Generating targeted sequence diversity in fusion proteins

1. A method of generating a fusion protein which is an antibody, an antigen-binding domain of the antibody or a T-cell receptor (TCR) into which a heterologous polypeptide comprising a natural or synthetic polypeptide ligand has been grafted into a complementarity determining region (CDR) of the fusion protein, the fusion protein retaining protein-binding functionality of the polypeptide ligand, wherein the polypeptide ligand is capable of binding to a G protein-coupled receptor (GPCR), the method comprising:

(i) providing recombination-competent host cells that are capable of expressing RAG-1 and RAG-2 and which comprise at least one nucleic acid for expression of the fusion protein following in vitro recombination, the at least one nucleic acid comprising:

a first nucleic acid sequence comprising a first coding sequence encoding a first portion of the antibody, the antigen-binding domain or the TCR and further comprising a first recombination signal sequence (RSS);

a second nucleic acid sequence comprising a second RSS capable of functional recombination with the first RSS, a second coding sequence encoding the polypeptide comprising the ligand and further comprising a third RSS; and

a third nucleic acid sequence comprising a third coding sequence encoding a second portion of the antibody, the antigen-binding domain or the TCR, the third nucleic acid sequence further comprising a fourth RSS capable of functional recombination with the third RSS;

wherein the first nucleic acid sequence, the second nucleic acid sequence and the third nucleic acid sequence together comprise a tripartite recombination substrate capable of recombining to insert the second coding sequence into a polynucleotide sequence encoding the CDR or to replace an immunoglobulin D segment-encoding sequence within the polynucleotide sequence encoding the CDR with the second coding sequence, the recombination generating sequence diversity at both recombination junctions;

(ii) culturing the recombination-competent host cells in vitro under conditions allowing:

(a) recombination of the first RSS with the second RSS and the third RSS with the fourth RSS to generate a library of chimeric polynucleotide variants comprising the first coding sequence, the second coding sequence and the third coding sequences; and

(b) expression of the library of chimeric polynucleotides so as to generate a library of fusion protein variants; and

(iii) screening the library of fusion protein variants using a functional assay to identify the fusion protein having retained protein binding functionality of the polypeptide ligand.

2. The method according to claim 1 , wherein the first nucleic acid sequence, the second nucleic acid sequence and the third nucleic acid sequence are comprised within a single polynucleotide.

3. The method according to claim 2 , wherein the first nucleic acid sequence and the second nucleic acid sequence further comprise a nucleotide sequence encoding a polypeptide linker and the first RSS and the second RSS are positioned within or proximal to the nucleotide sequence encoding the polypeptide linker, whereby recombination in step (ii) introduces sequence diversity into the nucleotide sequence encoding the polypeptide linker.

4. The method according to claim 1 , wherein the first portion and the second portion of the antibody, the antigen-binding domain or the TCR are from an antibody or an antigen-binding domain.

5. The method according to claim 4 , wherein the first portion and the second portion of the antibody, the antigen-binding domain or the TCR are from a heavy chain of the antibody or the antigen-binding domain.

6. The method according to claim 4 , wherein the first portion and the second portion of the antibody, the antigen-binding domain or the TCR are from a light chain of the antibody or the antigen-binding domain.

7. The method according to claim 4 , wherein the CDR is a CDR3 and the first RSS and the fourth RSS are positioned within a region encoding the CDR3.

8. The method according to claim 1 , wherein one or more of the first nucleic acid sequence, the second nucleic acid sequence and the third nucleic acid sequence further comprises flanking sequences adjacent to, respectively: the first RSS; the second RSS and/or the third RSS; and the fourth RSS.

9. The method according to claim 1 , wherein one or both of the first nucleic acid sequence and the third nucleic acid sequence further comprises additional coding sequences that encode a membrane anchor domain peptide.

10. The method according to claim 1 , wherein the recombination-competent host cells are capable of expressing TdT.

11. The method according to claim 2 , wherein the single polynucleotide is stably integrated into the genomes of the recombination-competent host cells.

12. The method according to claim 11 , wherein the single polynucleotide is stably integrated into the genome of each of the recombination-competent host cells as a single copy.

13. The method according to claim 1 , wherein expression of at least one of the RAG-1 and the RAG-2 is under inducible control in the recombination-competent host cells, and wherein step (ii) comprises inducing expression of the at least one of the RAG-1 and the RAG-2.

14. The method according to claim 1 , wherein the polypeptide ligand is a natural ligand.

15. The method according to claim 1 , wherein the polypeptide ligand that is capable of binding to the GPCR is derived from GLP-1, exendin-4 or TPO.

16. The method according to claim 1 , wherein the polypeptide ligand that is capable of binding to the GPCR has the sequence set forth in SEQ ID NO:38, 43, 71 or 73-80, or a fragment thereof.

17. The method according to claim 1 , wherein the fusion protein comprises a full length IgG scaffold.