of S-ACE2 (left panel) and the protein structure of S-ACE2 predicted by AlphaFold (right panel). FIG.22 provides a schematic diagram showing the construct design of ACE2-L (left panel) and the protein structure of ACE2-L predicted by AlphaFold (right panel). FIG.23 provides a schematic diagram showing the construct design of L-ACE2 (left panel) and the protein structure of L-ACE2 predicted by AlphaFold (right panel). FIG.24 provides a schematic diagram showing the construct design of 84S (left panel) and the protein structure of 84S predicted by AlphaFold (right panel). FIG.25 provides a schematic diagram showing the construct design of S84 (left panel) and the protein structure of S84 predicted by AlphaFold (right panel). FIG.26 provides a schematic diagram showing the construct design of 84L (left panel) and the protein structure of 84L predicted by AlphaFold (right panel). FIG.27 provides a schematic diagram showing the construct design of L84 (left panel) and the protein structure of L84 predicted by AlphaFold (right panel). Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 FIGs.28A-28B, Cryo-EM analysis of the soluble S trimer in complex with B3S / B3L. FIG.28A, Representative micrograph and 2D class averages (box dimension: 396 Å) of the cryo-EM particle images of the soluble S trimer in complex with B3S / B3L. FIG.28B, Data processing workflow for structure determination. FIG.29. A schematic showing that proximity is crucial for efficient bioluminescence generation at the concentration range of viral surface proteins in the patient’s body for clinical application. FIG.30. Illustration of the design of the HIV Luminescence Biosensor Detection System. Left panel is an illustration of the design for the HIV envelope glycoprotein trimer detection assay using the gp160 surface protein of HIV comprising a “double split” system employing the conformationally selective PCT145 antibody fragment (scFv) that binds to the quaternary epitope at the apex center of the native gp160 trimer of HIV. Right panel is a graphical representation of data showing that in the presence of the HIV trimer, these protein probes assemble an active luciferase and result in a 5-fold increase in the luminescence signal. FIG.31. (Left panel), Luminescence biosensor for the detection of RSV viruses. (Right panel), Evaluation of detection limit with varying concentrations of trimeric Fusion protein (F). FIG.32. Production of P1 and P2 in HEK293 cells. (left) SDS-PAGE showing P1 after purification using Ni beads. (Middle) Western Blot for P2. Anti-His tag antibody was used as a primary antibody. (Right) SDS-PAGE silver-stain for P2 purified using Ni beads followed by size exclusion chromatography. FIG.33. Binding of P1 and P2 fusion proteins to spike proteins. (top) Microscale thermophoresis (MST) binding of SARS-CoV-2 Spike (D614) protein with P1 protein. (bottom) Microscale thermophoresis (MST) binding of SARS-CoV-2 Spike protein (Delta, B.1.617.2) with P2 protein. FIG.34. Colorimetric assay of the wild-type Spike (D614) using P1 and P2 expressed in HEK293. (Left panel) Optical image of the samples containing different dilutions of the Spike protein. (Right panel) Colorimetric assay of the wild-type Spike protein. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 FIG.35. Colorimetric assay of S1 (D614) detection. The assay was performed using HEK-293 expressed P1 and P2 proteins. P1 and P2 were added to tubes containing different concentrations of S1 followed by addition of TMB, an HRP substrate. FIG.36, Examples of enveloped and non-enveloped viruses and their surface proteins. Viral surface attachment or fusion proteins assemble as homo-multimer. Enveloped viruses surface proteins usually assemble as homotrimer (Top panel), while non-enveloped viruses surface capsid proteins assemble as homopentamer (Bottom panel). The platform takes advantage of these assemblies to provide the spatial parameter required for the split enzyme fragments to be brought into proximity. FIGs.37A-37C, Analysis of the spike-B3L-B3S complex by negative-stain EM. FIG.37A shows a representative negative stain image of the soluble S trimer in complex with B3S / B3L (left), 2D class average of the soluble S trimer (right top) and B3S / B3L in complex with the soluble S trimer (right bottom). FIG.37B shows a 3D classification analysis of the S trimer in complex with B3S / B3L. There were extra densities near the RBD. FIG.37C shows flexible fitting of the AlphaFold predicted B3S / B3L structures and the soluble S trimer structure in the low-resolution map produced by negative stain EM. The three helices shared between B3S and B3L are marked with thick arrows, while the rest of B3S is marked with dashed arrow and the rest of B3L is marked with thin arrow. FIG.38 Cryo-EM structure validation of the soluble S trimer in complex with B3S / B3L. FSC curves and viewing direction distribution plot for the soluble S trimer- B3S / B3L complex. From top to bottom: the soluble S trimer-B3S / B3L complex in class I conformation; local refinement of the RBD-B3S / B3L interface from the S trimer in class I conformation using the different size of mask; the soluble S trimer-B3S / B3L complex in class II conformation; local refinement of the RBD-B3S / B3L interface from the S trimer in class II conformation using the different size of mask. FIGs.39A-39E, Analysis of 3D cryo-EM reconstruction of the soluble S trimer in complex with B3S / B3L. FIGs.39A-39B show 3D reconstructions of B3S / B3L in complex with the soluble S trimer in the class I (A) and class II (B) conformations, respectively, are colored according to local resolution estimated by cryoSPRAC. FIG. 39C-39D shows 3D reconstructions from masked local refinement of the RBD-B3S / B3L Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 from the soluble S trimer in the class I (C) and class II (D) conformations are colored according to local resolution estimated by cryoSPARC. FIG.39E shows representative density in surface representation shown in gray from the EM maps, primarily from the three-helix region of B3S / B3L and the binding interface with the RBD. FIG.40, Analysis of B3S / B3L in the complex with the S trimer. Left panel: top view of the EM map and model fitting of B3S / B3L bound to the RBDs. Right panel: close-up views of the map and model fitting of three B3S / B3L monomers in complex with one S trimer. Among three RBDs, there are extra densities from the N- and C- terminal ends of B3S / B3L, which were modeled based on the best locally refined map each B3S / B3L monomer, marked with thick arrows. Three B3S / B3L are marked. FIG.41, Cryo-EM characterization of the LCB1 (Left panel), LCB3 (Middle panel), and B3L / B3S (Right panel) in complex with SARS-CoV-2 spike proteins viewed along two orthogonal axes (top, side view; bottom, top view). The shortest distance between two LCB1, LCB3, and B3L / B3S is 37.8 Å, 23.9 Å, and 18.7 Å, respectively. The regions marked with thick arrows in Right panel represent the VSQ amino acids which realize the inward extension of B3L / B3S alpha helix III towards the apex center to achieve a shorter distance between B3L / B3S units. FIG.42, Detection of the trimeric SARS-CoV2 protein in plasma using B3L and B3S pair, shows evaluation of detection limit with varying concentrations of SARS-CoV- 2 spike protein in 10% plasma (right). FIG.43, Specificity evaluation of B3L and B3S. Only SARS-CoV-2 (strain: 2019-nCoV / USA-WA1 / 2020) was positive. Other viruses including human coronavirus NL63 and OC43 did not give positive signal (byond background). FIG.44, B3L and B3S retained activity after lyophilization in the presence of various cryoprotectants. FIG.45, Detection of TIM-1 using A5-L and A5-S paired fusion proteins. FIG.46, The user peels off the sealing tape and adds the biofluids, such as saliva, into the wells on the right (labelled T and C, contains lyophilized reagents / proteins). Next, the user will fold the plastic strip to cover the wells with the caps (contains lyophilized substrate). The biofluids will solubilize the lyophilized reagents in the wells and in the caps. The analytes generate luminescence signal. Since the wells Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 possesstransparent bottoms, the luminescence can be detected from the bottom by a smartphone camera. The plastic strip may be made of opaque material to avoid the interference of environmental light during the test. As shown in the lower left image, the strip can be adhered to a rear-facing camera of the smartphone and suspended from the device, to allow a user to view the camera images as they are being captured. FIG.47, The front camera or the back camera of a smartphone can be used to detect luminescence. FIG.48, A foldable strip prototype. Right panel: The top image is taken by a smart phone camera. The sample contains SARS-CoV-2 spike protein which generates luminescence when mixed with the lyophilized reagents in the wells and caps. The bottom image is for a negative sample (control). Left panel: an elongated rim is provided adjacent the wells to assist the user with directing fluid into the wells (e.g., the rim could be a “spitguard”). FIG.49, The folded test strip is held atop the front-facing lens of a mobile phone. As can be seen, both the test and control wells of the strip can be viewed at one time. It may be desirable, in some use environments, for the strip to be used with the front-facing lens to allow the user to view the camera image of the strip simultaneously with placing / moving the strip atop the mobile phone. FIG.50, This is a side view of the strip held (via gravity, an adhesive, or in any other desired manner) atop a front-facing camera of the mobile phone, as shown in FIG. 49. FIG.51, Illustration showing the procedure for luminescence detection from plastic strip using a light sensor integrated in the reader. The saliva sample is collected by a collection tube and added into the wells of a foldable plastic strip, which contains the lyophilized substrate and split enzyme in the caps and the wells, separately. T: test well, C: control well. FIG.52A, Illustrations showing the procedure for rapid and sensitive detection of any desired input (shown here as COVID-19, for example). The saliva sample is collected by a collection tube and added into the wells of a foldable plastic strip, which contains the lyophilized substrate and split enzyme in the caps and the wells, separately. T: test well, C: control well. A reader with embedded light sensors and analysis Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 algorithms is designed for luminescence readout. FIG.52B, Detection of varying concentrations of SARS-CoV-2 trimeric spike proteins using a light sensor. DETAILED DESCRIPTION The methods of the present disclosure relate to complementation assays that use biosensor fragments to detect analytes in a sample. Although the present disclosure exemplifies multimeric proteins as analytes, monomeric proteins can also be detected using the present methods. For example, the paired fusion proteins can include binders that bind to adjacent but non-overlapping epitopes on the monomer, in close proximity to each other, sufficient to bring the complementary fragments of the reporter protein together. In the present methods, a reporter protein is split into two fragments (e.g., two complementary fragments, also referred to herein as a split-reporter), which are reconstituted non-covalently to restore activity by other interacting proteins that are fused to the fragments. In some embodiments, the biosensor is a reporter protein, e.g., a luminescent protein. In some embodiments, the reporter protein is an enzyme, including but not limited to luciferase, beta-lactamase, beta-galactosidase, beta-glucuronidase, green fluorescent protein (GFP), horseradish peroxidase (HRP), chloramphenicol acetyl transferase (CAT), or alkaline phosphatase. For the successful use of split-reporter proteins in complementation assays, several criteria should be met. For example, each reporter protein fragment by itself should not exhibit any activity, the affinity of the reporter protein fragments in the absence of the attached interacting proteins should be negligible, and the reassembled reporter protein should provide an easily measurable read out. These criteria are met by the composition and methods of the present disclosure, thus providing a simple, rapid, and ultrasensitive diagnostic method. In some embodiments, the present disclosure provides a luciferase-based approach for the rapid and sensitive detection of multimeric analytes. Exemplary multimeric analytes include proteins expressed by, e.g., viruses, bacteria, parasaites, fungus, and mammalian cells. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Viral infections represent a major health threat to humankind. However, the lack of simple, rapid, and ultrasensitive methods to detect viruses limits large population screening. Described herein are fusion proteins, also referred to herein as highly-efficient and in-situ bioluminescent enzymatic (HiBlue) probes, for rapid and ultrasensitive detection of viruses based on split luciferase complementation on trimeric assembly of viral surface proteins. These fusion proteins (e.g., HiBlue probes) were established by engineering split enzyme pairs that bind to the apex of the trimeric assembly of SARS- CoV-2 spike proteins with very high affinity and ultra-efficiently reconstitute into a functional luciferase enzyme at close proximity (e.g., up to 5-10 nm, up to 2-5 nm, up to 1-2 nm, preferably less than 10050 Å or less than 5050 Å) to generate bright blue bioluminescence. As described in Example 3 hereinbelow, rapid and ultrasensitive detection of SARS-CoV-2 spike proteins was achieved at as low as 2-femtomolar concentration and HiBlue probes were validated using saliva samples from COVID-19 patients. Furthermore, as described in Example 4 hereinbelow, the HiBlue probes were extended for the universal detection of influenza A and B by fusing split luciferase fragments to multiple-domain binders, which target multiple non-overlapping and highly- conserved epitopes of trimeric hemagglutinin (HA) proteins with high sensitivity and specificity. The HiBlue probes of the present disclosure demonstrate the merit of targeting the trimeric assembly of viral surface proteins for rapid and ultrasensitive point- of-care diagnostics of infectious diseases. The rapid diagnostic tests described herein are an alternative as a faster, cheaper, and more user-friendly technology for individual or large-population screening by detecting viral antigens (active infection) or assessing host immune response IgG or IgM antibodies (past infection) (7). Although antigen / antibody test can provide diagnostic results with reduced turnaround time, minimal training, and low cost, it shows compromised sensitivity which may increase the risk of false-negative results (8, 9). Advanced transduction mechanisms, including field-effect-transistor-based biosensor (10, 11), CRISPR–Cas12-based lateral flow immunoassay (12), amplification-free CRISPR- Cas13a assay (13), RNA-extraction-free nanoamplified colorimetric test (14), and nanoplasmonic resonance sensor (15) have been developed to improve the sensitivity of Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the rapid diagnostic test. However, due to the lack of simplicity, most technologies are not readily adaptable for home diagnostics. Bioluminescent probes can evaluate the protein-protein interactions for biomedical applications, ranging from cellular assays (16-18), in-vivo imaging (19-22), to high-throughput screening for chemical biology and drug discovery (23-25). Split- luciferase bioluminescent probes have been developed for the detection of anti-SARS CoV-2 antibodies by fusing the split luciferase fragments to the viral proteins, such as nucleocapsid proteins (26), membrane proteins (27), spike proteins (28), and receptor- binding domains (RBD) of spike proteins (29), which bind to the separate binding sites of the same SARS-CoV-2 antibodies and force the split luciferase fragments into proximity to reconstitute an intact luciferase enzyme and generate bioluminescence. However, the reported bioluminescent probes suffer from low sensitivity due to the inefficient reconstitution of split luciferase fragments, and show limited ability for early diagnostics since it may take several days to weeks following symptom onset for a patient to accumulate a detectable antibody response (30). In contrast to methods known in the art, the present disclosure provides fusion proteins, also referred to herein as highly-efficient and in-situ bioluminescent enzymatic (HiBlue) probes, for the rapid and ultrasensitive detection of analytes, useful, e.g., for diagnostics of COVID-19 and influenza based on the split luciferase complementation on the trimeric assembly of viral surface proteins (FIGs.1A-1B). These fusion proteins utilize the close proximity between very-high-affinity binders on intrinsic trimeric assembly of the viral surface proteins, thus forming the functional luciferase enzyme with ultrahigh efficiency and generating bright blue bioluminescence, providing a new opportunity for rapid and ultrasensitive diagnostics of infectious diseases. The present disclosure takes advantage of the fact that most of the viral surface attachment or fusion proteins assemble as homo-trimers, and uses fusion proteins (e.g., HiBlue probes) targeting the trimeric assembly of viral surface proteins for the rapid and ultrasensitive detection of viral infections (e.g., SARS-CoV-2 and influenza). As highlighted here, simplicity is one of the critical factors that determine whether end-users widely adopt POC diagnostics. The fusion proteins of the present disclosure take advantage of the trimeric assembly of the SARS-CoV-2 spike proteins and influenza HA Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 to form a complex of two complementary luciferase probes with close proximity to directly generate the bright blue bioluminescence. The underlying working mechanism of these fusion proteins allows the bioluminescence assay to be performed rapidly in the homogenous liquid phase of clinical samples without the requirements of sample purification or amplification or other tedious procedures. With these advantages, the fusion proteins of the present disclosure show the potential to meet the demanding WHO “ASSURED” (Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipmentfree, and Deliverable end-users) criteria, enabling on-site and POC tests at low-resource settings such as in airports, clinics, and local emergency departments, and home. The continued emergence of SARS-CoV-2 variants can potentially impact the performance of the currently approved antigen tests which employ mainly antibody binders. Many of the antibodies are sensitive to viral escape through point mutations in their viral epitopes. Thus, there is a need for binders whose efficacy is not disrupted by antigenic drift. Our approach for generating multivalent spike protein binders can overcome the antigenic drift concern and yield designs that are resistant to escape mutants. Therefore, we developed multivalent HiBlue probes that contain ACE2-derived or mimic domains to remain effective as SARS-CoV-2 mutations continue to occur. We produced and evaluated the performance of the multivalent HiBlue probes against Omicron BA.1 and XBB.1 spike proteins. The multivalent HiBlue probes maintained strong binding and performance against the Omicron variants (FIG.5B), likely because of the combination of ACE2 receptor mimicry and multivalency. Since the early symptoms of COVID-19 and influenza patients are similar (fever, cough, and fatigue), it is challenging to distinguish SARS-CoV-2 infection from those of influenza based on symptoms alone. Therefore, making available rapid tests for both viruses is essential, particularly for high-risk patients. Fusion Protein(s) and Methods of Use thereof Described herein are fusion proteins that can be used for detection of analytes in test samples, and methods of use thereof. In some embodiments, the analyte is a monomeric or multimeric protein, such as a viral protein, or a protein expressed in a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 parasite, bacteria, fungus, or mammalian cell. Thus, fusion proteins of the present disclosure can be used for detection of multimeric proteins in test samples (e.g., biological samples or environmental samples). For detection of an analyte by the present methods, a pair of fusion proteins is used. One fusion protein from the pair is referred to herein as a first fusion protein, and the other fusion protein from the pair is referred to herein as a second fusion protein. Each of the first and second fusion proteins can comprise a fragment (e.g., one of the two complementary fragments) of a reporter protein. The reporter protein fragment in a first fusion protein can be referred to herein as a first reporter protein fragment, and the reporter protein fragment in a second fusion protein can be referred to herein as a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). “Close proximity,” as used herein, refers to a distance of up to 10 nm (e.g., up to 9 nm, up to 8 nm, up to 7 nm, up to 6 nm, up to 5 nm, up to 4 nm, up to 3 nm, up to 2 nm, or up to 1 nm), such as, up to 7-10 nm, up to 5-7 nm, up to 3-5 nm, up to 1-3 nm, up to 5- 10 nm, up to 2-5 nm, or up to 1-2 nm, or a distance of less than 100 Å (e.g., less than 95 Å, less than 90 Å, less than 85 Å, less than 80 Å, less than 75 Å, less than 70 Å, less than 65 Å, less than 60 Å, less than 55 Å, less than 50 Å, less than 45 Å, less than 40 Å, less than 35 Å, less than 30 Å, less than 25 Å, less than 20 Å, less than 15 Å, less than 10 Å, less than 5 Å, or less than 1 Å), such as, about 95 Å to about 100 Å, about 90 Å to about 95 Å, about 85 Å to about 90 Å, about 80 Å to about 85 Å, about 75 Å to about 80 Å, about 70 Å to about 75 Å, about 65 Å to about 70 Å, about 60 Å to about 65 Å, about 55 Å to about 60 Å, about 50 Å to about 55 Å, about 45 Å to about 50 Å, about 40 Å to about 45 Å, about 35 Å to about 40 Å, about 30 Å to about 35 Å, about 25 Å to about 30 Å, about 20 Å to about 25 Å, about 15 Å to about 20 Å, about 10 Å to about 15 Å, about Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 5 Å to about 10 Å, about 1 Å to about 5 Å (e.g., about 100 Å, about 95 Å, about 90 Å, about 85 Å, about 80 Å, about 75 Å, about 70 Å, about 65 Å, about 60 Å, about 55 Å, about 50 Å, about 45 Å, about 40 Å, about 35 Å, about 30 Å, about 25 Å, about 20 Å, about 15 Å, about 10 Å, about 5 Å, or about 1 Å). Thus, a first reporter protein fragment and a second reporter protein fragment, when in close proximity to each other (e.g., within a distance of up to 10 nm, ideally less than 50-60 Å from each other), can reconstitute a reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). For example, a full and functional reporter protein can be formed when a first reporter protein fragment and a second reporter protein fragment come together, e.g., come within a distance of less than 10 nm, preferably within 50-60 Å of each other. This is demonstrated, e.g., in FIG 2D, that shows a distance of 18.7 Å between a pair of fusion proteins, B3L and B3S, wherein one fusion protein comprises a first luciferase fragment and the other fusion protein comprises the second luciferase fragment, wherein the first luciferase fragment and the the second luciferase fragment are complementary to each other. This close distance results in efficient reconstitution of the reporter protein, resulting in robust and easily detectable signal. The importance of close proximity between the reporter protein fragments for generation of robust detectable signal by the reporter protein is also demonstrated, e.g., in FIG.29. Along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins can comprise one or more domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that comprise or mimic domains from one or more binders of the analyte; the binding domains bind to the analyte. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins can comprise one or more domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are binders of the analyte. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins can comprise one or more domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are fragments of binders of the analyte. For Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 example, for detection of a viral protein by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises one of the two complementary fragments of a reporter protein and one or more domains, wherein the domains: mimic or comprise domains from one or more binders of the viral protein; are one or more binders of the viral protein; and / or are fragments of one or more binders of the viral protein. These domains of the fusion proteins can be referred to herein as binding domains, minibinders, mini binders, binders, and / or binder domains. The binding domains can include receptors or receptor domains that bind the analytes. The methods include bringing the fusion proteins into contact with a sample, and then detecting a signal from the fusion proteins. For reporter proteins that require a substrate, the methods include making a mixture comprising the substrate, the fusion proteins, and the sample, and then detecting a signal from the reporter. In some embodiments, the substrate can be provided in a mixture with the fusion proteins in a single container, e.g., in a well of a test strip. Alternatively, the substrate can be provided separately, and the sample can be added to the fusion proteins and then the substrate added, or the sample can be added to the substrate and then mixed into the fusion proteins. In some embodiments, the sample can be added to the fusion proteins and then the substrate added, to allow sample and the fusion proteins to interact and then combine with the substrate, before the signal is detected. In some embodiments, the analytes that are detected by the fusion proteins of the present disclosure are multimeric analytes (e.g., multimeric proteins). In certain embodiments, binding domains of the fusion proteins mimic or comprise domains from binders that recognize non-overlapping epitopes at or near the apex of the multimeric analyte. Additionally, or in the alternative, binding domains of the fusion proteins can be binders that recognize non-overlapping epitopes at or near the apex of the multimeric analyte. Additionally, or in the alternative, binding domains of the fusion proteins can be fragments of binders that recognize non-overlapping epitopes at or near the apex of the multimeric analyte. In certain embodiments, the multimeric analyte is a protein expressed by (e.g., on or in) a virus, e.g., a multimeric protein on the surface of the virus. Viral surface proteins of enveloped viruses, such as SARS-CoV-2, respiratory syncytial virus infection (RSV), Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 measles, influenza, human immunodeficiency virus (HIV), ebola, hepatitis C, zika, monkeypox, and MERS-CoV, usually assemble as homo-trimers. Thus, for detection of enveloped viruses by the present methods, fusion proteins can be used, wherein the one or more binding domains of the fusion proteins mimic or comprise domains from binders that recognize non-overlapping epitopes at or near the apex of the trimeric assembly of the viral surface proteins. Additionally or in the alternative, for detection of enveloped viruses by the present methods, fusion proteins can be used, wherein the one or more binding domains of the fusion proteins are binders or fragments of binders that recognize non-overlapping epitopes at or near the apex of the trimeric assembly of the viral surface proteins. Viral surface proteins (e.g., capsid proteins) of non-enveloped viruses, such as poliovirus, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, and rotavirus, many of which generally assemble as homo-pentamers. Thus, for detection of non-enveloped viruses by the present methods, fusion proteins can be used, wherein the one or more binding domains of the fusion proteins mimic or comprise domains from binders that recognize non-overlapping epitopes at or near the apex of the pentameric assembly of the viral surface proteins. Additionally or in the alternative, for detection of non-enveloped viruses by the present methods, fusion proteins can be used, wherein the one or more binding domains of the fusion proteins are binders or fragments of binders that recognize non-overlapping epitopes at or near the apex of the pentameric assembly of the viral surface proteins. Binders of viral proteins or other analytes can be one or more antibodies. For example, for detection of an analyte by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises one of the two complementary fragments of a reporter protein and one or more binding domains, wherein the binding domains mimic or comprise domains from (or the binding domains are) one or more antibodies that bind to the analyte. In some embodiments, the binding domains of the fusion proteins mimic or comprise domains from (or the binding domains are) one or more antibodies that bind to non-overlapping epitopes at or near the apex of the analyte . Additionally, or in the alternative, binders of protein analytes can be one or more non-antibody binders. For example, for detection of a protein by the present methods, a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises one of the two complementary fragments of a reporter protein and one or more binding domains, wherein the binding domains mimic or comprise domains from (or the binding domains are) one or more receptors, e.g., in the case of viral protein analytes can be host receptors that bind to the vial protein. In some embodiments, the binding domains of the fusion proteins mimic or comprise domains from (or the binding domains are) one or more host receptors that bind to non- overlapping epitopes at or near the apex of a viral protein analyte. When a first fusion protein and a second fusion protein come in contact with a test sample (e.g., a biological sample or environmental sample) that contains the analyte of interest (e.g., analyte to which binding domains of the fusion proteins bind), the one or more binding domains of the fusion proteins bind to the analyte. Binding of these binding domains to the analyte brings the two fragments (e.g., the two complementary fragments) of the reporter protein into close proximity of one another, and results in formation of a functional reporter protein. For example, a functional reporter protein can be formed, when the binding domains of a first fusion protein and a second fusion protein bind to the protein analyte in a test sample (e.g., a biological sample or environmental sample), and bring the two complementary fragments of the reporter protein into close proximity of one another. As used herein, close proximity can refer to conditions wherein the two fragments from a reporter protein, which are separately fused to one or more binding domains in the first and second fusion proteins, are brought together in a suspension or solution or fluid mixture, so that interactions can occur between the two fragments, and a full reporter protein is formed and functionality of the reporter protein is restored. Such interaction between the two complementary fragments can be referred to herein as complementation. In some embodiments, the two complementary fragments of a reporter protein, e.g., a fragment present in a first fusion protein and a fragment present in a second fusion protein, can be referred to herein as a first fragment and a second fragment, respectively. In some embodiments, the fusion proteins comprise one of the two complementary fragments of the reporter protein (e.g., the first fragment or the second fragment) at the N-terminal. In some embodiments, the fusion proteins comprise one of Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the two complementary fragments of the reporter protein (e.g., the first fragment or the second fragment) at the C-terminal. In some embodiments, the reporter protein fragment (e.g., the first fragment or the second fragment) is fused to the one or more binding domains. In some embodiments, the reporter protein fragment (e.g., the first fragment or the second fragment) is fused directly to one of the binding domains. In some embodiments, the reporter protein fragment (e.g., the first fragment or the second fragment) is fused to one of the binding domains via a linker and / or other intervening sequence. In some instances, a fusion protein of the present disclosure further comprises a purification tag (e.g., FLAG, cMyc tag (EQKLISEEDL; SEQ ID NO: 92), streptavidin tag, His-tag (HHHHHH; SEQ ID NO: 65), or biotin tag). For example, fusion proteins of the present disclosure can comprise a purification tag at the N-terminal. Additionally, or in the alternative, fusion proteins of the present disclosure can comprise a purification tag at the C-terminal. First Fusion Proteins A first fusion protein described herein can comprise a first reporter protein fragment genetically fused to one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains. The first reporter protein fragment of a first fusion protein can be one of the complementary fragments of the reporter protein. When the first reporter protein fragment of a first fusion protein comes together with (e.g., come in close proximity to) its complementary reporter protein fragment (e.g., a second reporter protein fragment of a second fusion protein), a full and functional reporter protein is formed. In some embodiments, a first fusion protein comprises, from N-terminal to C- terminal: (a) a first reporter protein fragment; and (b) one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains. In some embodiments, a first fusion protein comprises, from N-terminal to C-terminal: (a) one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains; and (b) a first reporter protein fragment. In some embodiments, the first reporter protein fragment is directly fused to one of the binding domains. In some embodiments, the first reporter protein fragment is fused to one of the binding domains via a linker. Additionally, or in the alternative, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 linkers can be present between two binding domains. In some embodiments, a first fusion protein comprises one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) linkers. The linkers can be from 1 to 25 amino acids long, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids long. In some embodiments, the linker is GSG, or variations thereof (e.g., SGGGGSGGGGSGGGGS (SEQ ID NO: 70)). In some embodiments, a first fusion protein comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, a first fusion protein can comprise a purification tag at the N-terminal. Additionally, or in the alternative, a first fusion protein can comprise a purification tag at the C-terminal. In some embodiments, the reporter protein is luciferase. Thus, a first fusion protein of the present disclosure can comprise, from N-terminal to C-terminal: (a) a first luciferase fragment (also referred to herein as a first split luciferase fragment); and (b) one or more binding domains. Additionally, or in the alternative, a first fusion protein of the present disclosure can comprise, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a first luciferase fragment (also referred to herein as a first split luciferase fragment). In some embodiments, the first luciferase fragment is a small luciferase fragment, and the first fusion protein comprises, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) one or more binding domains. Examples of such first fusion proteins include, without limitation the following fusion proteins of the present disclosure: SB3, S-CTC, S-CT30, S-ACE2, S84, S-M97, S-CD155, S-A12, and S-F12. In some embodiments, the first luciferase fragment is a small luciferase fragment, and the first fusion protein comprises, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a small luciferase fragment. Examples of such first fusion proteins include, without limitation the following fusion proteins of the present disclosure: B3S, IS, CTC-S, CT30-S, ACE2-S, 84S, BN117-PGT145(heavy)-S, 10E8- BN117-PGT145(heavy)-S, 10E8-CD4-PGT145(heavy)-S, M97-S, CD155-S, A12-S, and F12-S. In some embodiments, the first luciferase fragment is a large luciferase fragment, and the first fusion protein comprises, from N-terminal to C-terminal: (a) a large Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 luciferase fragment; and (b) one or more binding domains. Examples of such first fusion proteins include, without limitation the following fusion proteins of the present disclosure: LB3, L-CTC, L-CT30, L-ACE2, L84, L-M97, L-CD155, L-A12, and L-F12. In some embodiments, the first luciferase fragment is a large luciferase fragment, and the first fusion protein comprises, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a large luciferase fragment. Examples of such first fusion proteins include, without limitation the following fusion proteins of the present disclosure: B3L, IL, CTC-L, CT30-L, ACE2-L, 84L, BN117-PGT145(light)-L, 10E8- BN117-PGT145(light)-L, 10E8-CD4-PGT145(light)-L, M97-L, CD155-L, A12-L, and A12-L. Second Fusion Proteins A second fusion protein described herein can comprise a second reporter protein fragment genetically fused to one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains. The second reporter protein fragment of a second fusion protein can be one of the complementary fragments of the reporter protein. When the second reporter protein fragment of a second fusion protein comes together with (e.g., come in close proximity to) its complementary reporter protein fragment (e.g., a first reporter protein fragment of a first fusion protein), a full and functional reporter protein is formed. In some embodiments, a second fusion protein comprises, from N-terminal to C- terminal: (a) a second reporter protein fragment; and (b) one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains. In some embodiments, a second fusion protein comprises, from N-terminal to C-terminal: (a) one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) binding domains; and (b) a second reporter protein fragment. In some embodiments, the second reporter protein fragment is directly fused to one of the binding domains. In some embodiments, the second reporter protein fragment is fused to one of the binding domains via a linker. Additionally, or in the alternative, linkers can be present between two binding domains. In some embodiments, a second fusion protein comprises one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) linkers. The linkers can be from 1 to 25 amino acids long, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids long. In some Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 embodiments, the linker is GSG, or variations thereof (e.g., SGGGGSGGGGSGGGGS (SEQ ID NO: 70)). In some embodiments, a second fusion protein comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, a second fusion protein can comprise a purification tag at the N-terminal. Additionally, or in the alternative, a second fusion protein can comprise a purification tag at the C-terminal. In some embodiments, the reporter protein is luciferase. Thus, a second fusion protein of the present disclosure can comprise, from N-terminal to C-terminal: (a) a second luciferase fragment (also referred to herein as a second split luciferase fragment); and (b) one or more binding domains. Additionally, or in the alternative, a second fusion protein of the present disclosure can comprise, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a second luciferase fragment (also referred to herein as a second split luciferase fragment). In some embodiments, the second luciferase fragment is a small luciferase fragment, and the second fusion protein comprises, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) one or more binding domains. Examples of such second fusion proteins include, without limitation the following fusion proteins of the present disclosure: SB3, S-CTC, S-CT30, S-ACE2, S84, S-M97, S-CD155, S-A12, and S-F12. In some embodiments, the second luciferase fragment is a small luciferase fragment, and the second fusion protein comprises, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a small luciferase fragment. Examples of such second fusion proteins include, without limitation the following fusion proteins of the present disclosure: B3S, IS, CTC-S, CT30-S, ACE2-S, 84S, BN117-PGT145(heavy)-S, 10E8-BN117-PGT145(heavy)-S, 10E8-CD4-PGT145(heavy)-S, M97-S, CD155-S, A12- S, and F12-S. In some embodiments, the second luciferase fragment is a large luciferase fragment, and the second fusion protein comprises, from N-terminal to C-terminal: (a) a large luciferase fragment; and (b) one or more binding domains. Examples of such second fusion proteins include, without limitation the following fusion proteins of the Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 present disclosure: LB3, L-CTC, L-CT30, L-ACE2, L84, L-M97, L-CD155, L-A12, and L-F12. In some embodiments, the second luciferase fragment is a large luciferase fragment, and the second fusion protein comprises, from N-terminal to C-terminal: (a) one or more binding domains; and (b) a large luciferase fragment. Examples of such second fusion proteins include, without limitation the following fusion proteins of the present disclosure: B3L, IL, CTC-L, CT30-L, ACE2-L, 84L, BN117-PGT145(light)-L, 10E8-BN117-PGT145(light)-L, 10E8-CD4-PGT145(light)-L, M97-L, CD155-L, A12-L, and A12-L. Multimeric Analytes The multimeric analytes detectable using the fusion proteins and methods described herein can include any multimeric proteins; non-limiting examples include multimeric proteins expressed by a pathogenic organism, e.g., multimeric proteins that are present on the surface of pathogenic organisms and / or intracellular multimeric proteins (e.g., wherein the method further includes a lysis step), or by a cell such as a mammalian cell and / or a cancer cell. Multimeric proteins are those that assemble into multimeric complexes (e.g., dimers, trimers, tetramers, pentamers, etc). Pathogenic organisms can include any virus, bacteria, parasites, archae, fungus, protozoa, or multicellular organism that utilize multimeric complexes on their surfaces or intracellularly (in which case the methods would include a lysis step to release the analytes into solution). Viral surface proteins of enveloped viruses (e.g., SARS-CoV-2, RSV, influenza, measles, HIV, ebola, hepatitis C, zika, monkeypox, and MERS-CoV) assemble as homo-trimers, and such homo-trimeric viral proteins, as such, can be detected by the fusion proteins of the present disclosure. Viral surface proteins (e.g., capsid proteins) of non-enveloped viruses (e.g., poliovirus, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, and rotavirus) assemble as homo- pentamers, and such homo-pentameric viral proteins can also be detected by the fusion proteins of the present disclosure. In some embodiments, multimeric analytes that are detected by the present methods are one or more multimeric eukaryotic proteins (e.g., multimeric eukaryotic Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 membrane proteins). Examples of such multimeric eukaryotic membrane proteins include, without limitation, T cell immunoglobulin mucin domain-1 (TIM-1), G protein- coupled receptor (GPCR) rhodopsin and β2 adrenergic receptor (6,7), ion channel acetylcholine receptor and P2X receptor (8,9), tumor necrosis factor receptor (TNFR) family (10), integrin family of cell adhesion receptors (11), and interleukin-2 receptor (IL-2R) (12). In some embodiments, multimeric analytes that are detceted by the present methods are one or more of multimeric eukaryotic proteins (e.g., multimeric eukaryotic membrane proteins). Many bacterial proteins (e.g., bacterial surface proteins) also assemble as multimers. Examples of multimeric bacterial proteins and their associated bacteria include: 1) Type IV Pilus (T4P) Proteins: Many bacteria, including Pseudomonas aeruginosa and Neisseria gonorrhoeae, produce Type IV pili, which are important for adherence and motility.2) Pilin (Fimbrial Protein) or Porin Proteins (PorB): important in Neisseria gonorrhoeae.3) CyaA (Adenylate Cyclase) Protein: This protein from Bordetella pertussis is involved in the pathogenic mechanisms of the bacterium. CyaA can form a trimeric structure, which is essential for its function as it interacts with host cells.4) Aerobactin Irom Aquisition System: In Escherichia coli, the component proteins that are part of the aerobactin system, which plays a role in iron acquisition, may assemble in trimers to facilitate transport and binding.5) Collagen-Binding Protein (CbpA): Found in Streptococcus pneumoniae, this surface protein that binds to collagen can also exist in a trimeric form, which is crucial for its function in host tissue adhesion. 6) Mannose-Binding Pili: In some strains of E. coli, proteins that form the mannose- binding pili can organize into trimers to enhance the linking efficiency to host tissues.7) Chlamydial major outer membrane protein (MOMP): in Chlamydia trachomatis MOMP forms homo‐trimers (see, Feher VA, Randall A, Baldi P, Bush RM, de la Maza LM, Amaro RE (2013) A 3-Dimensional Trimeric β-Barrel Model for Chlamydia MOMP Contains Conserved and Novel Elements of Gram-Negative Bacterial Porins. PLoS ONE 8(7): e68934. https: / / doi.org / 10.1371 / journal.pone.0068934 and Peterson EM, Cheng X, Markoff BA, Fielder TJ, de la Maza LM. Functional and structural mapping of Chlamydia trachomatis species-specific major outer membrane protein epitopes by use of neutralizing monoclonal antibodies. Infect Immun.1991 Nov;59(11):4147-53; each Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 incorporated herein by reference in their entireties).8) TprK (Treponema pallidum repeat protein K): in Treponema pallidum.9) NaP, MgPa, ML1 and ML2: in Mycoplasma genitalium. In some embodiments, multimeric analytes that are detected by the present methods are one or more multimeric bacterial proteins (e.g., multimeric bacterial surface proteins). Parasitic proteins (e.g., parasitic surface proteins) can also assemble into multimeric complexes. An exemplary parasite includes malarial parasite, plasmodium (e.g., the fusion proteins can target merozoite surface proteins, MSP-1 and MSP-2). Another example is Trichomonas vaginalis (Target: P270 or flagellar proteins). In some embodiments, multimeric analytes that are detected by the present methods are one or more multimeric parasitic proteins (e.g., multimeric parasitic surface proteins). Thus, multimeric analyte, as used herein, can include, without limitation, multimeric proteins expressed by (e.g., expressed in or on) one or more of a virus (e.g., a multimeric viral protein), a bacterium (e.g., a multimeric bacterial protein), a parasite (e.g., a multimeric parasitic protein), a fungus, and an eukaryotic cell (e.g., a multimeric eukaryotic protein). As used herein, an analyte can be a multimeric protein expressed by a pathogenic organism (e.g., virus, bacteria, parasite, fungus). For example, the fusion proteins of the present method can be used for detection of one or more multimeric protein expressed by viruses in test sample (e.g., in a biological sample from a subject). In some embodiments, the virus is a pathogenic virus, including, but not limited to, SARS CoV-2, MERS CoV, influenza virus (e.g., human influenza A virus or human influenza B virus), HIV, RSV, measles, poliovirus, Ebola, zika virus, BKV, monkeypox virus, poliovirus, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, and rotavirus. Thus, in some embodiments, the fusion proteins of the present disclosure can be used for detection of multimeric proteins expressed by SARS CoV-2, influenza virus (e.g., human influenza A virus or human influenza B virus), HIV, RSV, measles, poliovirus, poliovirus, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, and rotavirus in a test sample (e.g., a biological sample from a subject). The subject can be a mammalian subject, such as, a human or a non-human veterinary subject (e.g., a cat, dog, rabbit, horse, goat, or pig). Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Examples of binding domains for these multimeric analytes are described hereinbelow. Fusion Proteins for Detection of SARS-CoV-2 Described herein are fusion proteins for detection of SARS-CoV-2 (e.g., multimeric proteins expressed by SARS-CoV-2 in test samples (e.g., biological samples). In some embodiments, fusion proteins for detection of multimeric proteins expressed by SARS-CoV-2 can be referred to herein as fusion proteins for detection of SARS-CoV-2. For detection of multimeric proteins expressed by SARS-CoV-2 by the present methods, a pair of fusion proteins (e.g., a first fusion protein and a second fusion protein) can be used, wherein the first fusion protein comprises a first reporter protein fragment, and the second fusion protein comprises a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). Along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of these first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that mimic or comprise domains from one or more binders of SARS-CoV-2. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are one or more binders or fragments of one or more binders of SARS-CoV-2. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the reporter protein is luciferase. For example, for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split luciferase fragments (e.g., one fusion protein comprising a large luciferase fragment and the other fusion protein comprising a small luciferase fragment, wherein the small luciferase fragment is complementary to the large luciferase fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 1 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1. In some embodiments, a fusion protein (e.g., a fusion protein for detection of SARS-CoV-2 by the present methods) comprises a small luciferase fragment that comprises 11 amino acids. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) comprises a large luciferase fragment. In some embodiments, the large luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 2 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2. In some embodiments, a fusion protein (e.g., a fusion protein for detection of Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 SARS-CoV-2 by the present methods) comprises a large luciferase fragment that comprises 158 amino acids. In some embodiments, the reporter protein is HRP. For example, for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split HRP fragments (e.g., one fusion protein comprising a first HRP fragment and the other fusion protein comprising a second HRP fragment, wherein the first HRP fragment is complementary to the second HRP fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) comprises a first HRP fragment. In some embodiments, the first HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 71. In some embodiments, the first HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 71 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the first HRP fragment comprises the amino acid sequence of SEQ ID NO: 71. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) comprises a second HRP fragment. In some embodiments, the second HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 72. In some embodiments, the second HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 72 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the second HRP fragment comprises the amino acid sequence of SEQ ID NO: 72. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Binding Domains for SARS-CoV-2 Binding domains of these fusion proteins (e.g., fusion proteins that can be used for detection of multimeric proteins expressed by SARS-CoV-2 by the present methods) can mimic or comprise domains from angiotensin-converting enzyme 2 (ACE2) receptors on host cells that bind to the receptor binding domain (RBD) of SARS-CoV-2 spike (e.g., SARS-CoV-2 wild-type spike). Additionally, or in the alternative, these binding domains can be ACE2 receptor (from host cells) or fragments thereof that bind to the RBD of SARS-CoV-2 spike (e.g., SARS-CoV-2 wild-type spike). In some embodiments, binding domains of these fusion proteins can be binders (or fragments thereof) that recognize and bind to epitopes (e.g., non-overlapping epitopes) at or near the apex of the trimeric assembly of the SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). These fusion proteins (e.g., fusion proteins for detection of SARS-CoV-2 by the present methods) can comprise one or more of the following binding domains: B3, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 3; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 3 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 3; I, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 8; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 8 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 8; CTC, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 11; an amino acid sequence that differs from the amino acid Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 sequence of SEQ ID NO: 11 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 11; CT30, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 16; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 16 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 16; and ACE2, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 21; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 21 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 21. Detection of SARS-CoV-2 The fusion proteins of the present disclosure can be used for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample). If a test sample comprising SARS-CoV-2 (multimeric proteins expressed by SARS-CoV-2) is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). Binding of the binding domains to the RBD of the trimeric spike proteins brings the two complementary reporter protein fragments into close proximity to reconstitute a functional reporter protein. In some embodiments, the reporter protein is luciferase, and binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from one fusion protein and the small luciferase fragment from the other fusion protein) into close proximity to reconstitute a functional luciferase enzyme. When a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 substrate of the reporter protein is added to this composition, the functional reporter protein generates a detectable signal (e.g., luminescence, color, and / or precipitation). Thus, a pair of these fusion proteins (e.g., one fusion protein comprising one reporter protein fragment and the other fusion protein comprising its complementary reporter protein fragment) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Such an assay is rapid and straightforward to perform and involves only mixing a test sample (e.g., a biological sample from a subject) with the two fusion proteins, followed by the addition of a suitable substrate for detectable readout (e.g., luminescence, color, and / or precipitation). The whole detection process occurs in the liquid phase without any washing steps. Also, since persistent circulating SARS-CoV-2 spike trimer (and not the Nucleocapsid or S1) in the blood is associated with post-acute sequelae of COVID-19 or long COVID, the fusion proteins of the present disclosure can also be used in the diagnosis of long COVID. Two or more of the following fusion proteins can be used for detection of SARS- CoV-2 in biological samples. B3S In some embodiments, a fusion protein for detection of SARS-CoV-2 comprises a small luciferase fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS- CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a B3 domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as B3S. In some embodiments, the small luciferase fragment in B3S is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, B3S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, B3S can comprise a purification tag at the N- Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 terminal. Additionally, or in the alternative, B3S can comprise a purification tag at the C- terminal. B3S has a molecular weight of 12.5 kDa. In some embodiments, B3S is produced using E. coli expression system. For example, a nucleic acid sequence encoding B3S can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, B3S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, B3S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 4 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, B3S comprises the amino acid sequence of SEQ ID NO: 4. In some embodiments, B3S comprises a signal sequence. Exemplary construct design of B3S (left panel) and exemplary protein structure of B3S predicted by AlphaFold (right panel) are provided in FIG.6. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), B3S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, B3S can be used with B3L and / or LB3 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from B3S and the large luciferase fragment from B3L and / or LB3) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, B3S along with another fusion protein (e.g., B3L and / or LB3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 SB3 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) a B3 domain. Such a fusion protein can be referred to herein as SB3. In some embodiments, the small luciferase fragment in SB3 is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, SB3 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, SB3 can comprise a purification tag at the N- terminal. Additionally, or in the alternative, SB3 can comprise a purification tag at the C- terminal. SB3 has a molecular weight of 12.5 kDa. In some embodiments, SB3 is produced using E. coli expression system. For example, a nucleic acid sequence encoding SB3 can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, SB3 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 5. In some embodiments, SB3 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 5 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, SB3 comprises the amino acid sequence of SEQ ID NO: 5. In some embodiments, SB3 comprise a signal sequence. Exemplary construct design of SB3 (left panel) and exemplary protein structure of SB3 predicted by AlphaFold (right panel) are provided in FIG.7. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), SB3 can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, SB3 can be used with B3L and / or LB3 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from SB3 and the large luciferase fragment from B3L and / or LB3) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, SB3 along with another fusion protein (e.g., B3L and / or LB3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. B3L In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a B3 domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as B3L. In some embodiments, the large luciferase fragment in B3L is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, B3L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, B3L can comprise a purification tag at the N- Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 terminal. Additionally, or in the alternative, B3L can comprise a purification tag at the C- terminal. B3L has a molecular weight of 28.7 kDa. In some embodiments, B3L is produced using E. coli expression system. For example, a nucleic acid sequence encoding B3L can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, B3L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 6. In some embodiments, B3L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 6 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, B3L comprises the amino acid sequence of SEQ ID NO: 6. In some embodiments, B3L comprises a signal sequence. Exemplary construct design of B3L (left panel) and exemplary protein structure of B3L predicted by AlphaFold (right panel) are provided in FIG.8. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), B3L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, B3L can be used with B3S and / or SB3 for detection of SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from B3L and the small luciferase fragment from B3S and / or SB3) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, B3L along with another fusion protein (e.g., B3S and / or SB3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 LB3 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a large luciferase fragment; and (b) a B3 domain. Such a fusion protein can be referred to herein as LB3. In some embodiments, the large luciferase fragment in LB3 is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, LB3 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, LB3 can comprise a purification tag at the N- terminal. Additionally, or in the alternative, LB3 can comprise a purification tag at the C- terminal. LB3 has a molecular weight of 28.7 kDa. In some embodiments, LB3 is produced using E. coli expression system. For example, a nucleic acid sequence encoding LB3 can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, LB3 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 7. In some embodiments, LB3 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 7 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, LB3 comprises the amino acid sequence of SEQ ID NO: 7. In some embodiments, LB3 comprises a signal sequence. Exemplary construct design of LB3 (left panel) and exemplary protein structure of LB3 predicted by AlphaFold (right panel) are provided in FIG.9. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), LB3 can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, LB3 can be used with B3S and / or SB3 for detection of multimeric proteins expressed by SARS-CoV-2 in a test. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from LB3 and the small luciferase fragment from B3S and / or SB3) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, LB3 along with another fusion protein (e.g., B3S and / or SB3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. IS In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, I. The I domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the C- terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a I domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as IS. In some embodiments, the small luciferase fragment in IS is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some instances, IS further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, IS can comprise a purification tag at the N-terminal. Additionally, or in the alternative, IS can comprise a purification tag at the C-terminal. IS has a molecular weight of 69.8 kDa. In some embodiments, IS is produced using mammalian expression system (e.g., in HEK293 T cells). For example, a nucleic acid sequence encoding IS can be cloned into a pcDNA3.1 plasmid for protein expression. In some embodiments, IS comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 9. In some embodiments, IS comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 9 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, IS comprises the amino acid sequence of SEQ ID NO: 9. In some embodiments, IS comprises a signal sequence. Exemplary construct design of IS (left panel) and exemplary protein structure of IS predicted by AlphaFold (right panel) are provided in FIG.11. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), IS can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, IS can be used with IL for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS- CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from IL and the small luciferase fragment from IS) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, IS along with another fusion protein (e.g., IL) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 IL In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, I. The I domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the C- terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a I domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as IL. In some embodiments, the large luciferase fragment in IL is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, IL further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, IL can comprise a purification tag at the N-terminal. Additionally, or in the alternative, IL can comprise a purification tag at the C-terminal. IL has a molecular weight of 86 kDa. In some embodiments, IL is produced using mammalian expression system (e.g., in HEK293 T cells). For example, a nucleic acid sequence encoding IL can be cloned into a pcDNA3.1 plasmid for protein expression. In some embodiments, IL comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 10. In some embodiments, IL comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 10 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, IL comprises the amino acid sequence of SEQ ID NO: 10. In some embodiments, IL comprises a signal sequence. Exemplary construct design of IL (left panel) and exemplary protein structure of IL predicted by AlphaFold (right panel) are provided in FIG.10. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample, IL can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, IL can be used with IS for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from IL and the small luciferase fragment from IS) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, IL along with another fusion protein (e.g., IS) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CTC-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, CTC. The CTC domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a CTC domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as CTC-S. In some embodiments, the small luciferase fragment in CTC-S is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, CTC-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, CTC-S can comprise a purification tag at the N- Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 terminal. Additionally, or in the alternative, CTC-S can comprise a purification tag at the C-terminal. CTC-S has a molecular weight of 21.9 kDa. In some embodiments, CTC-S is produced using E. coli expression system. For example, a nucleic acid sequence encoding CTC-S can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, CTC-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 12. In some embodiments, CTC-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 12 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CTC-S comprises the amino acid sequence of SEQ ID NO: 12. In some embodiments, CTC-S comprises a signal sequence. In some embodiments, CTC-S includes a signal peptide and comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 15. In some embodiments, CTC-S includes a signal peptide and comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 15 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. Exemplary construct design of CTC-S (left panel) and exemplary protein structure of CTC-S predicted by AlphaFold (right panel) are provided in FIG.12. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CTC-S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, CTC-S can be used with CTC-L and / or L-CTC for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from CTC-S and the large luciferase fragment from CTC-L Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 and / or L-CTC) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, CTC-S along with another fusion protein (e.g., CTC-L and / or L-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. S-CTC In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, CTC. The CTC domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) a CTC domain. Such a fusion protein can be referred to herein as S-CTC. In some embodiments, the small luciferase fragment in S-CTC is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, S-CTC further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, S-CTC can comprise a purification tag at the N- terminal. Additionally, or in the alternative, S-CTC can comprise a purification tag at the C-terminal. S-CTC has a molecular weight of 22.2 kDa. In some embodiments, S-CTC is produced using E. coli expression system. For example, a nucleic acid sequence encoding S-CTC can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, S-CTC comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 SEQ ID NO: 13. In some embodiments, S-CTC comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 13 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, S-CTC comprises the amino acid sequence of SEQ ID NO: 13. In some embodiments, S-CTC comprises a signal sequence. Exemplary construct design of S-CTC (left panel) and exemplary protein structure of S-CTC predicted by AlphaFold (right panel) are provided in FIG.13. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), S-CTC can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, S-CTC can be used with CTC-L and / or L-CTC for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from S-CTC and the large luciferase fragment from CTC-L and / or L-CTC) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, S-CTC along with another fusion protein (e.g., CTC-L and / or L-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CTC-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, CTC. The CTC domain of the fusion protein mimics ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 N-terminal to C-terminal: (a) a CTC domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as CTC-L. In some embodiments, the large luciferase fragment in CTC-L is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, CTC-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, CTC-L can comprise a purification tag at the N- terminal. Additionally, or in the alternative, CTC-L can comprise a purification tag at the C-terminal. CTC-L has a molecular weight of 38.2 kDa. In some embodiments, CTC-L is produced using E. coli expression system. For example, a nucleic acid sequence encoding CTC-L can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, CTC-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 14. In some embodiments, CTC-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 14 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CTC-L comprises the amino acid sequence of SEQ ID NO: 14. In some embodiments, CTC-L comprises a signal sequence. Exemplary construct design of CTC-L (left panel) and exemplary protein structure of CTC-L predicted by AlphaFold (right panel) are provided in FIG.14. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CTC-L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, CTC-L can be used with CTC-S and / or S-CTC for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 large luciferase fragment from CTC-L and the small luciferase fragment from CTC-S and / or S-CTC) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, CTC-L along with another fusion protein (e.g., CTC-S and / or S-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. L-CTC In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, CTC. The CTC domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a large luciferase fragment; and (b) a CTC domain. Such a fusion protein can be referred to herein as L-CTC. In some embodiments, the large luciferase fragment in L-CTC is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, L-CTC further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, L-CTC can comprise a purification tag at the N- terminal. Additionally, or in the alternative, L-CTC can comprise a purification tag at the C-terminal. L-CTC has a molecular weight of 38.2 kDa. In some embodiments, L-CTC is produced using E. coli expression system. For example, a nucleic acid sequence encoding L-CTC can be cloned into a pET-28a(+) plasmid for protein expression. In some embodiments, L-CTC comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 15. In some embodiments, L-CTC comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 15 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, L-CTC comprises the amino acid sequence of SEQ ID NO: 15. In some embodiments, L-CTC comprises a signal sequence. Exemplary construct design of L-CTC (left panel) and exemplary protein structure of L-CTC predicted by AlphaFold (right panel) are provided in FIG.15. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), L-CTC can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, L-CTC can be used with CTC-S and / or S-CTC for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from L-CTC and the small luciferase fragment from CTC-S and / or S-CTC) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, L-CTC along with another fusion protein (e.g., CTC-S and / or S-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CT30-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, CT30 (also referred to herein as CTC-N30). The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 protein comprises the small luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N- terminal to C-terminal: (a) a CT30 domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as CT30-S. In some embodiments, the small luciferase fragment in CT30-S is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, CT30-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, CT30-S can comprise a purification tag at the N-terminal. Additionally, or in the alternative, CT30-S can comprise a purification tag at the C-terminal. CT30-S has a molecular weight of 35.8 kDa. In some embodiments, CT30-S is produced using E. coli expression system. For example, a nucleic acid sequence encoding CT30-S can be cloned into a pET-26b(+) plasmid for protein expression. In some embodiments, CT30-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 17. In some embodiments, CT30-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 17 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CT30-S comprises the amino acid sequence of SEQ ID NO: 17. In some embodiments, CT30-S comprises a signal sequence. Exemplary construct design of CT30-S (left panel) and exemplary protein structure of CT30-S predicted by AlphaFold (right panel) are provided in FIG.16. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CT30-S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, CT30-S can be used with CT30-L and / or L-CT30 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from CT30-S and the large luciferase fragment from CT30-L and / or L-CT30) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, CT30-S along with another fusion protein (e.g., CT30-L and / or L-CT30) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. S-CT30 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, CT30 (also referred to herein as CTC-N30). The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) a CT30 domain. Such a fusion protein can be referred to herein as S-CT30. In some embodiments, the small luciferase fragment in S-CT30 is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, S-CT30 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, S-CT30 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, S-CT30 can comprise a purification tag at the C-terminal. S-CT30 has a molecular weight of 36 kDa. In some embodiments, S-CT30 is produced using E. coli expression system. For example, a nucleic acid sequence Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 encoding S-CT30 can be cloned into a pET-26b(+) plasmid for protein expression. In some embodiments, S-CT30 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 18. In some embodiments, S-CT30 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 18 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, S-CT30 comprises the amino acid sequence of SEQ ID NO: 18. In some embodiments, S-CT30 comprises a signal sequence. Exemplary construct design of S-CT30 (left panel) and exemplary protein structure of S-CT30 predicted by AlphaFold (right panel) are provided in FIG.17. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), S-CT30 can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, S-CT30 can be used with CT30-L and / or L-CT30 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from CT30-S and the large luciferase fragment from CT30-L and / or L-CT30) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, CT30-S along with another fusion protein (e.g., CT30-L and / or L-CT30) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CT30-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 CT30 (also referred to herein as CTC-N30). The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N- terminal to C-terminal: (a) a CT30 domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as CT30-L. In some embodiments, the large luciferase fragment in CT30-L is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, CT30-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, CT30-L can comprise a purification tag at the N-terminal. Additionally, or in the alternative, CT30-L can comprise a purification tag at the C-terminal. CT30-L has a molecular weight of 52.3 kDa. In some embodiments, CT30-L is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding CT30-L can be cloned into a pcDNA3.4(+) plasmid for protein expression. In some embodiments, CT30-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 19. In some embodiments, CT30-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 19 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CT30-L comprises the amino acid sequence of SEQ ID NO: 19. In some embodiments, CT30-L comprises a signal sequence. Exemplary construct design of CT30-L (left panel) and exemplary protein structure of CT30-L predicted by AlphaFold (right panel) are provided in FIG.18. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CT30-L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, CT30-L Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 can be used with CT30-S and / or S-CT30 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from CT30-L and the small luciferase fragment from CT30-S and / or S-CT30) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, CT30-L along with another fusion protein (e.g., CT30-S and / or S-CT30) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. L-CT30 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, CT30 (also referred to herein as CTC-N30). The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a large luciferase fragment; and (b) a CT30 domain. Such a fusion protein can be referred to herein as L-CT30. In some embodiments, the large luciferase fragment in L-CT30 is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, L-CT30 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, L-CT30 can comprise a purification tag Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 at the N-terminal. Additionally, or in the alternative, L-CT30 can comprise a purification tag at the C-terminal. L-CT30 has a molecular weight of 52.3 kDa. In some embodiments, L-CT30 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding L-CT30 can be cloned into a pcDNA3.4(+) plasmid for protein expression. In some embodiments, L-CT30 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 20. In some embodiments, L-CT30 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 20 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, L-CTC-N30 comprises the amino acid sequence of SEQ ID NO: 20. In some embodiments, L-CT30 comprises a signal sequence. Exemplary construct design of L-CT30 (left panel) and exemplary protein structure of L-CT30 predicted by AlphaFold (right panel) are provided in FIG.19. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), L-CT30 can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, L-CT30 can be used with CT30-S and / or S-CT30 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from L-CT30 and the small luciferase fragment from CT30-S and / or S-CT30) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, L-CT30 along with another fusion protein (e.g., CT30-S and / or S-CT30) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 ACE2-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) an ACE2 domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as ACE2-S. In some embodiments, the small luciferase fragment in ACE2-S is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, ACE2-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, ACE2-S can comprise a purification tag at the N-terminal. Additionally, or in the alternative, ACE2-S can comprise a purification tag at the C-terminal. ACE2-S has a molecular weight of 74.1 kDa. In some embodiments, ACE2-S is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding ACE2-S can be cloned into a pcDNA3.1(+) plasmid for protein expression. In some embodiments, ACE2-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 22. In some embodiments, ACE2-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 22 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, ACE2-S comprises the amino acid sequence of SEQ ID NO: 22. In some embodiments, ACE2-S comprises a signal sequence. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Exemplary construct design of ACE2-S (left panel) and exemplary protein structure of ACE2-S predicted by AlphaFold (right panel) are provided in FIG.20. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), ACE2-S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, ACE2-S can be used with ACE2-L and / or L-ACE2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from ACE2-S and the large luciferase fragment from ACE2-L and / or L-ACE2) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, ACE2-S along with another fusion protein (e.g., ACE2-L and / or L-ACE2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. S-ACE2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a small luciferase fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the small luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) an ACE2 domain. Such a fusion protein can be referred to herein as S- ACE2. In some embodiments, the small luciferase fragment in S-ACE2 is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, S-ACE2 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, S-ACE2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, S-ACE2 can comprise a purification tag at the C-terminal. S-ACE2 has a molecular weight of 72.9 kDa. In some embodiments, S-ACE2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding S-ACE2 can be cloned into a pcDNA3.1(+) plasmid for protein expression. In some embodiments, S-ACE2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 23. In some embodiments, S-ACE2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 23 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, S-ACE2 comprises the amino acid sequence of SEQ ID NO: 23. In some embodiments, S-ACE2 comprises a signal sequence. Exemplary construct design of S-ACE2 (left panel) and exemplary protein structure of S-ACE2 predicted by AlphaFold (right panel) are provided in FIG.21. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), S-ACE2 can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, S-ACE2 can be used with ACE2-L and / or L-ACE2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the small luciferase fragment from S-ACE2 and the large luciferase fragment from ACE2-L and / or L-ACE2) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, S-ACE2 along with another fusion protein (e.g., ACE2-L and / or L-ACE2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. ACE2-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) an ACE2 domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as ACE2-L. In some embodiments, the large luciferase fragment in ACE2-L is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, ACE2-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, ACE2-L can comprise a purification tag at the N-terminal. Additionally, or in the alternative, ACE2-L can comprise a purification tag at the C-terminal. ACE2-L has a molecular weight of 90.3 kDa. In some embodiments, ACE2-L is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding ACE2-L can be cloned into a pcDNA3.1(+) plasmid for protein expression. In some embodiments, ACE2-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 24. In some embodiments, ACE2-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 24 by Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, ACE2-L comprises the amino acid sequence of SEQ ID NO: 24. In some embodiments, ACE2-L comprises a signal sequence. Exemplary construct design of ACE2-L (left panel) and exemplary protein structure of ACE2-L predicted by AlphaFold (right panel) are provided in FIG.22. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), ACE2-L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, ACE2-L can be used with ACE2-S and / or S-ACE2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the large luciferase fragment from ACE2-L and the small luciferase fragment from ACE2-S and / or S-ACE2) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, ACE2-L along with another fusion protein (e.g., ACE2-S and / or S-ACE2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. L-ACE2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a large luciferase fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the large luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a large luciferase Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 fragment; and (b) an ACE2 domain. Such a fusion protein can be referred to herein as L- ACE2. In some embodiments, the large luciferase fragment in L-ACE2 is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, L-ACE2 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, strep tag, or biotin tag). For example, L-ACE2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, L-ACE2 can comprise a purification tag at the C-terminal. L-ACE2 has a molecular weight of 89.2 kDa. In some embodiments, L-ACE2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding L-ACE2 can be cloned into a pcDNA3.1(+) plasmid for protein expression. In some embodiments, L-ACE2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 25. In some embodiments, L-ACE2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 25 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, L-ACE2 comprises the amino acid sequence of SEQ ID NO: 25. In some embodiments, L-ACE2 comprise a signal sequence. Exemplary construct design of L-ACE2 (left panel) and exemplary protein structure of L-ACE2 predicted by AlphaFold (right panel) are provided in FIG.23. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), L-ACE2 can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, L-ACE2 can be used with ACE2-S and / or S-ACE2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two split luciferase fragments (e.g., the Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 large luciferase fragment from L-ACE2 and the small luciferase fragment from ACE2-S and / or S-ACE2) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, L-ACE2 along with another fusion protein (e.g., ACE2-S and / or S-ACE2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. P1-AC2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P1 HRP fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P1 HRP fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a P1 HRP fragment; and (b) an ACE2 domain. Such a fusion protein can be referred to herein as P1-AC2. In some embodiments, the P1 HRP fragment in P1-AC2 is fused to the binding domain. In some embodiments, the P1 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, P1-AC2 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P1-AC2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, P1-AC2 can comprise a purification tag at the C-terminal. In some embodiments, P1-AC2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P1-AC2 can be cloned into a plasmid for protein expression. In some embodiments, P1-AC2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 100%) sequence identity to the amino acid sequence of SEQ ID NO: 73. In some embodiments, P1-AC2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 73 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P1-AC2 comprises the amino acid sequence of SEQ ID NO: 73. In some embodiments, P1-AC2 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P1-AC2 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P2). For example, P1-AC2 can be used with P2-AC2 and / or AC2-P2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS- CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from P1-AC2 and the P2 HRP fragment from P2-AC2 and / or AC2-P2) into close proximity to reconstitute a functional HRP enzyme. When (3,3',5,5'- tetramethylbenzidine (TMB), a substrate of HRP, is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P1-AC2 along with another fusion protein (e.g., P2-AC2 and / or AC2-P2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. P2-AC2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P2 HRP fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P2 HRP fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 can comprise, from N-terminal to C-terminal: (a) a P2 HRP fragment; and (b) an ACE2 domain. Such a fusion protein can be referred to herein as P2-AC2. In some embodiments, the P2 HRP fragment in P2-AC2 is fused to the binding domain. In some embodiments, the P2 HRP fragment is fused directly to the binding domain. In some embodiments, the P2 HRP fragment is fused to the binding domain via a linker. In some instances, P2-AC2 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P2-AC2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, P2-AC2 can comprise a purification tag at the C-terminal. In some embodiments, P2-AC2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P2-AC2 can be cloned into a plasmid for protein expression. In some embodiments, P2-AC2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 74. In some embodiments, P2-AC2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 74 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P2-AC2 comprises the amino acid sequence of SEQ ID NO: 74. In some embodiments, P2-AC2 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P2-AC2 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P1). For example, P2-AC2 can be used with P1-AC2 and / or AC2-P1 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS- CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P2 HRP fragment from P2-AC2 and the P1 HRP fragment from P1-AC2 and / or AC2-P1) into close proximity to reconstitute a functional HRP enzyme. When TMB (a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P2-AC2 along with another fusion protein (e.g., P1-AC2 and / or AC2-P1) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. AC2-P1 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P1 HRP fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P1 HRP fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) an ACE2 domain; and (b) a P1 HRP fragment. Such a fusion protein can be referred to herein as AC2-P1. In some embodiments, the P1 HRP fragment in AC2-P1 is fused to the binding domain. In some embodiments, the P1 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, AC2-P1 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, AC2-P1 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, AC2-P1 can comprise a purification tag at the C-terminal. In some embodiments, AC2-P1 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding AC2-P1 can be cloned into a plasmid for protein expression. In some embodiments, AC2-P1 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 75. In some embodiments, AC2-P1 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 75 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P1-AC2 comprises the amino acid sequence of SEQ ID NO: 75. In some embodiments, AC2-P1 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), AC2-P1 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P2). For example, AC2-P1 can be used with P2-AC2 and / or AC2-P2 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS- CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from AC2-P1 and the P2 HRP fragment from P2-AC2 and / or AC2-P2) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, AC2-P1 along with another fusion protein (e.g., P2-AC2 and / or AC2-P2) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. AC2-P2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P2 HRP fragment and a binding domain, ACE2. The ACE2 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P2 HRP fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) an ACE2 domain; and (b) a P2 HRP fragment. Such a fusion protein can be referred to herein as AC2-P2. In some embodiments, the P2 HRP fragment in AC2-P2 is fused to the binding domain. In some embodiments, the P2 HRP fragment is fused directly to the binding Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domain. In some embodiments, the P2 HRP fragment is fused to the binding domain via a linker. In some instances, AC2-P2 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, AC2-P2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, AC2-P2 can comprise a purification tag at the C-terminal. In some embodiments, AC2-P2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding AC2-P2 can be cloned into a plasmid for protein expression. In some embodiments, AC2-P2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 76. In some embodiments, AC2-P2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 76 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, AC2-P2 comprises the amino acid sequence of SEQ ID NO: 76. In some embodiments, AC2-P2 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), AC2-P2 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P1). For example, AC2-P2 can be used with P1-AC2 and / or AC2-P1 for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS- CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P2 HRP fragment from AC2-P2 and the P1 HRP fragment from P1-AC2 and / or AC2-P1) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, AC2-P2 along with another fusion protein (e.g., Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 P1-AC2 and / or AC2-P1) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CTC-N30-P1 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P1 HRP fragment and a binding domain, CT30. The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P1 HRP fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a CT30 domain; and (b) a P1 HRP fragment. Such a fusion protein can be referred to herein as CTC-N30-P1. In some embodiments, the P1 HRP fragment in CTC-N30-P1 is fused to the binding domain. In some embodiments, the P1 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, CTC-N30-P1 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, CTC-N30-P1 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, CTC-N30-P1 can comprise a purification tag at the C-terminal. In some embodiments, CTC-N30-P1 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding CTC- N30-P1 can be cloned into a plasmid for protein expression. In some embodiments, CTC-N30-P1 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 77. In some embodiments, CTC-N30-P1 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 77 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CTC-N30-P1 comprises the amino acid sequence of SEQ ID NO: 77. In some embodiments, CTC-N30-P1 comprises a signal sequence. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CTC-N30-P1 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P2). For example, CTC-N30-P1 can be used with CTC-N30-P2 and / or P2-N30-CTC for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from CTC-N30-P1 and the P2 HRP fragment from CTC-N30-P2 and / or P2-N30-CTC) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, CTC-N30-P1 along with another fusion protein (e.g., CTC-N30-P2 and / or P2-N30-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. CTC-N30-P2 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P2 HRP fragment and a binding domain, CT30. The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P2 HRP fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a CT30 domain; and (b) a P2 HRP fragment. Such a fusion protein can be referred to herein as CTC-N30-P2. In some embodiments, the P2 HRP fragment in CTC-N30-P2 is fused to the binding domain. In some embodiments, the P2 HRP fragment is fused directly to the binding domain. In some embodiments, the P2 HRP fragment is fused to the binding domain via a linker. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some instances, CTC-N30-P2 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, CTC-N30-P2 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, CTC-N30-P2 can comprise a purification tag at the C-terminal. In some embodiments, CTC-N30-P2 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding CTC- N30-P2 can be cloned into a plasmid for protein expression. In some embodiments, CTC-N30-P2 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 78. In some embodiments, CTC-N30-P2 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 78 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, CTC-N30-P2 comprises the amino acid sequence of SEQ ID NO: 78. In some embodiments, CTC-N30-P2 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), CTC-N30-P2 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P1). For example, CTC-N30-P2 can be used with CTC-N30-P1 and / or P1-N30-CTC for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P2 HRP fragment from CTC-N30-P2 and the P1 HRP fragment from CTC-N30-P1 and / or P1-N30-CTC) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, CTC-N30-P2 along with another fusion protein (e.g., CTC-N30-P1 and / or P1-N30-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 P1-N30-CTC In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P1 HRP fragment and a binding domain, CT30. The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P1 HRP fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a P1 HRP fragment; and (b) a CT30 domain. Such a fusion protein can be referred to herein as P1-N30-CTC. In some embodiments, the P1 HRP fragment in P1-N30-CTC is fused to the binding domain. In some embodiments, the P1 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, P1-N30-CTC further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P1-N30-CTC can comprise a purification tag at the N-terminal. Additionally, or in the alternative, P1-N30-CTC can comprise a purification tag at the C-terminal. In some embodiments, P1-N30-CTC is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P1-N30- CTC can be cloned into a plasmid for protein expression. In some embodiments, P1- N30-CTC comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 79. In some embodiments, P1-N30-CTC comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 79 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P1- N30-CTC comprises the amino acid sequence of SEQ ID NO: 79. In some embodiments, P1-N30-CTC comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P1-N30-CTC can be used with another fusion protein that Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 comprises similar binding domain but a complementary HRP fragment (e.g., P2). For example, P1-N30-CTC can be used with CTC-N30-P2 and / or P2-N30-CTC for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from P1-N30-CTC and the P2 HRP fragment from CTC-N30-P2 and / or P2-N30-CTC) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P1-N30-CTC along with another fusion protein (e.g., CTC-N30-P2 and / or P2-N30-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. P2-N30-CTC In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P2 HRP fragment and a binding domain, CT30. The CT30 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P2 HRP fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a P2 HRP fragment; and (b) a CT30 domain. Such a fusion protein can be referred to herein as P2-N30-CTC. In some embodiments, the P2 HRP fragment in P2-N30-CTC is fused to the binding domain. In some embodiments, the P2 HRP fragment is fused directly to the binding domain. In some embodiments, the P2 HRP fragment is fused to the binding domain via a linker. In some instances, P2-N30-CTC further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P2-N30-CTC can comprise a Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 purification tag at the N-terminal. Additionally, or in the alternative, P2-N30-CTC can comprise a purification tag at the C-terminal. In some embodiments, P2-N30-CTC is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P2-N30- CTC can be cloned into a plasmid for protein expression. In some embodiments, P2- N30-CTC comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 80. In some embodiments, P2-N30-CTC comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 80 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P2- N30-CTC comprises the amino acid sequence of SEQ ID NO: 80. In some embodiments, P2-N30-CTC comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P2-N30-CTC can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P1). For example, P2-N30-CTC can be used with CTC-N30-P1 and / or P1-N30-CTC for detection of multimeric proteins expressed by SARS-CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from P1-N30-CTC and the P2 HRP fragment from CTC-N30-P2 and / or P2-N30-CTC) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P1-N30-CTC along with another fusion protein (e.g., CTC-N30-P2 and / or P2-N30-CTC) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 P1-B3 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P1 HRP fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P1 HRP fragment at the N- terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a P1 HRP fragment; and (b) a B3 domain. Such a fusion protein can be referred to herein as P1-B3. In some embodiments, the P1 HRP fragment in P1-B3 is fused to the binding domain. In some embodiments, the P1 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, P1-B3 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P1-B3 can comprise a purification tag at the N-terminal. Additionally, or in the alternative, P1-B3 can comprise a purification tag at the C-terminal. In some embodiments, P1-B3 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P1-B3 can be cloned into a plasmid for protein expression. In some embodiments, P1-B3 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 82, SEQ ID NO: 83, or SEQ ID NO: 84. In some embodiments, P1-B3 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 82, SEQ ID NO: 83, or SEQ ID NO: 84 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P1-B3 comprises the amino acid sequence of SEQ ID NO: 82, SEQ ID NO: 83, or SEQ ID NO: 84. In some embodiments, SEQ ID NO: 82, SEQ ID NO: 83, and SEQ ID NO: 84 differ from one another based on the length of linker sequences comprised therein. In some embodiments, P1-B3 comprises a signal sequence. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P1-B3 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P2). For example, P1- B3 can be used with P2-B3 for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P1 HRP fragment from P1-B3 and the P2 HRP fragment from P2-B3) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P1-B3 along with another fusion protein (e.g., P2-B3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. P2-B3 In some embodiments, a fusion protein for detection of multimeric proteins expressed by SARS-CoV-2 comprises a P2 HRP fragment and a binding domain, B3. The B3 domain of the fusion protein mimics or comprises a domain of an ACE2 receptor that bind to RBD of of SARS-CoV-2 spike protein (e.g., SARS-CoV-2 wild-type spike). In some embodiments, the fusion protein comprises the P2 HRP fragment at the N- terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a P2 HRP fragment; and (b) a B3 domain. Such a fusion protein can be referred to herein as P2-B3. In some embodiments, the P2 HRP fragment in P2-B3 is fused to the binding domain. In some embodiments, the P2 HRP fragment is fused directly to the binding domain. In some embodiments, the P1 HRP fragment is fused to the binding domain via a linker. In some instances, P2-B3 further comprises a purification tag (e.g., FLAG, cMyc tag, strep, His-tag, or biotin tag). For example, P2-B3 can comprise a purification tag at Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the N-terminal. Additionally, or in the alternative, P2-B3 can comprise a purification tag at the C-terminal. In some embodiments, P2-B3 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding P2-B3 can be cloned into a plasmid for protein expression. In some embodiments, P2-B3 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 85, SEQ ID NO: 86, or SEQ ID NO: 86. In some embodiments, P2-B3 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 85, SEQ ID NO: 85, or SEQ ID NO: 85 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, P2-B3 comprises the amino acid sequence of SEQ ID NO: 85, SEQ ID NO: 86, or SEQ ID NO: 87. In some embodiments, SEQ ID NO: 85, SEQ ID NO: 86, and SEQ ID NO: 87 differ from one another based on the length of linker sequences comprised therein. In some embodiments, P2-B3 comprises a signal sequence. For detection of multimeric proteins expressed by SARS-CoV-2 in a test sample (e.g., a biological sample), P2-B3 can be used with another fusion protein that comprises similar binding domain but a complementary HRP fragment (e.g., P1). For example, P2- B3 can be used with P1-B3 for detection of multimeric proteins expressed by SARS- CoV-2 in a test sample. If a test sample comprising SARS-CoV-2 is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the RBD of SARS-CoV-2 spike. Binding of the binding domains to the RBD of the trimeric spike proteins brings the two HRP fragments (e.g., the P2 HRP fragment from P2-B3 and the P1 HRP fragment from P1-B3) into close proximity to reconstitute a functional HRP enzyme. When TMB (a substrate of HRP) is added to this composition, the functional HRP reduces hydrogen peroxide and oxidizes TMB, turning it from colorless to blue-green color, thus generating a detectable colorimetric signal. Thus, P2-B3 along with another fusion protein (e.g., P1-B3) can be used for ultrasensitive detection of SARS-CoV-2 spike proteins by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Fusion Proteins for Detection of Influenza Virus Described herein are fusion proteins for detection of influenza virus (e.g., multimeric proteins expressed by influenza virus) in test samples (e.g., biological samples). In some embodiments, fusion proteins for detection of multimeric proteins expressed by influenza virus can be referred to herein as fusion proteins for detection of influenza virus. For detection of multimeric proteins expressed by influenza virus (e.g., human influenza A and B virus) by the present methods, a pair of fusion proteins (e.g., a first fusion protein and a second fusion protein) can be used, wherein the first fusion protein comprises a first reporter protein fragment, and the second fusion protein comprises a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). Along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of these first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by influenza virus by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that mimic or comprise domains from one or more binders of influenza virus. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by influenza virus by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are one or more binders of influenza virus. In some embodiments, the reporter protein is luciferase. For example, for detection of multimeric proteins expressed by influenza virus (e.g., human influenza A and B virus) by the present methods, a first fusion protein and a second fusion protein can Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split luciferase fragments (e.g., one fusion protein comprising a large luciferase fragment and the other fusion protein comprising a small luciferase fragment, wherein the small luciferase fragment is complementary to the large luciferase fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 1 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a small luciferase fragment that comprises 11 amino acids. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a large luciferase fragment. In some embodiments, the large luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 2 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a large luciferase fragment that comprises 158 amino acids. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the reporter protein is HRP. For example, for detection of multimeric proteins expressed by influenza virus by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split HRP fragments (e.g., one fusion protein comprising a first HRP fragment and the other fusion protein comprising a second HRP fragment, wherein the first HRP fragment is complementary to the second HRP fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a first HRP fragment. In some embodiments, the first HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 71. In some embodiments, the first HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 71 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the first HRP fragment comprises the amino acid sequence of SEQ ID NO: 71. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by influenza virus by the present methods) comprises a second HRP fragment. In some embodiments, the second HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 72. In some embodiments, the second HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 72 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the second HRP fragment comprises the amino acid sequence of SEQ ID NO: 72. Binding Domains for Influenza Binding domains of these fusion proteins (e.g., fusion proteins that can be used for detection of multimeric proteins expressed by influenza virus by the present methods) Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 can mimic or comprise domains of multiple-domain binders that target multiple highly conserved epitopes of trimeric hemagglutinin (HA) proteins of influenza A and B viruses. Additionally, or in the alternative, these binding domains can be multiple-domain binders that target multiple highly conserved epitopes of trimeric HA proteins of influenza A and B viruses. In some embodiments, the multiple-domain binders comprise binders that target influenza A and binders that target influenza B. For example, a fusion protein can comprise one or more binding domains that mimic or comprise domains of multiple- domain binders, wherein the multiple-domain binders comprise two binders (e.g., SD36 and SD38) that target influenza A and two binders (e.g., SD83 and SD84) that target influenza B. Additionally, or in the alternative, a fusion protein can comprise one or more binding domains that are multiple-domain binders or fragments thereof, wherein the multiple-domain binders comprise two binders (e.g., SD36 and SD38) that target influenza A and two binders (e.g., SD83 and SD84) that target influenza B. One or more of these binders can recognize the HA stem and / or the HA head. For example, each of SD36, SD38, and SD83 recognizes the HA stem, and SD84 binds to a conserved epitope in the HA head. In some embodiments, binding domains of these fusion proteins can be binders (or fragments thereof) that recognize and bind to epitopes (e.g., non-overlapping epitopes) at or near the apex of the trimeric assembly of the influenza HA protein. These fusion proteins (e.g., fusion proteins for detection of multimeric proteins expressed by influenza virus by the present methods) can comprise one or more of the following binding domains: SD84, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 26; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 26 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 26. In some embodiments, SD84 targets influenza B. In some embodiments, SD84 binds to a conserved epitope in the HA head (e.g., the HA head of influenza B). SD83, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 27; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 27 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 27. In some embodiments, SD83 targets influenza B. In some embodiments, SD83 recognizes the HA stem (e.g., the HA stem of influenza B). SD36, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 28; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 28 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 28. In some embodiments, SD36 targets influenza A. In some embodiments, SD36 recognizes the HA stem (e.g., the HA stem of influenza A). SD38, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 29; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 29 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 29. In some embodiments, SD38 targets influenza A. In some embodiments, SD38 recognizes the HA stem (e.g., the HA stem of influenza A). Detection of Influenza The fusion proteins of the present disclosure can be used for detection of multimeric proteins expressed by influenza virus (e.g., human influenza A and B virus) in test samples (e.g., biological samples). The trimeric structure on HA proteins promotes the split reporter protein complementation of the two complementary reporter protein fragments to generate a detectable signal (e.g., luminescence, color, and / or precipitation) as readout. Meanwhile, since HA proteins are closely packed, the intermolecular complementation of a pair of fusion proteins (e.g., one fusion protein comprising one Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 reporter protein fragment and the other fusion protein comprising its complementary reporter protein fragment) between neighboring HA proteins can generate additional detectable signal (e.g., luminescence, color, and / or precipitation). If a test sample (e.g., a biological sample) comprising influenza virus is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to HA proteins of influenza A and B viruses. Binding of the binding domains to the HA proteins brings the two split reporter protein fragments into close proximity to reconstitute a functional reporter protein. When a suitable substrate is added to this composition, the functional reporter protein generates a detectable signal (e.g., luminescence, color, and / or precipitation). Thus, a pair of these fusion proteins (e.g., one fusion protein comprising one reporter protein fragment and the other fusion protein comprising its complementary reporter protein fragment) can be used for ultrasensitive detection of HA proteins of influenza A and B viruses by an assay of the present disclosure. Such an assay is rapid and straightforward to perform and involves only mixing a test sample (e.g., a biological sample from a subject) with the two fusion proteins, followed by the addition of a suitable substrate for detectable readout (e.g., luminescence, color, and / or precipitation). The whole detection process occurs in the liquid phase without any washing steps. Two or more of the following fusion proteins can be used for detection of influenza virus (e.g., human influenza A and B virus) in test samples (e.g., biological samples). 84S In some embodiments, a fusion protein for detection of multimeric proteins expressed by influenza A and B viruses comprises a small luciferase fragment and one or more binding domains that target influenza A and / or influenza B viruses. For example, the fusion protein can comprise SD84, SD83, SD36, and SD38 as binding domains. In some embodiments, the fusion protein comprises the small luciferase fragment at the C- terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) SD84; (b) SD83; (c) SD36; (d) SD38; and (e) a small luciferase fragment. Such a fusion protein can be referred to herein as 84S. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the small luciferase fragment in 84S is fused to one of the binding domains. In some embodiments, the small luciferase fragment is fused directly to one of the binding domains. In some embodiments, the small luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, there are one or more binders between the binding domains. For example, 84S can comprise one or more linkers, e.g., between SD84 and SD83, between SD83 and SD36, between SD36 and SD38, and / or between SD38 and the small luciferase fragment. In some instances, 84S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 84S can comprise a purification tag at the N- terminal. Additionally, or in the alternative, 84S can comprise a purification tag at the C- terminal. 84S has a molecular weight of 61.6 kDa. In some embodiments, 84S is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding 84S can be cloned into a pcDNA3.4 plasmid for protein expression. In some embodiments, 84S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 30. In some embodiments, 84S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 30 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 84S comprises the amino acid sequence of SEQ ID NO: 30. In some embodiments, 84S comprise a signal sequence. Exemplary construct design of 84S (left panel) and exemplary protein structure of 84S predicted by AlphaFold (right panel) are provided in FIG.24. For detection of multimeric proteins expressed by influenza virus in a test sample (e.g., a biological sample), 84S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, 84S can be used with 84L and / or L84 for detection of multimeric proteins expressed by influenza virus (e.g., HA proteins of influenza A and B virus) in a test sample. If a test sample comprising influenza virus (e.g., influenza A and B virus) is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the HA proteins of the virus (e.g., each of SD36, SD38, and SD83 can bind to the HA stem; and / or SD84 can bind to a conserved epitope in the HA head). Binding of the binding domains to the HA protein brings the two split luciferase fragments (e.g., the small luciferase fragment from 84S and the large luciferase fragment from 84L and / or L84) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 84S along with another fusion protein (e.g., 84L and / or L84) can be used for ultrasensitive detection of HA proteins of influenza A and B virus by an assay of the present disclosure. S84 In some embodiments, a fusion protein for detection of multimeric proteins expressed by influenza A and B viruses comprises a small luciferase fragment and one or more binding domains that target influenza A and / or influenza B viruses. For example, the fusion protein can comprise SD84, SD83, SD36, and SD38 as binding domains. In some embodiments, the fusion protein comprises the small luciferase fragment at the N- terminal and / or the binding domains at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; (b) SD84; (c) SD83; (d) SD36; and (e) SD38. Such a fusion protein can be referred to herein as S84. In some embodiments, the small luciferase fragment in S84 is fused to one of the binding domains. In some embodiments, the small luciferase fragment is fused directly to one of the binding domains. In some embodiments, the small luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, there are one or more binders between the binding domains. For example, S84 can comprise one or more linkers, e.g., between the small luciferase fragment and SD84, between SD84 and SD83, between SD83 and SD36, and / or between SD36 and SD38. In some instances, S84 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, S84 can comprise a purification tag at the N- Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 terminal. Additionally, or in the alternative, S84 can comprise a purification tag at the C- terminal. S84 has a molecular weight of 61.7 kDa. In some embodiments, S84 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding S84 can be cloned into a pcDNA3.4 plasmid for protein expression. In some embodiments, S84 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 31. In some embodiments, S84 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 31 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, S84 comprises the amino acid sequence of SEQ ID NO: 31. In some embodiments, S84 comprises a signal sequence. Exemplary construct design of S84 (left panel) and exemplary protein structure of S84 predicted by AlphaFold (right panel) are provided in FIG.25. For detection of multimeric proteins expressed by influenza virus in a test sampel (e.g., a biological sample), S84 can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, S84 can be used with 84L and / or L84 for detection of multimeric proteins expressed by influenza virus (e.g., HA proteins of influenza A and B virus) in a test sample. If a test sample comprising influenza virus (e.g., influenza A and B virus) is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the HA proteins of the virus (e.g., each of SD36, SD38, and SD83 can bind to the HA stem; and / or SD84 can bind to a conserved epitope in the HA head). Binding of the binding domains to the HA protein brings the two split luciferase fragments (e.g., the small luciferase fragment from S84 and the large luciferase fragment from 84L and / or L84) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, S84 along Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 with another fusion protein (e.g., 84L and / or L84) can be used for ultrasensitive detection of HA proteins of influenza A and B virus by an assay of the present disclosure. 84L In some embodiments, a fusion protein for detection of multimeric proteins expressed by influenza A and B viruses comprises a large luciferase fragment and one or more binding domains that target influenza A and / or influenza B viruses. For example, the fusion protein can comprise SD84, SD83, SD36, and SD38 as binding domains. In some embodiments, the fusion protein comprises the large luciferase fragment at the C- terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) SD84; (b) SD83; (c) SD36; (d) SD38; and (e) a large luciferase fragment. Such a fusion protein can be referred to herein as 84L. In some embodiments, the large luciferase fragment in 84L is fused to one of the binding domains. In some embodiments, the large luciferase fragment is fused directly to one of the binding domains. In some embodiments, the large luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, there are one or more binders between the binding domains. For example, 84L can comprise one or more linkers, e.g., between SD84 and SD83, between SD83 and SD36, between SD36 and SD38, and / or between SD38 and the large luciferase fragment. In some instances, 84L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 84L can comprise a purification tag at the N- terminal. Additionally, or in the alternative, 84L can comprise a purification tag at the C- terminal. 84L has a molecular weight of 77.9 kDa. In some embodiments, 84L is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding 84L can be cloned into a pcDNA3.4 plasmid for protein expression. In some embodiments, 84L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 32. In some embodiments, 84L comprises an amino acid sequence that Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 differs from the amino acid sequence of SEQ ID NO: 32 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 84L comprises the amino acid sequence of SEQ ID NO: 32. In some embodiments, 84L comprise a signal sequence. Exemplary construct design of 84L (left panel) and exemplary protein structure of 84L predicted by AlphaFold (right panel) are provided in FIG.26. For detection of multimeric proteins expressed by influenza virus in a test sample (e.g., a biological sample), 84L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, 84L can be used with 84S and / or S84 for detection of multimeric proteins expressed by influenza virus (e.g., HA proteins of influenza A and B virus) in a test sample. If a test sample comprising influenza virus (e.g., influenza A and B virus) is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the HA proteins of the virus (e.g., each of SD36, SD38, and SD83 can bind to the HA stem; and / or SD84 can bind to a conserved epitope in the HA head). Binding of the binding domains to the HA protein brings the two split luciferase fragments (e.g., the large luciferase fragment from 84L and the small luciferase fragment from 84S and / or S84) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 84L along with another fusion protein (e.g., 84S and / or S84) can be used for ultrasensitive detection of HA proteins of influenza A and B virus by an assay of the present disclosure. L84 In some embodiments, a fusion protein for detection of multimeric proteins expressed by influenza A and B viruses comprises a large luciferase fragment and one or more binding domains that target influenza A and / or influenza B viruses. For example, the fusion protein can comprise SD84, SD83, SD36, and SD38 as binding domains. In some embodiments, the fusion protein comprises the large luciferase fragment at the N- terminal and / or the binding domains at the C-terminal. For example, the fusion protein Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 can comprise, from N-terminal to C-terminal: (a) a large luciferase fragment; (b) SD84; (c) SD83; (d) SD36; and (e) SD38. Such a fusion protein can be referred to herein as L84. In some embodiments, the large luciferase fragment in L84 is fused to one of the binding domains. In some embodiments, the large luciferase fragment is fused directly to one of the binding domains. In some embodiments, the large luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, there are one or more binders between the binding domains. For example, L84 can comprise one or more linkers, e.g., between the large luciferase fragment and SD84, between SD84 and SD83, between SD83 and SD36, and / or between SD36 and SD38. In some instances, L84 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, L84 can comprise a purification tag at the N- terminal. Additionally, or in the alternative, L84 can comprise a purification tag at the C- terminal. L84 has a molecular weight of 78 kDa. In some embodiments, L84 is produced using mammalian expression system (e.g., in HEK293 cells). For example, a nucleic acid sequence encoding L84 can be cloned into a pcDNA3.4 plasmid for protein expression. In some embodiments, L84 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 33. In some embodiments, L84 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 33 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, L84 comprises the amino acid sequence of SEQ ID NO: 33. In some embodiments, L84 comprises a signal sequence. Exemplary construct design of L84 (left panel) and exemplary protein structure of L84 predicted by AlphaFold (right panel) are provided in FIG.27. For detection of multimeric proteins expressed by influenza virus in a test sample (e.g., a biological sample), L84 can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, L84 can be used with 84S and / or S84 for detection of multimeric proteins expressed by influenza virus Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 (e.g., HA proteins of influenza A and B virus) in a test sample. If a test sample comprising influenza virus (e.g., influenza A and B virus) is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the HA proteins of the virus (e.g., each of SD36, SD38, and SD83 can bind to the HA stem; and / or SD84 can bind to a conserved epitope in the HA head). Binding of the binding domains to the HA protein brings the two split luciferase fragments (e.g., the large luciferase fragment from L84 and the small luciferase fragment from 84S and / or S84) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, L84 along with another fusion protein (e.g., 84S and / or S84) can be used for ultrasensitive detection of HA proteins of influenza A and B virus by an assay of the present disclosure. Fusion Proteins for Detection of HIV Described herein are fusion proteins for detection of human immunodeficiency virus (HIV) (e.g., multimeric proteins expressed by HIV) in test samples (e.g., biological samples). In some embodiments, fusion proteins for detection of multimeric proteins expressed by HIV can be referred to herein as fusion proteins for detection of HIV. For detection of multimeric proteins expressed by HIV by the present methods, a pair of fusion proteins (e.g., a first fusion protein and a second fusion protein) can be used, wherein the first fusion protein comprises a first reporter protein fragment, and the second fusion protein comprises a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of these first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by HIV by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that mimic or comprise domains from one or more binders of HIV. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins (e.g., first and second fusion proteins for detection of HIV by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are one or more binders of HIV. In some embodiments, the reporter protein is luciferase. For example, for detection of multimeric proteins expressed by HIV by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split luciferase fragments (e.g., one fusion protein comprising a large luciferase fragment and the other fusion protein comprising a small luciferase fragment, wherein the small luciferase fragment is complementary to the large luciferase fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 1 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a small luciferase fragment that comprises 11 amino acids. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a large luciferase fragment. In some embodiments, the large luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 2 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a large luciferase fragment that comprises 158 amino acids. In some embodiments, the reporter protein is HRP. For example, for detection of multimeric proteins expressed by HIV by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split HRP fragments (e.g., one fusion protein comprising a first HRP fragment and the other fusion protein comprising a second HRP fragment, wherein the first HRP fragment is complementary to the second HRP fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a first HRP fragment. In some embodiments, the first HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 71. In some embodiments, the first HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 71 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the first HRP fragment comprises the amino acid sequence of SEQ ID NO: 71. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by HIV by the present methods) comprises a second HRP fragment. In some embodiments, the second HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 72. In some embodiments, the second HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 72 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the second HRP fragment comprises the amino acid sequence of SEQ ID NO: 72. Binding Domains for HIV HIV-1 envelope glycoprotein gp160 exists as a trimer of heterodimers on the viral surface. Binding domains of these fusion proteins (e.g., fusion proteins that can be used for detection of multimeric proteins expressed by HIV by the present methods) can mimic, or can be, one or more antibody fragments, e.g., single-chain variable fragments (scFvs), that bind to HIV envelope glycoprotein gp160. For example, binding domains of these fusion proteins can mimic, comprise, or can be, one or more antibody fragments (e.g., scFvs) that bind to quaternary epitopes at the apex center of the native gp160 trimer of HIV. Thus, in some embodiments, binding domains of these fusion proteins can comprise one or more fragments of PGT145. A PGT145 fragment for use in the present methods can be a variable light fragment of PGT145 (also referred to herein as PGT145 (light) or PGT145 variable light chain)) and / or a heavy fragment of PGT145 (also referred to herein as PGT145 (heavy) or PGT145 variable heavy chain)). In some embodiments, the binding domain PGT145 is split across a pair of fusion proteins, such that the variable heavy chain is the binding domain on a first fusion protein, and the variable light chain is the binding domain on a second fusion protein (or vice versa). Such double-split fusion proteins in which both the reporter protein and the first binding domain are split are used in pairs such that the binding of the first and second fusion protein results in reconstitution of the binding domain (forming scFv) and of the full and Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 functional reporter protein. A schematic representation of fusion proteins comprising the double split is provided in FIG.30. Additionally, fusion proteins for HIV can further comprise binding domains that can mimic, comprise, or can be, one or more domains of antibodies (e.g., BN117, 10E8), and / or, one or more domains from CD4 receptor, which is the host receptor that binds to HIV envelope glycoprotein gp120. These fusion proteins (e.g., fusion proteins for detection of multimeric proteins expressed by HIV by the present methods) can comprise one or more of the following binding domains: PGT145 (light), a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 34; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 34 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 34; PGT145 (heavy), a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 35; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 35 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 35; BN117 scFv, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 36; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 36 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 36; Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 10E8 scFv, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 37; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 37 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 37; and CD4 receptor, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 38; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 38 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 38. Detection of HIV The fusion proteins of the present disclosure can be used for detection of multimeric proteins expressed by HIV in test samples (e.g., biological samples). Trimeric structure of the HIV glycoprotein gp160 can promote the split reporter protein complementation of the two complementary reporter protein fragments to generate a detectable signal (e.g., luminescence, color, and / or precipitation) as readout. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the gp160 protein of HIV. Binding of the binding domains to the gp160 protein brings the two complementary split reporter protein fragments into close proximity to reconstitute a functional reporter protein. When a substrate of the reporter protein is added to this composition, the functional reporter protein generates a detectable signal (e.g., luminescence, color, and / or precipitation). Thus, a pair of these fusion proteins (e.g., one fusion protein comprising one reporter protein fragment and the other fusion protein comprising its complementary reporter protein fragment) can be used for ultrasensitive detection of multimeric proteins expressed by HIV (e.g., the gp160 protein of HIV) by an assay of the present disclosure. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, in addition to the split reporter protein system, fusion proteins for detection of HIV comprise an additional split system. For example, fusion proteins for detection of HIV can comprise one of the two fragments of PGT145 scFv, e.g., a light fragment of PGT145 or a heavy fragment of PGT145. If a test sample (e.g., a biological sample) comprising HIV is added to a composition comprising these fusion proteins, the light and heavy PGT145 fragments bind to the quaternary epitope at or near the apex center of the native gp160 trimer of HIV and bring the split reporter protein fragments into close proximity of each other. Thus, in presence of the HIV trimer, these fusion proteins assemble an active reporter protein and result in an increased detectable signal. Such an assay is rapid and straightforward to perform and involves only mixing a test sample (e.g., a biological sample from a patient) with the two fusion proteins, followed by the addition of a suitable substrate for detectable readout (e.g., luminescence, color, and / or precipitation). The whole detection process occurs in the liquid phase without any washing steps. Two or more of the following fusion proteins can be used for detection of HIV in test samples (e.g., biological samples). BN117-PGT145(light)-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a large luciferase fragment and one or more binding domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more light fragments (e.g., scFvs) of PGT145; and one or more fragments (e.g., scFvs) of the antibody, BN117. In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C- terminal: (a) BN117 scFv; (b) a PGT145 (light); and (c) a large luciferase fragment. Such a fusion protein can be referred to herein as BN117-PGT145(light)-L. In some embodiments, the large luciferase fragment in BN117-PGT145(light)-L is fused to one of the binding domains. For example, the large luciferase fragment in BN117-PGT145(light)-L can be fused to the light fragment of PGT145. In some embodiments, the large luciferase fragment is fused directly to one of the binding Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domains. In some embodiments, the large luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the light fragment of PGT145 and the fragment (e.g., scFv) of BN117. For example, BN117-PGT145(light)-L can comprise one or more linkers, e.g., between BN117 scFv and PGT145 (heavy), and / or between PGT145 (heavy) and the large luciferase fragment. In some instances, BN117-PGT145(light)-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, BN117-PGT145(light)-L can comprise a purification tag at the N-terminal. Additionally, or in the alternative, BN117-PGT145(light)-L can comprise a purification tag at the C-terminal. In some embodiments, BN117-PGT145(light)-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 39. In some embodiments, BN117- PGT145(light)-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 39 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, BN117- PGT145(light)-L comprises the amino acid sequence of SEQ ID NO: 39. In some embodiments, BN117-PGT145(light)-L comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test sample (e.g., a biological sample), BN117-PGT145(light)-L can be used with another fusion protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(heavy)) and a complementary luciferase fragment (e.g., a small luciferase fragment). For example, BN117-PGT145(light)-L can be used with BN117-PGT145(heavy)-S for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from BN117-PGT145(light)-L and the small luciferase fragment from BN117-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, BN117-PGT145(light)-L along with another fusion protein (e.g., BN117-PGT145(heavy)-S) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. BN117-PGT145(heavy)-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a small luciferase fragment and one or more binding domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more heavy fragments (e.g., scFvs) of PGT145; and one or more fragments (e.g., scFvs) of the antibody, BN117. In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C- terminal: (a) a BN117 scFv; (b) a PGT145 (heavy); and (c) a small luciferase fragment. Such a fusion protein can be referred to herein as BN117-PGT145(heavy)-S. In some embodiments, the small luciferase fragment in BN117-PGT145(heavy)-S is fused to one of the binding domains. For example, the small luciferase fragment in BN117-PGT145(heavy)-S can be fused to the heavy fragment of PGT145. In some embodiments, the small luciferase fragment is fused directly to one of the binding domains. In some embodiments, the small luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the heavy fragment of PGT145 and the fragment (e.g., scFv) of BN117. For example, BN117-PGT145(heavy)-S can comprise one or more linkers, e.g., between BN117 scFv and PGT145 (heavy), and / or between PGT145 (heavy) and the small luciferase fragment. In some instances, BN117-PGT145(heavy)-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, BN117-PGT145(heavy)-S can comprise a purification tag at the N-terminal. Additionally, or in the alternative, BN117-PGT145(heavy)-S can comprise a purification tag at the C-terminal. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, BN117-PGT145(heavy)-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 40. In some embodiments, BN117- PGT145(heavy)-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 40 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, BN117- PGT145(heavy)-S comprises the amino acid sequence of SEQ ID NO: 40. In some embodiments, BN117-PGT145(heavy)-S comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test sample (e.g., a biological sample), BN117-PGT145(heavy)-S can be used with another fusion protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(light)) and a complementary luciferase fragment (e.g., a large luciferase fragment). For example, BN117-PGT145(heavy)-S can be used with BN117-PGT145(light)-L for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from BN117-PGT145(light)-L and the small luciferase fragment from BN117-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, BN117-PGT145(heavy)-S along with a another fusion protein (e.g., BN117-PGT145(light)-L) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. 10E8-BN117-PGT145(light)-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a large luciferase fragment and one or more binding Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more light fragments (e.g., scFvs) of PGT145; one or more fragments (e.g., scFvs) of the antibody, BN117; and one or more fragments (e.g., scFvs) of the antibody, 10E8. In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a 10E8 scFv; (b) a BN117 scFv; (c) a PGT145 (light); and (d) a large luciferase fragment. Such a fusion protein can be referred to herein as 10E8-BN117-PGT145(light)-L. In some embodiments, the large luciferase fragment in 10E8-BN117- PGT145(light)-L is fused to one of the binding domains. For example, the large luciferase fragment in 10E8-BN117-PGT145(light)-L can be fused to PGT145 (light). In some embodiments, the large luciferase fragment is fused directly to one of the binding domains. In some embodiments, the large luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the light fragment of PGT145 and the fragment (e.g., scFv) of BN117; and / or between the fragment (e.g., scFv) of BN117 and the fragment (e.g., scFv) of 10E8. For example, 10E8-BN117- PGT145(light)-L can comprise one or more linkers, e.g., between 10E8 scFv and BN117 scFv, between BN117 scFv and PGT145 (light), and / or between PGT145 (light) and the large luciferase fragment. In some instances, 10E8-BN117-PGT145(light)-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 10E8-BN117- PGT145(light)-L can comprise a purification tag at the N-terminal. Additionally, or in the alternative, 10E8-BN117-PGT145(light)-L can comprise a purification tag at the C- terminal. In some embodiments, 10E8-BN117-PGT145(light)-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 41. In some embodiments, 10E8- BN117-PGT145(light)-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 41 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 10E8-BN117- PGT145(light)-L comprises the amino acid sequence of SEQ ID NO: 41. In some embodiments, 10E8-BN117-PGT145(light)-L comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test samplel (e.g., a biological sample), 10E8-BN117-PGT145(light)-L can be used with another fusion protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(heavy)) and a complementary luciferase fragment (e.g., a small luciferase fragment). For example, 10E8-BN117-PGT145(light)-L can be used with 10E8-BN117- PGT145(heavy)-S for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from 10E8-BN117-PGT145(light)-L and the small luciferase fragment from 10E8-BN117-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 10E8-BN117-PGT145(light)-L along with another fusion protein (e.g., 10E8-BN117-PGT145(heavy)-S) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. 10E8-BN117-PGT145(heavy)-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a small luciferase fragment and one or more binding domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more heavy fragments (e.g., scFvs) of PGT145; one or more fragments (e.g., scFvs) of the antibody, BN117; and one or more fragments (e.g., scFvs) of the antibody, 10E8. In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a 10E8 scFv; (b) a BN117 scFv; (c) a PGT145 (heavy); and (d) a small luciferase fragment. Such a fusion protein can be referred to herein as 10E8-BN117-PGT145(heavy)-S. In some embodiments, the small luciferase fragment in 10E8-BN117- PGT145(heavy)-S is fused to one of the binding domains. For example, the small luciferase fragment in 10E8-BN117-PGT145(heavy)-S can be fused to the heavy fragment of PGT145. In some embodiments, the small luciferase fragment is fused directly to one of the binding domains. In some embodiments, the small luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the heavy fragment of PGT145 and the fragment (e.g., scFv) of BN117; and / or between the fragment (e.g., scFv) of BN117 and the fragment (e.g., scFv) of 10E8. For example, 10E8-BN117-PGT145(heavy)-S can comprise one or more linkers, e.g., between 10E8 scFv and BN117 scFv, between BN117 scFv and PGT145 (heavy), and / or between PGT145 (heavy) and the small luciferase fragment. In some instances, 10E8-BN117-PGT145(heavy)-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 10E8- BN117-PGT145(heavy)-S can comprise a purification tag at the N-terminal. Additionally, or in the alternative, 10E8-BN117-PGT145(heavy)-S can comprise a purification tag at the C-terminal. In some embodiments, 10E8-BN117-PGT145(heavy)-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 42. In some embodiments, 10E8- BN117-PGT145(heavy)-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 42 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 10E8-BN117- PGT145(heavy)-S comprises the amino acid sequence of SEQ ID NO: 42. In some embodiments, 10E8-BN117-PGT145(heavy)-S comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test sample (e.g., a biological sample), 10E8-BN117-PGT145(heavy)-S can be used with another fusion Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(light)) and a complementary luciferase fragment (e.g., a large luciferase fragment). For example, 10E8-BN117-PGT145(heavy)-S can be used with 10E8- BN117-PGT145(light)-L for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from 10E8-BN117-PGT145(light)-L and the small luciferase fragment from 10E8-BN117-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 10E8-BN117-PGT145(heavy)-S along with another fusion protein (e.g., 10E8-BN117-PGT145(light)-L) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. 10E8-CD4-PGT145(light)-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a large luciferase fragment and one or more binding domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more light fragments (e.g., scFvs) of PGT145; one or more fragments (e.g., scFvs) of the antibody, 10E8; and one or more domains (e.g., extracellular domain) of CD4. In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a 10E8 scFv; (b) a CD4 domain; (c) a PGT145 (light); and (d) a large luciferase fragment. Such a fusion protein can be referred to herein as 10E8-CD4-PGT145(light)-L. In some embodiments, the large luciferase fragment in 10E8-CD4-PGT145(light)- L is fused to one of the binding domains. For example, the large luciferase fragment in Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 10E8-CD4-PGT145(light)-L can be fused to the light fragment of PGT145. In some embodiments, the large luciferase fragment is fused directly to one of the binding domains. In some embodiments, the large luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the light fragment of PGT145 and the domain (e.g., extracellular domain) of CD4; and / or between the domain (e.g., extracellular domain) of CD4 and the fragment (e.g., scFv) of 10E8. For example, 10E8- BN117-PGT145(light)-L can comprise one or more linkers, e.g., between 10E8 scFv and CD4 domain, between CD4 domain and PGT145 (light), and / or between PGT145 (light) and the large luciferase fragment. In some instances, 10E8-CD4-PGT145(light)-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 10E8-CD4- PGT145(light)-L can comprise a purification tag at the N-terminal. Additionally, or in the alternative, 10E8-CD4-PGT145(light)-L can comprise a purification tag at the C- terminal. In some embodiments, 10E8-CD4-PGT145(light)-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 43. In some embodiments, 10E8- CD4-PGT145(light)-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 43 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 10E8-CD4- PGT145(light)-L comprises the amino acid sequence of SEQ ID NO: 43. In some embodiments, 10E8-CD4-PGT145(light)-L comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test sampel (e.g., a biological sample), 10E8-CD4-PGT145(light)-L can be used with another fusion protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(heavy)) and a complementary luciferase fragment (e.g., a small luciferase fragment). For example, 10E8-CD4-PGT145(light)-L can be used with 10E8-CD4-PGT145(heavy)-S for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from 10E8-CD4- PGT145(light)-L and the small luciferase fragment from 10E8-CD4-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 10E8-CD4- PGT145(light)-L along with another fusion protein (e.g., 10E8-CD4-PGT145(heavy)-S) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. 10E8-CD4-PGT145(heavy)-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by HIV comprises a small luciferase fragment and one or more binding domains that target HIV. In some embodiments, the binding domains of these fusion protein comprise: one or more heavy fragments (e.g., scFvs) of PGT145; one or more fragments (e.g., scFvs) of the antibody, 10E8; and one or more domains (e.g., extracellular domain) of CD4. In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domains at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a 10E8 scFv; (b) a CD4 domain; (c) a PGT145 (heavy); and (d) a small luciferase fragment. Such a fusion protein can be referred to herein as 10E8-CD4-PGT145(heavy)-S. In some embodiments, the small luciferase fragment in 10E8-CD4- PGT145(heavy)-S is fused to one of the binding domains. For example, the small luciferase fragment in 10E8-CD4-PGT145(heavy)-S can be fused to the heavy fragment of PGT145. In some embodiments, the small luciferase fragment is fused directly to one of the binding domains. In some embodiments, the small luciferase fragment is fused to one of the binding domains via a linker. In some embodiments, the fusion protein comprises one or more linkers between the binding domains, e.g., between the heavy fragment of PGT145 and the domain (e.g., extracellular domain) of CD4; and / or between Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the domain (e.g., extracellular domain) of CD4 and the fragment (e.g., scFv) of 10E8. For example, 10E8-CD4-PGT145(heavy)-S can comprise one or more linkers, e.g., between 10E8 scFv and CD4 domain, between CD4 domain and PGT145 (heavy), and / or between PGT145 (heavy) and the small luciferase fragment. In some instances, 10E8-CD4-PGT145(heavy)-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, 10E8-CD4- PGT145(heavy)-S can comprise a purification tag at the N-terminal. Additionally, or in the alternative, 10E8-CD4-PGT145(heavy)-S can comprise a purification tag at the C- terminal. In some embodiments, 10E8-CD4-PGT145(heavy)-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 44. In some embodiments, 10E8- CD4-PGT145(heavy)-S comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 44 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, 10E8-CD4- PGT145(heavy)-S comprises the amino acid sequence of SEQ ID NO: 44. In some embodiments, 10E8-CD4-PGT145(heavy)-S comprise a signal sequence. For detection of multimeric proteins expressed by HIV in a test sample (e.g., a biological sample), 10E8-CD4-PGT145(heavy)-S can be used with another fusion protein that comprises the other PGT145 fragment as a binding domain (e.g., PGT145(light)) and a complementary luciferase fragment (e.g., a small luciferase fragment). For example, 10E8-CD4-PGT145(heavy)-S can be used with 10E8-CD4-PGT145(light)-L for detection of multimeric proteins expressed by HIV in a test sample. If a test sample comprising HIV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the virus (e.g., to an envelope glycoprotein (e.g., gp160 protein) of the virus). Binding of the binding domains to HIV brings the two split luciferase fragments (e.g., the large luciferase fragment from 10E8-CD4- PGT145(light)-L and the small luciferase fragment from 10E8-CD4-PGT145(heavy)-S) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, 10E8-CD4- PGT145(heavy)-S along with another fusion protein (e.g., 10E8-CD4-PGT145(light)-L) can be used for ultrasensitive detection of HIV (e.g., an envelope glycoprotein (e.g., gp160) of HIV) by an assay of the present disclosure. Fusion Proteins for Detection of RSV Described herein are fusion proteins for detection of respiratory syncytial virus (RSV) (e.g., multimeric proteins expressed by RSV) in test samples (e.g., biological samples). In some embodiments, fusion proteins for detection of multimeric proteins expressed by RSV can be referred to herein as fusion proteins for detection of RSV. For detection of RSV by the present methods, a pair of fusion proteins (e.g., a first fusion protein and a second fusion protein) can be used, wherein the first fusion protein comprises a first reporter protein fragment, and the second fusion protein comprises a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). Along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of these first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins expressed by RSV by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that mimic or comprise domains from one or more binders of RSV. Additionally, or in the alternative, along with a fragment (e.g., one of the two complementary fragments) of a reporter protein, each of the first and second fusion proteins (e.g., first and second fusion proteins for detection of multimeric proteins Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 expressed by RSV by the present methods) can comprise one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains) that are one or more binders of RSV. In some embodiments, the reporter protein is luciferase. For example, for detection of multimeric proteins expressed by RSV by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split luciferase fragments (e.g., one fusion protein comprising a large luciferase fragment and the other fusion protein comprising a small luciferase fragment, wherein the small luciferase fragment is complementary to the large luciferase fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a small luciferase fragment. In some embodiments, the small luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the small luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 1 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a small luciferase fragment that comprises 11 amino acids. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a large luciferase fragment. In some embodiments, the large luciferase fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the large luciferase fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 2 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the large Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a large luciferase fragment that comprises 158 amino acids. In some embodiments, the reporter protein is HRP. For example, for detection of multimeric proteins expressed by RSV by the present methods, a first fusion protein and a second fusion protein can be used, wherein each of the first and second fusion proteins comprises: one of the two complementary split HRP fragments (e.g., one fusion protein comprising a first HRP fragment and the other fusion protein comprising a second HRP fragment, wherein the first HRP fragment is complementary to the second HRP fragment); and one or more binding domains (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains). In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a first HRP fragment. In some embodiments, the first HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 71. In some embodiments, the first HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 71 by 1, 2, 3, 4, or 5 amino acids. In certain embodiments, the first HRP fragment comprises the amino acid sequence of SEQ ID NO: 71. In some embodiments, a fusion protein (e.g., a fusion protein for detection of multimeric proteins expressed by RSV by the present methods) comprises a second HRP fragment. In some embodiments, the second HRP fragment comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 72. In some embodiments, the second HRP fragment comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 72 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, the second HRP fragment comprises the amino acid sequence of SEQ ID NO: 72. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 Binding Domains for RSV Binding domains of these fusion proteins (e.g., fusion proteins that can be used for detection of multimeric proteins expressed by RSV by the present methods) can recognize and bind to the trimeric fusion glycoprotein (F), a surface protein of RSV. In some embodiments, these binding domains recognize and bind to the apex of the trimeric fusion glycoprotein (F) with high affinity. In some embodiments, binding domains of these fusion proteins are binders (or fragments thereof) that recognize and bind to epitopes (e.g., non-overlapping epitopes) at or near the apex of the trimeric assembly of the RSV fusion glycoprotein (F). These fusion proteins (e.g., fusion proteins for detection of multimeric proteins expressed by RSV by the present methods) can comprise the following binding domain: M97, a binding domain that comprises: an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 45; an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 45 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids; and / or the amino acid sequence of SEQ ID NO: 45. Detection of RSV The fusion proteins of the present disclosure can be used for detection of multimeric proteins expressed by RSV in test samples (e.g., biological samples). If a test sample comprising RSV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the trimeric fusion glycoprotein (F) of RSV. Binding of the binding domains to the trimeric fusion glycoprotein (F) of RSV brings the two complemenatry split reporter protein fragments into close proximity to reconstitute a functional reporter protein. When a substrate of the reporter protein is added to this composition, the functional reporter protein generates a detectable signal (e.g., luminescence, color, and / or precipitation). Thus, a pair of these fusion proteins (e.g., one fusion protein comprising one reporter protein fragment and the other fusion Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 protein comprising its complemenatry reporter protein fragment) can be used for ultrasensitive detection of multimeric proteins expressed by RSV by an assay of the present disclosure. Such an assay is rapid and straightforward to perform and involves only mixing a testsample (e.g., a biological sample from a subject with the two fusion proteins, followed by the addition of a suitable substrate for detectable (e.g., luminescence, color, and / or precipitation) readout. The whole detection process occurs in the liquid phase without any washing steps. Two or more of the following fusion proteins can be used for detection of multimeric proteins expressed by RSV in test samples (e.g., biological samples). M97-S In some embodiments, a fusion protein for detection of multimeric proteins expressed by RSV comprises a small luciferase fragment and a binding domain, M97. The binding domain of the fusion protein can recognize and bind to the trimeric fusion glycoprotein (F) of RSV. In some embodiments, the fusion protein comprises the small luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a M97 domain; and (b) a small luciferase fragment. Such a fusion protein can be referred to herein as M97-S. In some embodiments, the small luciferase fragment in M97-S is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, M97-S further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, M97-S can comprise a purification tag at the N- terminal. Additionally, or in the alternative, M97-S can comprise a purification tag at the C-terminal. In some embodiments, M97-S comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 46. In some embodiments, M97-S comprises an amino acid Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 sequence that differs from the amino acid sequence of SEQ ID NO: 46 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, M97-S comprises the amino acid sequence of SEQ ID NO: 46. In some embodiments, M97-S comprises a signal sequence. For detection of multimeric proteins expressed by RSV in a test sample (e.g., a biological sample), M97-S can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, M97-S can be used with M97-L and / or L-M97 for detection of multimeric proteins expressed by RSV in a test sample. If a test sample comprising RSV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the trimeric fusion glycoprotein (F) of RSV. Binding of the binding domains to the trimeric fusion glycoprotein (F) brings the two split luciferase fragments (e.g., the small luciferase fragment from M97-S and the large luciferase fragment from M97-L and / or L-M97) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, M97-S along with another fusion protein (e.g., M97-L and / or L-M97) can be used for ultrasensitive detection of RSV (e.g., trimeric fusion glycoprotein (F) of RSV) by an assay of the present disclosure. S-M97 In some embodiments, a fusion protein for detection of multimeric proteins expressed by RSV comprises a small luciferase fragment and a binding domain, M97. The binding domain of the fusion protein can recognize and bind to the trimeric fusion glycoprotein (F) of RSV. In some embodiments, the fusion protein comprises the small luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a small luciferase fragment; and (b) a M97 domain. Such a fusion protein can be referred to herein as S-M97. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, the small luciferase fragment in S-M97 is fused to the binding domain. In some embodiments, the small luciferase fragment is fused directly to the binding domain. In some embodiments, the small luciferase fragment is fused to the binding domain via a linker. In some instances, S-M97 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, S-M97 can comprise a purification tag at the N- terminal. Additionally, or in the alternative, S-M97 can comprise a purification tag at the C-terminal. In some embodiments, S-M97 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 47. In some embodiments, S-M97 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 47 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, S-M97 comprises the amino acid sequence of SEQ ID NO: 47. In some embodiments, S-M97 comprises a signal sequence. For detection of multimeric proteins expressed by RSV in a test sample (e.g., a biological sample), S-M97 can be used with another fusion protein that comprises similar binding domain but a large luciferase fragment. For example, S-M97 can be used with M97-L and / or L-M97 for detection of multimeric proteins expressed by RSV in a test sample. If a test sample comprising RSV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the trimeric fusion glycoprotein (F) of RSV. Binding of the binding domains to the trimeric fusion glycoprotein (F) brings the two split luciferase fragments (e.g., the small luciferase fragment from S-M97 and the large luciferase fragment from M97-L and / or L-M97) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, S-M97 along with another fusion protein (e.g., M97-L and / or L-M97) can be used for Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 ultrasensitive detection of RSV (e.g., trimeric fusion glycoprotein (F) of RSV) by an assay of the present disclosure. M97-L In some embodiments, a fusion protein for detection of multimeric proteins expressed by RSV comprises a large luciferase fragment and a binding domain, M97. The binding domain of the fusion protein can recognize and bind to the trimeric fusion glycoprotein (F) of RSV. In some embodiments, the fusion protein comprises the large luciferase fragment at the C-terminal and / or the binding domain at the N-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a M97 domain; and (b) a large luciferase fragment. Such a fusion protein can be referred to herein as M97-L. In some embodiments, the large luciferase fragment in M97-L is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, M97-L further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, M97-L can comprise a purification tag at the N- terminal. Additionally, or in the alternative, M97-L can comprise a purification tag at the C-terminal. In some embodiments, M97-L comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 48. In some embodiments, M97-L comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 48 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, M97-L comprises the amino acid sequence of SEQ ID NO: 48. In some embodiments, M97-L comprises a signal sequence. For detection of multimeric proteins expressed by RSV in a test sample (e.g., a biological sample), M97-L can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, M97-L can be used with Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 M97-S and / or S-M97 for detection of multimeric proteins expressed by RSV in a test sample. If a test sample comprising RSV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the trimeric fusion glycoprotein (F) of RSV. Binding of the binding domains to the trimeric fusion glycoprotein (F) brings the two split luciferase fragments (e.g., the large luciferase fragment from M97-L and the small luciferase fragment from M97-S and / or S-M97) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, M97-L along with another fusion protein (e.g., M97-S and / or S-M97) can be used for ultrasensitive detection of RSV (e.g., trimeric fusion glycoprotein (F) of RSV) by an assay of the present disclosure. L-M97 In some embodiments, a fusion protein for detection of multimeric proteins expressed by RSV comprises a large luciferase fragment and a binding domain (e.g., M97). The binding domain of the fusion protein can recognize and bind to the trimeric fusion glycoprotein (F) of RSV. In some embodiments, the fusion protein comprises the large luciferase fragment at the N-terminal and / or the binding domain at the C-terminal. For example, the fusion protein can comprise, from N-terminal to C-terminal: (a) a large luciferase fragment; and (b) a M97 domain. Such a fusion protein can be referred to herein as L-M97. In some embodiments, the large luciferase fragment in L-M97 is fused to the binding domain. In some embodiments, the large luciferase fragment is fused directly to the binding domain. In some embodiments, the large luciferase fragment is fused to the binding domain via a linker. In some instances, L-M97 further comprises a purification tag (e.g., FLAG, cMyc tag, His-tag, or biotin tag). For example, L-M97 can comprise a purification tag at the N- terminal. Additionally, or in the alternative, L-M97 can comprise a purification tag at the C-terminal. Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 In some embodiments, L-M97 comprises an amino acid sequence having at least 80% (e.g., at least 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%) sequence identity to the amino acid sequence of SEQ ID NO: 49. In some embodiments, L-M97 comprises an amino acid sequence that differs from the amino acid sequence of SEQ ID NO: 49 by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 amino acids. In certain embodiments, L-M97 comprises the amino acid sequence of SEQ ID NO: 49. In some embodiments, L-M97 comprises a signal sequence. For detection of multimeric proteins expressed by RSV in a test sample (e.g., a biological sample), L-M97 can be used with another fusion protein that comprises similar binding domain but a small luciferase fragment. For example, L-M97 can be used with M97-S and / or S-M97 for detection of multimeric proteins expressed by RSV in a test sample. If a test sample comprising RSV is added to a composition comprising these fusion proteins, the binding domains of these fusion proteins can bind to the trimeric fusion glycoprotein (F) of RSV. Binding of the binding domains to the trimeric fusion glycoprotein (F) brings the two split luciferase fragments (e.g., the large luciferase fragment from L-M97 and the small luciferase fragment from M97-S and / or S-M97) into close proximity to reconstitute a functional luciferase enzyme. When luciferin (a substrate of luciferase) is added to this composition, the functional luciferase oxidizes luciferin to produce an excited state oxyluciferin, which emits a photon when it relaxes to the ground state, thus generating a bright blue bioluminescence signal. Thus, L-M97 along with another fusion protein (e.g., M97-S and / or S-M97) can be used for ultrasensitive detection of RSV (e.g., trimeric fusion glycoprotein (F) of RSV) by an assay of the present disclosure. Fusion Proteins for Detection of Poliovirus Described herein are fusion proteins for detection of poliovirus (e.g., multimeric proteins expressed by poliovirus) in test samples (e.g., biological samples). In some embodiments, fusion proteins for detection of multimeric proteins expressed by poliovirus can be referred to herein as fusion proteins for detection of poliovirus. For detection of multimeric proteins expressed by poliovirus by the present methods, a pair of Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 fusion proteins (e.g., a first fusion protein and a second fusion protein) can be used, wherein the first fusion protein comprises a first reporter protein fragment, and the second fusion protein comprises a second reporter protein fragment. The first and second reporter protein fragments are complementary to each other, such as, when together, the first and second reporter protein fragments comprise the reporter protein in its entirety and restore its function (e.g., the reporter protein can generate a detectable signal in presence of a suitable substrate when the first and second reporter protein fragments come together). Thus, a full and functional reporter protein is formed when a first reporter protein fragment and a second reporter protein fragment come together (e.g., come in close proximity of each other). Along ...

Claims

Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 WHAT IS CLAIMED IS:

1. A method for detecting the presence or absence of an analyte, optionally a multimeric analyte, optionally a multimeric protein expressed by a pathogenic organism, in a test sample, the method comprising: a) combining the test sample with a first fusion protein and a second fusion protein, wherein the first fusion protein comprises a first reporter protein fragment and one or more binding domains, and the second fusion protein comprises a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to the analyte, the first reporter protein fragment and second reporter protein fragment, when together, comprise the reporter protein in its entirety and restore its function, and wherein the test sample, the first fusion protein, and the second fusion protein are combined under conditions effective for the reporter protein to generate a detectable signal; and b) detecting the presence or absence of the analyte in the test sample based on presence or absence of the detectable signal generated by the reporter protein.

2. The method of claim 1, wherein the first fusion protein comprises the first reporter protein fragment at the N-terminal or at the C-terminal; and / or the second fusion protein comprises the second reporter protein fragment at the N-terminal or at the C- terminal.

3. The method of claim 1 or 2, wherein the first reporter protein fragment is fused to a binding domain via a linker; and / or the second reporter protein fragment is fused to a binding domain via a linker.

4. The method of any one of claims 1-3, wherein the first fusion protein comprises one or more linkers between two binding domains; and / or the second fusionAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 protein comprises one or more linkers between two binding domains.

5. The method of any one of claims 1-4, wherein the one or more binding domains are binders that recognize and bind at or near the apex of the analyte.

6. The method of any one of claims 1-5, wherein the conditions effective for the reporter protein to generate a detectable signal comprises combining the test sample, the first fusion protein, and the second fusion protein in a composition comprising a substrate for the reporter protein.

7. The method of any one of claims 1-6, wherein the reporter protein is luciferase, beta-lactamase, beta-galactosidase, beta-glucuronidase, green fluorescent protein (GFP), horseradish peroxidase, chloramphenicol acetyl transferase, or alkaline phosphatase.

8. The method of any one of claims 1-7, wherein the reporter protein is luciferase.

9. The method of claim 8, wherein the conditions effective for the reporter protein to generate a detectable signal comprises combining the test sample, the first fusion protein, and the second fusion protein in a composition comprising luciferin or a derivative of luciferin.

10. The method of any one of claims 1-9, wherein the first fusion protein comprises a small luciferase fragment and the second fusion protein comprises a large luciferase fragment, or the first fusion protein comprises a large luciferase fragment and the second fusion protein comprises a small luciferase fragment.

11. The method of claim 10, wherein the small luciferase fragment comprises an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 1.Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 12. The method of claim 10, wherein the small luciferase fragment comprises the amino acid sequence of SEQ ID NO:

1.

13. The method of claim 10, wherein the small luciferase fragment comprises 10- 14, optionally 11, amino acids starting from the C terminus of luciferase, optionally deep sea shrimp luciferase or a variant thereof.

14. The method of claim 10, wherein the large luciferase fragment comprises an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

2.

15. The method of claim 10, wherein the large luciferase fragment comprises the amino acid sequence of SEQ ID NO:

2.

16. The method of claim 10, wherein the large luciferase fragment comprises 156- 160, optionally 158, amino acids, starting from the N terminus of luciferase, optionally deep sea shrimp luciferase or a variant thereof.

17. The method of any one of claims 1-16, wherein the first fusion protein further comprises a purification tag; and / or the second fusion protein further comprises a purification tag.

18. The method of claim 17, wherein the purification tag is FLAG, streptavidin, cMyc tag, His-tag, or biotin tag.

19. The method of any one of claims 1-19, wherein the analyte is a mammalian protein, optionally TIM1.

20. The method of any one of claims 1-19, wherein the analyte is a multimeric protein expressed by a pathogenic virus.Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 21. The method of claim 20, wherein the pathogenic virus is SARS CoV-2, influenza virus, human immunodeficiency virus (HIV), respiratory syncytial virus (RSV), measles, ebola, hepatitis C, zika, monkeypox, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, rotavirus or poliovirus.

22. The method of claim 20 or 21, wherein the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 3; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 8; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 11; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

21.

23. The method of claim 20 or 21, wherein the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 3; a binding domain comprising the amino acid sequence of SEQ ID NO: 8; a binding domain comprising the amino acid sequence of SEQ ID NO: 11; a binding domain comprising the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising the amino acid sequence of SEQ ID NO:

21.

24. The method of claim 20 or 21, wherein the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 26; a binding domain comprising an amino acid sequence having at least 85% sequence identity to theAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 amino acid sequence of SEQ ID NO: 27; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

29.

25. The method of claim 20 or 21, wherein the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 26; a binding domain comprising the amino acid sequence of SEQ ID NO: 27; a binding domain comprising the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising the amino acid sequence of SEQ ID NO:

29.

26. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

38.

27. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and the second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34; a binding domain comprising the amino acid sequence of SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 36; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 38.Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 28. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

36.

29. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising the amino acid sequence of SEQ ID NO:

36.

30. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO:

38.

31. The method of claim 20 or 21, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO:

38.

32. The method of claim 20 or 21, wherein the pathogenic virus is RSV, and theAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 one or more binding domains of the first and second fusion proteins comprise a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

45.

33. The method of claim 20 or 21, wherein the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising the amino acid sequence of SEQ ID NO:

45.

34. The method of claim 20 or 21, wherein the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 50; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

60.

35. The method of claim 20 or 21, wherein the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 50; a binding domain comprising the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising the amino acid sequence of SEQ ID NO:

60.

36. The method of any one of claims 1-35, wherein the test sample is a biological sample.

37. The method of claim 36, wherein the biological sample is pus, mucus, blood, plasma, serum, semen, sputum, nasal fluid, urine, saliva, or milk.

38. A system or kit for detecting an analyte, optionally a multimeric analyte in a test sample, the system or kit comprising: i) a first fusion protein comprising a firstAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 reporter protein fragment and one or more binding domains; and ii) a second fusion protein comprising a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to the analyte, and the first reporter protein fragment and second reporter protein fragment, when together, comprise the reporter protein in its entirety and restore its function.

39. The system or kit of claim 38, wherein the first fusion protein and the second fusion protein are lyophilized.

40. The system or kit of claim 38 or 39, further comprising a substrate for the reporter protein.

41. The system or kit of any one of claims 38-40, further comprising a negative control sample.

42. The system or kit of claim 41, wherein the negative control sample comprises 1-10% PBS.

43. The system or kit of any one of claims 38-42, further comprising a positive control sample.

44. The system or kit of claim 43, wherein the positive control sample comprises the analyte or a component thereof that is known to specifically bind to the first and second fusion proteins and reconstitute the function of the reporter protein.

45. The system or kit of any one of claims 38-44, wherein the one or more binding domains are binders that recognize and bind at or near the apex of the analyte.

46. The system or kit of any one of claims 38-45, wherein:Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 the first fusion protein comprises the first reporter protein fragment at the N- terminal or at the C-terminal; and / or the seocnd fusion protein comprises the second reporter protein fragment at the N- terminal or at the C-terminal.

47. The system or kit of any one of claims 38-46, wherein: the first reporter protein fragment is fused to a binding domain via a linker; and / or the second reporter protein fragment is fused to a binding domain via a linker.

48. The system or kit of any one of claims 38-47, wherein: the first fusion protein comprises one or more linkers between two binding domains; and / or the second fusion protein comprises one or more linkers between two binding domains.

49. The system or kit of any one of claims 38-48, wherein the reporter protein is luciferase, beta-lactamase, beta-galactosidase, beta-glucuronidase, green fluorescent protein (GFP), horseradish peroxidase, chloramphenicol acetyl transferase, or alkaline phosphatase.

50. The system or kit of any one of claims 38-49, wherein the reporter protein is luciferase.

51. The system or kit of any one of claims 38-50, further comprising luciferin or a derivative of luciferin.

52. The system or kit of any one of claims 38-51, wherein: the first fusion protein comprises a small luciferase fragment and the second fusion protein comprises a large luciferase fragment; or the first fusion protein comprises a large luciferase fragment and the secondAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 fusion protein comprises a small luciferase fragment.

53. The system or kit of any one of claims 38-52, wherein the small luciferase fragment comprises the amino acid sequence of SEQ ID NO: 1, or an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

1.

54. The system or kit of any one of claims 38-53, wherein the small luciferase fragment comprises 10-14, optionally 11, amino acids starting from the C terminus of luciferase, optionally deep sea shrimp luciferase or a variant thereof.

55. The system or kit of any one of claims 38-54, wherein the large luciferase fragment comprises the amino acid sequence of SEQ ID NO: 2, or an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

2.

56. The system or kit of any one of claims 38-55, wherein the large luciferase fragment comprises 156-160, optionally 158, amino acids, starting from the N terminus of luciferase, optionally deep sea shrimp luciferase or a variant thereof.

57. The system or kit of any one of claims 38-56, wherein: the first fusion protein further comprises a purification tag; and / or the second fusion protein further comprises a purification tag.

58. The system or kit of claim 57, wherein the purification tag is FLAG, cMyc tag, His-tag, streptavidin, or biotin tag.

59. The system or kit of claim 57 or 58, wherein the analyte is a mammalian protein, optionally TIM1.

60. The system or kit of any one of claims 38-59, wherein the analyte is aAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 multimeric protein expressed by a pathogenic virus.

61. The system or kit of claim 60, wherein the pathogenic virus is SARS CoV-2, influenza virus, human immunodeficiency virus (HIV), respiratory syncytial virus (RSV), measles, ebola, hepatitis C, zika, monkeypox, hepatitis E, BK polyomavirus (BKV), norovirus, coxsackievirus, parvovirus, rotavirus or poliovirus.

62. The system or kit of claim 60 or 61, wherein the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 3; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 8; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 11; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

21.

63. The system or kit of claim 60 or 61, wherein the pathogenic virus is SARS CoV-2, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 3; a binding domain comprising the amino acid sequence of SEQ ID NO: 8; a binding domain comprising the amino acid sequence of SEQ ID NO: 11; a binding domain comprising the amino acid sequence of SEQ ID NO: 16; and a binding domain comprising the amino acid sequence of SEQ ID NO:

21.

64. The system or kit of claim 60 or 61, wherein the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 26; a bindingAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 27; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

29.

65. The system or kit of claim 60 or 61, wherein the pathogenic virus is influenza virus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 26; a binding domain comprising the amino acid sequence of SEQ ID NO: 27; a binding domain comprising the amino acid sequence of SEQ ID NO: 28; and a binding domain comprising the amino acid sequence of SEQ ID NO:

29.

66. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

38.

67. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34; a binding domain comprising the amino acid sequence of SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 36; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domainAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 comprising the amino acid sequence of SEQ ID NO:

38.

68. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

36.

69. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; and a binding domain comprising the amino acid sequence of SEQ ID NO:

36.

70. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO:

38.

71. The system or kit of claim 60 or 61, wherein the pathogenic virus is HIV, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 34 or SEQ ID NO: 35; a binding domain comprising the amino acid sequence of SEQ ID NO: 37; and a binding domain comprising the amino acid sequence of SEQ ID NO: 36 or SEQ ID NO: 38.Attorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 72. The system or kit of claim 60 or 61, wherein the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

45.

73. The system or kit of claim 60 or 61, wherein the pathogenic virus is RSV, and the one or more binding domains of the first and second fusion proteins comprise a binding domain comprising the amino acid sequence of SEQ ID NO:

45.

74. The system or kit of claim 60 or 61, wherein the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 50; a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising an amino acid sequence having at least 85% sequence identity to the amino acid sequence of SEQ ID NO:

60.

75. The system or kit of claim 60 or 61, wherein the pathogenic virus is poliovirus, and the one or more binding domains of the first and second fusion proteins comprise one or more of: a binding domain comprising the amino acid sequence of SEQ ID NO: 50; a binding domain comprising the amino acid sequence of SEQ ID NO: 55; and a binding domain comprising the amino acid sequence of SEQ ID NO:

60.

76. The system or kit of any one of claims 38-75, wherein the test sample is a biological sample.

77. The system or kit of claim 76, wherein the biological sample is pus, mucus, blood, plasma, serum, semen, sputum, nasal fluid, urine, saliva, or milk.

78. The method of any one of claims 1-37, or the system or kit of any one ofAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 claims 38-77, wherein the reporter protein generates the detectable signal, and / or the method comprises detecting presence or absence of a detectable signal generated by the reporter protein, in less than 180 seconds, less than 120 seconds, less than 60 seconds, less than 30 seconds, less than 20 seconds, less than 10 seconds, less than 5 seconds, or less than 3 seconds after contacting the first fusion protein and the second fusion protein with the test sample, when the analyte is present in the test sample.

79. The method of any one of claims 1-37, or the system or kit of any one of claims 38-77, wherein the first and second fusion proteins are configured to bind to the analyte to bring the first and second reporter protein fragments together within 50 Å of each other.

80. A test strip apparatus, comprising: an elongate strip body, including first and second longitudinally separated strip ends; at least one well extending laterally downward from the strip body proximate the first strip end, the well having a cup shape with an at least partially optically accessible bottom well surface spaced laterally from the strip body by an at least partially opaque cup sidewall; at least one cap extending laterally upward from the strip proximate the second strip end; at least one of the well and the cap configured to maintain a test reagent therein; at least one of the well and the cap being configured to selectively receive a biofluid therein for reaction with the test reagent to produce an optically detectable test result, wherein the strip body is manipulable to superimpose the cap over the well, and wherein the strip body and at least one of the well and the cap comprise a single unitary structure.

81. The test strip apparatus of claim 80, wherein the test reagent comprises: i) a first fusion protein comprising a first reporter protein fragment and one or more binding domains; and ii) a second fusion protein comprising a second reporter protein fragment and one or more binding domains, wherein: the one or more binding domains are binders that recognize and bind to a analyte, and the first reporter protein fragment and second reporter protein fragment, whenAttorney Docket No.29618-0448WO1 / Client Ref. BWH 2021-497 together, comprise the reporter protein in its entirety and restore its function.

82. A system for optically detecting biofluid test results, comprising: the test strip apparatus of claim 80 or 81; an optical detector configured to detect an optical property of the biofluid through the optically accessible portion of the well bottom and responsively produce a test result signal; and a user-perceptible result indicator configured to selectively display the test result signal to a user.

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