Polypeptide tags and their use in in vitro protein synthesis - Patents.com

Abstract

A polypeptide tag is provided, having the amino acid sequence Xaa1Xaa2Xaa3PHDYNXaa4Xaa5Xaa6, where Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently an amino acid or absent, and the polypeptide tag is used to label a target protein. A second aspect provides a polypeptide fusion protein, comprising two structures: (1) any of the polypeptide tags of the first aspect and (2) a target protein linked to the polypeptide tag. A cell-free protein synthesis system and its use in in vitro protein synthesis are also provided. By constructing a fusion protein from a polypeptide tag and a target protein, the expression level of the labeled target protein can be effectively increased even if the polypeptide tag is not removed.

Description

[Technical field]

[0001] [Detailed Description of the Invention] This application claims priority from Chinese patent application No. 2019112066163, filed on November 30, 2019. This application cites the above Chinese patent application in its entirety.

[0002] The present invention is in the field of biochemistry, specifically, it relates to polypeptide tags and their use in in vitro protein synthesis. [Background technology]

[0003] Proteins are important molecules in cells and are involved in the execution of almost all the functions of cells. Different protein sequences and structures determine their different functions (1). Within cells, proteins can catalyze various biochemical reactions as enzymes, coordinate various activities of the body as signaling molecules, support the body's morphology, store energy, transport molecules, and move the body (2). In the biomedical field, protein antibodies, as targeted drugs, are an important means of treating diseases such as cancer (1, 2).

[0004] In addition to humans' understanding of intracellular protein synthesis, protein synthesis can also be carried out outside cells. In vitro protein synthesis systems generally refer to systems that add components such as mRNA or DNA templates, RNA polymerase, amino acids, and ATP to the decomposition system of bacteria, fungi, plant cells, or animal cells to complete rapid and highly efficient translation of foreign proteins (3, 4). In vitro cell-free protein synthesis systems have several advantages over traditional in vivo recombinant expression systems, such as the ability to express special proteins that are toxic to cells or contain unnatural amino acids (e.g., D-amino acids), and the ability to directly use PCR products as templates to simultaneously synthesize multiple proteins in parallel for high-throughput drug screening and proteomic research (3, 5).

[0005] According to literature reports, fusion protein technology is currently one of the common methods for optimizing the production process of recombinant proteins, and fusion tags can achieve the purpose of promoting the expression of target proteins. For example, researchers have found that protein fusion tag policies are effective in increasing the expression of target proteins and suppressing the formation of inclusion bodies (6). Tags widely used in protein fusion include, but are not limited to, MBP (maltose binding protein) (7), TrxA (thioredoxin) (8), NUSA (nitrogen utilization substance A) (9), GST (glutathione-S-transferase) (10), and SUMO (small ubiquitin-like modifier) ​​(11). Here, the tags used in most protein fusions are polypeptide chains consisting of 20 to 300 amino acids, the disadvantage of which is that the chain length is relatively long, the molecule is relatively large, and in many cases, it needs to be removed from the target protein after expression is completed to prevent it from interfering with the structure and function of the target protein and reducing the efficiency of protein synthesis. Therefore, in order to reduce the impact and interference of the introduced fusion tag on the target protein, there is a need in the art to develop a polypeptide tag that has no or little effect on the protein structure and function and improves the production efficiency of the target protein. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention solves the problems existing in the conventional techniques of protein synthesis, and provides a polypeptide tag with a short polypeptide chain, a low molecular weight, and no effect on the spatial structure of the target protein. Therefore, there is no need to cleave the polypeptide tag, which simplifies the process flow, improves production efficiency, and reduces costs. Furthermore, even if the polypeptide tag is not cleaved, the labeled target protein can effectively improve the protein expression level.

[0007] A first aspect of the present invention provides a polypeptide tag, the amino acid sequence of which is Xaa1Xaa2Xaa3PHDYNXaa4Xaa5Xaa6 (SEQ ID NO:37) and wherein Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently an amino acid or absent, and the polypeptide tag is used to label a target protein, preferably for intracellular or extracellular protein expression based on a Kluyveromyces lactis source.

[0008] Preferably, said Xaa1 in the general formula of the amino acid sequence of said polypeptide tag is V or absent.

[0009] Preferably, said Xaa2 in the general formula of the amino acid sequence of said polypeptide tag is S or absent.

[0010] Preferably, said Xaa3 in the general formula of the amino acid sequence of said polypeptide tag is E or absent.

[0011] Preferably, said Xaa4 in the general formula of the amino acid sequence of said polypeptide tag is Y or absent.

[0012] Preferably, said Xaa5 in the general formula of the amino acid sequence of said polypeptide tag is E or G or is absent.

[0013] Preferably, said Xaa6 in the general formula of the amino acid sequence of said polypeptide tag is P or K or is absent.

[0014] Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; The Xaa6 is P or absent. Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being G or absent; The Xaa6 is K or absent.

[0015] Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being absent, said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; The Xaa6 is P or K or is absent.

[0016] Preferably, Xaa1 is absent, Xaa2Xaa3 is SE or absent, and more preferably, Xaa4Xaa5Xaa6 is YEK.

[0017] Preferably, said Xaa1 is absent and Xaa4Xaa5 is YE, YG or absent, more preferably, Xaa4Xaa5Xaa6 is YEP or YGK.

[0018] Preferably, Xaa1Xaa2Xaa3 is VSE, more preferably, Xaa4Xaa5 is YE or YG, and Xaa6 is absent.

[0019] Preferably, Xaa1Xaa2Xaa3 is VSE and Xaa4 is absent, and more preferably, Xaa5Xaa6 is EP or GK.

[0020] Preferably, at least three of Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are not absent, and more preferably, Xaa2Xaa3 and Xaa4 are not absent, or Xaa3 and Xaa4Xaa5 are not absent.

[0021] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is P.

[0022] Preferably, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is G, and Xaa6 is K in the general formula of the amino acid sequence of the polypeptide tag.

[0023] Preferably, the Xaa1 in the general formula of the amino acid sequence of the polypeptide tag is absent, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K.

[0024] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is absent.

[0025] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is absent, Xaa2 is absent, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K.

[0026] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is absent, and Xaa6 is absent.

[0027] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is absent, Xaa2 is absent, Xaa3 is absent, Xaa4 is Y, and Xaa 5 is E and Xaa6 is K.

[0028] Preferably, in the general formula of the amino acid sequence of the polypeptide tag, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is absent, Xaa5 is absent, and Xaa6 is absent.

[0029] The second aspect of the present invention provides a polypeptide fusion protein, which is a fusion protein formed by labeling a target protein with any of the polypeptide tags provided by the first aspect of the present invention. The polypeptide fusion protein comprises two structures: (1) any of the polypeptide tags provided by the first aspect of the present invention, and (2) a target protein connected to the polypeptide tag. The structure of the polypeptide fusion protein comprises a target protein and a polypeptide tag for labeling the target protein, and a fusion protein is formed between them.

[0030] Furthermore, the C-terminus of the polypeptide tag is joined to the N-terminus of the target protein.

[0031] Furthermore, the target protein is one or a combination of a fluorescent protein, an enhanced fluorescent protein, and firefly luciferase.

[0032] The third aspect of the present invention is (1) a cell extract; (2) a DNA or mRNA encoding any one of the polypeptide fusion proteins provided by the second aspect of the present invention; The present invention provides an in vitro cell-free protein synthesis system comprising the above-mentioned

[0033] Preferably, the cell extract is a yeast cell extract, more preferably a Kluyveromyces lactis cell extract.

[0034] Furthermore, the in vitro cell-free protein synthesis system further comprises one or more of an amino acid mixture, dNTPs, and RNA polymerase.

[0035] Furthermore, the in vitro cell-free protein synthesis system further comprises one or more of a DNA polymerase, an energy supply system, polyethylene glycol, and an aqueous solvent.

[0036] The fourth aspect of the present invention provides a use of any one of the polypeptide tag-encoding genes provided in the first aspect of the present invention, or any one of the polypeptide fusion protein-encoding genes provided in the second aspect of the present invention, or any one of the cell-free protein synthesis systems provided in the third aspect of the present invention, in in vitro protein synthesis. The synthesized protein is an exogenous protein. Preferably, the use is based on a Kluyveromyces lactis cell extract in in vitro protein synthesis.

[0037] The present invention further discloses the use of said polypeptide tag in the synthesis of intracellular proteins, particularly preferably in the synthesis of Kluyveromyces lactis intracellular proteins.

[0038] The present invention has the following positive advances: The polypeptide tag provided by the present invention overcomes the problems in the prior art, such as when the molecular weight of the tag is large, if it is not removed, it is likely to affect the spatial structure and function of the target protein, and if it is removed, it will increase the process and cost. The polypeptide tag provided by the present invention has a short polypeptide chain, a length of less than 11 amino acids, a small molecular weight, and can easily identify the target protein. It does not affect the spatial structure and biological function, and does not need to be excised. By constructing a polypeptide tag and a target protein into a fusion protein, the expression level of the labeled target protein can be effectively increased even if the polypeptide tag is not excised. [Brief description of the drawings]

[0039] [Figure 1] The schematic diagram of the carrier structure in which the DNA coding sequence of the polypeptide tag of the present invention is connected to the coding sequence of the target protein (eGFP is taken as an example), where eGFP is the coding sequence of enhanced green fluorescent protein, and the representative example of the target protein is not limited to eGFP. In the figure, AUG is the start codon, and the "-" between AUG and the coding sequence of the polypeptide tag is the coding sequence of the connecting peptide. [Diagram 2] The present invention shows a comparison of the protein expression effects of multiple N-terminal fused polypeptide tag eGFPs in different in vitro cell-free protein synthesis systems, where BC (Blank control) is a blank control of the N-terminal unfused polypeptide tag sequence of enhanced green fluorescent protein eGFP, PC (Positive control) is a positive control of the N-terminal fused wild-type polypeptide tag sequence of enhanced green fluorescent protein eGFP, and NC (Negative control) is a negative control without adding a DNA template of the coding protein. Here, the cell extract used in Figure 2 is YY1904102, the cell extract used in Figure 3 is YY1908191, and the cell extract used in Figure 4 is YY1904224, all of which are different strains obtained by genetic recombination of Kluyveromyces lactis, including recombination of endogenous expression of RNA polymerase. [Diagram 3]The present invention shows a comparison of the protein expression effects of multiple N-terminal fused polypeptide tag eGFPs in different in vitro cell-free protein synthesis systems, where BC (Blank control) is a blank control of the N-terminal unfused polypeptide tag sequence of enhanced green fluorescent protein eGFP, PC (Positive control) is a positive control of the N-terminal fused wild-type polypeptide tag sequence of enhanced green fluorescent protein eGFP, and NC (Negative control) is a negative control without adding a DNA template of the coding protein. Here, the cell extract used in Figure 2 is YY1904102, the cell extract used in Figure 3 is YY1908191, and the cell extract used in Figure 4 is YY1904224, all of which are different strains obtained by genetic recombination of Kluyveromyces lactis, including recombination of endogenous expression of RNA polymerase. [Figure 4] The present invention shows a comparison of the protein expression effects of multiple N-terminal fused polypeptide tag eGFPs in different in vitro cell-free protein synthesis systems, where BC (Blank control) is a blank control of the N-terminal unfused polypeptide tag sequence of enhanced green fluorescent protein eGFP, PC (Positive control) is a positive control of the N-terminal fused wild-type polypeptide tag sequence of enhanced green fluorescent protein eGFP, and NC (Negative control) is a negative control without adding a DNA template of the coding protein. Here, the cell extract used in Figure 2 is YY1904102, the cell extract used in Figure 3 is YY1908191, and the cell extract used in Figure 4 is YY1904224, all of which are different strains obtained by genetic recombination of Kluyveromyces lactis, including recombination of endogenous expression of RNA polymerase. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Nouns and Terms The "polypeptide tag" as used herein is composed of multiple amino acids linked by peptide bonds and is used to label proteins.

[0041] The term "polypeptide fusion protein" as used herein refers to a fusion protein that is ... This is a protein obtained by expression with a polypeptide tag fused to its end.

[0042] The "seamless cloning" of the present invention is different from the conventional cloning of PCR products, and the only difference is that the carrier end and the primer end should have 15-20 homologous bases, so that the resulting PCR product has 15-20 bases homologous to the carrier sequence at both ends, which are complementarily paired by the interaction between the bases, and can be directly used to transform the host bacteria without the need for enzyme connection, and the linear or circular plasmid that enters the host bacteria will repair the notch by the host bacteria's self-enzyme system.

[0043] As used herein, an "NT tag" is a small peptide consisting of multiple amino acids or amino acid residues at the N-terminus of a polypeptide or protein, and the number of amino acids in the small peptide is indicated by the number following NT, e.g., NT11 (undecapeptide), NT8 (octapeptide), and NT6 (hexapeptide).

[0044] As used herein, "eGFP" is an enhanced green fluorescent protein.

[0045] As used herein, an "amino acid mixture" is a mixture of the 20 naturally occurring amino acids or other unnatural amino acids.

[0046] The "N-terminus" as referred to in the present invention is the amino terminus of the amino acid chain of a peptide or protein.

[0047] The "C-terminus" according to the present invention is the carboxyl terminus of the amino acid chain of a peptide or protein.

[0048] The "energy supply system" as used herein refers to a combination of substances that release ATP through hydrolysis or enzymatic degradation to provide energy for extracorporeal protein synthesis.

[0049] As used herein, "dNTP" refers to a mixture containing adenosine trinucleotide (ATP), thymine trinucleotide (TTP), guanosine trinucleotide (GTP) and cytidine trinucleotide (CTP).

[0050] The present invention will now be described in more detail with reference to specific embodiments, examples and Figures 1 to 4. The embodiments of the present invention are described using a few examples to specifically explain how the gist of the present invention is realized, and are not intended to limit the present invention in any way. In the following examples, the experimental reagents used are not described, and all are of conventional commercially available origin.

[0051] Polypeptide tag screening The inventors screened for a polypeptide that is advantageous for improving the synthesis efficiency of an extracorporeal protein as a tag through a large amount of experimental design and experimental verification, and the amino acid sequence of the polypeptide tag is: Xaa1Xaa2Xaa3PHDYNXaa4Xaa5Xaa6 (SEQ ID NO:37) and wherein Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently an amino acid or absent, and the polypeptide tag is used to label a target protein.

[0052] Preferably, said Xaa1 in the general formula of the amino acid sequence of said polypeptide tag is V or absent.

[0053] Preferably, said Xaa2 in the general formula of the amino acid sequence of said polypeptide tag is S or absent.

[0054] Preferably, said Xaa3 in the general formula of the amino acid sequence of said polypeptide tag is E or absent.

[0055] Preferably, said Xaa4 in the general formula of the amino acid sequence of said polypeptide tag is Y or absent.

[0056] Preferably, said Xaa5 in the general formula of the amino acid sequence of said polypeptide tag is E or G or is absent.

[0057] Preferably, said Xaa6 in the general formula of the amino acid sequence of said polypeptide tag is P or K or is absent.

[0058] Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; The Xaa6 is P or absent.

[0059] Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being G or absent; The Xaa6 is K or absent.

[0060] Preferably, Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being absent, said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; The Xaa6 is P or K or is absent.

[0061] Preferably, Xaa1 is absent, Xaa2Xaa3 is SE or absent, and more preferably, Xaa4Xaa5Xaa6 is YEK.

[0062] Preferably, said Xaa1 is absent and Xaa4Xaa5 is YE, YG or absent, more preferably, Xaa4Xaa5Xaa6 is YEP or YGK.

[0063] Preferably, Xaa1Xaa2Xaa3 is VSE, more preferably, Xaa4Xaa5 is YE or YG, and Xaa6 is absent.

[0064] Preferably, Xaa1Xaa2Xaa3 is VSE and Xaa4 is absent, and more preferably, Xaa5Xaa6 is EP or GK.

[0065] Preferably, at least three of Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are not absent, and more preferably, Xaa2Xaa3 and Xaa4 are not absent, or Xaa3 and Xaa4Xaa5 are not absent.

[0066] In another embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is P.

[0067] In another embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is G, and Xaa6 is K.

[0068] In another embodiment, Xaa1 is absent, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E and Xaa6 is K.

[0069] In another embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is absent.

[0070] In other embodiments, Xaa1 is absent, Xaa2 is absent, Xaa3 is E, Xaa4 is Y, Xaa5 is E and Xaa6 is K.

[0071] In another embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is absent, and Xaa6 is absent.

[0072] In other embodiments, Xaa1 is absent, Xaa2 is absent, Xaa3 is absent, Xaa4 is Y, Xaa5 is E, and Xaa6 is K.

[0073] In another embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is absent, Xaa5 is absent, and Xaa6 is absent.

[0074] Plasmid construction and sequencing identification of polypeptide fusion proteins Using a pair of primers, the coding sequence of the screened polypeptide tag was fused to the coding sequence position of the N-terminus of the target protein of the pD2P plasmid by seamless cloning method. In the fusion protein expressed in this way, the C-terminus of the polypeptide tag and the N-terminus of the target protein were connected by a peptide bond. For example, a gene sequence encoding a polypeptide tag was connected to the position of the N-terminus gene sequence of eGFP of the pD2P-eGFP plasmid (a pD2P plasmid into which a gene sequence encoding eGFP was introduced). Specifically, a plasmid containing a gene sequence encoding a series of polypeptide fusion proteins was constructed, for example, a plasmid numbered pD2P-1.07-001 was constructed.

[0075] The basic structure of the pD2P plasmid can be seen in the specification and drawings of Chinese patent application CN201910460987.8.

[0076] Primer pairs were designed based on the cloning technique employed, and each primer was PCR amplification was performed using a plasmid containing a peptide fusion protein as a template, and 5 μL of the amplified product was taken and subjected to 1% agarose electrophoresis identification. 5 μL of DpnI was added to 10 μL of the amplified product and incubated at 37 ° C for 6 h. DH5α competent cells were added to the centrifuge tube containing the product after DpnI treatment, mixed gently and uniformly, left on ice for 30 min, then heat shocked at 42 ° C for 45 s, immediately left on ice for 3 min, and 700 μL of LB liquid medium was added to the centrifuge tube. The centrifuge tube was left in a cradle at 37 ° C and cultured with vibration for 1 h, and then 200 μL of the culture solution was taken and applied to solid LB medium containing 100 mmol / L ampicillin and cultured at 37 ° C for 14 to 16 h. After that, white colonies were selected and sequenced, and after confirming that the gene sequence was correct, the plasmid was extracted and stored at -20 ° C.

[0077] In vitro cell-free protein synthesis system In one embodiment, the in vitro cell-free protein synthesis system comprises a cell extract and DNA encoding a polypeptide fusion protein.

[0078] In one embodiment, the in vitro cell-free protein synthesis system includes a cell extract, DNA encoding a polypeptide fusion protein, and one or more of an amino acid mixture, dNTPs, and an RNA polymerase.

[0079] In one embodiment, the in vitro cell-free protein synthesis system includes a cell extract, DNA encoding a polypeptide fusion protein, and one or more of an amino acid mixture, dNTPs, RNA polymerase, DNA polymerase, an energy supply system, polyethylene glycol, and an aqueous solvent.

[0080] In one embodiment, the in vitro cell-free protein synthesis reaction system is as follows: Tris(hydroxymethyl)aminomethane (Tris-HCl) with a final concentration of 9.78 mM and pH 8.0, 80 mM potassium acetate, 5.6 mM magnesium ion, 1.5 mM nucleoside triphosphate mixture (containing dNTP, adenine nucleoside triphosphate, guanine nucleoside triphosphate, cytosine nucleoside triphosphate, and uracil nucleoside triphosphate, each nucleoside triphosphate having a concentration of 1.5 mM), amino acid mixture (glycine, alanine, The concentrations of each of the following amino acids are 0.7 mM: valine, leucine, isoleucine, phenylalanine, proline, tryptophan, serine, tyrosine, cysteine, methionine, asparagine, glutamine, threonine, aspartic acid, glutamic acid, lysine, arginine, and histidine (0.7 mM), 0.7 mM dithiothreitol, 1.7 mM polyethylene glycol, energy supply system, 24 mM tripotassium phosphate, 50% by volume cell extract, and 0.33 µg / µL DNA template.

[0081] In one embodiment, the magnesium salt derived from magnesium ions is selected from any one of magnesium glutamate, magnesium acetate, or a combination thereof.

[0082] In one embodiment, the amino acid mixture includes the 20 naturally occurring amino acids and other unnatural amino acids.

[0083] In one embodiment, the molecular weight of the polyethylene glycol is 200 to 12,000 Da, and preferably 400, 600, 800, 2,000, 4,000, or 8,000 Da. It is calculated as a weight average molecular weight.

[0084] In one embodiment, the energy delivery system comprises glucose, maltose, trehalose, maltodextrin, starch dextrin, creatine phosphate, and quinine phosphate. The preferred maltodextrin is 320 mM, 6% trehalose.

[0085] In one embodiment, the cell extract is selected from eukaryotic cells, yeast cells, and Kluyveromyces cells, preferably Kluyveromyces lactis cells, more preferably Kluyveromyces lactis cells having T7 RNA polymerase integrated into their genome or Kluyveromyces lactis cells having T7 RNA polymerase introduced into a plasmid.

[0086] In another embodiment, the cell extract is selected from Kluyveromyces lactis cells artificially bred for high protein yield, specifically Kluyveromyces lactis cells found with DNA polymerase integrated into their genome, more preferably Kluyveromyces lactis cells with phi29 polymerase integrated and expressed into their genome.

[0087] In one embodiment, the DNA template is a DNA encoding a polypeptide fusion protein, including a polypeptide fusion fluorescent protein, a polypeptide fusion firefly luciferase, and preferably a DNA encoding a polypeptide fusion enhanced fluorescent protein (eGFP).

[0088] Example 1 Identification of Polypeptide Tag Sequences 1.1 The origin and identification of the amino acid sequence of the polypeptide tag have been reported in the literature. Researchers have found through experiments that the first 11 amino acid residues (the amino acid sequence of NT11 is shown in SEQ ID NO:18 and its DNA sequence is shown in SEQ ID NO:9) in the N-terminal half-domain of Dunaliella carbonic anhydrase (dca) can be connected to the N-terminus of a foreign protein to perform fusion expression, thereby improving the translation level of proteins such as YFP (yellow fluorescent protein) in BL21(DE3) E.coli cells (Thi Khoa My Nguyen, et al. The NT11, a novel fusion tag for enhancing protein expression in Escherichia coli. 2019;103(5): 2205-2216.). In this embodiment, partial site deletion or random point mutation was performed on the amino acid sequence of NT11, and further experiments were carried out to screen for amino acid sequences that have a significant effect on improving foreign protein expression.

[0089] Specifically, stepwise deletion or random point mutation is performed on the amino acid sequence of NT11 (PC) to obtain a series of different amino acid sequences, and the corresponding nucleotide sequences are obtained by codon optimization. The amino acid and nucleotide sequences are numbered, and the obtained partial sequences are referred to in Table 1 and SEQ ID NOs: 1 to 18 in the sequence listing.

[0090] [Table 1]

[0091] In a preferred embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is P, the nucleotide sequence is, for example, SEQ ID NO: 1, and the amino acid sequence is, for example, SEQ ID NO: 10.

[0092] In another preferred embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is G, and Xaa6 is K, the nucleotide sequence is, for example, SEQ ID NO: 2, and the amino acid sequence is, for example, SEQ ID NO: 11.

[0093] In another preferred embodiment, Xaa1 is absent, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K, and the nucleotide sequence is, for example, SEQ ID NO:3, and the amino acid sequence is, for example, SEQ ID NO:12.

[0094] In another preferred embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is absent, the nucleotide sequence is, for example, SEQ ID NO: 4, and the amino acid sequence is, for example, SEQ ID NO: 13.

[0095] In another preferred embodiment, Xaa1 is absent, Xaa2 is absent, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K, the nucleotide sequence is, for example, SEQ ID NO:5, and the amino acid sequence is, for example, SEQ ID NO:14.

[0096] In another preferred embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is absent, and Xaa6 is absent, the nucleotide sequence is, for example, SEQ ID NO:6, and the amino acid sequence is, for example, SEQ ID NO:15.

[0097] In another preferred embodiment, Xaa1 is absent, Xaa2 is absent, Xaa3 is absent, Xaa4 is Y, Xaa5 is E, and Xaa6 is K, the nucleotide sequence is, for example, SEQ ID NO: 7, and the amino acid sequence is, for example, SEQ ID NO: 16.

[0098] In another preferred embodiment, Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is absent, Xaa5 is absent, and Xaa6 is absent, the nucleotide sequence is, for example, SEQ ID NO:8, and the amino acid sequence is, for example, SEQ ID NO:17.

[0099] The above eight types of newly obtained polypeptide tags are merely some preferred examples provided by the present invention, and embodiments of the present invention include, but are not limited to, the above preferred examples.

[0100] Example 2 Plasmid construction of N-terminal fusion polypeptide tagged eGFP The plasmids were constructed by connecting the coding gene of the polypeptide tag to the N-terminal coding sequence of eGFP in the pD2P-eGFP plasmid using a pair of primers by seamless cloning method, and the gene structure is shown in FIG. 1. The names of the nine plasmids are pD2P-1.07-(001 to 008) and PC, respectively (see Table 1). The amplification primer sequences of the nine plasmids are SEQ ID NOs: 19 to 36, respectively.

[0101] The specific construction process is as follows:

[0102] Design a pair of primers based on cloning technology (see Table 2, except that the tail is PF, which corresponds to the forward primer, and the tail is PR, which corresponds to the reverse primer. PCR amplification is performed using the above nine plasmids pD2P-1.07-(001-008) and PC as templates, respectively, take 5 μL of the amplified product and perform 1% agarose electrophoresis identification, add 5 μL of DpnI to 10 μL of the amplified product, incubate at 37 °C for 6 h, and add 50 μL of DH5α competent cells to the centrifuge tube containing the product after DpnI treatment. The tubes were then mixed gently and left on ice for 30 min, then heat-shocked at 42°C for 45 s, and immediately left on ice for 3 min, after which 700 μL of LB liquid medium was added to the centrifuge tube. The centrifuge tubes were placed in a cradle at 37°C and cultured with shaking for 1 h, after which 200 μL of the culture medium was taken and spread onto solid LB medium containing 100 mmol / L ampicillin and cultured at 37°C for 14 to 16 h. White colonies were then selected and sequenced for identification. After confirming that the gene sequence was correct, the plasmids from the corresponding colonies were extracted and stored at -20°C.

[0103] [Table 2]

[0104] Examples 3 to 5 Application of an in vitro cell-free protein synthesis system in which a polypeptide tag coding gene, an N-terminal fusion polypeptide tag eGFP coding gene, and the DNA template of the system contain the coding genes in in vitro protein synthesis S1: DNA amplification was performed using the constructed plasmid as a DNA template. The amplification system was as follows: Random primers (NNNNNNN (SEQ ID NO:38), indicating that the seven bases are randomly composed), the above plasmid template 1.14ng / μL, dNTP 0.5-1mM, BSA 0.1mg / mL, phi29 DNA polymerase 0.05-0.1mg / mL, 1×phi29 reaction buffer (components are Tris-HCl 200mM, MgCl2 20mM, (NH4)2SO4 10mM, KCl 10mM, pH 7.5). After the above reaction system was mixed uniformly, it was left in an environment of 30°C to react for 3h. After the reaction was completed, the DNA concentration was measured using an ultraviolet spectrophotometer.

[0105] The experimental group (treatment mode) was supplemented with a DNA template, which contained a nucleotide sequence encoding the polypeptide tag-fused enhanced green fluorescent protein eGFP.

[0106] Group BC (treatment method) is BC (Blank Control), i.e., a blank control, in which a DNA template is added, and the DNA template does not contain a nucleotide sequence encoding a polypeptide tag, but contains a nucleotide sequence encoding enhanced green fluorescent protein eGFP.

[0107] The PC group (treatment regime) was a positive control (PC), which included a DNA template containing a nucleotide sequence encoding a wild-type polypeptide tag and fusing it with enhanced green fluorescent protein (eGFP).

[0108] The NC group (treatment mode) is a negative control (NC), ie, no exogenous DNA template is added.

[0109] S2: eGFP expressing an N-terminal fused polypeptide tag in an in vitro protein synthesis system The DNA fragment amplified in S1 above was added to an in vitro protein synthesis system. The in vitro cell-free protein synthesis reaction system was as follows: Tris(hydroxymethyl)aminomethane (Tris-HCl) with a final concentration of 9.78 mM and pH 8.0, 80 mM potassium acetate, 5.6 mM magnesium ion, 1.5 mM nucleoside triphosphate mixture (containing adenine nucleoside triphosphate, guanine nucleoside triphosphate, cytosine nucleoside triphosphate, and uracil nucleoside triphosphate, each nucleoside triphosphate having a concentration of 1.5 mM), amino acid mixture (glycine, alanine, valine, leucine, isoleucine, phenylalanine, proline, tryptophan, serine, The concentrations of tyrosine, cysteine, methionine, asparagine, glutamine, threonine, aspartic acid, glutamic acid, lysine, arginine, and histidine were 0.7 mM, 1.7 mM dithiothreitol, 2% (w / v) polyethylene glycol 8000, 320 mM maltodextrin, 6% trehalose, 24 mM tripotassium phosphate, 50% by volume yeast cell extract, and 0.33 μg / μL of DNA template (obtained by DNA amplification as described above).

[0110] Three different yeast cell extracts were used to construct different in vitro protein synthesis systems, and the effects of the constructed polypeptide fusion proteins in different systems were verified. Here, the cell extract used in Example 3 is YY1904102, the cell extract used in Example 4 is YY1908191, and the cell extract used in Example 5 is YY1904224, all of which are Kluyveromyces lactis ATCC8585 and its different genetically modified strains, including the endogenous expression of RNA polymerase, and the preparation method is referred to in the Chinese patent application CN201710768550.1.

[0111] The reaction system was left at 30°C for approximately 20 hours. After the reaction was completed, the plate was immediately placed on an Envision2120 multifunction microplate reader (Perkin Elmer) and the intensity of the eGFP fluorescent signal was measured. The relative fluorescence unit (RFU) was used as the activity unit.

[0112] The experimental results are as follows:

[0113] As can be seen from the fluorometry measurement results, in the in vitro protein synthesis system, the wild-type polypeptide tag does not enhance protein expression, but rather reduces the amount of protein expression. See Figures 2 to 4, the RFU values ​​of the PC group are all lower than those of the BC group, and the inhibitory effect is particularly significant in Figure 4. This is the opposite of the promoting effect of the wild-type polypeptide tag in E. coli cells.

[0114] The polypeptide tags screened by the inventors all had a promoting effect on the in vitro expression of eGFP. As can be seen from the fluorometry measurement results, the RFU values ​​of eGFP expressed in an in vitro protein synthesis system using multiple N-terminal fusion polypeptide tags were all improved, which explains the improved expression of eGFP by the polypeptide tag of the present invention, as shown in Figures 2 to 4.

[0115] Regarding Example 3, the results are shown in Figure 2. All of the different experimental groups exceeded the blank control group, and in particular, pD2P-1.07-003 (RFU value reached 7361 after 20 hours of reaction) had an RFU improvement of 41.86% compared with the blank control in which no polypeptide tag-related sequence was introduced (RFU value was 5189).

[0116] As can be seen from Example 4, all of the different experimental groups exceeded the blank control group. Referring to Figure 3, the effects of four experimental groups including pD2P-1.07-003, pD2P-1.07-004, pD2P-1.07-006 and pD2P-1.07-008 were significant, in particular, pD2P-1.07-008 reached an RFU value of 5532 after reacting for 20 hours, which was improved by 65.93% compared with the blank control in which no polypeptide tag-related sequence was introduced (RFU value of 3334).

[0117] In Example 5, all of the different experimental groups exceeded the blank control group. Referring to Figure 4, after 20 hours of reaction, the RFU value measured for the experimental group pD2P-1.07-002 was 3580, which was an improvement of 34.89% compared with the blank control (RFU value of 2654) in which no polypeptide tag-related sequence was introduced.

[0118] As a result of the experiments in the above examples, by introducing the polypeptide tag of the present invention, particularly the polypeptide tag sequence fused to the N-terminus of a target protein, a polypeptide fusion protein constructed using an in vitro cell-free protein synthesis system can be synthesized, thereby improving the translation efficiency and production amount of the target protein, and greatly improving the applicability of the in vitro protein synthesis system.

[0119] All documents mentioned in the present invention are incorporated herein by reference as if each document were incorporated by reference alone. In addition, it should be understood that after reading the above content of the present invention, a person skilled in the art may make various changes or modifications to the present invention, and the equivalent forms thereof are within the scope of the appended claims of this application. The present invention includes the following aspects. <1> A polypeptide tag, the amino acid sequence of which is Xaa1Xaa2Xaa3PHDYNXaa4Xaa5Xaa6; wherein Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently an amino acid or absent, and the polypeptide tag is used to label a target protein. <2> Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; Xaa6 is P, K or absent. <1> The polypeptide tag according to claim 1. <3> Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; Xaa6 is P or absent. <1> The polypeptide tag according to claim 1. <4> Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being V or absent; said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being G or absent; Xaa6 is K or absent. <1> The polypeptide tag according to claim 1. <5> Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are each independently said Xaa1 being absent, said Xaa2 being S or absent; said Xaa3 being E or absent; said Xaa4 being Y or absent; said Xaa5 being E or G or absent; Xaa6 is P, K or absent. <1> The polypeptide tag according to claim 1. <6> Xaa1 is absent, Xaa2Xaa3 is SE or absent, and more preferably, Xaa4Xaa5Xaa6 is YEK. <1> The polypeptide tag according to claim 1. <7> Xaa1 is absent, Xaa4Xaa5 is YE or YG or is absent, and more preferably, Xaa4Xaa5Xaa6 is YEP or YGK. <1> The polypeptide tag according to claim 1. <8> Xaa1Xaa2Xaa3 is VSE, more preferably, Xaa4Xaa5 is YE or YG, and Xaa6 is absent. <1> The polypeptide tag according to claim 1. <9> Xaa1Xaa2Xaa3 is VSE, Xaa4 is absent, and more preferably, Xaa5Xaa6 is EP or GK. <1> The polypeptide tag according to claim 1. <10> At least three of Xaa1, Xaa2, Xaa3, Xaa4, Xaa5, and Xaa6 are not absent, and more preferably, Xaa2, Xaa3, and Xaa4 are not absent, or Xaa3 and Xaa4, Xaa5 are not absent. <1> The polypeptide tag according to claim 1. <11> Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is P. <1> The polypeptide tag according to claim 1. <12> Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is G, and Xaa6 is K. <1> The polypeptide tag according to claim 1. <13> Xaa1 is absent, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K. <1> The polypeptide tag according to claim 1. <14> Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is absent. <1> The polypeptide tag according to claim 1. <15> Xaa1 is absent, Xaa2 is absent, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K. <1> The polypeptide tag according to claim 1. <16> wherein Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is absent, and Xaa6 is absent. <1> The polypeptide tag according to claim 1. <17> Xaa1 is absent, Xaa2 is absent, Xaa3 is absent, Xaa4 is Y, Xaa5 is E, and Xaa6 is K. <1> The polypeptide tag according to claim 1. <18> Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is absent, Xaa5 is absent, and Xaa6 is absent. <1> The polypeptide tag according to claim 1. <19> (1) <1> ~ <18> A polypeptide fusion protein comprising two structures: (1) a polypeptide tag described in any one of (1) to (3) a target protein connected to the polypeptide tag. <20> the C-terminus of the polypeptide tag is connected to the N-terminus of the target protein. <19> A polypeptide fusion protein as described in claim 1. <21> The target protein is one or a combination of a fluorescent protein, an enhanced fluorescent protein, and a firefly luciferase. <19> or <20> A polypeptide fusion protein as described in claim 1. <22> (1) a cell extract; (2) <19> or <20> or <21> and a DNA or mRNA encoding the polypeptide fusion protein according to any one of claims 1 to 5. <23> The cell extract is a yeast cell extract, preferably a Kluyveromyces lactis cell extract. <22> The in vitro cell-free protein synthesis system described in <24> The method further comprises one or more of an amino acid mixture, dNTP, and RNA polymerase. <22> or <23> The in vitro cell-free protein synthesis system described in <25> The method further comprises one or more of a DNA polymerase, an energy supply system, polyethylene glycol, and an aqueous solvent. <22> or <23> or <24> The in vitro cell-free protein synthesis system described in <26> In vitro protein synthesis <1> ~ <18> A gene encoding the polypeptide tag according to any one of the preceding claims, or <19> ~ <21> A gene encoding a polypeptide fusion protein according to any one of claims 1 to 4, <22> ~ <25> Use of the cell-free protein synthesis system described in any one of claims 1 to 5.

[0120] References 1.Garcia RA, Riley MR. Applied biochemistry and biotechnology. Humana Press,; 1981. 263-264 p. 2. Fromm HJ, Hargrove M. Essentials of Biochemistry. 2012; 3. Katzen F, Chang G, Kudlicki W. The past, present and future of cell-free pro tein synthesis. Trends Biotechnol. 2005;23(3):150-6. 4. Gan R, Jewett MC. A combined cell-free transcription-translation system from Saccharomyces cerevisiae for rapid and robust protein synthesis. Biotechnol J. 2014;9(5):641-51. 5. Lu Y. Cell-free synthetic biology: Engineering in an open world. Synth Syst Biotechnol. 2017;2(1):23-7. 6. Esposito D, Chatterjee DKEnhancement of soluble protein ex-pression through the use of fusion tags. Curr Opin Biotechnol.2006;17(4):353-358. 7.Kapust RB, Waugh DS Escherichia coli maltose-binding protein is uncommonly effective at promoting the solubility of polypeptides to which it is fused. Protein Science. 1999;8(8):1668-1674. 8.Liangfan Zhou, Zhihui Zhao,Baocun Li,Yufeng Cai,Shuangquan Zhang.TrxA mediating fusion expression of antimicrobial peptide CM4 from multiple joined genes in Escherichia coli.Protein Expression and Purification.2009;64(2):225-230. 9.Kohl T, schmidt C, Wiemann S, Poustka A, Korf U Automated production of recombinant human proteins as resource for proteome research. Proteome Science. 2008; 6:4. 10.Hu J, Qin H, Sharma M, Cross TA, Gao FP Chemical cleavage of fusion proteins for high-level production of transmembrane peptides and protein domains containing conserved methionines. Biochim Biophys Acta.2008;1778(4):1060-1066. 11.Da Sol Kim, Seon Woong Kim, Jae Min Song, Soon Young Kim&Kwang-Chul Kwon.A new prokaryotic expression vector for the expression of antimicrobial peptide abaecin using SUMO fusion tag.BMC Biotechnology.2019;19:13. 12. Thi Khoa My Nguyen, Mi Ran Ki, Ryeo Gang Son, Seung Pil Pack1.the NT11, a novel fusion tag for enhancing protein expression in Escherichia coli. Applied Microbiology and Biotechnology. 2019;103(5): 2205-2216.

Claims

1. A polypeptide tag, the amino acid sequence of which is Xaa1Xaa2Xaa3PHDYNXaa4Xaa5Xaa6 (SEQ ID NO: 37); provided that said polypeptide tag is used to tag a target protein; said Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is P; or said Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is G, and Xaa6 is K; or said Xaa1 is absent, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K; or said Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is absent; or said Xaa1 is absent, Xaa2 is absent, Xaa3 is E, Xaa4 is Y, Xaa5 is E, and Xaa6 is K; or said Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is Y, Xaa5 is absent, and Xaa6 is absent; or said Xaa1 is absent, Xaa2 is absent, Xaa3 is absent, Xaa4 is Y, Xaa5 is E, and Xaa6 is K; or A polypeptide tag, wherein Xaa1 is V, Xaa2 is S, Xaa3 is E, Xaa4 is absent, Xaa5 is absent, and Xaa6 is absent.

2. A polypeptide fusion protein, comprising two structures: (1) a polypeptide tag according to claim 1; and (2) a target protein connected to the polypeptide tag.

3. The polypeptide fusion protein of claim 2, wherein the C-terminus of the polypeptide tag is connected to the N-terminus of the target protein.

4. 4. The polypeptide fusion protein of claim 2 or claim 3, wherein the target protein is one or a combination of a fluorescent protein, an enhanced fluorescent protein, and firefly luciferase.

5. (1) a cell extract; (2) An in vitro cell-free protein synthesis system, comprising a DNA or mRNA encoding the polypeptide fusion protein according to any one of claims 2 to 4.

6. The in vitro cell-free protein synthesis system according to claim 5 , wherein the cell extract is a yeast cell extract.

7. The in vitro cell-free protein synthesis system according to claim 6, wherein the cell extract is a Kluyveromyces lactis yeast cell extract.

8. The in vitro cell-free protein synthesis system according to any one of claims 5 to 7, further comprising one or more of an amino acid mixture, dNTP, and RNA polymerase.

9. The in vitro cell-free protein synthesis system according to any one of claims 5 to 8, further comprising one or more of a DNA polymerase, an energy supply system, polyethylene glycol, and an aqueous solvent.

10. Use of a gene encoding the polypeptide tag according to claim 1, or a gene encoding the polypeptide fusion protein according to any one of claims 2 to 4, or an in vitro cell-free protein synthesis system according to any one of claims 5 to 9, in in vitro protein synthesis.

11. A polynucleotide encoding the polypeptide tag according to claim 1 or the polypeptide fusion protein according to any one of claims 2 to 4.

12. The polynucleotide of claim 11, wherein the sequence of the polynucleotide is as set forth in any one of SEQ ID NOs: 1 to 8.

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