Fusion partners for peptide production

1. A recombinant fusion protein comprising:

an N-terminal fusion partner, wherein the N-terminal fusion partner is selected from: P. fluorescens DnaJ-like protein wherein the P. fluorescens DnaJ-like protein has the amino acid sequence set forth as SEQ ID NO: 2; P. fluorescens FklB protein wherein the P. fluorescens FklB protein has the amino acid sequence set forth as SEQ ID NO: 4, 28, 61, or 62 ; P. fluorescens FrnE protein wherein the P. fluorescens FrnE protein has the amino acid sequence set forth as SEQ ID NO: 3, 63, or 64 ; P. fluorescens FkbP protein wherein the P. fluorescens FkbP protein has the amino acid sequence set forth as SEQ ID NO: 25; and P. fluorescens EcpD protein wherein the P. fluorescens EcpD protein has the amino acid sequence set forth as SEQ ID NO: 7, 65, 66, or 67;

a polypeptide of interest selected from: hPTH1-34, a proinsulin that is processed to insulin or an insulin analog; Glp1; Glp2; IGF-1; Exenatide (SEQ ID NO: 37); Teduglutide (SEQ ID NO: 39); Pramlintide (SEQ ID NO: 40); Ziconotide (SEQ ID NO: 41); Becaplermin (SEQ ID NO: 42); Enfuvirtide (SEQ ID NO: 43); Nesiritide (SEQ ID NO: 44); a N-met-GCSF; GCSF; P. falciparum circumsporozoite protein; and IFN-β; and

a linker of 10 to 50 amino acids in length, comprising a cleavage site between the N-terminal fusion partner and the polypeptide of interest.

2. The recombinant fusion protein of claim 1 , wherein the polypeptide of interest is a proinsulin, wherein the proinsulin is processed to insulin or an insulin analog, and wherein the proinsulin comprises a C-peptide that has an amino acid sequence selected from: SEQ ID NO: 97; SEQ ID NO: 98; SEQ ID NO: 99; and SEQ ID NO: 100.

3. The recombinant fusion protein of claim 1 , wherein the insulin analog is insulin glargine, insulin aspart, lispro, glulisine, detemir, or degludec.

4. The recombinant fusion protein of claim 1 , wherein the cleavage site is recognized by a cleavage enzyme in the group consisting of: enterokinase, trypsin, Factor Xa, and furin.

5. The recombinant fusion protein of claim 1 , wherein the linker comprises an affinity tag.

6. The recombinant fusion protein of claim 1 , wherein the linker has an amino acid sequence selected from: from the N to C terminus, a (G4S) 2 spacer sequence (SEQ ID NO: 59), a hexahistidine affinity tag (SEQ ID NO: 242), and a protease cleavage site DDDDK (SEQ ID NO: 13); from the N to C terminus, a (G4S) 2 spacer sequence (SEQ ID NO: 59), a hexahistidine affinity tag (SEQ ID NO: 242), and a protease cleavage site RKR; from the N to C terminus, a (G4S) 2 spacer sequence (SEQ ID NO: 59), a hexahistidine affinity tag (SEQ ID NO: 242), and a protease cleavage site RRR; from the N to C terminus, a (G4S) 2 spacer sequence (SEQ ID NO: 59), a hexahistidine affinity tag (SEQ ID NO: 242), and a protease cleavage site LVPR; and SEQ ID NO: 226.

7. The recombinant fusion protein of claim 1 , wherein the polypeptide of interest is hPTH1-34, and wherein the recombinant fusion protein comprises an amino acid sequence selected from: SEQ ID NO: 45; SEQ ID NO: 46; and SEQ ID NO: 47.

8. The recombinant fusion protein of claim 1 , wherein the N-terminal fusion partner is P. fluorescens FklB protein, wherein the P. fluorescens FklB protein has the amino acid sequence set forth as SEQ ID NO: 4, 28, 61, or 62, and wherein the polypeptide of interest is hPTH1-34.

9. The recombinant fusion protein of claim 8 , wherein the linker further comprises a spacer and an affinity tag.

10. The recombinant fusion protein of claim 9 , wherein:

a. the spacer is selected from: (G4S) 1 , (G4S) 2 , (G4S) 3 , (G4S) 4 , and (G4S) 5 ;

b. the affinity tag is selected from: a Maltose Binding Protein tag, a Polyhistidine tag, a FLAG tag, a Myc tag, an HA-tag, and a Nus tag; and

c. the cleavage site is selected from: an enterokinase cleavage site, a trypsin cleavage site, a chymotrypsin cleavage site, a Factor Xa cleavage site, and a Furin cleavage site.

11. The recombinant fusion protein of claim 1 , wherein the N-terminal fusion partner is P. fluorescens FklB protein, wherein the P. fluorescens FklB protein has the amino acid sequence set forth as SEQ ID NO: 4, 28, 61, or 62, and wherein the polypeptide of interest is a proinsulin that is processed to insulin or an insulin analog.

12. The recombinant fusion protein of claim 11 , wherein the linker further comprises an affinity tag.

13. The recombinant fusion protein of claim 12 , wherein the linker further comprises a spacer.

14. The recombinant fusion protein of claim 13 , wherein:

a. the spacer selected is from: (G4S) 1 , (G4S) 2 , (G4S) 3 , (G4S) 4 , and (G4S) 5 ;

b. the affinity tag is selected from: a Maltose Binding Protein tag, a Polyhistidine tag, a FLAG tag, a Myc tag, an HA-tag, and a Nus tag; and

c. the cleavage site is selected from: an enterokinase cleavage site, a trypsin cleavage site, a chymotrypsin cleavage site, a Factor Xa cleavage site, and a Furin cleavage site.

15. The recombinant fusion protein of claim 14 , wherein the polypeptide of interest is a proinsulin that is processed to insulin or an insulin analog, and wherein the recombinant fusion protein comprises an amino acid sequence selected from SEQ ID NOS: 130-153.

16. A method for producing a polypeptide of interest, comprising:

(i) culturing a microbial host cell transformed with an expression vector comprising an expression construct, wherein the expression construct comprises a nucleotide sequence encoding the recombinant fusion protein of claim 1 ;

(ii) inducing the host cell of step (i) to express the recombinant fusion protein;

(iii) purifying the recombinant fusion protein expressed in the induced host cells of step (ii); and

(iv) cleaving the purified recombinant fusion protein of step (iii) by incubation with a cleavage enzyme that recognizes the cleavage site in the linker, to release the polypeptide of interest;

thereby obtaining the polypeptide of interest.

17. The method of claim 16 , further comprising measuring the expression level of the fusion protein expressed in step (ii), measuring the amount of the recombinant fusion protein purified in step (iii), or measuring the amount of the polypeptide of interest obtained in step (iv) that has been properly released, or a combination thereof.

18. The method of claim 17 , wherein the amount of the polypeptide of interest obtained in step (iii) or step (iv) is about 0.1 g/L to about 25 g/L.

19. The method of claim 17 , wherein the properly released polypeptide of interest obtained is soluble, intact, or both.

20. An expression vector for expression of the recombinant fusion protein of claim 1 , wherein the expression vector comprises a nucleotide sequence encoding the recombinant fusion protein.