Biologically Active PDGF Derived a-Chain Homodimers

The present invention relates to proteins having chemotactic and mitogenic activity, therapeutic compositions containing these proteins, and methods for enhancing the wound-healing process in warm-blooded animals utilizing the therapeutic compositions.

Human platelet-derived growth factor (PDGF) has been shown to be the major mitogenic protein in serum for mesenchymal-derived cells. This is well documented by numerous studies of platelet extracts or purified PDGF induction of either cell multiplication or DNA synthesis (a prerequisite for cell division) in cultured smooth muscle cells, fibroblasts and glial cells (Ross et al., PNAS 71:1207, 1974; Kohler and Lipton, Exp. Cell Res. 87: 297,1974; Westermark and Wasteson, Exp. Cell Res. 98: 170, 1976; Heldin et al., J. Cell Physiol. 105: 235, 1980; Raines and Ross, J. Biol. Chem. 257: 5154, 1982). Furthermore, PDGF is a potent chemoattractant for cells that are responsive to it as a mitogen (Grotendorst et al., J. Cell Physiol. 113: 261, 1982; Seppa et al., J. Cell Biol. 92: 584, 1982). It is not generally the case that mitogens also act as chemotactic agents. Due to its mitogenic activity, PDGF is useful as an important component of a defined medium for the growth of mammalian cells in culture, making it a valuable research reagent with multiple applications in the study of animal cell biology.

In vivo, PDGF normally circulates stored in the alpha granules of platelets. Injury to arterial endothelial linings causes platelets to adhere to the exposed connective tissue and release their granules. The released PDGF is understood to chemotactically attract fibroblasts and smooth muscle cells to the site of injury and to induce their focal proliferation as part of the process of wound repair (Ross and Glomset, N. Eng. J. of Med. 295: 369, 1976).

It has been postulated that as a part of this response to injury, PDGF released by platelets may play a causative role in the development of the proliferative lesions of atherosclerosis (Ross and Glomset, ibid.) which is one of the principal causes of myocardial and cerebral infarction. Strategies for the prophylaxis and treatment of atherogenesis in the past have been narrowly directed toward reducing risk factors for the disease, such as lowering blood pressure in hypertensive subjects and reducing elevated cholesterol levels in hypercholesterolemic subjects.

Recent studies have shown that at least one of the two protein chains comprising PDGF and the putative transforming protein of simian sarcoma virus (SSV), an acute transforming retrovirus, appear to have arisen from the same or closely related cellular genes. In particular, computer analysis of a partial amino acid sequence of PDGF has revealed extensive homology with the gene product, p28.sup.sis, of SSV (Doolittle et al., Science 221: 275, 1983; Waterfield et al., Nature 304: 35, 1984; and Johnson et al., EMBO 3: 921, 1984). Further, more recent studies have illustrated that p28.sup.sis and PDGF show antigenic as well as structural similarities (Robbins et al., Nature 305: 605, 1983; Niman, Nature 307: 180, 1984).

Although previous attempts, such as that summarized in Devare et al. (Cell 36: 43, 1984), have been made to express the v-sis gene in a transformed microorganism, they have not been successful in producing mitogenic material. More recently, investigators have described the production of p28.sup.sis in E. coli as a fusion protein (Wang et al., J. Biol. Chem. 259: 10645, 1984). This protein appears to compete with PDGF for binding to PDGF receptor sites. While SSV transformed rodent cells have been shown to exhibit a mitogenic activity similar to PDGF (Deuel et al., Science 221: 1348, 1983; Owen et al., Science 225: 54, 1984), it is not clear that this activity is due to a gene product from SSV (i.e., p28.sup.sis). Furthermore, cells transformed by a variety of viruses other than SSV produce a PDGF-like mitogen into the culture medium (Bowen-Pope et al., PNAS 81: 2396, 1984; Bleibers et al., J. Cell Phys. 123: 161-166, 1985).

While natural PDGF may be isolated from human plasma or platelets as starting material, it is a complex and expensive process, in part due to the limited availability of the starting material. In addition, it is difficult to purify PDGF with high yield from other serum components due to its extremely low abundance and biochemical properties. Furthermore, the therapeutic use of products derived from human blood carries the risk of disease transmission due to contamination by, for example, hepatitis virus, cytomegalovirus, or the causative agent of Acquired Immune Deficiency Syndrome (AIDS).

In view of PDGF's clinical applicability in the treatment of injuries in which healing requires the proliferation of fibroblasts or smooth muscle cells and its value as an important component of a defined medium for the growth of mammalian cells in culture, the production of useful quantities of protein molecules similar to authentic PDGF which possess mitogenic activity is clearly invaluable.

In addition, the ability to produce relatively large amounts of PDGF or PDGF analogs would be a useful tool for elucidating the putative role of the v-sis protein, p28.sup.sis, in the neoplastic process.

Further, since local accumulation of smooth muscle cells in the intamal layer of an arterial wall is central to the development of atherosclerotic lesions (Ross and Glomset, ibid.), one strategy for the prophylaxis and treatment of atherosclerosis would be to suppress smooth muscle cell proliferation. The ability to produce large amounts of PDGF would be useful in developing inhibitors or designing specific approaches which prevent or interfere with the in vivo activity of PDGF in individuals with atherosclerosis.

Briefly stated, the present invention discloses a variety of proteins which have substantially the same biological activity as PDGF. In one aspect of the present invention, a protein is disclosed having two substantially identical polypeptide chains, each of said chains being substantially homologous to the A-chain of PDGF. The polypeptide chains may also be substantially identical to the A-chain of PDGF. For purposes of the present invention, "substantially identical polypeptide chains" are those chains that are at least eighty percent homologous to one another at the amino acid level. Within the present invention, the phrase "substantially homologous" refers to those sequences that are at least 30% homologous to one another.

In addition, proteins comprising polypeptides that are variants and derivatives of the A-chain of PDGF are also disclosed. These modifications to the A-chain fall basically into two broad classes, amino acid deletions and amino acid substitutions. In regard to the former, polypeptide chains are disclosed that are substantially identical to the A-chain of PDGF from (a) amino acid 9 to amino acid 104; (b) amino acid 23 to amino acid 104; (c) amino acid 9 to amino acid 95; (d) amino acid 23 to amino acid 95; or (e) amino acid 1 to amino acid 95, the B-chain itself consisting of amino acids 1 to 104. Removal of amino- and/or carboxy-terminal amino acids as described herein results in smaller biologically active molecules which may have broader therapeutic utility. In addition, the protein described above may have the amino acid sequence of FIG. 9, from A-chain amino acid 1 to amino acid 104.

Preferred amino acid substitutions include the replacement of selected cysteine residues with another amino acid, as well as the replacement of other amino acids, the substitution of which does not destroy the biological activity of the resultant molecule. In a particular embodiment of the present invention, proteins are disclosed that include the substitution of A-chain cysteine residue at position 10.

In another aspect of the present invention, a therapeutic composition is disclosed comprising a protein having two substantially identical polypeptide chains, each of said chains being substantially homologous to the A-chain of PDGF, and a physiologically acceptable carrier or diluent. As noted above, the polypeptide chains may also be substantially identical to the A-chain of PDGF. In addition, proteins comprising variants and derivatives of the A-chain of PDGF as described above are also suitable for use in the therapeutic compositions of the present invention.

A related aspect of the present invention is directed toward a method for enhancing the wound-healing process in warm-blooded animals. The method generally comprises administering to the animal a therapeutically effective amount of one or more of the proteins described above, and a physiologically acceptable carrier or diluent.

Other aspects of the invention will become evident upon reference to the following detailed description and attached drawings.

FIG. 1A is a schematic restriction map of the proviral genome of SSV.

FIG. 1B depicts the nucleotide sequence and predicted amino acid sequence encoded by the v-sis region of the SSV genome.

FIG. 2 illustrates the construction of a plasmid which contains the MF.alpha.1 promoter and secretory signal sequence upstream of the v-sis gene.

FIG. 3 illustrates the construction of plasmid p192.

FIG. 4 illustrates the oligonucleotide-directed deletion mutagenesis of the amino terminal 66 v-sis codons.

FIG. 5 illustrates the construction of plasmid p270.

FIG. 6 illustrates the insertion of v-sis expression units upstream of the TPI terminator.

FIG. 7 illustrates the construction of plasmid pTVS2 T.

FIG. 8 illustrates the construction of a B-chain expression unit VSB and its introduction into the pMPOT2 vector.

FIG. 9 depicts the amino acid sequences of the mature A- and B-chains of PDGF.

FIG. 10 is a dose response curve of PDGF receptor binding by media concentrates from yeast transformants containing plasmids pVSBm and pMPOT2 compared to authentic PDGF.

Prior to setting forth the invention, it may be helpful to an understanding thereof to set forth definitions of certain terms to be used hereinafter.

Polypeptide: A polymer of amino acids.

Reading Frame: The arrangement of nucleotide codons which encode an uninterrupted stretch of amino acids. During translation of an mRNA, the proper reading frame must be maintained. For example, the sequence GCUGGUUGUAAG may be translated into three reading frames or phases, depending on whether one starts with G, with C, or with U, and thus may yield three different peptide products. Translation of the template begins with an AUG codon, continues with codons for specific amino acids, and terminates with one of the translation termination codons.

Coding Sequence: DNA sequences which in the appropriate reading frame directly code for the amino acids of a protein.

Complementary DNA: or cDNA. A DNA molecule or sequence which has been enzymatically synthesized from the sequences present in an mRNA template.

Secretory Signal Sequence: That portion of a gene or cDNA encoding a signal peptide. A signal peptide is the amino acid sequence in a secretory protein which signals its translocation into the secretory pathway of the cell. Signal peptides generally occur at the beginning (amino terminus) of the protein and are 20-40 amino acids long with a stretch of 9-10 hydrophobic amino acids in their center. Very often the signal sequence is proteolytically cleaved from the protein during the process of secretion.

Cell Surface Receptor: A protein molecule at the surface of a cell which specifically interacts with or binds a molecule approaching the cell's surface. Once the receptor has bound the cognate molecule, it effects specific changes in the physiology of the cell.

Mitogen: A molecule which stimulates cells to undergo mitosis. Mitosis is asexual somatic cell division leading to two daughter cells, each having the same number of chromosomes as the parent cell.

Transformation: The process of stably and hereditably altering the genotype of a recipient cell or microorganism by the introduction of purified DNA. This is typically detected by a change in the phenotype of the recipient organism.

Transcription: The process of producing a mRNA template from a structural gene. As used herein, the term "gene" is understood to include cDNA sequences.

Expression: The process, starting with a structural gene or cDNA, of producing its polypeptide, being a combination of transcription and translation. An expression vector is a plasmid-derived construction designed to enable the expression of a gene or cDNA carried on the vector.

Plasmid: An extrachromosomal, double-stranded DNA sequence comprising an intact "replicon" such that the plasmid is replicated in a host cell. When the plasmid is placed within a unicellular organism, the characteristics of that organism may be changed or transformed as a result of the expression of the DNA sequences of the plasmid. For example, a plasmid carrying the gene for tetracycline resistance (tet.RTM.) transforms a cell previously sensitive to tetracycline into one which is resistant to it.

Yeast Promoter: DNA sequences upstream from a yeast gene which promote its transcription.

Biological Activity: Some function or set of activities performed by a molecule in a biological context i.e., in an organism or an in vitro facsimile). In the case of PDGF, these biological activities include inducing chemotaxis and/or inducing mitogenesis of responsive cell types, following the binding of PDGF to specific cell surface receptors. Other biological effects of PDGF may include: phospholipase activation; increased phosphatidylinositol turnover and prostaglandin metabolism; stimulation of both collagen and collagenase synthesis by responsive cells; an indirect proliferative response of cells lacking PDGF receptors; and potent vasoconstrictor activity.

In its biologically active form, PDGF is a heatstable protein composed of heterogeneously sized species ranging between 28,000 and 31,000 Daltons, all of the individual species being active in stimulating DNA synthesis (Raines and Ross, ibid.; Deuel et al., J. Biol. Chem. 256: 8896, 1981; Antoniades, PNAS 78: 7314, 1981). Where individual species with molecular sizes of 27,000; 28,500; 29,000; and 31,000 Daltons have been isolated and analyzed, they show extensive tryptic peptide homology and have been found to have comparable mitogenic activity and amino acid composition (Raines and Ross, ibid.) The slight variations in size among the species are most probably due to differences in carbohydrate composition and minor proteolysis.

Through studies of PDGF which has been extensively purified from platelet-rich human plasma, PDGF has been shown to be composed of two polypeptide chains, an A-chain (14,000 Daltons) and a B-chain (16,000 Daltons), which are disulfide bonded together to form the biologically active dimer molecule (Raines and Ross; Deuel et al.; Antoniades, ibid.). The PDGF nomenclature found in the literature is not consistent (Doolittle et al.; Waterfield et al.; Raines and Ross; Johnsson et al., ibid.). The nomenclature of Johnsson et al. (ibid.), wherein the two polypeptides found in pure PDGF are called "A-chain" and "B-chain," is adopted herein. The B-chain is homologous to p28.sup.sis and was previously called "peptide I" (Waterfield et al., ibid.) or "1a" (Doolittle et al., ibid.). The A-chain was previously termed "peptide II" (Waterfield et al., ibid.) or "2a" (Doolittle et al., ibid.). Data derived from a partial amino acid sequence of PDGF indicate that the two polypeptide chains (A-chain and B-chain) show extensive homology (Doolittle et al., ibid.; Waterfield et al., ibid.; and Johnsson et al., ibid.; Antoniades and Hunkapiller, Science 220: 963, 1983).

Following complete chemical reduction, the single A-chain and B-chain polypeptides alone do not exhibit any mitogenic activity (Raines and Ross, ibid.), and attempts to reconstitute activity by reoxidation of the reduced polypeptides have not been successful. Recently, the amino acid sequence of the B-chain has been determined and shown to share homology with a portion of the v-sis gene product, p28.sup.sis (Doolittle et al., ibid.; Waterfield et al., ibid.; and Johnson et al., ibid.). The homology between these two proteins strongly suggests that they are derived from the same or closely related cellular genes.

As shown in FIG. 9, there is 56% amino acid identity between the A-chain and B-chain. In addition, there are several blocks of perfect homology between the two chains. Further, both of the chains contain eight cysteine residues at identical positions, suggesting that each polypeptide folds into a similar three-dimensional structure. It appears that these two polypeptides are closely related members of a small family. The blocks of perfect homology between the A- and B-chains reflect regions of the protein which may contribute to function, while the less homologous regions may reflect portions of the protein which are less important to its function.

Given the fact that a single reduced A-chain polypeptide is not biologically active and that previous attempts directed toward expressing v-sis sequences in E. coli did not yield mitogenic material, it would not be expected that merely expressing a sequence encoding a PDGF-like molecule in a microorganism would result in a molecule which exhibited biological activity. The present invention, however, unlike the previous attempts noted above, was designed to express A-chain like sequences, or portions thereof, absent of heterologous sequences, such that the expressed molecules exhibit biological activity characteristic of PDGF. Further, the expression system of the present invention was designed to produce the gene product via a eucaryotic secretory pathway. This enables the expressed polypeptide molecules to be properly processed and places them in a cellular environment which allows them to be correctly folded and assembled into biologically active dimers. Indeed, the present invention, in contrast to previous efforts, results in the secretion of A-chain homologous dimers which are biologically active in established assays for PDGF activity, i.e., radioreceptor assay (RRA), mitogenesis assay, and chemotaxis assay.

As noted above, human platelet-derived growth factor has been shown to be a major mitogenic protein in serum. PDGF, as it is isolated from platelets, is a different molecule from the novel proteins of the present invention. Purified platelet PDGF contains two amino acid sequences, one A-chain and the other B-chain (Antoniades and Hunkapiller Science 220: 963-965, 1983; Waterfield, et al. Nature 304: 35-39, 1983), which are held together by disulfide bonds to form the biologically active heterodimer molecule. This structure has been confirmed by immunoprecipitation experiments (Hart et al., Heldin et al. unpublished). These investigators used monoclonal antibodies directed specifically against the A-chain or the B-chain to immunoprecipitate PDGF. Their results indicate that the PDGF can be removed from solution with antibodies which recognize either chain alone. This confirms the structure of PDGF as a heterodimer of two different polypeptide chains. In addition, naturally occurring PDGF contains carbohydrate (Deuel et al. J. Biol. Chem. 256:8896-8899, 1981).

In contrast to naturally occurring PDGF, one particular aspect of the present invention discloses protein products that are disulfide-bonded dimers of two A-chain-like polypeptides. One such dimer comprising chains having complete homology to the 104 amino acids of PDGF A-chain, migrates on polyacrylamide gels with an apparent molecular weight of ca. 31,000 Daltons. When the dimer is chemically reduced, the component chains migrate to a position consistent with a polypeptide of 104 amino acids. The amino acid composition of the pure protein has been determined and the results show that the composition is substantially identical to the A-chain sequence shown in FIG. 9. The amino acid sequence of this pure, yeast-expressed protein was determined on a gas-phase sequenator (Applied Biosystems). All of the amino terminal sequence obtained could be accounted for by the sequence information shown for the A-chain in FIG. 9. These results indicate that the proteins of this aspect of the present invention are homodimers consisting of polypeptide chains homologous to the A-chain of PDGF. The amino acid sequence of the A-chain produced in yeast contains no N-linked glycosylation sites and there is no evidence, based on polyacrylamide gel electrophoresis, that the product contains carbohydrate.

As noted above, another aspect of the present invention discloses proteins comprising polypeptides which are variants and derivatives of the A-chain of PDGF. These modifications to the A-chain sequence fall basically into two classes: amino acid deletions and amino acid substitutions.

In regard to the deletion of amino acids, it has been found that the PDGF A-chain may be truncated at either or both the amino- and carboxy-terminal ends and will still form a biologically active molecule. Removal of these amino- and/or carboxy-terminal amino acids results in smaller biologically active molecules which may have broader therapeutic utility. Amino acids which may be deleted without destroying the biological activity of the resultant molecule include residues 1 through 22 and residues 96 through 104. Particularly preferred truncated A-chain analogs consist of amino acids 1 through 95, 9 through 95, 23 through 95, 9 through 104, and 23 through 104, although it will be evident to those skilled in the art that other polypeptides may also be constructed while still providing a molecule having biological activity.

In addition, a variety of amino acid substitutions are possible. Preferred amino acid substitutions include replacement of selected cysteine residues with another amino acid, e.g. serine, as well as the replacement of other amino acids, the substitution of which does not destroy the biological activity of the resultant molecule. While the dimerization of the proteins of the present invention involves disulfide bonding between the component chains, it has been found that not all of the cysteine residues participate in the formation of disulfide bonds necessary for biological activity. Cysteine residues at positions 54 and 93 of the A-chain are essential for the formation of active dimers. Cys 91 may also contribute to proper structure. The cysteine at position 10 is not required for the formation of active dimers. The remaining cysteines at positions 37, 43, 46, and 47 may not be required for the formation of active dimers. Therefore, proteins having amino acid substitutions at residues 10, 37, 43, 46 or 47 may also be suitable for use within the present invention, such as within a method for enhancing the wound-healing process in warm-blooded animals.

The v-sis gene, as mentioned above, is the transforming gene of simian sarcoma virus (SSV). The v-sis gene has been cloned and its DNA sequence determined (Devare et al., PNAS 79: 3179, 1982; Devare et al., PNAS 80: 731, 1983). Analysis of this sequence revealed an open reading frame which could encode a 28,000 Dalton protein, designated p28.sup.sis. Subsequently, such a protein was immunologically identified in SSV infected cells (Niman, ibid.; Robbins, ibid.). The predicted amino acid sequence of the v-sis gene product, p28.sup.sis, was found to have a high degree of homology with the actual amino acid sequence of a portion of the B-chain of PDGF (Johnsson, ibid.). The homology of the PDGF B-chain to the v-sis gene product begins at amino acid 67 of p28.sup.sis, a serine, and continues for 109 amino acids to a threonine residue at amino acid 175. The amino acid sequences preceding and following the B-chain homologous region of p28.sup.sis are not homologous to either the A- or B-chains of mature PDGF (Johnsson, ibid.) and represent portions of the B-chain precursor. In addition, PDGF and p28.sup.sis have been shown to be similar immunologically (Niman, ibid.; Robbins, ibid.). The v-sis gene product, p28.sup.sis, a protein of 226 amino acids, dimerizes and is proteolytically processed to a dimeric protein of approximately 20,000 Daltons (p20sis) in SSV infected cells (Niman, ibid.; Robbins, ibid.). This 20,000 Dalton protein can be immunoprecipitated with antiserum against PDGF.

The mature B-chain homologous region of v-sis encodes a 109 amino acid polypeptide which is almost identical to the human B chain. The four amino acid differences . between these two gene products occur at positions 6, 7, 91 and 97. The mature human A-chain sequence is 104 amino acids in length, and is 56 percent homologous to the B-chain, therefore having a degree of homology to the v-sis product similar to its homology to the B-chain.

As noted above, previous attempts at expressing PDGF related sequences in prokaryotes did not yield biologically active material. The v-sis gene product p28.sup.sis, as well as PDGF itself, are secreted mammalian proteins. Within the present invention, it has been found that by utilizing the secretory pathway of eucaryotic cells to express proteins substantially homologous or substantially identical to the A-chain of PDGF, biologically active material may be obtained. Expression and secretion of these gene products from a eucaryotic cell enable processing and assembly, which result in molecules with native and biologically active conformation, i.e., in one aspect, A-chain like dimers.

The secretory pathways of eucaryotes are believed to be quite similar. In particular, mammalian cell and yeast cell secretory pathways are well characterized and are homologous. The presence of a secretory signal sequence on the expressed polypeptide is an important element in eucaryotes, due to its role in directing the primary translation product into the secretory pathway, thereby leading to proper processing and assembly. Provided that appropriate transcriptional promoter and secretory signal sequences are utilized, generally any eucaryote could express and secrete the A-chain like product in a biologically active form.

An easily manipulable and well-characterized eucaryote is the yeast cell. For these reasons, yeast was chosen as a model example of an appropriate eucaryotic cell within the present invention. In accordance with the present invention, the yeast promoter is followed downstream by a DNA sequence which encodes a protein having substantially the same biological activity as PDGF. For example, DNA sequences encoding the 109 amino acids of the PDGF B-chain or the 104 amino acids of the A-chain, or other DNA sequences encoding amino acids with homology to the A-chain, were inserted into yeast extrachromosomal elements containing a yeast promoter capable of directing their expression. These extrachromosomal elements were transformed into yeast cells capable of expression and secretion of these biologcally active proteins. In addition, variants and derivatives of the PDGF A-chain were also inserted into such a yeast extrachromosomal element.

DNA sequences which encode a protein having substantially the same structure and/or biological activity as PDGF include the v-sis gene or derivatives of the v-sis gene, or portions thereof, or the human A-chain or portions thereof. Specifically, DNA sequences encoding polypeptides substantially homologous or substantially identical to the A-chain of PDGF are preferred. In addition, suitable DNA sequences include those which encode variants and derivatives of the A-chain. The genes or sequences to be utilized in the extrachromosomal element may be isolated using standard recombinant DNA techniques.

The human A-chain cDNA may be isolated from a human cDNA library made from an appropriate source of messenger RNA by using the v-sis gene or a fragment thereof as a hybridization probe, or through use of oligonucleotide probes designed from the A-chain DNA sequence. Preferred sources of mRNA are human transformed cell lines, e.g. U2-OS and T-24. These cells can be cultured in vitro and are known to secrete a protein having PDGF-like activity. (Heldin et al. Nature 319:511-514, 1986). The identity of this cDNA as that encoding A-chain may be verified by DNA sequencing.

Once an appropriate DNA sequence encoding a protein exhibiting PDGF-like biological activity is identified, the sequence is ligated to an appropriate promoter and secretory signal fragment. Promoters which may be utilized in yeast include the yeast alpha-factor (MF.alpha.1) promoter and the yeast triose phosphate isomerase (TPI) promoter. Promoters may also be obtained from other yeast genes, e.g., Alcohol Dehydrogenase 1 (ADH1), Alcohol Dehydrogenase 2 (ADH2). Appropriate promoters for other eucaryotic species may also be used and will be apparent to those skilled in the art. The constructions described herein were designed such that the PDGF-related gene products would be secreted from the yeast cell into the media. This was accomplished through use of the prepro secretory signal sequence of the yeast mating pheromone alpha-factor (Kurjan and Herskowitz, Cell 30: 933, 1982; Julius et al., Cell 36: 309, 1984; and Brake et al., PNAS 81: 4642, 1984), although other secretion signals may be used. To ensure the efficient transcription termination and polyadenylation of mRNA, a yeast terminator sequence, such as the triose phosphate isomerase terminator, was added. (Alber and Kawasaki, J. Molec. Genet. Appl. 1: 419, 1982.) Methods of ligation of DNA fragments have been amply described (Maniatis et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory 1982) and are well within the skill of those of ordinary skill in the art to perform. After preparation of the expression unit constructions, the constructs are inserted into an appropriate expression vector.

It is preferable to use an expression vector which is stably maintained within the host cell in order to produce more biological activity per culture. Suitable yeast expression vectors in this regard are the plasmids pCPOT and pMPOT2, which include the Schizosaccharomyces pombe gene encoding the glycolytic enzyme triose phosphate isomerase (POT1 gene). Inclusion of the POT1 gene ensures the stable maintenance of the plasmid in an appropriate host cell due to its ability to complement the corresponding gene deletion present within this host cell.

After preparation of the DNA construct incorporating the POT1 selectable marker, the TPI promoter, the alpha-factor secretory signal sequences, the appropriate DNA sequence encoding a molecule having PDGF-like biological activity, and the TPI terminator in an appropriate vector, the construct is transformed into a yeast host with a TPI deletion. Procedures for transforming yeast are well known in the literature.

The transformed yeast cells may be selected by growth on conventional complex medium containing glucose when the pCPO or pMPOT2 vector is utilized. A conventional medium, such as YEPD (20 grams glucose, 20 grams Bacto-peptone, 10 grams yeast extract per liter), may be used. Once selected, transformants containing the appropriate expression constructions are grown to stationary phase on conventional complex media, the cells removed by centrifugation or filtration, and the medium concentrated. Noting that authentic human PDGF is a highly cationic and hydrophobic protein (Raines and Ross, ibid.; Antoniades, ibid.; Deuel et al., 1981, ibid.), it was expected that the recombinant products would possess similar characteristics, allowing the use of ion exchange chromatography to be used in their purification.

Using a variety of assays, it can be demonstrated that spent media from yeast cultures expressing the proteins possess biological activities substantially identical to authentic human PDGF.

Expression of biologically active proteins in eucaryotic cells other than yeast cells can be achieved by a person skilled in the art through use of appropriate expression/regulatory signals. Transcriptional promoters capable of directing the expression of these sequences are chosen for their ability to give efficient and/or regulated expression in the particular eucaryotic cell type. Signal sequences capable of directing the gene product into the cell's secretory pathway are chosen for their function in the appropriate cell type. Other useful regulatory signals, such as transcription termination signals, polyadenylation signals and transcriptional enhancer sequences, are also chosen for their function in the appropriate cell type, the selection of which would be apparent to an individual skilled in the art.

According to the present invention, it is possible to produce recombinant PDGF-like molecules which are homodimers or heterodimers of substantially identical polypeptide chains. To produce heterodimers, two different expression units are introduced into the same cell and heterodimers are identified among the biologically active products. The expression units may be on different expression vectors with different selectable markers or, preferably, on a single expression vector. The second strategy offers the advantage of providing equal copy numbers of the two expression units.

The techniques of cell culture have advanced considerably in the last several years as have the number and varieties of mammalian cells which will grow in culture. Central to these advances is a better understanding of the nutritional requirements (i.e., hormones and growth factors) of cultured cells (Barnes and Sato, Cell 22: 649, 1980). The types of cells able to grow in culture can be crudely classified in two groups: normal and transformed. So-called "normal" cells are generally not immortal in culture, they do not form tumors when injected into animals, and they retain a normal diploid karyotype. Normal cells may also retain much of their differentiated character in culture. Within the category of normal cells are those which will only grow for a limited number of generations in culture, termed "cell strains" or "primary cultures." Some normal cell lines, while not meeting all the criteria of transformation, may grow indefinitely in culture. Transformed cells are immortalized for growth in culture, typically have lost their differentiated phenotype, and have acquired karyotypic aberrations. They may also be independent of anchorage for growth and induce tumors when injected into the appropriate host animal. Cells in any of these categories which grow in vitro and possess PDGF receptors will be responsive to the PDGF analogs of this invention.

As noted above, the proteins described herein are suitable for use within therapeutic compositions for enhancing the wound-healing process in warm-blooded animals. The normal wound-healing process in warm-blooded animals proceeds by an orderly series of events involving the interaction of chemoattractants, growth factors, and a variety of specialized cell types. This process includes an ordered migration and, in some cases, the subsequent proliferation of a number of these specialized cell types into the wound space, and involves the complex interaction of a variety of biologically active factors. This process is discussed in detail in Hunt et al., eds., Soft and Hard Tissue Repair; Biological and Clinical Aspects, Praeger Publishers, New York, 1984, which is hereby incorporated by reference. Briefly, tissue injury results in the release of chemotactic factors which attract particular cell types, which then release additional and/or other chemoattractant or mitogenic factors. These factors, in turn, affect additional specialized cells, ultimately restoring the injured tissue. Further, there is evidence that the rate at which this process normally proceeds is limited by the levels of chemoattractants and growth factors at the wound site, and may be enhanced by the addition of these agents (Grotendorst et al., J. Clin. Invest. 76: 2323-2329, 1985, herein incorporated by reference).

The wound-healing process in the dermis begins with the formation of a clot from the blood which flows into the wound. This results in a cross-linked network of fibrin molecules binding the wound together. During this process, platelets adhere to the injured tissue, becoming activated, and release the contents of their alpha granules. The disruption of the dermal tissue, the blood coagulation reactions, and platelet activation all generate molecules which cause the migration of a series of new cells into the wound, thereby initiating the repair process.

Among the contents of the alpha granules released by the platelets is PDGF. In addition, other contents of the alpha granules and by-products of the coagulation reactions induce the appearance of macrophages. Macrophages are a second important source of PDGF in the wound. The deposition of PDGF at the site of an injury provides a chemotactic stimulus for fibroblasts to enter the wound space and a mitogenic stimulus for the fibroblasts to subsequently proliferate therein, thereby participating in the process of repair. An important role of the fibroblast is the regeneration of connective tissue at the wound site. The fibroblasts proliferate in the wound and deposit collagen types I and II and other extracellular proteins to the connective tissue matrix. The presence of new fibroblasts and their protein products reconstitutes the dermal architecture such that it can be re-epithelialized and the wound thereby healed.

Similarly, the wound-healing process in relation to the repair of connective tissue also requires fibroblast infiltration and proliferation, leading to subsequent collagen deposition.

The proteins of the present invention have been shown to possess substantially the same biological activity as authentic PDGF. The basic biological activity of PDGF, particularly the induction of chemotaxis and mitogenesis in responsive cell types (inlcuding fibroblasts and smooth muscle cells), underlies many of the physiological roles of this protein, including its role in tissue repair.

Because the chemotactic and mitogenic properties of PDGF are central to its role in the wound-healing process, the biologically active proteins of the present invention will have similar therapeutic utility. These biologically active proteins are therefore expected to have clinical applicability in the treatment of wounds in which healing requires the migration and/or proliferation of fibroblasts. In addition, PDGF acts as a chemotactic and mitogenic agent for smooth muscle cells, the proliferation of which may contribute to the healing of certain wounds. Smooth muscle cells will be affected by PDGF in a manner similar to that described above for fibroblasts, thereby contributing to the healing process.

In individuals with normal healing capacity, exogenous proteins having the biological activity of PDGF accelerate the rate of appearance of fibroblasts in the wound and their subsequent proliferation. In addition, there are a large number of individuals who have substantially impaired wound healing capacity, and thereby lack the ability to provide to the wound site endogenous growth factors which are necessary for the process of wound healing. In these individuals, the addition of exogenous proteins having the biological activity of PDGF enables wound healing to proceed in a normal manner.

The proteins of the present invention are expected to accelerate the healing process in a broad spectrum of wound conditions. For purposes of the present invention, the terms "wound" or "wound condition" include any disruption of the dermal layer of the skin. Examples of disruptions to the dermal layer include chronic non-healing dermal ulcers (which can have a variety of causes), superficial wounds and lacerations, abrasions, surgical wounds, and some burns. In addition, wounds may also result in damage to connective tissue, the repair of which involves fibroblast proliferation and collagen deposition. The proteins of the present invention are useful in enhancing the healing process of all of these wounds, and will also be useful in the treatment of other wounds in which healing requires the migration and/or proliferation of fibroblasts. Furthermore, normal wound-healing may be retarded by a number of factors, including advanced age, diabetes, cancer, and treatment with anti-inflammatory drugs or anticoagulants, and the proteins described herein may be used to offset the delayed wound-healing effects of such treatments. Lawrence et al. (Ann. Surgery 203: 142-147, 1986) demonstrated that PDGF restored the wound-healing process to normal in diabetic rats. Knighton et al. (Ann. Surgery 204: 322-330, 1986) used a mixed growth factor preparation comprising PDGF on chronic non-healing dermal wounds of human patients and observed dramatic positive results. Their results indicate that some of the activity in their preparation is due to PDGF and that PDGF contributes to the rapid healing they see in humans as it does in animal experiments. PDGF acts synergistically with other components of the preparation.

For therapeutic use in the applications described herein, the proteins of the present invention are preferably administered topically in combination with a physiologically acceptable carrier or diluent. Further, it is preferable to use a substantially pure preparation of the protein, that is, one which is generally free of impurities or contaminants which would interfere with its therapeutic use. Particularly preferred are those preparations which are free of toxic, antigenic, inflammatory or other deleterious substances, and are greater than 80% pure. Typically, the proteins desired herein will be in a concentration of about 1 to 50 ug/ml of total volume, although it will be apparent that concentrations in the range of 10 ng/ml-100 ug/ml may be used. However, it should be noted that concentrations in excess of 50 ug/ml may result in reduced therapeutic effectiveness. A therapeutically effective amount sufficient to accelerate the rate of appearance and increase the number of new fibroblasts in the wound space and to stimulate DNA synthesis in and collagen deposition by those fibroblasts, will typically be in the range of one to five milliliters of the preparation, depending upon the characteristics of the wound.

Therapeutic compositions according to the present invention comprise the proteins described herein in combination with suitable carriers, as well as adjuvants, diluents, or stabilizers. Suitable adjuvants include collagen or hyaluronic acid preparations, fibronectin, factor XIII, or other proteins or substances designed to stabilize or otherwise enhance the active therapeutic ingredient(s). Diluents include albumins, saline, sterile water, etc. Other stabilizers, antioxidants, or protease inhibitors may also be added. Alternatively, the proteins may be applied to wound dressings as aqueous solutions. The therapeutic compositions according to the present invention may be reapplied at one to several-day-intervals until healing is complete.

The therapeutic compositions of the present invention may also contain other pharmaceutically active ingredients, for example, heparin, which has been shown to accelerate the healing of thermal burns. Other growth factors, such as TGF-.alpha., TGF-.beta., EGF, FGF, platelet factor 4, insulin or somatomedins (see Grotendorst et al., 1985) and angiogenesis factor, may also work synergistically with the PDGF analogs described herein. Antibiotics may also be included to keep the wound free of infection.

To summarize the examples which follow, EXAMPLE I demonstrates the construction of a v-sis subclone of pSSV-11 in the E. coli replicating plasmid pUC13, subsequently designated pVSIS/Pst. EXAMPLE II demonstrates the construction of the plasmid pVS.alpha., which includes the ligation of v-sis to the MF.alpha.1 promoter and secretory signal sequence. EXAMPLE III demonstrates the oligonucleotide-directed deletion mutagenesis of the first 195 base pairs of the v-sis gene using a technique which employs single stranded bacteriophage M13 in order to eliminate the first sixty-six amino acids of the v-sis gene product, p28.sup.sis, which are not homologous to the B-chain of PDGF. A resulting phage with the correct deletion was designated m11vs2. EXAMPLE IV demonstrates the construction of the expression vector pVSBm. EXAMPLE V demonstrates the transformation of yeast host cells. EXAMPLE VI demonstrates the construction of pSB1. EXAMPLE VII demonstrates the construction of variants and derivatives of the A-chain. EXAMPLE VIII demonstrates the construction of yeast expression vectors for A- and B-chain variants and derivatives. EXAMPLE IX demonstrates the concentration of the spent yeast growth media from transformed cultures and subsequent analysis for PDGF-like material. Clear evidence is presented that these yeast media containing the proteins described herein possess substantially the same biological activity as authentic human PDGF.

The following examples are offered by way of illustration, and not by way of limitation.

The SSV retroviral genome was cloned from SSV-11 nonproductively infected normal rat kidney (NRK) cells which had SSV integrated into their genome (Devare et al., 1982, ibid.). The SSV DNA was isolated as a 5.8 kilobase (kb) Eco RI fragment and subsequently inserted into the plasmid pBR322, resulting in the clone pSSV-11. This clone was obtained from S. Aaronson (National Institutes of Health, Bethesda, Md.).

FIG. 1A is a schematic restriction map of the 5.8 kilobase proviral genome of SSV. Only the restriction sites relevant to the present invention are indicated. The open box designates the p28.sup.sis coding portion of the v-sis gene.

FIG. 1B depicts the nucleotide sequence of the v-sis gene and some flanking SSV sequences. The v-sis gene is inserted 19 nucleotides 3' of the putative ATG initiation codon of the envelope (env) gene of SSV (Devare et al., 1982, ibid.). It is believed that transcription and translation of v-sis sequences are directed by SSV sequences resulting in an env-sis fusion protein. The nucleotide sequence shown in FIG. 1B is corrected from that published by Devare et al. in 1982 (ibid.). The corrections include those made by Devare et al. in 1983 (ibid.) and by the inventors herein. The original numbering scheme of Devare et al. (1982, ibid.) is retained here for ease of reference. The numbers assigned to the restriction sites in FIG. 1A are from FIG. 1B.

A subclone of pSSV-11 (FIG. 2) containing a portion of the v-sis gene was constructed in the E. coli replicating plasmid pUC13 (Vieira and Messing, Gene, 19: 259, 1982; and Messing, Meth. in Enzymology 101: 20, 1983). Five micrograms (ug) of pSSV-11 was digested with the restriction endonuclease Pst I and the 1.2 kb fragment containing sequences numbered 454-1679 (FIG. 1) was purified by agarose gel electrophoresis (0.9%) and extracted from the gel with cetyltrimethylammonium bromide (CTAB) plus butanol (Langridge et al., ibid.). Two ug of pUC13 was also digested with Pst I, phenol/chloroform (CHCl.sub.3) extracted and ethanol (EtOH) precipitated. Forty ng of the 1.2 kb v-sis fragment and 50 ng of Pst I cut pUC13 were ligated overnight at room temperature with 40 units (u) of T.sub.4 DNA ligase. The ligation mixture was used to transform E. coli K-12 strain JM83 (Messing, Recombinant DNA Technical Bulletin, NIH Publication No. 79-009, 2, No. 2, 43-48, 1979) in the presence of 5-bromo,4-chloro, 3-indolyl-.beta.-D-galactoside (X-gal) and isopropyl .beta.-D-thiogalactoside (IPTG). Plasmid DNA prepared from ampicillin-resistant white colonies was digested with Pst I to verify the presence of the insert and the resulting plasmid was designated pVSIS/Pst.

Six hundred ug of plasmid pSSV-11 (FIG. 2) was digested with restriction endonucleases Bam HI and Pvu II in 200 microliters (ul) of 50 mM NaCl, 10 mM MgCl.sub.2, 10 mM Tris pH 7.5 (medium salt buffer), and 100 ug/ml bovine serum albumin (BSA), overnight at 37.degree. C. The digestion products were electrophoresed through a 1.1% agarose gel and the 1100 base pair (bp) Bam HI--Pvu II fragment (FIG. 2) cut out, extracted and EtOH precipitated. The DNA pellet was dissolved in 75 ul Hph I buffer to which was added 20 ul of 1 mg/ml BSA and 5 ul Hph 1. After overnight digestion at 37.degree. C., the mixture was electrophoresed through a 1.25% agarose gel and the 396 bp Hph I--Pvu II fragment isolated from the gel and EtOH precipitated. The DNA pellet was dissolved in 30 ul of Klenow buffer (6mM Tris pH 7.5, 6 mM MgCl.sub.2, 60 mM NaCl) and the 3' overhanging nucleotide at the Hph I cleavage site removed by treatment with 5 u of Klenow polymerase for 5 minutes at 37.degree. C. One ul of a mixture containing all four deoxyribonucleotides each at 1 mM was added and the reaction mixture incubated an additional 10 minutes. After phenol/CHCl.sub.3 /ether (Et.sub.2 O) extraction and EtOH precipitation, the DNA pellet was dissolved in 30 ul of medium salt buffer and digested with 5 u of Bgl II for three hours at 37.degree. C. The DNA was electrophoresed through a 1.25% agarose gel and the 269 bp Hph I--Bgl II fragment extracted and EtOH precipitated. The Hph I cleavage terminus of this Klenow blunted fragment begins with the tri-nucleotide sequence ##STR1##

Plasmid p192 (FIG. 3) comprises a portion of the gene for the yeast mating pheromone .alpha.-factor (MF.alpha.1 gene) cloned in the bacterial plasmid pUC13 (Vieira and Messing, ibid.; and Messing, Meth. in Enzymology 101: 20, 1983). Cloning of the MF.alpha.1 gene from a genomic library has been described by Kurjan and Herskowitz (ibid.). The gene was isolated in this laboratory in a similar manner, using as starting material a yeast genomic library of partial Sau 3A fragments cloned into the Bam HI site of Yep13 (Nasmyth and Tatchell, Cell 19: 753, 1980). From this library, a plasmid was isolated which expressed .alpha.-factor in a diploid strain of yeast homozygous for the mat.alpha.2-34 mutation (Manney et al., J. Cell Biol 96: 1592, 1983). The clone contained an insert overlapping with the MF.alpha.1 gene characterized by Kurjan and Herskowitz (ibid). This plasmid, known as pZA2 (FIG. 3), was cut with Eco RI and the 1700 bp fragment comprising the MF.alpha.1 gene was purified. This fragment was then subcloned into the Eco RI site of pUC13 to produce the plasmid p192.

Fifteen ug of plasmid p192 was digested in 30 ul of medium salt buffer with 20 units of Hind III overnight at 37.degree. C. The reaction mixture was diluted to 60 ul with Klenow buffer and the four deoxyribonucleotides added to a final concentration of 50 uM each. Ten units of Klenow polymerase were added to the ice-cold mixture and incubation allowed to proceed 12 minutes at 15.degree. C. Following phenol/CHCl.sub.3 /Et.sub.2 O extraction, the aqueous phase was concentrated by lyophilization to a volume of 10 ul and digested with 20 units of Eco RI for 70 minutes at 37.degree. C. The products were electrophoresed through a 0.9% agarose gel and the 1.2 kb Eco RI--Hind III (blunted) MF 1 fragment extracted and EtOH precipitated. This DNA fragment contains the transcriptional promoter and secretory signal sequences of ME.alpha.1.

Twenty ug of plasmid pVSIS/Pst was digested with Bgl II and Xba I in 40 ul of medium salt buffer. Subsequent electrophoresis through 1% agarose, extraction of the DNA and EtOH precipitation provided the purified v-sis 756 bp Bgl II--Xba I fragment (FIG. 2). E. coli replicating plasmid pUC12 (5 ug) was digested with Eco RI and Xba I and gel-purified as above (FIG. 2).

Referring to FIG. 2, equimolar amounts of the four DNA fragments described above, adjusted to 10 ng of the 296 bp Hph I--Bgl II v-sis fragment, were mixed in 15 ul of ligase buffer (6 mM Tris pH 7.6, 6.6 mM MgCl.sub.2, 0.4 mM ATp, 2 mM spermidine, 20 mM DTT, and 100 ug/ml BSA) and ligated with 40 units of T.sub.4 DNA ligase overnight at 14.degree. C. The reaction mixture was brought to room temperature, an additional 150 units of T.sub.4 ligase added, and incubated 10 more hours. Seven ul of the ligation mix was used to transform E. coli K-12 RR1 (ATCC #31343; Bolivar, E. et al., Gene 2: 95, 1977), and ampicillin-resistant transformants selected. Plasmid DNA was prepared from twelve such bacterial colonies and digested with Xba I. Two clones gave a 2.2 kb band predicted by the proper fragment alignment (FIG. 2). Further analysis of these by Bgl II--Xba I restriction mapping gave expected bands of approximately 1.5 kb from the MF.alpha.1/v-sis fusion and 760 bp for the Bgl II--Xba I v-sis fragment. DNA sequence analysis verified the desired nucleotide sequence at the MF.alpha.1/v-sis junction. The resultant plasmid was designated pVS.alpha..

Homology between the v-sis protein p28.sup.sis and PDGF begins at amino acid 67 of p28.sup.sis, a serine residue corresponding to the NH.sub.2 terminal residue of the PDGF B-chain (Johnsson, ibid.)

Proteolytic processing of the MF.alpha.1 primary translation product occurs at the Lys-Arg cleavage signal 85 amino acids from the initiator methionine (Kurjan and Herskowitz, ibid.). A v-sis derivative was constructed in which the first 66 codons of p28.sup.sis were removed such that serine residue 67 of v-sis immediately follows the MF 1 Lys-Arg processing signal.

Referring to FIG. 4, approximately 40 ng of the gel purified 2.2 kb Xba I fragment of pVS.alpha. was ligated with 120 ng of Xba I digested, alkaline phosphatase-treated M13mp11 DNA (Messing, Meth. in Enzymology, ibid.). The ligation mixture was used to transform E. coli K-12 strain JM101 (ATCC 33876) in the presence of X-gal and IPTG. Isolated white plaques were picked and used to infect 3 ml cultures of log phase growth JM101 cells. Replicative Form (RF) DNA was prepared and clones identified which carried the insert fragment in the same orientation as the positive (+) strand form of the single-stranded mature phage. Single-stranded phage DNA was prepared from one such clone and designated m11VS.alpha..

To precisely remove codons 1-66 of v-sis, oligo-nucleotide-directed mutagenesis was performed essentially according to the two-primer method of Zoller et al. (Manual for Advanced Techniques in Molecular Cloning Course, Cold Spring Harbor Laboratory, 1983). Oligonucleotide ZC 130 3' AGAAACCTATTTTCCTCGGACCCA 5' was synthesized on an Applied Biosystems 380-A DNA synthesizer. Fifty pmoles of ZC 130 was kinased in 10 ul of kinase buffer (BRL) with 4 units of T.sub.4 polynucleotide kinase for 45 minutes at 37.degree. C. The enzyme was inactivated by heating at 65.degree. C. for 10 minutes.

One-half pmole of m11VS.alpha. was annealed with 1 pmole of kinased ZC 130 and 1.5 pmoles of universal sequencing primer (BRL) using conditions described (Zoller et al., ibid.), except that the annealing mixture was first heated to 65.degree. C. for 10 minutes, shifted to 37.degree. C. for 10 minutes, and then quickly chilled on ice. The annealed mixture was then treated with Klenow polymerase as described by Zoller et al. (ibid.) to create circular duplex DNA. Portions of the elongation mixture were used to transform E. coli K12 JM101 cells. The resulting phage plaques were screened for the proper deletion by transfer onto nitrocellulose filters and subsequent hybridization with .sup.32 P phosphorylated ZC 130 at 65.degree. C. Correctly juxtaposed sequences formed stable duplexes with the radioactive probe at the stringent hybridization temperature employed. Approximately 1% of the transformants screened gave positive signals by autoradiography. Ten clones were plaque-purified and R(DNA was prepared for restriction enzyme analysis. Five isolates showed the expected decrease in size of 195 bp to the 1450 bp Hind III--Bgl II fragment (FIG. 4). DNA sequence analysis of two isolates confirmed the correct fusion junction had been made, thus maintaining the proper translational reading frame. One of these phage was designated m11VS2.alpha..

Yeast Replicating Vector YEp13 (Broach et al., Gene 8: 121, 1979) was used as an expression vehicle for v-sis-derived constructions described in Examples II and III. YEp13 is a multicopy extrachromosomal plasmid containing a 2 micron replication origin and the yeast LEU2 gene. This allows for selection of the plasmid in yeast strains possessing a defective chromosomal LEU2 gene when grown on synthetic medium lacking leucine. Addition of yeast terminator sequences to foreign genes expressed in yeast ensures efficient transcription termination and polyadenylation of mRNA. The v-sis expression units VS.alpha. and VS2.alpha. were placed adjacent to the TPI terminator fragment which was previously cloned into YEp13 (below).

Plasmid p270 (see FIG. 5) contains the transcription terminator region of the yeast triose phosphate isomerase (TPI) gene. It was constructed in the following manner. The yeast TPI terminator fragment was obtained from plasmid pFGl (Albert and Kawasaki, ibid.). It encompasses the region from the penultimate amino acid codon of the TPI gene to the Eco RI site approximately 700 base pairs downstream. A Bam HI site was substituted for this unique Eco RI site of pFGl by first cutting the plasmid with Eco RI, then blunting the ends with DNA polymerase I (Klenow fragment), adding synthetic Bam HI linkers (CGGATCCA), and re-ligating to produce plasmid p136. The TpI terminator was then excised from p136 as a Xba I--Bam HI fragment. This fragment was ligated into YEp13 (Broach et al., ibid.), which had been linearized with Xba I and Bam HI. The resulting plasmid is known as p213. The Hind III site was then removed from the TPI terminator region of p213 by digesting the plasmid with Hind III, blunting the resultant termini with DNA polymerase I (Klenow fragment), and recircularizing the linear molecule using T.sub.4 DNA ligase. The resulting plasmid is p270.

Alternatively, p270 may be constructed by digesting plasmid pM220 (see below) with Xba I and Bam HI, purifying the TPI terminator fragment 700 bp) and inserting this fragment into Xba I and Bam HI digested YEp13.

Referring to FIG. 6, plasmid p270 DNA was digested with Xba I and treated with calf alkaline phosphatase to prevent religation of the cohesive vector ends. V-sis expression units VS.alpha. and VS2.alpha. were prepared by Xba I digestion and agarose gel purification of pVS.alpha. and m11vs2.alpha. respectively. Each of the isolated fragments was ligated with an approximately equimolar amount of phosphatased p270 vector in the presence of 40 units of T.sub.4 DNA ligase and the ligation mixtures transformed into E. coli K-12 RR1. Plasmid DNA was prepared from ampicillin-resistant colonies and restriction enzyme analysis performed in order to identify clones which possessed the TPI terminator adjacent to 3' v-sis sequences. presence of 3.3 kb or 3.1 kb Bgl II fragments after gel electrophoresis indicated the correct orientation of YEpVS.alpha. and YEpVS2.alpha., respectively.

Because the product encoded by pVS2.alpha. is larger than authentic human PDGF B-chain and because a smaller product might result in higher expression levels in a transformed yeast host cell, a vector was constructed comprising the v-sis sequence of pVS2.alpha. truncated at the 3' end. The polypeptide encoded by this sequence comprises amino acids 67 to 175 of p28.sup.sis and is homologous to the B-chain of PDGF.

An expression vector containing this "B-chain" sequence was constructed by combining elements of the pVS2.alpha. expression unit with a partial v-sis gene and a synthetic double-stranded DNA fragment encoding amino acids 158 to 175 of p28.sup.sis. This synthetic fragment was designed to substitute preferred yeast codons for many of the 13 v-sis codons it replaces, and to supply a stop codon at the end of the coding sequence. The construction of this vector is illustrated in FIGS. 7 and 8.

Plasmid YEpVS2.alpha. was digested with Pst I and Bam HI; and the 1.8 kb fragment, comprising the partial MF.alpha.1, v-sis, and TPI terminator sequences, was purified by agarose gel electrophoresis. Plasmid pIC19R (Marsh et al., Gene 32: 481-486, 1984), comprising the polylinker shown in Chart 1 inserted into the Hind III site of pUC19 (Norrander et al., Gene 26: 101-106, 1983), was digested with Pst I and Bam HI, and the vector fragment was gel-purified and joined to the 1.8 kb fragment from pVS2 to produce plasmid pVS2.alpha.T. ##STR2##

The S. cerevisiae TPI promoter was used to control expression of VS2 sequences in a yeast expression vector. Plasmid pM220 contains the TPI promoter fused to the MF.alpha.1 signal sequence. E. coli RRI transformed with pM220 has been deposited with American Type Culture Collection under accession number 39853.

Plasmid pM220 was digested with Bgl II and Pst I (FIG. 7), and the ca. 1 kb fragment comprising the TPI promoter and the 5' portion of the MF.alpha.1 sequence was isolated and cloned in Bgl II+Pst I digested pIC19R. The resultant plasmid was digested with Cla I and Pst I, and the TPI promoter--MF.alpha.1 fragment was gel-purified. Plasmid pVS2.alpha.T was then cut with Cla I and Pst I and joined to the TPI promoter--MF.alpha.1 fragment. The correct construct was identified by the presence of a 2.6 kb Cla I--Bam HI fragment and was designated pTVS2.alpha.T.

Ten ug of plasmid pVS.alpha. was digested with Xma I and Sph I (FIG. 8) to completion. The resulting ca. 4.9 kb vector fragment, which also comprises most of the v-sis sequence, was purified by agarose gel electrophoresis, extraction of the DNA and EtOH precipitation.

In order to supply a new 3' terminus for the v-sis sequence, a double-stranded DNA fragment was constructed from oligonucleotides synthesized on an Applied Biosystems Model 380-A DNA synthesizer. 0.7 pmole of oligonucleotide ZC299 (Table 1) was heated with an equimolar amount of oligonucleotide ZC300 in a volume of 10 ul containing 40 mM NaCl for 5 minutes at 65.degree. C.

The mixture was then incubated at 37.degree. C. for 5 minutes and allowed to cool to room temperature. 0.2 pmole of the purified 4.9 kb vector fragment was added, the mixture ligated for 18 hours at 12.degree. C. and used to transform E. coli HB101 (ATCC 33694) to ampicillin resistance. DNA was prepared from ampicillin-resistant colonies and digested with Bgl II and Xba I. After electrophoresis through agarose, the desired clone (known as pVS.alpha.B) was identified by loss of a ca. 750 bp Bgl II--Xba I fragment and appearance of two smaller fragments of approximately 500 and 260 bp.

Approximately 8 ug of plasmid pTVS2.alpha.T (FIG. 8) were digested to completion with Xba I in a volume of 10 ul. The volume was increased to 40 ul with Bgl 11 buffer, and 6 units of Bgl II were added and the mixture was incubated at 37.degree. C. Ten ul aliquots were removed to a stop buffer containing 50 mM EDTA at 15 and 30 minutes, and the remaining 20 ul stopped at 45 minutes. The resulting mixtures were separated by electrophoresis through 0.7% agarose. The ca. 4.6 kb Bgl II--Xba I vector fragment was cut out, extracted from the gel, and EtOH precipitated. Plasmid pVS.alpha.B was digested with Bgl II and Xba I, and the ca. 260 bp fragment containing the synthetic 3' terminus and stop codon was isolated by electrophoresis through agarose, subsequent extraction from the gel, and EtOH precipitation.

The 4.6 kb Bgl II--Xba I vector fragment from pTVS2.alpha.T and the 260 bp Bgl II--Xba I fragment from pVS.alpha.B were ligated in the presence of T.sub.4 DNA ligase for 7 hours at room temperature. The reaction mixture was used to transform E. coli HB101 to ampicillin resistance. DNA was prepared from transformants and the presence of the desired insert was confirmed by screening for a 550 bp Pst I--Xba I band on an agarose gel. A plasmid having the correct configuration was designated pVSB.

There are several alternative approaches which can be used to construct plasmid pVSB. The essential elements of pVSB include: the TPI promoter/alpha-factor fusion, which can be obtained from plasmid pM220, the B-chain coding sequence (base 551 through 877 of FIG. 1B) of the v-sis gene, which is widely available, and the TPI terminator, which can be obtained from plasmid p270. Someone skilled in the art could develop several strategies to arrive at pVSB using these elements.

In order to achieve maximal protein production from a yeast culture, it is desirable to use expression vehicles which are very stably maintained in the host cell. Plasmid pCPOT is such a preferred expression vehicle.

E. coli HB101 transformed with pCPOT has been deposited with American Type Culture Collection under accession number 39685. Plasmid pCPOT comprises the 2 micron circle genome (Hartley and Donelson, Nature 286: 860, 1980), E. coli plasmid pBR322 replication and selection sequences, and the Schizosaccharomyces pombe DNA sequences encoding the glycolytic enzyme triose phosphate isomerase (POT1). Presence of the POT1 gene in pCPOT ensures stable maintenance of the plasmid in the appropriate host background during growth on nonselective medium utilizing glucose as a carbon source.

For expression of the v-sis derivatives in yeast, a stable expression vector comprising the REP1, REP2, REP3 and ori sequences from yeast 2 micron DNA and the Schizosaccharomyces pombe triose phosphate isomerase (POT1) gene was constructed. The POT1 gene provides for plasmid maintenance in a transformed yeast host grown in complex media if such host is defective for triose phosphate isomerase.

The POT1 gene was obtained from the plasmid pFATPOT. S. cerevisiae strain E18 transformed with pFATPOT has been deposited with ATCC under accession number 20699. The plasmid may be purified from the host cells by conventional techniques. The POT1 sequence was removed from pFATPOT by digestion of the plasmid with Sal I and Bam HI. This 1600 bp fragment was then ligated to pIC19R, which had first been linearized by digestion with Sal I and Bam HI. The Bam HI, Pst I and Sal I sites in the resultant plasmid were destroyed in two steps to produce plasmid pICPOT*. The Pst I and Sal I sites were removed by cutting with pst I and Sal I; the ends were blunted by digesting the Pst I 3' overhang with DNA polymerase I (Klenow fragment) and filling in the Sal I 5' overhang with Klenow fragment. The blunt ends were then ligated. The Bam HI site was then removed by cutting the plasmid with Bam HI, filling in the ends with DNA polymerase I (Klenow fragment) and religating the blunt ends.

The 2u sequences were obtained from the plasmids YEp13 (Broach et al., Gene 8: 121-133, 1979) and Cl/l. Cl/l was constructed from pJDB248 (Beggs, Nature 275: 104-109, 1978) by removal of the pMB9 sequences by partial digestion with Eco RI and replacement by Eco RI-cut pBR322. The REP3 and ori sequences were removed from YEp13 by digestion with Pst I and Xba I and gel purification. REP2 was obtained from Cl/l by digestion with Xba I and Sph I and gel purification. The two fragments were then joined to pUC18 (Norrander et al., Gene 26: 101-106, 1983) which had been linearized with Pst I and Sph I to produce plasmid pUCREP2,3. REP1 was obtained from Cl/l by digestion with Eco RI and Xba I and gel purification of the 1704 bp fragment. The Eco RI--Xba I fragment was cloned into pUC13 which had been linearized with Eco RI and Xba I. The resultant plasmid was designated pUC13+REP1. The pUC13+REP1 plasmid was cut with Hind II and ligated in the presence of Eco RI linkers (obtained from Bethesda Research Laboratories). The REP1 gene was then removed as an Eco RI fragment of approximately 1720 bp. This Eco RI fragment was cloned into pIC7 (Marsh et al., ibid.), which had been linearized with Eco RI and Xba I. The resultant plasmid was designated pICREP1#9.

To construct the final expression vector pMPOT2 (FIG. 8), pICPOT* was linearized by a partial Hind III digestion and complete Sst I digestion. Plasmid pUCREP2,3 was cut with Hind III and Sst I, and the fragment comprising REP2, REP3 and ori sequences was gel-purified and joined to the linearized pICPOT*. The resultant plasmid, comprising REP2, REP3, ori, POT1 and amp.sup.r sequences, was designated pMPOT1. REP1 was then removed from pICREP1 as a Bgl II--Nar I fragment and was ligated to pMPOT1, which had been cleaved with Bgl II and Nar I. The product of this ligation was designated pMPOT2 (deposited with ATCC, accession number 20744). Plasmid pMPOT2 was digested with Cla I and Bam HI, and the vector fragment was purified as above.

Plasmid pVSB was digested with Cla I and Bam HI, and the 2.2 kb fragment containing the "B-chain" expression unit purified by agarose gel electrophoresis and EtOH precipitation. Plasmid pMPOT2 was also digested with Cla I and Bam HI. The fragments were ligated overnight at room temperature in the presence of T.sub.4 DNA ligase and the reaction mixture used to transform E. coli HB101 to ampicillin resistance. DNA was prepared from transformants and the presence of the insert verified by digestion with Cla I and Bam HI and agarose gel electrophoresis. The resulting expression vector was designated pVSBm (FIG. 8).

Plasmids pVSBm and pMPOT2 were used to transform S. cerevisiae strain E18 #9 by conventional methods. Strain E18-#9 is a diploid produced by crossing strains E11-3c (ATCC No. 20727) (.DELTA.tpi::LEU2 pep4 leu2 MAT.alpha.) and .DELTA.tpi29 (.DELTA.tpi::LEU2 pep4 leu2 his MATa). .DELTA.tpi29 is produced by disrupting the triose phosphate isomerase gene of strain E2-7b (ATCC No. 20689), essentially as described by Rothstein (Meth. in Enzymology 101: 202-210, 1983).

In order to begin replacing B-chain coding sequence with A-chain sequence in the pVSB vector, a convenient Sst I restriction endonuclease site was created close to the .alpha.-factor prepro-B-chain boundary (FIG. 8). This was accomplished by oligonucleotide-directed mutagenesis (Zoller and Smith, DNA 3: 479-488, 1984) on a single-stranded pVSB template using established techniques. The mutagenic oligonucleotide used is termed ZC506 and can be seen in Table 2.

A Pst I--Xba I fragment of pVSB (FIG. 8) was subcloned into the M13 phage vector mp19. Single-stranded template DNA was prepared from E. coli JM107 cultures infected with this recombinant phage and used in the following mutagenesis reaction. Five ul of M13 template DNA (0.5 picomole) were combined with 2 ul of oligonucleotide ZC506 (1.8 pmole) plus 2.5 ul of water and 1.5 ul of 10X annealing buffer A (0.2 M Tris-HCl, 0.0 M MgCl.sub.2, 0.01 M DTT pH 7.5; Zoller and Smith, DNA 3: 479-488, 1984). This mixture was annealed by heating to 70.degree. C. for 5 minutes, cooled slowly to room temperature and then placed on ice. To this cold annealing mixture was added 1.5 ul of 10X elongation buffer B (0.2 M Tris-HCl, 0.1 M MgCl.sub.2, 0.1 M DTT pH 7.5, Zoller and Smith, ibid.), 6 ul of deoxynucleotide triphosphates (2.5 mM each dNTP), 1 ul of T.sub.4 DNA ligase, 1 ul of DNA polymerase Klenow fragment, 1 ul ATP (10 mM), and 5 ul of water. This mixture was incubated for 16 hours at 18.degree. C. This reaction mixture was then diluted 20-fold with water, and 2 ul of the dilute mixture was used to transform E. coli JM107 cells. The resulting phage plaques were transferred to nitrocellulose discs by the procedure of Benton and Davis (Science 196: 180, 1977) and screened with 32p-labeled ZC506 which was labeled with T.sub.4 polynucleotide kinase under standard conditions. The hybridization of the .sup.32 P-ZC506 to the filters was performed at 37.degree. C. in 6X SSC (0.9 M NaCl, 0.09 M Na Citrate, pH 7.2), 100 ug/ml carrier DNA, 0.05% sodium pyrophosphate. Following hybridization, the filters were washed at 54.degree. C. in 6X SSC, 0.1% SDS. Phage plaques giving strong autoradiographic signals were picked and RF DNA made and analyzed for the presence of a new Sst I restriction endonuclease site. The sequence around the Sst I site was also confirmed by DNA sequence analysis. The Pst I-Xba I subclone now containing an Sst I site was ligated back into Pst I-Xba I digested pVSB and the resulting plasmid termed pSB1. Plasmid pSB1 encodes two amino acid changes (Leu to Glu and Asp to Leu) in the alphafactor leader just upstream of the Lys-Arg. The resulting junction sequence is: .alpha.-factor . . . Glu Leu Lys Arg Ser . . . B-chain. The B-chain coding sequences of pSB1 are thus flanked by an Sst I site at the 5' end and an Xba site at the 3' end.

The A-chain coding sequences were inserted into the pSB1 vector as short synthetic oligonucleotide duplexes designed to encode known A-chain amino acid sequence (Johnson et al., EMBO J. 3: 921-928, 1984). ZC545 and ZC546 (Table 2) were annealed, creating a short duplex DNA fragment with a 5' Sst I cohesive end, a unique Mlu I restriction site, and a 3' Bgl II cohesive end. This duplex was cloned into Sst I and Bgl II digested pSB1. One ul of pSB1 vector (0.15 pmole) was combined with 1 ul of ZC546 (.about.1.6 pmole) and 0.6 ul of ZC545 (.about.1.5 pmole), plus 0.25 ul of 0.3 M NaCl (final NaCl concentration in the annealing reaction is 30 mM) and the mixture was heated to 60.degree. C. for five minutes. After heating, the mixture was brought to room temperature and then placed on ice. Then 0.5 ul of 10X ligase buffer (0 5 M Tris-HCl, 0.1 M MgCl.sub.2, 2 M DTT, 0.01 M ATP, pH 7.8), 0.1 ul of T.sub.4 DNA ligase (New England Biolabs) and 2.5 ul of water were added and this ligation mixture was diluted and used to transform E. coli HB101 cells. Ampicillin-resistant, plasmid-bearing colonies were picked, grown up and plasmid DNA isolated by the "miniprep" method of Ish-Horowicz and Burke (Nuc. Acid Res. 9: 2989-2998, 1981). The plasmids were analyzed for the presence of an Sst I-Bgl II insert and a new Mlu I restriction site and confirmed by DNA sequence analysis. The ZC545-546 duplex encoded A-chain amino acids alanine 8 through tryosine 17 (FIG. 9) and the resulting plasmid was termed pA1.

ZC547 and ZC548 (Table 2) were annealed to create a second short Sst 1--Bgl II fragment encoding A-chain amino acids serine 1 through arginine 13 (FIG. 9) and also containing an Mlu I restriction site. The ZC547-548 duplex was separately cloned into Sst I and Bgl II digested pSB1. One ul of pSB1 (1.5 pmole) digested with Sst I and Bgl II was combined with 2 ul of ZC547 (1 pmole) and 2 ul of ZC548 (1 pmole) plus 0.25 ul of 0.3 M NaCl and the mixture was heated to 50.degree. C. for five minutes After heating, this annealing mixture was brought to room temperature and then placed on ice. Then 0.6 ul of 10X ligase buffer and 0.1 ul of T.sub.4 DNA ligase (New England Biolabs) were added and the reaction was incubated overnight at 12.degree. C. An aliquot of this ligation reaction was diluted and used to transform E. coli HB101 cells and the resulting plasmids were screened and analyzed as described above for pA1. In this case, the resulting plasmid was termed pA2.

The overlapping pA1 and pA2 A-chain coding regions were joined at the unique Mlu I restriction site using conventional techniques. Plasmid pA2 was digested with Mlu I and Bam HI and the .about.1.4 kb vector (pUC containing) fragment was isolated by agarose gel electrophoresis and extracted from the agarose with CTAB (Langridge et al., Anal. Biochem. 103: 264-271, 1980). Plasmid pA1 was also digested with Mlu I and Bam HI and the .about.800 base pair fragment, encoding A-chain amino acids 13 through 17 fused to B-chain amino acids 24 through 109 followed by the TPI terminator, was isolated and extracted as above. Equimolar amounts of these two fragments were ligated under standard conditions and an aliquot used to transform E. coli HB101 cells. Plasmids obtained from ampicillin-resistant colonies were analyzed by restriction enzyme digestion for the correct fragments and confirmed by DNA sequencing. The resulting plasmid termed pA3 thus encoded a hybrid protein beginning with A-chain amino acids 1 through 17 followed in frame by B-chain amino acids 24 through 109. The Cla 1--Bam HI fragment of pA3 containing the entire expression unit was cloned into pMPOT2 and the resulting plasmid pA3m was transformed into yeast.

Further addition of A-chain amino acids to the A-B hybrid was accomplished in a similar fashion. Plasmid pA3 was digested first with Asp718, which cuts the plasmid once in the A-chain sequence at proline codon 7, and with Bam HI, and the hybrid amino acid coding fragment subcloned into pUC118. This subclone was termed pA3N and was subsequently digested with Bgl II and Bst XI. Bgl II cuts at the boundary of the A- and B-chain sequences in the hybrid and Bst XI cuts approximately 40 base pairs downstream in the B-chain. The vector fragment (pUC containing) from this digest was isolated by agarose gel electrophoresis and extracted with CTAB. One picomole each of oligonucleotides ZC692 and ZC693 (Table 2) was annealed to form a short DNA duplex with a 5' Bgl II end and a 3' Bst XI end. This duplex encoded A-chain glutamic acid 18 through phenylanine 31 and was ligated with 0.1 picomole of Bgl II-Bst XI digested pA3N The ligation was performed overnight and the ligated products transformed into E. coli MV1193 cells. The resulting plasmid termed pA6N now has extended the A-chain amino acid sequence to the Bst XI site at amino acid A31 followed by B-chain amino acids B38 through B109.

Plasmid pA6N was then digested with Asp718 and Bam HI and the A-B hybrid fragment cloned back into Asp718Bam HI digested pA3m. This new A-B hybrid plasmid is termed pA6m and encodes A-chain amino acid sequence up to amino acid 40 because the Bst XI site lies at the start of a region of high homology between A- and B-chains

As can be seen from FIG. 9, both the A- and B-chains of PDGF contain eight cysteine residues which are capable of forming disulphide bonds. It can also be seen from FIG. 9 that these cysteine residues are in analogous positions in the two polypeptides and hence may participate in similar disulfide arrangements in and between the two chains and even between two different chains (A and B). It has been known for several years that chemical reduction of the disulfide-bonded PDGF dimer to monomers destroys its biological activity. It is of interest to know which of the cysteine residues in question are involved in disulfide bonds of both the intra- and intermolecular type. It is very likely that the role of each cysteine will be analogous in both A- and B-chains.

The first cysteine residue in the A-chain occurs at position #10, which is analogous to #16 in the B-chain (FIG. 9). The A B hybrid pA3 (described above) encodes A-chain amino acids 1-17, followed by B-chain. The synthetic strategy leading to construction of pA3 incorporated unique restriction sites flanking the cysteine at residue A10. An Asp718 and an Mlu I restriction site were placed 5' and 3', respectively, to the A10 cysteine codon approximately 20 base pairs apart. Two oligonucleotides (ZC671 and ZC672, Table 2) were synthesized and annealed to form a short DNA duplex with a 5' Asp718 cohesive end and a 3' Mlu I cohesive end. This duplex encodes a serine residue in place of cysteine A10. Plasmid pA3 was digested with Asp718 and Mlu I and the large vector (pUC containing) fragment isolated by agarose gel electrophoresis. Equimolar amounts of the vector and the ZC671-672 duplex were ligated under standard conditions as described above and then transformed into E. coli MV1193 cells. Plasmid (miniprep) DNA was prepared from the resulting transformants and screened for a new Pvu II site present in the ZC671-672 duplex. The duplex region of the plasmid is then confirmed by DNA sequence analysis. The resulting plasmid, termed pA5, encodes an A-B hybrid protein with A-chain amino acids 1-17 at the amino terminus, but residue 10 is a serine instead of a cysteine. The remaining amino acids of the pA5 hybrid are the normal B-chain residues (Glu 24 through Thr 109).

The remainder of the A-chain gene was synthesized with oligonucleotides in a fashion very similar to that described above. Many strategies could be designed to accomplish this task. One such strategy is described below. The oligonucleotides used in this strategy are shown in Table 2 and their design reflects optimal codon usage for Saccharomyces cerevisiae. In this strategy, the remainder of the A-chain gene was synthesized with unique restriction sites introduced in order to facilitate subcloning and sequencing the synthetic oligonucleotide sequences. All the oligonucleotides were synthesized on an Applied Biosystems 380-A DNA synthesizer Oligonucleotides ZC752 and ZC753, each 87mers, were annealed and subcloned as a Hind III--Xba I fragment encoding A-chain amino acids 77-104. ZC752 and ZC753 (1.25 picomole each) were annealed in 5 ul of 40 mM NaCl by heating to 65.degree. C. for 15 minutes and then allowing the mixture to come to room temperature and putting on ice. One tenth of this annealed duplex (0.0125 picomole) was ligated into both pUC118 (0.07 pmole) and M13 mp18 (0.02 picomole) which were previously digested with Hind III and Xba I. The ligated mixtures were used to transform the appropriate E. coli host strain (JM107 in the case of M13 mp18 and MV1193 in the case of pUC118) and the resulting plasmid or RF DNAs analyzed by restriction endonuclease digestion and DNA sequencing.

The oligonucleotides ZC746+747, 748+749, and 750+751 were designed to form short duplexes with cohesive ends which when joined would constitute the sequence between the Bst XI site at A31 and the Hind III site at A77. The oligonucleotides were phosphorylated with 32P and T.sub.4 polynucleotide kinase under standard conditions. The pairs ZC746+ZC747, ZC748+ZC749, and ZC750+ZC751 were each annealed by combining 2.5 pmole of each oligonucleotide in 5 ul of 40 mM NaCl, heating to 65.degree. C. for 15 minutes, allowing to come to room temperature, and putting on ice. The three annealing mixtures were combined (now 15 ul) and ligated in a final volume of 20 ul. The ligated products were electrophoresed in a 4% NuSieve agarose gel (FMC Corporation) in TBE buffer (90 mM Tris, 90 mM boric acid, 2 mM disodium EDTA) followed by autoradiography. The .about.140 base pair fragment corresponding to the three correctly ligated duplexes was cut out of the gel and extracted with CTAB. This fragment, together with the previously cloned Hind IIl-Xba I fragment, was ligated into the Bst XI-Xba I digested pA6N vector The resulting plasmid was termed pA6N+. Plasmid pA6N+ was then digested with Asp718 and Xba I and the A-chain coding fragment cloned back into pA3 This plasmid pA7 encodes the entire mature A-chain

For purposes of yeast expression, a preferred embodiment would employ oligonucleotides ZC748 and ZC749. These encode a glutamine at position A-48 instead of an asparagine. This change destroys the N-linked glycosylation site which can be aberrantly glycosylated in yeast. Oligonucleotides designed to preserve the N-linked glycosylation site could also be used.

The strategy employing total gene synthesis described above is desirable because the amino acid sequence of the A-chain is known and the codon usage can be optimized for yeast. Alternatively, an A-chain cDNA sequence could be expressed in yeast or other eukaryotic cells, provided the cDNA was appropriately incorporated into a suitable expression vector An A-chain cDNA could be obtained from a variety of mammalian cell lines by conventional techniques (Betsholtz, C. et al., Nature 320, 695-699, 1986.)

During biosynthesis of the A-chain protein in the yeast expression system, the .alpha.-factor prepro-peptide is removed from the A-chain by proteolytic processing at the basic dipeptide Lys-Arg, alpha factor amino acid residues #84 and #85. In order to drive the proteolytic processing to occur at an internal A-chain site, the Lys-Arg at the .alpha.-factor-A-chain boundary is removed and an internal Arg-Arg created by oligonucleotide directed mutagenesis.

The Lys-Arg removal mutagenesis is performed essentially as described for the construction of pSB1 above. The Pst I--Xba I fragment of pVSB (FIG. 8) is subcloned into the M13 phage vector mp19 and single-stranded template DNA is prepared. In this case, the mutagenic oligonucleotide is designed to change the .alpha.-factor Lys-Arg residues to Gly-Leu and to introduce a new Pvu II restriction site. The mutagenesis reactions are carried out as described above for pSB1 and the resulting mutants are screened for the new Pvu II site and then confirmed by DNA sequence analysis The mutagenized Pst 1-Xba fragment is subcloned back into the A-chain expression unit (designated pA7) into

In order to introduce a dibasic peptide site the A-chain coding sequence, oligonucleotide directed mutagenesis is employed as described above. Amino acid residue #22 in the A-chain is a serine, while #21 is an Arg. In this case, the mutagenic oligonucleotide is designed to change the Ser #22 to an Arg, creating the sequence Arg-Arg at positions #21 and #22. This new dibasic site in the A-chain occurs in a position precisely analogous to one which is normally present in the B-chain (FIG. 9) By expressing this mutant construction from the .alpha.-factor leader lacking the dibasic processing site, the resultant A-chain molecule should be processed internally at the new Arg-Arg and be secreted as a truncated polypeptide.

Each of the molecules constructed in Examples VI-VIII above was introduced back into the basic expression unit pVSB or pSB1 if the Sst I site was employed. Then each of them was ultimately cloned into the yeast plasmid pMPOT2 (Example IV). In each case, this was done by removing the expression unit as a single fragment from pVSB or pSB1 by Cla I-Bam HI digestion. The Cla I--Bam HI fragment of each was isolated by agarose gel electrophoresis and cloned into pMPOT2 which had been digested with Cla I and Bam HI. The names of the resulting plasmids are then amended with a lower case "m," e.g., pA2 becomes pA2m.

Each of the mPOT constructions was then transformed into the yeast strain E18-#9 (Example V)

The radioreceptor assay for PDGF (Bowen-Pope and Ross, J. Biol. Chem. 257 5161, 1982) is a specific and sensitive (0.2-2 ng/ml PDGF) method for detecting biologically active PDGF-like material in yeast In this assay, PDGF-like material is tested for its ability to compete with purified, radio-labeled .sup.125 I PDGF for binding sites on cell surface PDGF receptors Results are interpreted by comparison to a standard curve generated with purified, unlabeled PDGF. Comparison of results obtained with other assay methods (e.g., ELISA) provides an indication of the strength of the receptor/ligand interaction in addition to quantitation of the material bound. The assay is conducted as follows: Subconfluent monolayers of diploid human fibroblasts are prepared by plating 1.5.times.10.sup.4 cells per 2 cm.sup.2 culture well in Costar 24 well cluster trays in Dulbecco's Modified Eagles Medium (DMEM) supplemented with 1% human plasma-derived serum (PDS). Cultures are set on an ice tray and rinsed once with ice-cold binding rinse (Ham's medium F-12 buffered at pH 7.4 with 25 mM HEPES and supplemented with 0.25% BSA). One ml/well of test substance in binding medium is added and the cultures incubated in a refrigerated room on an oscillating platform for 3-4 hours. The trays are then placed on ice, aspirated, rinsed once with cold binding rinse and incubated for one hour as above with 1 ml/well binding medium containing 0.5 ng/ml .sup.125 I-PDGF. Labeling is terminated with four rinses of binding rinse and cell-associated .sup.125 I-PDGF determined by extraction with solubilization buffer. Standard curves are obtained using 0, 0.05, 0.1, 0.2, 0.4, and 0.8 ng/ml purified PDGF and test samples compared to these values.

PDGF receptor binding by CM-Sephadex media concentrates from yeast transformants containing plasmids pVSBm and pMPOT2 was compared to receptor binding by authentic PDGF. After concentration by binding to and elution from CM-Sephadex, the pVSBm concentrate was normalized to PDGF equivalents in an ELISA using polyclonal goat antibody to PDGF. The RRA results were interpreted by comparison to a standard curve generated with purified, unlabeled PDGF, as shown in FIG. 10. Media from cultures transformed with the pVSBm constructions are shown to compete with .sup.125 I-PDGF for binding to the PDGF receptor. Media from yeast cells transformed with pMPOT2 do not compete with radio-labeled PDGF for receptor binding.

The ability of PDGF to stimulate DNA synthesis and cell growth in culture was the basis for its definition and discovery. .sup.3 H-Thymidine incorporation into DNA of cultured cells responsive to PDGF (Raines and Ross, Meth. in Enzomology 109: in press) is a preferred method for demonstrating the biological activity of PDGF-like molecules produced in yeast.

Straight spent media test samples or concentrates of spent media or test samples in 10 mM acetic acid (up to 100 ul/well) are added to quiescent cultures of mouse 3T3 cells in 2cm.sup.2 Costar 24-well culture dishes (2-3.times.10.sup.8 cells/well in 1 ml). Quiescent test cultures can be obtained by plating the cells in 10% serum and allowing them to deplete the medium, 4-5 days. The test samples are removed from the wells at 20 hours and replaced with 0.5 ml of fresh medium per well containing 2 uCi/ml [3H]-Thymidine and 5% (v/v) calf serum. After an additional two-hour incubation at 37.degree. C. the cells are harvested by: aspirating off the medium, washing the wells twice each with 1 ml of ice-cold 5% TCA; solubilizing TCA-insoluble material in 0.8 ml 0.25N NaOH with mixing; and counting 0.6 ml of this solution in 5 ml Aquasol in a liquid scintillation counter. Fold stimulation over control wells (100 ul of 10 mM acetic acid alone) is determined (normally 30-50 fold maximal stimulation) and compared to a standard curve obtained using purified PDGF preparations.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.