The present invention provides a method for producing a secretable polypeptide in a host cell, comprising overexpressing a peptidyl prolyl cis-trans isomerase in the host cell, thereby increasing the yield of the secreted polypeptide, and a novel peptidyl prolyl cis-trans isomerase from A. niger useful in such a method.
Claims 1. A method for producing a secretable polypeptide in a host cell, comprising overexpressing a peptidyl prolyl cis-trans isomerase in the host cell, thereby increasing the yield of the secreted polypeptide. 2. A method according to claim 1, comprising cotransfecting the cell with a first coding sequence encoding the polypeptide and a second coding sequence encoding a peptidylprolyl cis-trans isomerase. 3. A method according to claim 2, wherein the polypeptide and the peptidyl prolyl cis-trans isomerase are encoded on separate vectors. 4. A method according to any preceding claim, wherein the coding sequence encoding that peptidyl prolyl cis-trans isomerase is integrated into the genome of the host cell. 5. A method according to any preceding claim, wherein the cell is a fungal cell. 6. A method according to any preceding claim, wherein the cyclophilin peptidyl prolyl cis-trans isomerase is of fungal origin. 7. A method according to any preceding claim, wherein the polypeptide comprises a an ER retention signal. 8. A method according to claim 7, wherein the ER retention signal is H D E L, H E E L or K D E L. 9. A polypeptide possessing foldase activity characterised by having a capability to catalyse the cis-trans isomerisation of a peptide bond on the N terminal side of proline residues in polypeptides, having a signal sequence at the N-terminus and an <Desc/Clms Page number 35> endoplasmic reticulum retention signal at the C-terminus, and molecular weight of 20.7 kDa and a deduced isoelectric point of 6. 27. 10. A polypeptide according to claim 9, which is of fungal origin. 11. A polypeptide according to claim 10, which is derived from the genus Aspergillus. 12. A polypeptide according to claim 11, which is derived from Aspergillus niger. 13. A polypeptide possessing foldase activity characterised by having a capability to catalyse the cis-trans isomerisation of a peptide bond on the N terminal side of proline residues in polypeptides, encoded by a nucleic acid capable of hybridising under low stringency conditions with a 20 base oligonucleotide derived from SEQ ID No. 2. 14. A polypeptide possessing foldase activity characterised by having a capability to catalyse the cis-trans isomerisation of a peptide bond on the N terminal side of proline residues in polypeptides, encoded by a nucleic acid capable of hybridising under low stringency conditions with a 17 base oligonucleotide derived from SEQ ID No. 1. 15. A polypeptide possessing foldase activity characterised by having a capability to catalyse the cis-trans isomerisation of a peptide bond on the N terminal side of proline residues in polypeptides, which is at least 40% to SEQ. ID. 2. 16. A method according to any one of claims 1 to 8, wherein the cyclophilin peptidyl prolyl cis-trans isomerase is a polypeptide according to any one of claims 9 to 15. 17. 17. A nucleic acid vector encoding a polypeptide according to any one of claims 9 to 15. <Desc/Clms Page number 36> 18. A host cell transformed with a vector according to claim 17. 19. A host cell according to claim 18, which is a fungal host cell. 20. A host cell according to claim 19, which is an Aspergillus cell. 21. A process for producing a polypeptide foldase activity characterised by having a capability to catalyse the cis-trans isomerisation of a peptide bond on the N terminal side of proline residues in polypeptides, comprising transforming a host cell with a vector according to claim 17.