Cytoplasmic expression system
The present invention provides host cells having an oxidizing cytoplasm, which are capable of growing to high cell densities and producing recombinant proteins in soluble form. Also provided are methods for growing such host cells to high densities, methods for optionally modulating the growth rates of the host cells, and methods for inducing the host cells to produce the desired gene product in soluble form. Further aspects of the invention relate to host cell preparation and storage methods, which utilize the advantages of producing gene products in host cell cytoplasm to provide methods for long-term storage and/or transport of expressed gene products that are retained in a stable and soluble form within the host cells.
1. A method of producing an insulin polypeptide with correctly-formed disulfide bonds, the method comprising:
providing one or more host cells comprising at least one expression construct comprising at least one inducible promoter and at least one polynucleotide sequence encoding an insulin polypeptide to be transcribed from at least one said inducible promoter;
wherein the host cells have an altered gene function of at least one gene that makes the reduction/oxidation environment of the host cell cytoplasm more oxidizing;
growing the host cells until the host cells reach a density greater than 145 (OD 600 );
inducing expression of the insulin polypeptide with an inducer selected from the group consisting of L-arabinose, propionate, rhamnose, xylose, and lactose;
lysing the host cells; and
purifying the insulin polypeptide with correctly-formed disulfide bonds from a soluble cell lysate fraction.
2. The method of claim 1 , wherein the at least one inducible promoter is selected from the group consisting of an L-arabinose-inducible promoter, a propionate-inducible promoter, a rhamnose-inducible promoter, a xylose-inducible promoter, and a lactose-inducible promoter.
3. The method of claim 2 , wherein the at least one inducible promoter is selected from the group consisting of an araBAD promoter, a prpBCDE promoter, a rhaSR promoter, a xlyA promoter, and a lacZYA promoter.
4. The method of claim 1 , further comprising adding an inducer to the host cells per host cell density (OD 600 ) unit, wherein the inducer is selected from the group consisting of: L-arabinose at a concentration between 200 micromolar and 1 pM; L-rhamnose at a concentration between 1 M and 1 pM; D-xylose at a concentration between 1 M and 1 pM; and propionate at a concentration between 1 M and 1 nM.
5. The method of claim 1 wherein the at least one gene that makes the reduction/oxidation environment of the host cell cytoplasm more oxidizing is selected from the group consisting of gor, gshA, gshB, and trxB.
6. The method of claim 5 wherein the host cells further have an altered gene function of ahpC, wherein the ahpC gene is ahpC Δ .
7. The method of claim 1 wherein the host cells further have a reduced level of gene function of at least one gene encoding a protein that metabolizes an inducer of at least one said inducible promoter.
8. The method of claim 7 wherein the at least one gene encoding a protein that metabolizes an inducer of at least one said inducible promoter is selected from the group consisting of araA, araB, araD, prpB, prpD, rhaA, rhaB, rhaD, xylA, and xylB.
9. The method claim 1 wherein the host cells have an alteration of gene function of at least one gene encoding a transporter protein for an inducer of at least one said inducible promoter.
10. The method of claim 9 wherein the at least one gene encoding a transporter protein is selected from the group consisting of araE, araF, araG, araH, rhaT, xyIF, xvIG, and xylH.
11. The method of claim 10 wherein araE is expressed from a constitutive promoter.
12. The method of claim 1 wherein the host cells further express a polypeptide selected from the group consisting of: cDsbA, cDshC, a protein disulfide isomerase, Ervip, a chaperone, and a transporter protein.
13. The method of claim 1 wherein the host cells comprise an expression construct comprising the polynucleotide sequence of pSOL (SEQ ID NO:3).
14. The method of claim 1 wherein the host cells comprise at least two expression constructs.
15. The method of claim 1 wherein the insulin polypeptide to be transcribed from the at least one said inducible promoter is a polypeptide that lacks a signal sequence.
16. The method of claim 1 wherein the insulin polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, and SEQ ID NO:12.
17. The method of claim 1 wherein the insulin polypeptide to be transcribed from the at least one said inducible promoter further comprises a tag selected from the group consisting of: polyhistidine, pestivirus N pro , CSFV N pro , CSFV N pro (strain Alfort), BDV N pro , BVDV N pro , SEQ ID NO: 13, SEQ ID NO: 14, and SEQ ID NO: 15.
18. The method of claim 1 wherein the insulin polypeptide to be transcribed from the at least one said inducible promoter forms a number of disulfide bonds selected from the group consisting of: one, two, three and four disulfide bonds.
19. The method of claim 1 wherein the host cells are grown until the host cells reach a density greater than a cell density selected from the group consisting of: 150 (OD 600 ); 155 (OD 600 ); 160 (OD 600 ); 165 (OD 600 ); 170 (OD 600 ); and 175 (OD 600 ).
20. The method of claim 1 , wherein the host cells are prokaryotic cells.
21. The method of claim 20 , wherein the host cells are E. coli cells.
22. The method of claim 21 , wherein the host cells are E. coli EB0001 cells.
23. The method of claim 1 further comprising collecting the host cells and storing them prior to lysis.