Polyomavirus middle T protein encoded by a retrovirus transforms nonestablished chicken embryo cells

Polyomavirus middle T-antigen NPTY mutants

A polyomavirus middle T-antigen (MTAg) mutant containing a substitution of Leu for Pro at amino acid 248 has previously been described as completely transformation defective (B. J. Druker, L. Ling, B. Cohen, T. M. Roberts, and B. S. Schaffhausen, J. Virol. 64:4454-4461, 1990). This mutant had no alterations in associated proteins or associated kinase activities compared with wild-type MTAg. Pro-248 lies in a tetrameric sequence, NPTY, which is reminiscent of the so-called NPXY sequence in the low-density-lipoprotein receptor. In the low-density-lipoprotein receptor, mutations in the NPXY motif but not in the surrounding amino acids abolish receptor function, apparently by decreasing receptor internalization (W. Chen, J. L. Goldstein, and M. S. Brown, J. Biol. Chem. 265:3116-3123, 1990). To determine whether this sequence represents a functional motif in MTAg as well, a series of single amino acid substitutions was constructed in this region of MTAg. All of the mutations of N, P, T, or Y, including the relatively conservative substitution of Ser for Thr at amino acid 249, resulted in a transformation-defective MTAg, whereas mutations outside of this sequence allowed mutants to retain near-wild-type transformation capabilities. Transformation-defective mutants with mutations in the NPTY region behaved similarly to the mutant with the original Pro-248-to-Leu-248 mutation when assayed for associated proteins and activities in vitro; that is, they retained a full complement of wild-type activities and associated proteins. Further, insertion of the tetrameric sequence NPTY downstream of the mutated motif restored transforming abilities to these mutants. Thus, the tetrameric sequence NPTY in MTAg appears to represent a well-defined functional motif of MTAg.

Generation of a large library of point mutations in polyoma middle T antigen

Polyoma middle T antigen (MTAg) transforms cells by associating with and activating a variety of intracellular proteins, including src family members and a phosphatidylinositol-3 kinase. In order to assist in the study of the relative importance of the various associated biochemical activities for transformation by polyomavirus MTAg, a library of MTAg mutants was constructed. Chemically mutagenized MTAg DNA was purified from wild-type DNA by separation on denaturing gradient gels and placed into a recombinant retrovirus vector. Utilizing the resultant library of MTAg-expressing retroviruses, fibroblast cell lines expressing retroviruses, fibroblast cell lines expressing individual MTAg mutants were generated and screened for a non-transformed morphology. Of the first seven non-transformed clones tested, all express the MTAg protein. We estimate that approximately 24% of the G418-resistant colonies contain a transformation-defective MTAg mutant. A more thorough evaluation of one such clone revealed four single base-pair changes as compared to wild-type. Further genetic dissection of this mutant reveals that substituting leucine for proline at amino acid 248 results in a completely transformation defective MTAg. The utility of this mutagenesis and screening procedure as well as the description of several new MTAg mutants is described. This library of mutations should be of general interest for studying the transforming ability of MTAg.

A completely transformation-defective point mutant of polyomavirus middle T antigen which retains full associated phosphatidylinositol kinase activity

By using a random mutagenesis procedure combined with a recombinant retrovirus vector, mutants of polyomavirus middle T antigen (MTAg) were generated. Three new MTAg mutants with various degrees of transformation competence were more thoroughly characterized. All of the mutants produced a stable MTAg, as assessed by metabolic labeling or immunoblotting, and each mutant possessed wild-type levels of associated tyrosine kinase activity and associated phosphatidylinositol-3 (PI-3) kinase activity. One of these mutants, with a substitution of leucine for proline at amino acid 248 of MTAg (248m) was completely transformation defective, as measured in a focus-forming assay. Furthermore, the pattern of phosphorylation of 248m in vivo was identical to that of wild-type MTAg, and the kinetics of association of MTAg with an 85-kilodalton protein, the putative PI kinase, was not altered. Similarly, the pattern of PI derivatives obtained in an in vitro kinase assay was not altered by the substitution at amino acid 248. Since the single base pair mutation at amino acid 248 resulted in an MTAg that was completely transformation defective despite possessing wild-type levels of kinase activities, this suggests that neither tyrosine kinase nor PI-3 kinase activity nor the combination of both are sufficient for transformation by MTAg.

Characterization of pp85, a target of oncogenes and growth factor receptors

An 85,000-molecular-weight polypeptide (85K polypeptide) has previously been identified as a common substrate for tyrosine phosphorylation upon polyomavirus middle T transformation or upon platelet-derived growth factor stimulation of 3T3 cells. In each case, pp85 has an associated phosphatidylinositol kinase activity. The tissue distribution of pp85 was determined by middle T blotting experiments; the highest levels were found in brain, lung, and spleen tissues. High-resolution examination of 85K by isoelectric focusing demonstrated that there are at least 10 different forms. These were resolved into two families, 85K and 86K; the ratio of the two families changed in different cells. Similar forms were found for pp85 associated with pp60v-src. Individual species within each family differed by phosphorylation. Analysis of pp85 and pp86 by immunoprecipitation with anti-phosphotyrosine antibody showed increasing phosphorylation in response to middle T or pp60v-src transformation. The association of middle T with pp85 and pp60c-src was examined in pulse-chase experiments. Association of middle T with pp60c-src was slow and was accompanied by progressive modification of middle T. pp85 formed a dissociable complex with middle T within 2.5 min.

Characterization of pp85, a target of oncogenes and growth factor receptors

An 85,000-molecular-weight polypeptide (85K polypeptide) has previously been identified as a common substrate for tyrosine phosphorylation upon polyomavirus middle T transformation or upon platelet-derived growth factor stimulation of 3T3 cells. In each case, pp85 has an associated phosphatidylinositol kinase activity. The tissue distribution of pp85 was determined by middle T blotting experiments; the highest levels were found in brain, lung, and spleen tissues. High-resolution examination of 85K by isoelectric focusing demonstrated that there are at least 10 different forms. These were resolved into two families, 85K and 86K; the ratio of the two families changed in different cells. Similar forms were found for pp85 associated with pp60v-src. Individual species within each family differed by phosphorylation. Analysis of pp85 and pp86 by immunoprecipitation with anti-phosphotyrosine antibody showed increasing phosphorylation in response to middle T or pp60v-src transformation. The association of middle T with pp85 and pp60c-src was examined in pulse-chase experiments. Association of middle T with pp60c-src was slow and was accompanied by progressive modification of middle T. pp85 formed a dissociable complex with middle T within 2.5 min.

Association of p62c-yes with polyomavirus middle T-antigen mutants correlates with transforming ability

A number of mutants of polyomavirus middle T antigen (MTag) were constructed into replication-competent avian retroviruses. To assess the ability of these MTag variants to transform and to associate with the avian p60c-src and p62c-yes proto-oncogene products, we used these viruses to infect chicken embryo fibroblasts. We found that the ability of individual mutant MTags to associate with p62c-yes correlated well with the ability of these mutants to transform, as has been previously shown for the association of MTag with p60c-src. All transformation-competent mutant MTags retained the ability to complex with p62c-yes. Two transformation-defective mutants, RX67 and RX68, which could weakly associate with p60c-src, were unable to associate with p62c-yes.dl1015, a transformation-defective mutant which could associate with p60c-src and with a phosphatidylinositol kinase activity, was also able to associate with p62c-yes. Therefore, some as yet unmeasured biochemical property is defective in this mutant.

pp60c-src activation in human colon carcinoma

We measured the in vitro protein-tyrosine kinase activity of pp60c-src from human colon carcinoma cell lines and tumors. The activity of pp60c-src from six of nine carcinoma cell lines was higher (on average, fivefold as measured by enolase phosphorylation, or eightfold as measured by autophosphorylation) than that of pp60c-src from normal colonic mucosal cells, or human or rodent fibroblasts. Similarly, the activity of pp60c-src from 13 of 21 primary colon carcinomas was five- or sevenfold higher than that of pp60c-src from normal colonic mucosa adjacent to the tumor. The increased pp60c-src activity did not result solely from an increase in the level of pp60c-src protein, suggesting the specific activity of the pp60c-src kinase is elevated in the tumor cells. pp60c-src from colon carcinoma cells and normal colonic mucosal cells was phosphorylated at similar sites. We used immunoblotting with antibodies to phosphotyrosine to identify substrates of protein-tyrosine kinases in colonic cells. Three phosphotyrosine-containing proteins were detected at significantly higher levels in most colon carcinoma cell lines than in normal colonic mucosal cells or human or rat fibroblasts. All colon carcinoma cell lines with elevated pp60c-src in vitro kinase activity, showed increased phosphorylation of proteins on tyrosine in vivo, suggesting the presence of an activated protein-tyrosine kinase(s).

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.

p56lck protein-tyrosine kinase is cytoskeletal and does not bind to polyomavirus middle T antigen

p56lck and p60c-src are closely related protein-tyrosine kinases that are activated by similar oncogenic mutations. We have used fibroblast cell lines that express p56lck from introduced DNA molecules to compare the subcellular localizations of p60c-src and p56lck and their abilities to bind polyomavirus middle T antigen (mT). p56lck is associated with the detergent-insoluble matrix, as defined by extraction with solutions containing nonionic detergents, whereas p60c-src is soluble under these conditions. p56lck is also associated with detergent-insoluble structures in a lymphoid cell line, LSTRA. p60c-src binds to mT, but p56lck does not bind detectably. In terms of both solubility and mT interactions, the nononcogenic p56lck more closely resembles oncogenically activated p60c-src mutants than it resembles p60c-src. Because published results show that an intact carboxy terminus is required for p60c-src to bind mT and be soluble, we tested whether the different localization and mT binding properties of p56lck and p60c-src were dictated by their different carboxy termini. A protein consisting largely of p60c-src sequences but carrying a p56lck carboxy terminus was soluble and bound to mT. We suggest that both the solubility and mT-binding properties of p60c-src not only require sequences common to the carboxy termini of p60c-src and p56lck, but also require sequences unique to the body of p60c-src.

Recombinant retroviruses that transduce middle T antigen cDNAs derived from polyomavirus mutants: separation of focus formation and soft-agar growth in transformation assays and correlations with kinase activities in vitro

To study correlations between cellular transformation and the biochemical properties of polyomavirus middle T antigen, middle T cDNAs have been derived from the polyomavirus mutants dl1015, dl23, and NG59b and have been introduced into rodent fibroblast cell lines by using a retrovirus vector. It was found that all three mutants are completely defective in inducing growth in soft agar but possess a range of activities in assays of focus formation on cell monolayers. Furthermore, when assays of middle T antigen-associated kinase activities were performed in vitro, a correlation between the level of associated phosphatidylinositol kinase activity and the ability of mutant middle T antigens to induce focus formation was observed. However, the association of this activity with middle T antigen does not appear to be sufficient to bring about full transformation, since the middle T antigen derived from dl1015 is completely defective for soft-agar growth but is associated with a level of phosphatidylinositol kinase activity which is comparable to that of the wild type. Therefore, some other unidentified middle T antigen function may also be required for full transformation.

Mutation of a site of tyrosine phosphorylation in the lymphocyte-specific tyrosine protein kinase, p56lck, reveals its oncogenic potential in fibroblasts

p56lck, a cellular tyrosine protein kinase (EC 2.7.1.112) of the src family, is expressed in essentially all T cells and in some B cells. Expression in nonlymphoid cells is observed only rarely. We have found that mutation of a carboxyl-terminal phosphorylation site, tyrosine-505, reveals an oncogenic activity of this protein. Infection of fibroblasts with a retrovirus encoding wild-type p56lck is without consequence. In contrast, infection with a virus encoding the mutant protein leads to greatly increased phosphorylation of cellular proteins on tyrosine, morphological transformation, and anchorage-independent growth. This suggests that the tyrosine protein kinase activity and the oncogenic potential of p56lck are normally suppressed in vivo by phosphorylation of tyrosine-505. Since similar results were obtained previously with an analogous mutant of c-src, our results suggest that the protein kinase activity of all members of the src family of cytoplasmic tyrosine protein kinases will prove to be regulated by tyrosine phosphorylation at a conserved residue near the carboxyl terminus. Because p56lck is normally expressed only in lymphoid cells, it was possible that p56lck would be without effect in other tissues. The transformation of fibroblasts by mutant p56lck shows that this lymphoid protein can interact productively with nonlymphoid polypeptide substrates.

A retrovirus carrying the polyomavirus middle T gene induces acute thrombocythemic myeloproliferative disease in mice

Mice inoculated with an artificially constructed retrovirus carrying the middle T gene of polyomavirus develop acute myeloproliferative disease with severe thrombotic and hemorrhagic disorder and impaired platelet function. The megakaryocytic lineage appears to be a target for polyoma-murine leukemia virus infection and middle T gene expression. This newly described disease represents a unique model system for studying disorders of the megakaryocytic lineage.

Interactions between polyomavirus medium T antigen and three cellular proteins of 88, 61, and 37 kilodaltons

Affinity-purified medium T antigen of wild-type polyomavirus and dl8, a transforming mutant with a deletion in the medium T gene, is associated with three cellular proteins with apparent molecular weights of 88,000 (88K protein), 61,000 (61K protein), and 37,000 (37K protein). Medium T antigen encoded by the nontransforming hrt mutants fails to associate with these proteins, whereas medium T antigen of the nontransforming mutant dl1015 is able to do so. Medium T antigen of the nontransforming mutant dl23 binds to the 61K and 37K proteins; however, binding to the 88K protein is uncertain. The pattern of complex formation between these proteins and medium T antigen resembles that of pp60c-src and medium T antigen. The binding of medium T antigen to the 88K, 61K, and 37K proteins, as well as to pp60c-src, might represent a necessary but insufficient step in transformation. By mixing extracts from infected and uninfected cells, complex formation between medium T antigen and the 88K, 61K, and 37K proteins can be demonstrated in vitro. Pulse-chase experiments indicated that in vivo the association between medium T antigen and the 61K and 37K proteins is a slow process. The latter two proteins are probably bound to each other in uninfected cells. On two-dimensional gels of whole-cell extract, the 61K protein comigrated with a minor protein with an isoelectric point of 5.2. The 61K protein was neither phosphorylated nor glycosylated. Polyomavirus tumor serum precipitated the 61K and 37K proteins independently of medium T antigen. Therefore, the 61K protein or the 37K protein or both have the properties of a cellular tumor antigen.

Mutants of polyomavirus middle-T antigen

Polyomavirus middle-T antigen induces the transformation of established cell lines in culture and is known to interact with and/or modulate the activity of several enzymes (pp60c.src, protein kinase C and phosphatidylinositol kinase) in vitro. This review is a compilation of the reported mutants of middle-T antigen and their biochemical and biological properties as they relate to the transformation event. The mutants of polyomavirus middle-T antigen have been previously classified phenotypically. Given the now large number of mutants, the classification presented here is based upon the position within the molecule. A model of middle-T is presented in which the protein is considered as consisting of three domains: a hydrophobic domain (the putative membrane-binding domain), the amino-terminal half of the molecule (the putative pp60c.src-binding domain) and the intervening amino acids (the putative modulatory domain). A current model for the induction of transformation by polyomavirus middle-T is presented.

A study of mechanisms of carcinogenesis by gene transfer of oncogenes into mammalian cells

Recent work has shown that individual oncogenes can be involved in several steps of the multistage process of carcinogenesis. Evidence comes from studies on the expression of cloned oncogenes transfected into early passage mammalian cells and into immortalized non-tumorigenic cell lines. Transformation of epithelial cells in vitro with cloned cellular and viral oncogenes is of special interest since most human tumors are of epithelial origin. An important aspect of cell transformation by oncogenes is the induction of transforming growth factors (TGFs). The role of oncogenes in differentiation has been examined by introducing the human myc and mutant T24 Ha-ras1 genes into mouse erythroleukemic cells which were then induced to differentiate. In several clones differentiation was inhibited by myc or ras genes. Studies are reported using oncogenes linked to transcriptional control elements that can be regulated in vitro, such as the human metallothionein (hMT-IIA) promoter region, by cadmium and dexamethasone. Phenotypic properties of transfectants including morphological transformation, anchorage dependence and TGF release are shown to be dependent on the regulators of the hMT-IIA control region.

Cell transformation by pp60c-src mutated in the carboxy-terminal regulatory domain

We introduced two mutations into the carboxy-terminal regulatory region of chicken pp60c-src. One, F527, replaces tyrosine 527 with phenylalanine. The other, Am517, produces a truncated pp60c-src protein lacking the 17 carboxy-terminal amino acids. Both mutant proteins were phosphorylated at tyrosine 416 in vivo. The specific activity of the Am517 mutant protein kinase was similar to that of wild-type pp60c-src whereas that of the F527 mutant was 5- to 10-fold higher. Both mutant c-src genes induced focus formation on NIH 3T3 cells, but the foci appeared at lower frequency, and were smaller than foci induced by polyoma middle tumor antigen (mT). The wild-type or F527 pp60c-src formed a complex with mT, whereas the Am517 pp60c-src did not. The results suggest that one, inability to phosphorylate tyrosine 527 increases pp60c-src protein kinase activity and transforming ability; two, transformation by mT involves other events besides lack of phosphorylation at tyrosine 527 of pp60c-src; three, activation of the pp60c-src protein kinase may not be required for transformation by the Am517 mutant; and four, the carboxyl terminus of pp60c-src appears to be required for association with mT.

Overproduction of polyomavirus middle T antigen in mammalian cells through the use of an adenovirus vector

To overproduce biologically active polyomavirus middle T antigen, we used an adenovirus vector and human 293 cells as hosts. Two helper-independent recombinant adenoviruses were isolated that contain a hybrid transcription unit, in differing orientations, at a site in the adenovirus genome from which the E1a and most of the E1b transcription units have been deleted. The hybrid transcription unit consists of the adenovirus type 2 major late promoter and tripartite leader and a cDNA segment capable of encoding polyomavirus middle T antigen and accompanying 3' RNA-processing signals. Both recombinant viruses were stable and replicated to high titers in human 293 cells. The polyomavirus sequences were expressed, predominantly at late times after infection of 293 cells, to yield mRNAs that encoded middle T antigen. One of the recombinant viruses also expressed a middle T antigen-related protein in 293 cells. The latter was translated from one of several novel mRNA species that resulted from aberrant splicing and incomplete RNA processing of precursor RNA transcripts. Comparison of the amount of middle T antigen produced in 3T6 cells infected with polyomavirus with that in 293 cells infected with either of the recombinant adenoviruses, under optimal conditions for each system, revealed at least a 10-fold greater yield of the protein on a per-cell basis in the latter system than in the former. The recombinant-virus-encoded middle T antigen was biologically active, as evidenced by its ability to associate with and serve as a substrate for human pp60c-src. The functionality of the middle T antigen was further confirmed by demonstrating that both recombinant viruses efficiently transformed Rat-1 cells. These recombinant viruses will be useful to overproduce middle T antigen and to introduce the polyomavirus oncogene into a wide variety of mammalian cells.

Transformation of chicken embryo fibroblasts and tumor induction by the middle T antigen of polyomavirus carried in an avian retroviral vector

The middle T antigen of polyomavirus transformed primary chicken embryo fibroblasts when expressed from a replication-competent avian retrovirus. This in vitro-constructed retrovirus, SRMT1, is a variant of Rous sarcoma virus that encodes the middle T antigen in place of v-src. Inoculation of SRMT1 into 1-week-old chickens rapidly induced hemangiomas and hemangiosarcomas. As shown with mammalian cells infected with polyomavirus, polyomavirus middle T antigen appears to be associated with p60c-src in chicken cells infected with SRMT1. When lysates of SRMT1-infected cells immunoprecipitated with either a monoclonal antibody against p60src or anti-T serum were assayed in an in vitro kinase reaction, the middle T antigen was heavily phosphorylated. To see whether an excess of p60c-src could alter the extent of phosphorylation of the middle T protein or the process of cell transformation by middle T, cells were doubly infected with SRMT1 and NY501, a virus which overexpresses p60c-src. Doubly infected chicken embryo fibroblasts transformed with the same kinetics and were morphologically indistinguishable from chicken embryo fibroblasts infected with SRMT1 alone. Phosphorylation of the middle T antigen was elevated two- to fivefold relative to cells infected only with SRMT1.

Altered sites of tyrosine phosphorylation in pp60c-src associated with polyomavirus middle tumor antigen

We characterized the tyrosine phosphorylation sites of free pp60c-src and of pp60c-src associated with the polyomavirus middle tumor antigen (mT) in transformed avian and rodent cells. The sites of tyrosine phosphorylation in the two populations of pp60c-src were different, both in vitro and in vivo. Free pp60c-src was phosphorylated in vitro at a single site, tyrosine 416. pp60c-src associated with mT was phosphorylated in vitro on tyrosine 416 and on one or more additional tyrosine residues located in the amino-terminal region of the molecule. Free pp60c-src in polyomavirus mT-transformed cells was phosphorylated in vivo on tyrosine 527. In contrast, pp60c-src associated with mT was phosphorylated in vivo on tyrosine 416 and not detectably on tyrosine 527. Thus, the in vivo phosphorylation sites of pp60c-src associated with mT in transformed cells are identical to those of pp60v-src, the Rous sarcoma virus transforming protein. The results suggest that altered phosphorylation of pp60c-src associated with mT may play a role in the enhancement of the pp60c-src protein kinase activity and in cell transformation by polyomavirus.

Association of the polyomavirus middle-T antigen with c-yes protein

Expression of the middle-T antigen of polyomavirus is sufficient to induce transformation of fibroblasts in culture and tumour formation in whole animals. Middle-T can form a complex with the cellular src gene product (p60c-src) and can be phosphorylated by p60c-src in vitro. Studies using middle-T mutants have suggested that the association of middle-T with p60c-src may be necessary but not sufficient for transformation. Therefore, we addressed the possibility that middle-T could interact with other tyrosine protein kinases structurally related to p60c-src. Using antibody raised against a fusion protein between beta-galactosidase and amino-terminal sequences of p90gag-yes from Y73 virus (anti-yes antibody), we have found that middle-T can associate with and be phosphorylated by the c-yes proto-oncogene product, a protein of relative molecular mass (Mr) 62,000 (62K). This raises the possibility that the middle-T-p62c-yes complex contributes to transformation by polyomavirus.

Retrovirus shuttle vector for study of kinase activities of pp60c-src synthesized in vitro and overproduced in vivo

We have constructed a recombinant murine retrovirus which efficiently transduces avian pp60c-src into murine cells and which is easily rescued from infected cells in plasmid form. To characterize the virus, several randomly selected NIH 3T3 lines were isolated after infection with recombinant retroviral stocks. All lines overproduced avian pp60c-src and appeared morphologically normal. Immunoprecipitates made from these lines with antisera specific for pp60c-src were tested for their kinase activities in vitro. We find that both autokinase and enolase kinase activities increase proportionately with the level of pp60c-src in the immunoprecipitates. To further test the authenticity of the pp60c-src encoded by the retroviral vector, these analyses were repeated in the presence of polyomavirus middle T antigen. Avian pp60c-src was activated as a protein kinase, indicating that the virally encoded pp60c-src interacts normally with middle T antigen. Interestingly, by increasing the intracellular levels of pp60c-src 15-fold over normal endogenous levels, we were unable to obtain a proportionate increase in the amount of middle-T-antigen-pp60c-src complex. Finally, using the shuttle features designed into the vector, we have isolated the first fully processed cDNA encoding functional avian pp60c-src X pp60c-src synthesized in vitro with this cDNA had intrinsic protein kinase activity and no detectable phosphatidylinositol kinase activity.

Retrovirus shuttle vector for study of kinase activities of pp60c-src synthesized in vitro and overproduced in vivo

We have constructed a recombinant murine retrovirus which efficiently transduces avian pp60c-src into murine cells and which is easily rescued from infected cells in plasmid form. To characterize the virus, several randomly selected NIH 3T3 lines were isolated after infection with recombinant retroviral stocks. All lines overproduced avian pp60c-src and appeared morphologically normal. Immunoprecipitates made from these lines with antisera specific for pp60c-src were tested for their kinase activities in vitro. We find that both autokinase and enolase kinase activities increase proportionately with the level of pp60c-src in the immunoprecipitates. To further test the authenticity of the pp60c-src encoded by the retroviral vector, these analyses were repeated in the presence of polyomavirus middle T antigen. Avian pp60c-src was activated as a protein kinase, indicating that the virally encoded pp60c-src interacts normally with middle T antigen. Interestingly, by increasing the intracellular levels of pp60c-src 15-fold over normal endogenous levels, we were unable to obtain a proportionate increase in the amount of middle-T-antigen-pp60c-src complex. Finally, using the shuttle features designed into the vector, we have isolated the first fully processed cDNA encoding functional avian pp60c-src X pp60c-src synthesized in vitro with this cDNA had intrinsic protein kinase activity and no detectable phosphatidylinositol kinase activity.

Transformation of chicken embryo fibroblasts and tumor induction by the middle T antigen of polyomavirus carried in an avian retroviral vector

The middle T antigen of polyomavirus transformed primary chicken embryo fibroblasts when expressed from a replication-competent avian retrovirus. This in vitro-constructed retrovirus, SRMT1, is a variant of Rous sarcoma virus that encodes the middle T antigen in place of v-src. Inoculation of SRMT1 into 1-week-old chickens rapidly induced hemangiomas and hemangiosarcomas. As shown with mammalian cells infected with polyomavirus, polyomavirus middle T antigen appears to be associated with p60c-src in chicken cells infected with SRMT1. When lysates of SRMT1-infected cells immunoprecipitated with either a monoclonal antibody against p60src or anti-T serum were assayed in an in vitro kinase reaction, the middle T antigen was heavily phosphorylated. To see whether an excess of p60c-src could alter the extent of phosphorylation of the middle T protein or the process of cell transformation by middle T, cells were doubly infected with SRMT1 and NY501, a virus which overexpresses p60c-src. Doubly infected chicken embryo fibroblasts transformed with the same kinetics and were morphologically indistinguishable from chicken embryo fibroblasts infected with SRMT1 alone. Phosphorylation of the middle T antigen was elevated two- to fivefold relative to cells infected only with SRMT1.

Altered sites of tyrosine phosphorylation in pp60c-src associated with polyomavirus middle tumor antigen

We characterized the tyrosine phosphorylation sites of free pp60c-src and of pp60c-src associated with the polyomavirus middle tumor antigen (mT) in transformed avian and rodent cells. The sites of tyrosine phosphorylation in the two populations of pp60c-src were different, both in vitro and in vivo. Free pp60c-src was phosphorylated in vitro at a single site, tyrosine 416. pp60c-src associated with mT was phosphorylated in vitro on tyrosine 416 and on one or more additional tyrosine residues located in the amino-terminal region of the molecule. Free pp60c-src in polyomavirus mT-transformed cells was phosphorylated in vivo on tyrosine 527. In contrast, pp60c-src associated with mT was phosphorylated in vivo on tyrosine 416 and not detectably on tyrosine 527. Thus, the in vivo phosphorylation sites of pp60c-src associated with mT in transformed cells are identical to those of pp60v-src, the Rous sarcoma virus transforming protein. The results suggest that altered phosphorylation of pp60c-src associated with mT may play a role in the enhancement of the pp60c-src protein kinase activity and in cell transformation by polyomavirus.