| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 31.12.02 |
| PATENT TITLE |
N-terminally extended proteins expressed in yeast |
| PATENT ABSTRACT | The present invention relates to polypeptides expressed and processed in yeast, a DNA construct comprising a DNA sequence encoding such polypeptides, vectors carrying such DNA fragments and yeast cells transformed with the vectors, as well as a process of producing heterologous proteins in yeast |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | June 2, 1999 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED |
Roberds et al. Primary Structure and Muscle-specific Expression of the 50-kDa Dystrophin-associated Glycoprotein (Adhalin) Journal of Biological Chemistry 268 (32): 23739-23742, Nov. 1993.* Julius et al. (1984) Cell 37:1075-1089. Pfeffer et al. (1987) Ann. Rev. Biochem. 56:829-52. Kurjan et al. (1982) Cell 30:933-943. Egel-Mitani et al. (1990) 6:127-137. Ohara et a. (1989) J. Biol. Chem. 264(34):20625-20631 |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
We claim: 1. A DNA construct encoding a precursor polypeptide having the following structure; signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg, X.sup.1 and X.sup.2 together defining a yeast processing site; X.sup.3 is Glu or Asp; X.sup.4 is Glu or Asp; X.sup.5 is a peptide bond or is 1-9 amino acids which may be the same or different; X.sup.6 is Pro; and X.sup.7 is Lys or Arg, and wherein expression of said DNA construct in yeast results in secretion of a polypeptide having the structure X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein. 2. A DNA construct encoding a precursor polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg, X.sup.1 and X.sup.2 together defining a yeast processing site; X.sup.3 is Glu or Asp; X.sup.4 is Glu or Asp; X.sup.5 is a peptide bond or 1-9 amino acid residues selected from the group of Glu, Ala, Pro, Lys, Arg, Leu, Ile, Gly, and Thr; X.sup.6 is Pro; and X.sup.7 is Lys or Arg, and wherein expression of said DNA construct in yeast results in secretion of a polypeptide having the structure X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein. 3. A DNA construct encoding a precursor polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Ala Glu Xaa Xaa Xaa Arg Ala Pro Arg (SEQ ID NO:94). 4. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Ala Glu Ala Glu Pro Arg (SEQ ID NO:44). 5. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Ala Glu Ala Glu Pro Lys (SEQ ID NO:45). 6. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Ala Glu Ala Pro Lys (SEQ ID NO:72). 7. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Ala Pro Lys (SEQ ID NO:51). 8. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Pro Lys (SEQ ID NO:52). 9. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Glu Glu Pro Lys (SEQ ID NO:54). 10. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Glu Pro Lys (SEQ ID NO:55). 11. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Pro Lys (SEQ ID NO:57). 12. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Ala Glu Pro Lys (SEQ ID NO:1). 13. DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg; X.sup.1 and X.sup.2 together define a yeast processing site; and Y is Glu Glu Gly Glu Pro Lys (SEQ ID NO:2). 14. A process for producing a heterologous protein, said process comprising; (i) cultivating a yeast cell comprising a DNA construct, wherein said DNA construct encodes a precursor polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.1 is Lys or Arg, X.sup.2 is Lys or Arg, X.sup.1 and x.sup.2 together defining a yeast processing site; X.sup.3 is Glu or Asp; X.sup.4 is Glu or Asp: X.sup.5 is a peptide bond or is 1-9 amino acids which may be the same or different; X.sup.6 is Pro; and X.sup.7 is Lys or Arg. and wherein said cultivating is in a suitable medium to obtain expression and secretion of the X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 N-terminally extended heterologous protein; (ii) isolating the expressed protein from the culture medium, and (iii) removing the X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 sequence from the protein by proteolytic cleavage in vitro with a proteolytic enzyme specific for a basic amino acid residue. 15. The process of claim 14, wherein the proteolytic enzyme is selected from the group consisting of trypsin, Achromobacter lyticus protease I, Enterokinase, Fusarium oxysporum trypsin-like protease, and YAP3. 16. The DNA construct of claim 1, wherein the leader peptide is selected from the group consisting of SEQ ID NOs:31-41. 17. The DNA construct of claim 12, wherein the leader peptide is selected from the group consisting of SEQ ID NOs:31-41. 18. The DNA construct of claim 13, wherein the leader peptide is selected from the group consisting of SEQ ID NOs:31-41. 19. A DNA construct encoding a polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -Y-heterologous protein wherein the leader peptide is SEQ ID NO:31, X.sup.1 is Lys or Arg, X.sup.2 is Lys or Arg, and X.sup.1 and X.sup.2 together define a yeast processing site, and Y is SEQ ID NO:1. 20. The process of claim 14, wherein X.sup.3 and X.sup.4 are each Glu. 21. The process of claim 14, wherein the signal peptide is .alpha.-factor signal peptide, yeast aspartic protease 3 signal peptide, mouse salivary amylase signal peptide, carboxypeptidase signal peptide, or yeast BAR1 signal peptide. 22. The process of claim 21, wherein the signal peptide is .alpha.-factor signal peptide. 23. The process of claim 14, wherein tie leader peptide is a natural leader or a synthetic leader peptide. 24. The process of claim 23, wherein the leader peptide is a synthetic leader selected from the group consisting of SEQ ID Nos: 31-41. 25. The process of claim 14, wherein the heterologous protein is selected from the group consisting of aprotinin, tissue factor pathway inhibitor or other protease inhibitors, insulin-like growth factor I or II, human or bovine growth hormone, interleukin, tissue plasminogen activator, glucagon, gliucagon-like peptide-1, Factor VII, Factor VIII, Factor XIII, platelet-derived growth factor, enzymes, insulin or an insulin precursor, and a functional analogue of any of the foregoing. 26. The process of claim 25, wherein the heterologous protein is insulin or an insulin precursor or a functional analogue thereof. 27. A process for producing a heterologous protein, comprising: (i) cultivating a yeast cell comprising a DNA construct, wherein said DNA construct encodes a precursor polypeptide having the following structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg, X.sup.1 and X.sup.2 together defining a yeast processing site; X.sup.3 is Glu or Asp; X.sup.4 is Glu or Asp; X.sup.5 is a peptide bond or 1-9 amino acid residues selected from the group of Glu, Ala, Pro, Lys, Arg, Leu, Ile, Gly, and Thr; X.sup.6 is Pro; and X.sup.7 is lys or Arg, and wherein said cultivating is in a suitable medium to obtain expression and secretion of the X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 N-terminally extended heterologous protein, (ii) isolating th e expressed protein from the culture medium, and (iii) removing the X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 sequence from the protein by proteolytic cleavage in vitro with a proteolytic enzyme specific for a basic amino acid residue. 28. The process of claim 27, wherein X.sup.3 and X.sup.4 are each Glu. 29. The process of claim 27, wherein the signal peptide is .alpha.-factor signal peptide, yeast aspartic protease 3 signal peptide, mouse salivary amylase signal peptide, carboxypeptidase signal peptide, or yeast BAR1 signal peptide. 30. The process of claim 29, wherein the signal peptide is .alpha.-factor signal peptide. 31. The process of claim 27, wherein the leader peptide is a natural leader or a synthetic leader peptide. 32. The process of claim 31, wherein the leader peptide is a synthetic leader selected from the group consisting of SEQ ID Nos: 31-41. 33. The process of claim 27, wherein the heterologous protein is selected from the group consisting of aprotinin, tissue factor pathway inhibitor or other protease inhibitors, insulin-like growth factor I or II, human or bovine growth hormone, interleukin, tissue plasminogen activator, glucagon, glucagon-like peptide-1, Factor VII, Factor VIII, Factor XIII, platelet-derived growth factor, enzymes, insulin or an insulin precursor, and a functional analogue of any of the foregoing. 34. The process of claim 33, wherein the heterologous protein is insulin or an insulin precursor or a functional analogue thereof. 35. The process of claim 27, wherein the proteolytic enzyme is selected from the group consisting of trypsin, Achromobacter lyticus protease I, Enterokinase, Fusarium oxysporum trypsin-like protease, and YAP3. |
| PATENT DESCRIPTION |
FIELD OF INVENTION The present invention relates to polypeptides produced in yeast, a DNA construct comprising a DNA sequence encoding such polypeptides, vectors carrying such DNA fragments and yeast cells transformed with the vectors, as well as a process of producing heterologous proteins in yeast. BACKGROUND OF THE INVENTION Yeast organisms produce a number of proteins synthesized intracellularly, but having a function outside the cell. Such extracelluar proteins are referred to as secreted proteins. These secreted proteins are expressed initially inside the cell in a precursor or a pre-form containing a pre-peptide sequence ensuring effective direction of the expressed product across the membrane of the endoplasmic reticulum (ER). The pre-peptide, normally named a signal peptide, is generally cleaved off from the desired product during translocation. Once entered in the secretory pathway, the protein is transported to the Golgi apparatus. From the Golgi the protein can follow different routes that lead to compartments such as the cell vacuole or the cell membrane, or it can be routed out of the cell to be secreted to the external medium (Pfeffer et al. (1987) Ann. Rev. Biochem. 56:829-852). Several approaches have been suggested for the expression and secretion in yeast of proteins heterologous to yeast. European publication 088632A describes a process by which proteins heterologous to yeast are expressed, processed and secreted by transforming a yeast organism with an expression vector harbouring DNA encoding the desired protein and a signal peptide, preparing a culture of the transformed organism, growing the culture and recovering the protein from the culture medium. The signal peptide may be the desired protein's heterologous signal peptide, or a hybrid of a homologous and a heterologous signal peptide. A problem encountered with the use of signal peptides heterologous to yeast may be that the heterologous signal peptide does not ensure efficient translocation and/or cleavage after the signal peptide. The Saccharomyces cerevisiae MF.alpha.1 (.alpha.-factor) is synthesized as a pre-pro form of 165 amino acids comprising a 19 amino acids long signal- or pre-peptide followed by a 64 amino acids long "leader" or pro-peptide, (Kurjan et al. (1982) Cell 30:933-943). Use of signal/leader peptides homologous to yeast is described in U.S. Pat. No. 4,546,082; EP publications 0116201A, 0123294A, 0123544A, 0163529A, 0123289A, EP No. 0100561B, and PCT Publication WO 95/02059. In EP 0123289A utilization of the S. cerevisiae .alpha.-factor precursor is described whereas EP 0100561 describes the utilization of the S. cerevisiae PHO5 signal and WO 95/02059 describes the utilization of YAP3 signal peptide for secretion of foreign proteins. U.S. Pat. No. 4,546,082 and European Publication Nos. 0016201A, 0123294A, 0123544A and 0163529A describe processes by which the .alpha.-factor signal-leader from S. cerevisiae (MF.alpha.1 or MF.alpha.2) is utilized in the secretion process of expressed heterologous proteins in yeast. Secretion and processing of the desired protein was demonstrated by fusing a DNA sequence encoding the S. cerevisiae MF.alpha.1 signal/leader peptide at the 5' end of the gene for the desired protein. EP 0206783 discloses a system for the secretion of polypeptides from S. cerevisiae whereby the .alpha.-factor signal/leader sequence has been truncated to eliminate the four .alpha.-factor peptides present on the native sequence so as to leave the signal/leader peptide itself fused to a heterologous polypeptide via the .alpha.-factor processing site Lys-Arg-Glu-Ala-Glu-Ala (SEQ ID NO:93). It is indicated that this construction leads to an efficient process for production of smaller peptides (less than 50 amino acids). For the secretion and processing of larger polypeptides, the native .alpha.-factor leader sequence has been truncated to leave one or two .alpha.-factor peptides between the leader peptide and the polypeptide. A number of secreted proteins are routed so that the precursor is exposed to a proteolytic processing system which can cleave the peptide bond at the carboxy end of two consecutive basic amino acids. This enzymatic activity is in S. cerevisiae encoded by the KEX 2 gene (Julius et al. (1984) Cell 37:1075). Processing of the product by the KEX 2 protease is needed for the secretion of active S. cerevisiae mating factor .alpha.1 (MF.alpha.1 or .alpha.-factor) but is not involved in the secretion of active S. cerevisiae mating factor a. Secretion and correct processing of a polypeptide intended to be secreted is obtained in some cases when culturing a yeast organism which is transformed with a vector constructed as indicated in the references given above. In many cases, however, the level of secretion is very low or there is no secretion, or the proteolytic processing may be incorrect or incomplete. As described in WO 90/10075, this is believed to be ascribable, to some extent, to an insufficient exposure of the processing site present between the C-terminal end of the leader peptide and the N-terminal end of the heterologous protein so as to render it inaccessible, or less accessible, to proteolytic cleavage, for example, by the KEX 2 protease. WO 90/10075 describes a yeast expression system with improved processing of a heterologous polypeptide obtained by providing certain modifications near the processing site at the C-terminal end of the leader peptide and/or the N-terminal end of a heterologous polypeptide fused to the leader peptide. SUMMARY OF THE INVENTION The present invention describes modifications of the N-terminal end of the heterologous polypeptide designed as extensions which can be cleaved off either by naturally occurring yeast proteases before purification from the culture media or by in vitro proteolysis during or subsequently to purification of the product from the culture media. In one aspect, the present invention is drawn to a DNA construct encoding a polypeptide having the structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.1 is Lys or Arg; X.sup.2 is Lys or Arg, X.sup.1 and X.sup.2 together defining a yeast processing site; X.sup.3 is Glu or Asp; X.sup.4 is a sequence of amino acids with the following structure (A-B).sub.n wherein A is Glu or Asp, B is Ala, Val, Leu or Pro, and n is 0 or an integer from 1 to 5, and when n.gtoreq.2 each A and B is the same or different from the other A(s) and B(s); or X.sup.4 is a sequence of amino acids with the following structure (C).sub.m wherein C is Glu or Asp, and m is 0 or an integer from 1 to 5; X.sup.5 is a peptide bond or is one or more amino acids which may be the same or different; X.sup.6 is a peptide bond or an amino acid residue selected from the group consisting of Pro, Asp, Thr, Glu, Ala and Gly; and X.sup.7 is Lys or Arg. A specific embodiment of the present invention is drawn to a DNA construct encoding a polypeptide having the structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 are the sequence Glu Glu Ala Glu Pro Lys (SEQ ID NO: 1). The sequence Glu Glu Ala Glu Pro Lys (SEQ ID NO: 1) forms an extension at the N-terminal of the heterologous polypeptide. This extension not only increases the fermentation yield but is protected against dipeptidyl aminopeptidase (DPAP A) processing, resulting in a homogenous N-terminal of the polypeptide. The extension is constructed in such a way that it is resistant to proteolytic cleavage during fermentation so that the N-terminally extended heterologous protein product can be purified from the culture media for subsequent in vitro maturation, e.g. by trypsin or Achromobacter lyticus protease I. The desired in vitro removal of the N-terminal extension of SEQ ID NO:1 is readily achieved by either trypsin or Achromobacter lyticus protease I, presumably due to flexibility of the N-terminal extension peptide resulting in an improved yield of the matured heterologous protein. Another specific embodiment of the present invention is drawn to a DNA construct encoding a polypeptide having the structure: signal peptide-leader peptide-X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 -heterologous protein wherein X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 are the sequence Glu Glu Gly Glu Pro Lys (SEQ ID NO:2). The sequence Glu Glu Gly Glu Pro Lys (SEQ ID NO:2) forms an extension at the N-terminal of the heterologous polypeptide. Without being bound by any specific theory, it is surprising shown that the location of a glycine (G) in the N-terminal extension compared to the repeated Glu Ala of the .alpha.-factor leader results in improved heterologous protein yield which may reflect an improved translocation and/or secretion, since glycine with only a hydrogen atom as a side chain can adopt a much wider range of conformations than other amino acid residues, thus allowing unusual main chain conformations, and a possible more unstable precursor polypeptide and secretion process. The term "signal peptide" is understood to mean a pre-peptide which is present as an N-terminal sequence on the precursor form of an extracellular protein expressed in yeast. The function of the signal peptide is to allow the heterologous protein to be secreted to enter the endoplasmic reticulum. The signal peptide is normally cleaved off in the course of this process. The signal peptide may be heterologous or homologous to the yeast organism producing the protein. A preferred signal peptide in this invention is yeast aspartic protease 3 (YAP3) signal peptide or any functional analogue thereof. YAP 3 has been cloned and characterised by Egel-Mitani et al. (1990) YEAST 6:127-137. The term "leader peptide" means a polypeptide sequence whose function is to allow the heterologous protein to be secreted to be directed from the endoplasmic reticulum to the Golgi apparatus and further to a secretory vesicle for secretion into the medium. Preferably the leader peptide used in the present invention is selected from the following group of leader peptides Gln Pro Ile Asp Glu Asp Asn Asp Thr Ser Val Asn Leu Pro Ala (SEQ ID NO:3); Gln Pro Ile Asp Asp Glu Asn Thr Thr Ser Val Asn Leu Pro Ala (SEQ ID NO:4); Gln Pro Ile Asp Asp Glu Ser Asn Thr Thr Ser Val Asn Leu Pro Ala(SEQ ID NO:5); Gln Pro Ile Asp Asp Glu Asn Thr Thr Ser Val Asn Leu Pro Val (SEQ ID NO:6); Gln Pro Ile Asp Asp Thr Glu Asn Thr Thr Ser Val Asn Leu Pro Ala (SEQ ID NO:7); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Pro Ala (SEQ ID NO:8); Gln Pro Ile Asp Asp Glu Asn Thr Thr Ser Val Asn Leu Met Ala (SEQ ID NO:9); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Pro Gly Ala (SEQ ID NO:10); Gln Pro Ile Asp Asp Thr GIu Ser Asn Thr Thr Ser Val Asn Leu Met Ala (SEQ ID NO:11); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Val Pro Thr (SEQ ID NO:12; Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Leu Val Asn Val Pro Thr (SEQ ID NO:13; Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Pro Thr (SEQ ID NO:14); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Leu Val Asn Val Pro Gly Ala (SEQ ID NO:15); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Pro Ala Val Ala (SEQ ID NO:16); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Asp Leu Ala Val Gly Leu Pro Gly Ala (SEQ ID NO:17); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ile Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:18); Gln Pro Ile Asp Asp Thr Glu Ser Ile Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:19); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:20); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Val Asn Leu Pro Leu (SEQ ID NO:21); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ile Asn Thr Thr Leu Val Asn Leu Ala Asn Val Ala Met Ala (SEQ ID NO:22); Gln Pro Ile Asp Asp Thr Glu Ser Ala Ile Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:23); Gln Pro Ile Asp Asp Thr Glu Ser Phe Ala Thr Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:24); Gln Pro Ile Asp Asp Thr Glu Ser Ile Asn Thr Thr Leu Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Val Asn Leu Pro Leu (SEQ ID NO:25); Gln Pro Ile Asp Asp Thr Glu Ser Ile Asn Thr Thr Leu Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Asp Val Val Asn Leu Pro Gly Ala (SEQ ID NO:26); Gln Pro Ile Asp Asp Thr Glu Ser Ala Ala Ile Asn Thr Thr Leu Val Asn Leu Pro Gly Ala (SEQ ID NO:27); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Val Asn Leu Ala Asn Val Ala Met Ala (SEQ ID NO:28); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Asp Val Val Asn Leu Ile Ser Met Ala (SEQ ID NO:29); Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asn Thr Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Asp Val Val Asn Leu Ile Ser Met Ala (SEQ ID NO:30); identified in PCT/DK95/00249 and all C-terminally followed by a Lys-Arg sequence and any functional analogue thereof, and more preferably the leader peptide has an amino acid sequence of 43 or more amino acids, such as the leader peptide (LA19): Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Ala Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:31), identified in PCT/DK95/00249 and which includes the C-terminal Lys-Arg processing site, or any functional analogue thereof. In the DNA construct of the present invention the leader peptide preferably contains an endopeptidase processing site at the C-terminal end, such as a Lys-Arg sequence. Even more preferred leader peptides encoded by the DNA constructs of the invention are: Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Ala Gly Gly Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:32), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ala Phe Ala Thr Asn Thr Thr Leu Ala Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:33), Ser Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Ala Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:34), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Asn Ser Gly Gly Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:35), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Ser Val Gly Gly Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:36), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Arg Phe Ala Thr Asn Thr Thr Leu Ala Gly Gly Leu Asp Val Val Gly Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:37), Gln Pro Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ala Phe Ala Thr Asn Thr Thr Ser Val Gly Gly Leu Asp Val Val Gly Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:38), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ala Phe Ala Thr AsnThr Thr Leu Ala Gly Gly Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:39), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ala Phe Ala Thr Asn Thr Thr Asn Ser Gly Gly Leu Asp Val Val Asn Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:40), Gln Pro Ile Asp Asp Thr Glu Ser Asn Thr Thr Ser Val Asn Leu Met Ala Asp Asp Thr Glu Ser Ala Phe Ala Thr Asn Thr Thr Leu Ala Gly Gly Leu Asp Val Val Gly Leu Ile Ser Met Ala Lys Arg (SEQ ID NO:41). The term "heterologous protein" means a protein or polypeptide which is not produced by the host yeast organism in nature. In a still further aspect, the invention relates to a process for producing a heterologous protein in yeast, comprising cultivating the transformed yeast strain in a suitable medium to obtain expression and secretion of the heterologous protein, after which the protein is isolated from the medium. The invention further relates to a recombinant expression vector which is capable of replicating in a eucaryotic cell, preferably a yeast cell, and which carries a DNA construct of the invention. Preferably, the DNA construct comprises a synthetic leader peptide, preferably the LA19 leader peptide. Besides, the invention relates to the DNA construct described in FIG. 2 herein. The invention also relates to a eucaryotic cell, preferably a yeast cell, which is capable of expressing a heterologous protein and which is transformed with a vector of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a general scheme for the construction of plasmids containing genes expressing N-terminally extended polypeptides. 1 denotes the TPI gene promoter sequence from S. cerevisiae; 2 denotes the region encoding a signal/leader peptide (e.g. from the .alpha.-factor gene of S. cerevisiae); 3 denotes the region encoding a heterologous polypeptide; 3* denotes the region encoding a N-terminal extended heterologous polypeptide; 4 denotes the TPI gene terminator sequence of S. cerevisiae; P1 denotes a synthetic oligonucleotide PCR primer determining the structure of the N-terminal extension; P2 denotes a universal PCR primer for the amplification of region 3. POT denotes TPI gene from S. pombe; 2.mu. Ori denotes a sequence from S. cerevisiae 2.mu. plasmid including its origin of DNA replication in S. cerevisiae; Ap.sup.R is the sequence from pBR322 /pUC13 including the ampicilli resistance gene and an origin of DNA replication in E. coli. FIGS. 2-3 shows the DNA sequence in pJB59 encoding the insulin precursor B.sub.chain (1-27)-Asp Lys Ala Ala Lys-A.sub.chain (1-21) N-terminally fused to the 85 residues which make up the .alpha.-factor signal/leader peptide in which Leu in position 82 and Asp in position 83 have been substituted by Met and Ala, respectively (SEQ ID NOS.95 and 96). FIG. 4 shows the DNA sequence of pAK623 encoding GLP-1.sub.7-36Ala N-terminally fused to the synthetic signal/leader sequence "YAP3/S1.sub.PAVA " (SEQ ID NOS.97 and 98). FIGS. 5-6 shows the DNA sequence of pKV142 encoding B.sub.chain (1-29)-Ala-Ala-Arg-A.sub.chain (1-21) N-terminally fused to the 85 residues which make up the .alpha.-factor signal/leader peptide in which Leu in position 82 and Asp in position 83 have been substituted by Met and Ala, respectively (SEQ ID NOS.95 and 96). FIGS. 7-8 shows the DNA sequence of pAK679 encoding B.sub.chain (1-29)-Ala-Ala-Lys-A.sub.chain (1-21) N-terminally fused to the synthetic signal/leader sequence "YAP3/LA19" (SEQ ID NOS.99 and 100). FIGS. 9-11 shows HPLC chromatograms of culture supernatants containing the insulin precursor B.sub.chain (1-27)Asp Lys Ala Ala Lys-A.sub.chain (1-21) with or without N-terminal extensions; and with or without in vivo or in vitro processing of the N-terminal extensions. FIG. 12 shows the effect of the presence of YAP3 coexpression on the yield derived from the HPLC data in pJB176 compared to pJB64. FIG. 13 shows the expression plasmid pAK729 containing genes expressing the N-terminally extended polypeptides of the invention. TPI-PROMOTER: Denotes the TPI gene promoter sequence from S. cerevisiae; 2: Denotes the region encoding a signal/leader peptide; TPI-TERMINATOR: Denotes TPI gene terminator sequence of S. cerevisiae; TPI-POMBE: Denotes TPI gene from S. pombe; Origin: Denotes a sequence from S. cerevisiae 2.mu. plasmid including its origin of DNA replication in S. cerevisiae; AMP-R: Sequence from pBR322 /pUC13 including the ampicillin resistance gene and an origin of DNA replication in E. coli. FIG. 14 is the DNA and amino acid sequences in pAK729 encoding the YAP3 signal peptide (amino acid No. 1 through 21), LA19 leader peptide (amino acid No. 22 through 64), N-terminal extension Glu Glu Ala Glu Pro Lys (amino acid No. 65 through 70), MI3 insulin precursor B.sub.chain (1-29)-Ala-Ala-Lys-A.sub.chain (1-21) (amino acid No. 71 through 123) (SEQ ID NOS. 101 and 102). FIG. 15 shows the expression plasmid pAK773 containing genes expressing the N-terminally extended polypeptides of the invention. The symbols used are as described in the legend of FIG. 13, with 2: Denotes the region encoding a signal/leader peptide (e.g. from the YAP3 signal peptide and LA19 leader peptide in conjunction with the Glu Glu Gly Glu Pro Lys (SEQ ID NO:2) N-terminally extended MI3 insulin precursor. FIG. 16 shows the DNA and amino acid sequences in pAK773 encoding the YAP3 signal peptide, LA19 leader peptide, N-terminal extension Glu Glu Gly Glu Pro Lys, MI3 insulin precursor B.sub.chain (1-29)-Ala Ala Lys-A.sub.chain (1-21) (SEQ ID NOS. 103 and 104). FIG. 17 is the DNA and amino acid sequences in pAK749 encoding the YAP3 signal petide-LA19 leader Glu Glu Ala Glu Pro Lys-MI5 insulin precursor complex (SEQ ID NOS.105 and 106). FIG. 18 is the DNA and amino acid sequences in pAK866 encoding the YAP3 signal petide-LA19 leader Glu Glu Ala Glu Pro Lys-X14 insulin precursor complex (SEQ ID NOS.107 and 108). FIG. 19 is the LA19 leader DNA sequence (SEQ ID NO.109). FIG. 20 shows the expression plasmid pAK721 containing genes expressing the N-terminally extended polypeptides of the invention. The symbols are as described in the legend of FIG. 13, with the exception that 2: Denotes the region encoding a signal/leader peptide (e.g. from the YAP3 signal peptide and LA19 leader peptide in conjunction with the Glu Glu Ala Glu Pro Lys N-terminally extended MI3 insulin precursor. FIG. 21 is the DNA sequence encoding the YAP3 signal peptide-LA19 leader Glu Glu Gly Glu Pro Lys-MI3 insulin precursor complex (Example 17 below) (SEQ ID NOS.110 and 111). DETAILED DESCRIPTION OF THE INVENTION In the peptide structure (A-B).sub.n, n is preferably 2-4 and more preferably 3. In preferred polypeptides according to the invention X.sup.3 may be Glu, A may be Glu, B may be Ala, X.sup.5 may be a peptide bond or Glu, or Glu Pro Lys Ala, or X.sup.6 may be Pro or a peptide bond. Examples of possible N-terminal extensions X.sup.3 -X.sup.4 -X.sup.5 -X.sup.6 -X.sup.7 are: Glu Glu Ala Glu Ala Glu (Pro/Ala) (Glu/Lys) (Ala/Glu/Lys/Thr) Arg Ala Pro Arg (SEQ ID NO:42), Glu Glu Ala Glu Ala Glu Pro Lys Ala (Thr/Pro) Arg (SEQ ID NO:43), Glu Glu Ala Glu Ala Glu Ala Glu Pro Arg (SEQ ID NO:44), Glu Glu Ala Glu Ala Glu Ala Glu Pro Lys (SEQ ID NO:45), Glu Glu Ala Glu Ala Glu Ala Glu Arg (SEQ ID NO:46), Glu Glu Ala Glu Ala Glu Ala (Asp/Ala/Gly/Glu) Lys (SEQ ID NO:47), Glu Glu Ala Glu Ala Glu Ala (Pro/Leu/Ile/Thr) Lys (SEQ ID NO:48), Glu Glu Ala Glu Ala Glu Ala Arg (SEQ ID NO:49), Glu Glu Ala Glu Ala Glu (Glu/Asp/Gly/Ala) Lys (SEQ ID NO:50), Glu Glu Ala Glu Ala Pro Lys (SEQ ID NO:51), Glu Glu Ala Pro Lys (SEQ ID NO:52), Asp Asp Ala Asp Ala Asp Ala Asp Pro Arg (SEQ ID NO:53), Glu Glu Glu GIu Pro Lys (SEQ ID NO:54), Glu Glu Glu Pro Lys (SEQ ID NO:55), Asp Asp Asp Asp Asp Lys (SEQ ID NO:56), and Glu Glu Pro Lys (SEQ ID NO:57). The N-terminally extended heterologous protein produced by the method of the invention may be any protein which may advantageously be produced in yeast. Examples of such proteins are aprotinin, tissue factor pathway inhibitor or other protease inhibitors, and insulin or insulin precursors, insulin analogues, insulin-like growth factors, such as IGF I and IGF II, human or bovine growth hormone, interleukin, tissue plasminogen activator, glucagon, glucagon-like peptide-1 (GLP 1), glucagon-like peptide-2 (GLP 2), GRPP, Factor VII, Factor VIII, Factor XIII, platelet-derived growth factor, enzymes, such as lipases, or a functional analogue of any one of these proteins. Preferred proteins are precursors of insulin and insulin like growth factors, and peptides of the proglucagon family, such as glucagon, GLP 1, GLP 2, and GRPP, including truncated forms, such as GLP-1(1-45), GLP-1(1-39), GLP-1(1-38), GLP-1(1-37), GLP-1(1-36), GLP-1(1-35), GLP-1(1-34), GLP-1(7-45), GLP-1(7-39), GLP-1(7-38), GLP-1(7-37), GLP-1(7-36), GLP-1(7-35), and GLP-1(7-34). In the present context, the term "functional analogue" is meant to indicate a polypeptide with a similar biological function as the native protein. The polypeptide may be structurally similar to the native protein and may be derived from the native protein by addition of one or more amino acids to either or both the C- and N-terminal end of the native protein, substitution of one or more amino acids at one or a number of different sites in the native amino acid sequence, deletion of one or more amino acids at either or both ends of the native protein or at one or several sites in the amino acid sequence, or insertion of one or more amino acids at one or more sites in the native amino acid sequence. Such modifications are well known for several of the proteins mentioned above. The precursors of insulin, including proinsulin as well as precursors having a truncated and/or modified C-peptide or completely lacking a C-peptide, precursors of insulin analogues, and insulin related peptides, such as insulin like growth factors, may be of human origin or from other animals and recombinant or semisynthetic sources. The cDNA used for expression of the precursors of insulin, precursors of insulin analogues, or insulin related peptides in the method of the invention include codon optimised forms for expression in yeast. By "a precursor of insulin" or "a precursor an insulin analogue" is meant a single-chain polypeptide which by one or more subsequent chemical and/or enzymatical processes can be converted to a two-chain insulin or insulin analogue molecule having the correct establishment of the three disulphide bridges as found in natural human insulin. Preferred insulin precursors are MI1, B(1-29)-A(1-21); MI3, B(1-29)-Ala-Ala-Lys-A(1-21) (as described in e.g. EP 163 529); X14, B(1-27-Asp-Lys)-Ala Ala Lys-A(1-21) (as described in e.g. PCT publication No. 95/00550); B(1-27-Asp-Lys)-A(1-21); B(1-27-Asp-Lys)-Ser Asp Asp Ala Lys-A(1-21); B(1-29)-Ala Ala Arg-A(1-21) (described in PCT Publication No. 95/07931); MI5, B(1-29)-Ser Asp Asp Ala Lys-A(1-21); and B(1-29)-Ser-Asp Asp Ala Arg-A(1-21), and more preferably MI1, B(1-29)-A(1-21), MI3, B(1-29)-Ala Ala Lys-A(1-21) and MI5, B(1-29)-Ser Asp Asp Ala Lys-A(1-21). Examples of insulins or insulin analogues which can be produced in this way are human insulin, preferably des(B30) human insulin, porcine insulin; and insulin analogues wherein at least one Lys or Arg is present, preferably insulin analogues wherein Phe.sup.B1 has been deleted, insulin analogues wherein the A-chain and/or the B-chain have an N-terminal extension and insulin analogues wherein the A-chain and/or the B-chain have a C-terminal extension. Other preferred insulin analogues are such wherein one or more of the amino acid residues, preferably one, two, or three of them, have been substituted by another codable amino acid residue. Thus in position A21 a parent insulin may instead of Asn have an amino acid residue selected from the group comprising Ala, Gln, Glu, Gly, His, Ile, Leu, Met, Ser, Thr, Trp, Tyr or Val, in particular an amino acid residue selected from the group comprising Gly, Ala, Ser, and Thr. The insulin analogues may also be modified by a combination of the changes outlined above. Likewise, in position B28 a parent insulin may instead of Pro have an amino acid residue selected from the group comprising Asp and Lys, preferably Asp, and in position B29 a parent insulin may instead of Lys have the amino acid Pro. The expression "a codable amino acid residue" as used herein designates an amino acid residue which can be coded for by the genetic code, i. e. a triplet ("codon") of nucleotides. The DNA construct of the invention encoding the polypeptide of the invention may be prepared synthetically by established standard methods, e.g. the phosphoamidite method described by Beaucage et al. (1981) Tetrahedron Letters 22:1859-1869, or the method described by Matthes et al. (1984) EMBO Journal 3:801-805. According to the phosphoamidite method, oligonucleotides are synthesized, e.g. in an automatic DNA synthesizer, purified, duplexed and ligated to form the synthetic DNA construct. A currently preferred way of preparing the DNA construct is by polymerase chain reaction (PCR), e.g. as described in Sambrook et al. supra. The DNA construct of the invention may also be of genomic or cDNA origin, for instance obtained by preparing a genomic or cDNA library and screening for DNA sequences coding for all or part of the polypeptide or the invention by hybridization using synthetic oligonucleotide probes in accordance with standard techniques. In this case, a genomic or cDNA sequence encoding a signal and leader peptide may be joined to a genomic or cDNA sequence encoding the heterologous protein, after which the DNA sequence may be modified at a site corresponding to the amino acid extension sequence of the polypeptide, by inserting synthetic oligonucleotides encoding the desired amino acid sequence for homologous recombination in accordance with well-known procedures or preferably generating the desired sequence by PCR using suitable oligonucleotides. Finally, the DNA construct may be of mixed synthetic and genomic, mixed synthetic and cDNA or mixed genomic and cDNA origin prepared by annealing fragments of synthetic, genomic or cDNA origin (as appropriate), the fragments corresponding to various parts of the entire DNA construct, in accordance with standard techniques. Thus, it may be envisaged that the DNA sequence encoding the heterologous protein may be of genomic or cDNA origin, while the sequence encoding the signal and leader peptide as well as the sequence encoding the N-terminal extension may be prepared synthetically. In a further aspect, the invention relates to a recombinant expression vector which is capable of replicating in yeast and which carries a DNA construct encoding the above-defined polypeptide. The recombinant expression vector may be any vector which is capable of replicating in yeast organisms. In the vector, the DNA sequence encoding the polypeptide of the invention should be operably connected to a suitable promoter sequence. The promoter may be any DNA sequence which shows transscriptional activity in yeast and may be derived from genes encoding proteins either homologous or heterologous to yeast. The promoter is preferably derived from a gene encoding a protein homologous to yeast. Examples of suitable promoters are the Saccharomyces cerevisiae M.alpha.1, TPI, ADH or PGK promoters. The DNA sequence encoding the polypeptide of the invention may also be operably connected to a suitable terminator, for example the TPI terminator (Alber et al. (1982) J. Mol. Appl. Genet. 1:419-434). The recombinant expression vector of the invention comprises a DNA sequence enabling the vector to replicate in yeast. Examples of such sequences are the yeast plasmid 2.mu. replication genes REP 1-3 and origin of replication. The vector may also comprise a selectable marker, such as the Schizosaccharomyces pompe TPI gene (Russell (1985) Gene 40:125-130). The procedures used to ligate the DNA sequences coding for the polypeptide of the invention, the promoter and the terminator, respectively, and to insert them into suitable yeast vectors containing the information necessary for yeast replication, are well known to persons skilled in the art (see for example, Sambrook et al. supra). It will be understood that the vector may be constructed either by first preparing a DNA construct containing the entire DNA sequence coding for the polypeptide of the invention and subsequently inserting this fragment into a suitable expression vector, or by sequentially inserting DNA fragments containing genetic information for the individual elements (such as the signal, leader or heterologous protein) followed by ligation. The yeast organism used in the process of the invention may be any suitable yeast organism which, on cultivation, produces large amounts of the heterologous protein or polypeptide in question. Examples of suitable yeast organisms may be strains selected from the yeast species Saccharomyces cerevisiae, Saccharomyces kluyveri, Schizosaccharomyces pombe, Sacchoromyces uvarum, Kluyveromyces lactis, Hansenula polymorpha, Pichia pastoris, Pichia methanolica, Pichia kluyveri, Yarrowia lipolytica, Candida sp., Candida utilis, Candida cacaoi, Geotrichum sp., and Geotrichum fermentans. The transformation of the yeast cells may for instance be effected by protoplast formation followed by transformation in a manner known per se. The medium used to cultivate the cells may be any conventional medium suitable for growing yeast organisms. The secreted heterologous protein, a significant proportion of which will be present in the medium in correctly processed form, may be recovered from the medium by conventional procedures including separating the yeast cells from the medium by centrifugation or filtration, precipitating the proteinaceous components of the supernatant or filtrate by means of a salt, e.g. ammonium sulphate, followed by purification by a variety of chromatographic procedures, e.g. ion exchange chromatography, affinity chromatography, or the like. The transformation of the yeast cells may for instance be effected by protoplast formation followed by transformation in a manner known per se. The medium used to cultivate the cells may be any conventional medium suitable for growing yeast organisms. The secreted heterologous protein, a significant proportion of which will be present in the medium in correctly processed form, may be recovered from the medium by conventional precedures including separating the yeast cells from the medium by centrifugation or filtration, precipitating the proteinaceous components of the supernatant or filtrate by means of a salt, e.g. ammonium sulphate, followed by purification by a variety of chromatographic procedures, e.g. ion exchange chromatography, affinity chromatography, or the like. After secretion to the culture medium, the protein may be subjected to various precedures to remove the extension sequence. The extension is found to be stably attached to the heterologous protein during fermentation, protecting the N-terminal of the heterologous protein against the proteolytic activity of yeast proteases such as DPAP. The presence of an N-terminal extension on the heterologous protein may also serve as a protection of the N-terminal amino group of the heterologous protein during chemical processing of the protein, i.e. it may serve as a substitute for a BOC (t-butyl-oxycarbonyl) or similar protecting group. In such cases the amino acid extension sequence may be removed from the recovered heterologous protein by means of a proteolytic enzyme which is specific for a basic amino acid (i.e. K (Lys)) so that the terminal extension is cleaved off at the K. Examples of such proteolytic enzymes are trypsin or Achromobacter lyticus protease I. The present invention is described in further detain in the following examples which are not in any way intended to limit the scope of the invention as claimed |
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