Title of Invention

DNA ENCODING SUPERSECRETABLE PEPTIDES AND PROCESSES FOR THEIR PRODUCTION

Abstract

The invention relates to A DNA-molecule of the form: P<SUB>x</SUB> -S<SUB>x</SUB> -B<SUB>n</SUB> -(ZR)-transport peptide- (ZJZ2)-protein(Y)-(Z<SUB>1</SUB> Z<SUB>2</SUB> )-protein(Y<SUB>m</SUB> )- T; wherein the DNA-molecule codes for a transport peptide linked via.a sequence Z<SUB>1</SUB> Z<SUB>2</SUB> to a second protein which in turn is linked via Z<SUB>1</SUB> Z<SUB>2</SUB> to a protein Y 1 which either corresponds to Y or can be different from Y and the transport peptide improves the rate of secretion of Y and/or Y<SUB>m</SUB> , where P<SUB>x</SUB> is any promoter DNA sequence selected in such a w~y that optimal yields of the protein of interest become obtainable; S<SUB>x</SUB> is any DNA which, accordingly, encodes any signal or leader sequence which allows optimal yields; B<SUB>n</SUB> is 1-15 genetically encodable amino acids or a chemical bond; Z is the codon of an amino acid selected from the group comprising Lys and Arg; Z<SUB>1</SUB> is the codon of an amino acid selected from the group comprising Lys and Arg; Z<SUB>2</SUB> is the codon of an amino acid selected from the group comprising Lys and Arg; protein Y m is a DNA sequence encoding any protein which can be produced and secreted by yeast (m = 1-5) or is a chemical bond (m = 0); R is ~n arginine codon; transport peptide is a DNA sequence encoding Hirudin or a Hirudin derivative; protein Y is a DNA sequence encoding any protein which can be produced and secreted by yeast and whose biological activity, when Y <SUB>m</SUB> is not a chemical bond, is not impaired by a basic dipeptide extension or allows degradation of the extension by carboxypeptidases; T is an untranslated DNA sequence advantageous to the expression.

Full Text

Description Use of supersecretable peptides in processes for their production and for parallel improvement of the exported forms of one or more other polypeptides of interest In view of economic viability, processes for the production of pharmaceutically relevant proteins must lead to biologically active products of the highest possible purity. The expression of such relevant proteins in yeasts is widely used here. The production of proteins such as insulin, GM-CSF (Leukine ®) and hirudin (Refludan ®) is an example of the successful development of genetic engineering processes which are based on the synthesis of the particular protein or precursors thereof in yeast. Generally, yeasts can directly synthesize particularly hirudins with good yields which are on the gram scale when using Hansenula polymorpha (Weydemann et a!., Appl. Microbiol Biotechnol. 44: 377 -385,1995) or Pichia pastoris (Rosenfeld et ah, Protein Expr. Purif :4, 476-82, 1996). EP-A 0 324 712 describes the hirudin derivative (Refludan®) whose N-terminal amino acid is leucine and its constitutive expression in Saccharomyces cerevisiae strain Y79. EP-A 0 347 781 describes a mini-proinsulin and, by way of example, its expression in bakers' yeast. Refludan® and insulin are produced by carrying out two separate expressions. Surprisingly, we have now found that hirudin derivatives and mini-proinsulin derivatives can be obtained from a common precursor protein by fusing the precursor protein to a signal or leader sequence, which is recognized by yeasts as a secretion signal, via a basic dipeptide, preferably Lys - Arg, and likewise introducing between the N-terminal hirudin derivative and the mini-proinsulin derivative a cleavage site which is recognized by a yeast endoprotease. Here too, preference is given to a basic dipeptide, for example Lys - Arg. After expression, a hirudin derivative extended by Lys - Arg and the mini-proinsulin derivative starting with the first amino acid of the insulin B chain are found in the supernatant. Surprisingly, we have found here that the yield of mini-proinsulin markedly improves compared with the yield achievable by direct signal - mini-proinsulin expression, whereas the yield of the hirudin derivative remains nearly the same. Surprisingly, hirudin thus acts as a kind of enhancer peptide with respect to the yield of mini-proinsulin. Peptides which can act as enhancer proteins are usually those which are relatively small and which are secreted naturally in large amounts over a short period, for example from glandular tissue. Peptides of this type, which include, for example, snake venom or eglin C or TAP (tick anticoagulant peptide), are distinguished by extremely good export compatibility. The invention relates to such proteins, too. Another advantage may result from the hirudin derivative having equal or better pharmaceutical properties compared with hirudin which is already used in pharmaceuticals. In this case, it becomes possible to produce two or even more pharmaceuticals from one and the same fermentation. As a consequence, less fermentation capacity is required. This is directly beneficial to production costs. However, the production of a plurality of products is optional. The amount needed of Refludan, for example, is less than that of insulin, and this may result in processes in which one of the pharmaceutically interesting substances is discarded. To improve the yield, it is possible, as suggested in patent application EP-A 0 200 655, to place a short peptide sequence via Lys- Arg N-terminally in front of the hirudin derivative, as a linker to the signal or leader sequence. It is also obvious to the skilled worker that the choice of signal or leader sequence directly affects the yield of ' the protein of interest. The selection of such a sequence is the subject of further optimizations. The sequence located at the 3* end of the expression cassette, too, directly affects the yield by influencing mRNA stability. Here too, it is obvious to the skilled worker that said sequence can be optimized for each protein of interest to be expressed. This is also true for the choice of a suitable promoter, which can be ) inducible or constitutively active. The choice of vector system and host system is equally important for the yield. Thus, instead of bakers' yeast which has been used by way of example, it is also possible to use the yeasts Pichia pastoris, Hansenula polymorpha or K. lactis together with vectors or expression cassettes, which have in each case been optimized for the different physiology. Another advantage of processes allowing secretion into the medium is the simpler protein-chemical workup of the protein of interest. Surprisingly, we have found that mini-proinsulin can be concentrated in the presence of hinjdin by filtration through membranes having an exclusion limit for molecules with a molecular weight of greater than 10 kDa. The molecules are found almost exclusively in the retentate. It is obvious to the skilled worker that the development of novel separation techniques and new combinations of process steps always make it possible to improve purification processes. This is directly beneficial to the yield and therefore to production costs. The invention thus relates to a DNA-molecule (alternative term: expression cassette) of the form: Px-Sx-Bn-(ZR)-transportpeptide-{ZiZ2)-protein(YHZiZ2)-protein(Yrn)-T; wherein the expression cassette codes for a transport peptide linked via a sequence Z1Z2to a second protein which in turn is linked via Z1Z2to a protein Y1 which either corresponds to Y or can be different from Y and the transport peptide improves the rate of secretion of Y and/or Ym, where: Px is any promoter DNA sequence selected in such a way that optimal yields of the protein of interest become obtainable; Sx is any DNA which, accordingly, encodes any signal or leader sequence which allows optimal yields; Bn is 1-15 genetically encoded amino acids or a chemical bond; Z is the codon of an amino acid selected from the group comprising Lys and Arg; Zi is the codon of an amino acid selected from the group comprising Lys and Arg; Z2 is the codon of an amino acid selected from the group comprising Lys and Arg; R is an Arg codon; transport peptide is a DNA sequence encoding a peptide which can be transported efficiently and which can pass membranes, such as hinjdin or a hinjdin derivative, for example; protein Y is a DNA sequence encoding any protein which can be produced and secreted by yeast; protein Ym is a DNA sequence encoding any protein which can be produced and secreted by yeast (m = 1-5) or is a chemical bond (m = 0); T is an untranslated DNA sequence advantageous to the expression. Another embodiment of the invention is a fusion protein encoded by any of the above-mentioned DNA molecules. A further embodiment of the invention is a multicopy vector and a plasmid comprising the above-mentioned DNA-molecule. An additional embodiment of the invention is a host cell comprising the above-mentioned DNA-molecule, or the above-mentioned multicopy vector or the above-mentioned plasmid. as a part of its chromosome, as a part of a mini-chromosome, or extra-chromosomally, wherein preferrentially said host cell is a yeast, in particular selected from the group comprising of S. cerevisiae, K. lactis. H, polymorpha and P. pastoris. Another embodiment of the invention is a process of fermenting the above-mentioned proteins, in which (a) the above-mentioned DNA-molecule, the above-mentioned multicopy vector, or the above-mentioned plasmid is expressed in an above-mentioned host cell, and (b) the expressed proteins are isolated from the supernatant of the cell culture, wherein in particular after completion of fermentation, the pH is adjusted to 2,5-3,5 in order to precipitate non-desired proteins and the expressed proteins are isolated from the supernatant of the precipitation. Another embodiment of the invention is the above mentioned process, in which process after separating the fermentation supernatant from the host cells, the host cells are repeatedly cultured in fresh medium, and the released fusion protein is isolated from each supernatant obtained during cultivation. Another embodiment of the invention is the above mentioned process, wherein a process step for concentrating the expressed protein in the supernatant after precipitation is selected from a group comprising microfiltration, hydrophobic interaction chromatography and ion exchange chromatography. An additional embodiment of the invention is a process for preparing insulin, in which (a) proinsulin is expressed as protein (Y) of the above-mentioned expression cassette in the above-mentioned process; (b) the proinsulin of step (a) is isolated and treated with trypsin and carboxypeptidase B; and (c) insulin is isolated from the reaction mixture of step (b). in particular, wherein the transport peptide is hirudin or a hirudin derivative which is destroyed or biologically inactivated after step (a) or (b). A further embodiment of the invention is a protein wherein the protein is a hirudin-derivative with two basic amino acid residues at its C-terminal end. Leeches of the type Hirudo have developed, for example, various isoforms of the thrombin inhibitor hirudin. Himdin has been optimized for pharmaceutical requirements by artificial variation of the molecule, for example exchange of the N-terminal amino acid (e.g. EP 0 324 712). The invention includes the use of hirudin and hirudin variants. Particular embodiments of the invention use one of the natural hirudin isoforms (the natural isoforms are together denoted "hirudin"). A natural isoform is, for example, Val-Val-hirudin or lle-Thr-hirudin. Other embodiments of the invention use a variant of a natural hirudin isoform. A variant is derived from a natural hirudin isoform but contains, for example, additional amino acids and/or amino acid deletions and/or amino acid exchanges compared with the natural isoform. A hirudin variant may contain alternating peptide segments of natural hirudin isoforms and new amino acids. Hirudin variants are known and are described, for example, in DE 3 430 556. Hirudin ^ variants are commercially available in the form of proteins (Calbiochem Biochemicals, cat. no. 377-853, -950, -960). The term "hirudin derivative" denotes sequences which are at least 40% homologous to natural hinjdin. The expression cassette is preferably introduced into yeasts such as S. cerevisae, K. ) lactis, H. polymorpha or P. pastoris. Said expression cassette may have one or more copies stably integrated into the particular yeast genome or may be present extrachromosomally on a multicopy vector. It is obvious to the skilled worker that this technique is also applicable to other systems such as animal cell culture or plant cells. This is also a subject of the invention. . The expression system described below serves as an example. It is obvious to the skilled worker that, in order to introduce the expression cassette into said selected system, the appropriate recombinant DNA constructs must be made depending on the type of host system selected. Accordingly, industrial fermentation can be optimized in ) relation to the selected host/vector system. Accordingly, the examples are nonrestrictive. Example 1: Constnjction of a yeast expression piasmid encoding hirudin (Refludan) -Lys-Arg - mini-proinsulin Starting materials are the plasmids pK152 (PCT/EPOO/08537), pSW3 (EP-A 0 347 781) and the recombinant yeast piasmid derivative coding for bovine interieukin 2 (Price et al. Gene 55, 1987). The yeast piasmid is distinguished by the fact that it carries the a factor leader sequence under the control of the yeast ADH2 I promoter. This sequence is followed by the bovine interieukin 2 cDNA sequence which is connected via a Kpni restriction enzyme recognition site and which contains, after manipulation, an Ncol restriction enzyme recognition site in the untranslated 3' end which is unique in the vector. Thus, the cDNA sequence can readily be removed from the plasmid via KpnI/Ncol cleavage. Since good expression yields have been reported, it can be assumed that the remaining 3' interleukin 2 sequence (as T) has a stabilizing effect on the mRNA and thus need not be deleted or replaced by a yeast terminator sequence . Plasmid pK152 carries the DNA sequence coding for Leu-hirudin (Refludan) and plasmid pSW3 carries the DNA sequence for mini-proinsulin. The gene sequence which is to encode hirudin - Lys Arg - mini-proinsulin is first prepared by means of PCR technology. For this purpose, 4 primers are prepared with the aid of the Expedite^^ DNA synthesis system: i. hirjnsfkr (SEQ ID NO: 1, encoded protein segment: SEQ ID NO: 2) I PEEYLQKRFVNQHLC 5 " - ATCCCTGAGGAATACCTTCAGAAGCGATTTGTTAACCAACACTTGTGTGG- 3 ' 59 60 61 62 63 64 65 Bl B2 B3 B4 B5 B6 B7 ij. hirjnsrevkr (SEQ ID NO: 3) 5^- CCTCACAAGTG TTGGTTAACA AATCGCTTCT GAAGGTATTC CTCAGGGAT-3^ I iji. hirf1 (SEQ ID NO: 4, encoded protein segment: SEQ ID NO: 5) L T Y T D C 5 ' - TTTTTTTGGATCCTTTGGATAAAAGACTTACGTATACTGACTGCAC ; iv. insncolrev (SEQ ID NO: 6) 5"" TTTTTTCCAT GGGTCGACTATCAG Primer hirjnsfkr describes the junction between codons for the terminal amino acids of ) hirudin (59 - 65) and the insulin sequence B1 - B7 via the Lys - Arg linker. Primer hirjnsrevkr is 100% complementary thereto. Primer hirf1 codes for the start of the hirudin gene extended to the Kpnl cleavage site as described in EP-A 0 324 712. Primer insncoirev marks the 3' end of the synthetic mini-proinsulin according to EP-A 0 347 781. Two standard polymerase chain reactions are carried out using the primer pairs hirfl/hirjnsrevkr with DNA of piasmid pK152 as template and hirjnsfkr/insncoirev with DNA of piasmid pSW3 as template. The reactions are carried out in 100 pi of PCR buffer with, in each case, 200 nmol of primer, 1 jj\ of polymerase and 100 ng of vector. Step 1 is a 2-minute incubation at 95°C. This is then followed by 25 cycles of 30" at 95°C, 30" at SS'^C and 30" at 72'C. The last cycle is followed by an incubation at 72 °C for 3 minutes, and the reaction is subsequently stopped. Since the primers hir_ insrevkr and hirjnsfkr are 100% complementary, the DNA products of the two products overiap according to said sequence so that in a third reaction, using the products of the first two reactions as templates and the primers hirf1 and insncoirev, a DNA fragment is formed, which encodes hirudin and mini-proinsulin separated by Lys - Arg. The PCR fragment is digested by the enzymes Kpnl und Ncol and then, in a T4 ligase reaction, inserted into the paADH2 vector opened by Kpn1 / Ncol. In analogy to example 7 of EP-A 0 347 781. competent E. coli strain MM294 cells are then transfonned with the ligation mixture. Piasmid DNA is then isolated from two clones for characterization by means of DNA sequence analysis. After confirmation of the inserted DNA sequence, DNA of a piasmid preparation is used to transform cells of bakers' yeast strain Y79, according to said example. However, when using the paADH2 vector, introduction of the vector is followed by selecting for complementation of the trp1-1 mutation, in contrast to said example. For another control, piasmid DNA is reisolated from yeast transformants and analyzed by means of restriction analysis. The expression vector constructed is denoted pADH2Hir_KR_lns. Expression is carried out according to example 4. Example 2; Construction of a yeast expression piasmid encoding hirudin (Refludan) -Lys-Arg - insulin B chain - Lys-Arg - insulin A chain Patent application EP-A 0 195 691 describes proinsulin derivatives which can contain the dipeptide XY, where X and Y each correspond to either Lys or Arg, as linker between the B and A chains of insulin. The following example describes the preparation of an expression vector for proinsulin derivatives of this kind. A DNA sequence which codes for a proinsulin derivative of the form B chain - Lys-Arg - A chain is selected by way of example and synthesized accordingly. The synthesis of the gene segment is carried out according to example 1. The oligonucleotide sequences used are hirf1 and insncoirev. The oligonucleotides B_KR_Af1 and B_KR_Arev1 are synthesized de novo. The part shown in bold type of the two primers depicted indicates the partially overlapping sequence. Both primers pair exactly with the sequence of the mini-) proinsulin gene of EP-A 0 347 781, apart from the 6 underlined nucleotides. The underlined part corresponds to codons for Lys and Arg. DNA of the plasmid pADH2Hir_KRJns constructed according to example 1 serves as template in the PCR. As described in example 1, two polymerase chain reactions are carried out using the 5 primer pairs hirf1 / B_KR_Arev and insncoirev / B_KR_Af 1. The template in each case is DNA of the plasmid pADH2Hir_KRJns constructed in example 1. The products of both reactions sen/e as template in a third PCR using the primer pair hirf1 and insncol. The reaction product from PCR 3 is cleaved with Ncol/ Sal! and inserted into the opened paADH2 vector. After sequence and restriction analysis, the correct ) plasmid is referred to as pADHHirKR_B_KR_A. Example 3: Construction of a yeast plasmid coding for hirudin - Lys-Arg - simian proinsulin Patent application EP-A 489 780 describes a plasmid, plNT90d, which contains cDNA of simian proinsulin (Wetekam et al., Gene 19, p,179"183,1982). DNA of said plasmid and DNA of plasmid pK152 serve as templates. The primer hirf1 described in example 1 is used and three further primers are synthesized. Primer insncorev reversely binds to the 3' region of the insulin gene cloned in pINT90d and has the sequence; . . 5'-TTTTTTCCATGGTCATGTTTGACAGCTTATCAT-3^ (SEQ ID NO: 9) The underiined sequence indicates the recognition site for the restriction enzyme Ncol. Primer hirjnsfkr has the sequence: 5"-ATCCCTGAGG AATACCTTCA GAAGCGATTT GTGAACCAGC ACCTGTGCGG C-3^ (SEQ ID NO: 10) Here, the nucleotides in bold type indicate the Lys-Arg linker between hirudin and proinsulin. Primer hirjnsrevkr is completely complementary to primer hirjnskr and has the sequence: 5^ -GCCGCACAGG TGCTGGTTCA CAAATCGCTT CTGAAGGTAT TCCTCAGGGA T-3 ^ (SEQ ID NO: 11) Corresponding to example 1, two polymerase chain reactions are carried out. The primer pair hirf1 / hirjnsrevkr is reacted with DNA of plasmid pKI 52 and the primer pair hirjnsfkr / insncorev is reacted with DNA of plasmid plNT91d. As described in example 1, the products of both reactions serve as template in a third PCR using the primer pair hirf1 / insncorev. The DNA product of this reaction includes the sequence for hirudin_Lys-Arg_proinsulin, It is subsequently cleaved with the enzymes Ncol and Kpnl and, corresponding to example 1, inserted into the plasmid paADH2. Accordingly expression vector for any natural proinsulin derivatives may be constructed. Example 4: Expression of the recombinant products The expression is divided into two phases. Firstly, a preculture is cultivated in yeast minimal medium. The medium has the following composition per 1 I: 6.7 g - yeast nitrogen base (without amino acids) 5.0 g - casamino acids (vitamin-free) 0.008% - adenine 0.008% - uracil 2% - glucose The main or expression culture is inoculated with an aliquot of the preculture. The main culture medium contains per liter: 10 g - yeast extract 20 g - peptone 0.008% - adenine 0.008% " uracil 4% - glucose Using the media described, expression is carried out in a shaken flask in the following way: 0.3 ml of a preculture which has been cultivated overnight is diluted with 80 ml of prewarmed medium and incubated with vigorous shaking at 30°C for approx. 24 h. In I each case, 1 ml of the culture produced in this way is then centrifuged, after determining the optical density, and, after removing the cells, the supernatant is lyophilized and analyzed by means of SDS -PAGE. The biologically active hirudin content is determined by carrying out a thrombin inhibition assay. An alternative fermentation protocol provides for the cells to be removed by filtration or careful centrifugation. While isolating the protein of interest from the medium, the cells i are provided with fresh prewarmed main culture medium containing alcohol and not more than 0.5% glucose as carbon source, and thus fermentation is continued without interruption. This step can be repeated up to 5 times. ) Example 5; Thrombin inhibition test The hirudin concentration is determined according to the method of GrieBbach et a\, (Thrombosis Research 37, pp. 347 -350 . 1985 ). For this purpose, specific amounts of a Refludan standard are included in the measurements in order to establish a ! calibration curve from which the yield in mg/l can be determined directly. Example 6: Cloning and expression of the hirudin - Lys-Arg - mini-proinsulin fusion protein in the P. pastoris system Invitrogen® sells a cloning and expression kit for preparing recombinant proteins using P. pastoris as host system. For this, a detailed technical protocol regarding preparation and subsequent expression of a P. pastoris system for the production of a desired recombinant protein is provided so that only the construction of the expression vector encoding the desired protein has to be described when following said protocols. The EasySelect™ Pichia expression kit (catalog no. K1740-01) is used. The pPICZaA vector is part of the kit. Opening the vector by the restriction enzymes Xhol and Sacll makes it possible to append, similar to example 1, a protein of interest to the alpha factor leader sequence and to test for secretion into the supernatant. Cloning requires two primers. Primer pichia_HJf1 (SEQ ID NO: 12) has the sequence: The template used is DNA of plasmid pADH2Hir_KRJns. A standard PCR with both primers produces a DNA product which contains-the sequence -hirudin - Lys-Arg -mini-proinsulin extended by the Xhol and Sacll integration sites. If the DNA product is cleaved appropriately and the fragment is isolated, said fragment can be inserted into the opened vector DNA in a T4 DNA ligase reaction. In deviation from the manufacturer's protocol, E. coli strain MM294, described in example 1, is transformed with the ligation mixture and recombinant colonies are screened for on zeocine selection plates. Plasmid DNA is reisolated from clones and then characterized by means of restriction and DNA sequence analysis. Using the plasmid constructed in this way. a P. pastoris expression clone for production of the peptides is then prepared, following the manufacturer's instructions. Example 7: Purification of mini-proinsulin and himdin The purification requires separation of the two proteins at an early stage. After completion of the expression, the medium is analyzed by means of analytical RP-HPLC. In contrast to most other polypeptides found in the supernatant due to either spontaneous lysis of yeast ceils or secretion, the two proteins, hirudin and mini-proinsulin, are not precipitated at pH 2.5-3. The culture medium is therefore acidified appropriately and then, after completion of the precipitation, the precipitate and the I cells are removed by centrifugation. After centrifugation. the medium is adjusted to pH 3.5-7 and the two components hirudin and mini-proinsulin are separated from one another by means of hydrophobic interaction chromatography, for example by using a chromatography column filled with Diaion HP20® material. Hirudin can then be isolated from the hirudin-containing fractions according to EP-A 0 549 915 and insulin can be isolated from the mini-proinsulin-containing fractions according to EP-A 0 347 781. Example 8: Preparation of insulin from mini-proinsulin At the end of the expression period, the culture medium is adjusted to pH 6.8 and trypsin is then added with stirring so that a final concentration of 4-8 mg per liter is established. After incubation for approx. 4 hours, the fermentation broth treated in this way is adjusted to pH 2.5-3. After 1-6 hours of precipitation, the precipitate is removed. The mono-Arg-insuiin formed is then isolated via ion exchange chromatography, by way of example on S - Sepharose® in a buffer of 50 mM lactic acid and 30% isopropanol (pH 3.5). Elution is carried out by means of an NaCI gradient of 0.05-0,5 M salt The product-containing fractions are diluted 1:1 with H2O and then ZnCl2 is added, so that a 0.1% strength ZnCb solution is formed. Mono-Arg-insulin precipitates at pH 6.8 and by way of example is converted to insulin according toEP"A0 324 712. Example 9: Preparation of insulin from mini-proinsulin after filtration At the end of the expression period, cells and supernatant components are removed by precipitation at pH 2.5 to 3. Then the medium is concentrated via filtration through membranes having an exclusion limit of 10 kDa. Like the hirudin derivative, mini-proinsulin is found quantitatively in the retentate and can then be processed to insulin according to example 8. ] 1. A DNA-molecule of the form: Px-Sx-Bn-(ZR}-transportpeptide-(Z1Z2)-protein(Y)-(Z1Z2)-protein(Ym)-T; wherein the DNA-molecule codes for a transport peptide linked via a sequence Z1Z2to a second protein which in turn is linked via Z1Z2to a protein Y1 which either corresponds to Y or can be different from Y and the transport peptide improves the rate of secretion of Y and/or Y^, wherR Px is any promoter DNA sequence selected in such a way that optimal yields of the protein of interest become obtainable; Sx is any DNA which, accordingly, encodes any signal or leader sequence which allows optimal yields; Bn is 1-15 genetically encodable amino acids or a chemical bond; Z is the codon of an amino acid selected from the group comprising Lys and Arg; Zi is the codon of an amino acid selected from the group comprising Lys and Arg; Z2 is the codon of an amino acid selected from the group comprising Lys and Arg; protein Ym is a DNA sequence encoding any protein which can be produced and secreted by yeast (m = 1-5) or is a chemical bond (m = 0); R is an arginine codon; transport peptide is a DNA sequence encoding a peptide which can be transported efficiently and which can pass membranes; protein Y is a DNA sequence encoding any protein which can be produced and secreted by yeast and whose biological activity, when Ym is not a chemical bond, is not impaired by a basic dipeptide extension or allows degradation of the extension by carboxypeptidases; T is an untranslated DNA sequence advantageous to the expression. 2. A DNA-molecule as claimed in claim 1, wherein the transport peptide is hirudin or a hirudin derivative. A DNA-molecule according to any of claims 1 or 2, wherein (Y) is a pharmaceutically relevant protein selected from a group comprising proinsulin and derivatives thereof, interleukins, lymphokines, interferons and factors derived from blood clotting systems. Proteins encoded by any of the DNA molecules according to claims 1 to 3. Multicopy vector comprising the DNA-molecule of any of claims 1 to 3. Plasmid comprising the DNA-molecule of any of claims 1 to 3. Host cell comprising a DNA-molecule of any of claims 1 to 3, a multicopy vector of claim 5 and/or a plasmid of claim 6. as a part of its chromosome, as a part of a mini-chromosome, or extra-chromosomally. Host cell according to claim 7. wherein said host cell is a yeast Host cell according to claim 8 selected from the group comprising of S. cerevisiae, K. lactis, H. polymorpha and P. pastoris. Process of fermenting proteins according to claim 4, in which (a) a DNA-molecule of any of claims 1 to 3, a multicopy vector of claim 5, or a plasmid of claim 6 is expressed in a host cell according to any of claims 7 to 9; and (b) the expressed proteins are isolated from the supernatant of the cell culture. Process according to claim 10, wherein, after completion of fermentation, the pH is adjusted to 2,5-3,5 in order to precipitate non-desired proteins and the expressed proteins are isolated from the supernatant of the precipitation. Process according to claim 11, in which process after separating the fermentation supematant from the host cells, the host cells are repeatedly cultured in fresh medium, and the released fusion protein is isolated from each supernatant obtained during cultivation. A process according to any of the claims 10 to 12. wherein a process step for concentrating the expressed protein in the supernatant after precipitation is selected from a group comprising microfiltration, hydrophobic interaction chromatography and ion exchange chromatography. Process for the preparation of insulin, in which (a) proinsulin is expressed as protein (Y) of the expression cassette of claim 1 in a process according to claims 10 or 11; (b) the proinsulin of step (a) is isolated and treated with trypsin and carboxypeptidase B; and (c) insulin is isolated from the reaction mixture of step (b). Process according to claim 14, wherein the transport peptide is hirudin or a hinjdin derivative which is destroyed or biologically inactivated after step (a) or (b). Protein according to claim 4 wherein the protein is a hirudin-derivative with two basic amino acid residues at its C-terminal end. A DNA molecule, substantially as hereinabove described and exemplified.

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