Potential positive and negative autoregulation of p60c-src by intermolecular autophosphorylation.

The product of the protooncogene c-src is a protein-tyrosine kinase, p60c-src, that is normally inhibited by phosphorylation at a tyrosine residue close to the C terminus (Tyr-527). If activated by dephosphorylation of Tyr-527, or by other means, p60c-src becomes phosphorylated at a tyrosine residue in the catalytic domain (Tyr-416). To test whether either or both of these tyrosines can be phosphorylated by p60c-src itself, we have created four mutations in c-src. One mutant product can receive but cannot donate phosphate, and other mutants are capable of catalysis but lack phosphorylation sites. The mutant genes were expressed singly or in combination in yeast. Analysis of the phosphorylation of mutant p60c-src in the yeast cells and in immunoprecipitates showed that p60c-src molecules can phosphorylate each other at Tyr-416 and -527. Prohibiting intramolecular phosphorylation had little effect on reaction rates and extents, suggesting that intermolecular phosphorylation predominates. If the same situation pertains in the milieu of the vertebrate fibroblast, phosphorylation of one p60c-src by another at Tyr-416 or -527 could permit positive or negative autoregulation.     

Phosphorylation of tyrosine in the carboxyl-terminal tryptic peptide of pp60c-src.

The major site of tyrosine phosphorylation of the transforming protein of Rous sarcoma virus, pp60v-src (tyrosine-416), is different from the major site of tyrosine phosphorylation of its nontransforming normal cellular counterpart, pp60c-src. We have shown that antibodies against a synthetic peptide modeled on the carboxyl-terminal 13 residues of pp60c-src specifically immunoprecipitate the major phosphotyrosine tryptic peptide of pp60c-src from both chicken and rat fibroblasts. These experiments localize the major site of tyrosine phosphorylation to one or more of the three tyrosine residues in the carboxyl-terminal tryptic peptide at positions 511, 519, and 527 of the amino acid sequence of chicken pp60c-src. Tyrosines-519 and -527 are in the carboxyl-terminal 19-amino acid segment of pp60c-src that is deleted and replaced by an unrelated sequence in pp60v-src. It is possible that phosphorylation of tyrosine in the carboxyl-terminal tryptic peptide may be involved in the normal regulation of pp60c-src. The absence of this phosphorylation site in pp60v-src may, in part, contribute to its oncogenic properties.     

Selective binding of activated pp60c-src by an immobilized synthetic phosphopeptide modeled on the carboxyl terminus of pp60c-src.

Phosphorylation of the carboxyl terminus of pp60c-src, the product of the c-src protooncogene, at Tyr-527 suppresses its tyrosine kinase activity and transforming potential. It has been proposed that the phosphorylated carboxyl terminus of pp60c-src inhibits kinase activity by binding to the SH2 (src homology 2) domain. We have synthesized peptides corresponding to the carboxyl-terminal 13 residues of pp60c-src phosphorylated and nonphosphorylated at Tyr-527. A highly transforming mutant, pp60c-src(F527), in which Tyr-527 is mutated to Phe, bound to the phosphorylated peptide immobilized to Affi-Gel 10. Binding of the phosphorylated peptide was abolished by deletion of residues 144-175 in the SH2 domain but not by deletion of residues 93-143, which removes most of the SH3 domain. The phosphorylated peptide also bound to pp60v-src, the transforming protein of Rous sarcoma virus. Only traces of pp60v-src and pp60c-src(F527) bound to the corresponding nonphosphorylated c-src peptide. Normal pp60c-src bound much less efficiently to the phosphorylated peptide than did pp60c-src(F527). A phosphorylated peptide corresponding to the carboxyl terminus of the c-fgr protein also bound to pp60c-src(F527), but with weaker affinity. Furthermore, the phosphorylated synthetic carboxyl-terminal pp60c-src peptide markedly inhibited phosphorylation of pp60c-src(F527) during cytoskeletal kinase assays. These results provide direct evidence for models in which the phosphorylated carboxyl terminus of pp60c-src binds intramolecularly or intermolecularly to the SH2 domain of the c-src protein.     

GTPase-activating protein interactions with the viral and cellular Src kinases.

GTPase-activating protein (GAP), which regulates the activities of Ras proteins, is implicated in mitogenic signal transduction by growth-factor receptors and oncoproteins with tyrosine kinase activity. Oncogenic viral Src (p60v-src) encoded in Rous sarcoma virus possesses elevated tyrosine kinase activity compared with its nononcogenic normal homolog, cellular Src (p60c-src). To examine molecular interactions between GAP and the two Src kinases, immunoprecipitates of Src or GAP prepared from cell lystates were resolved by gel electrophoresis and analyzed by an immunoblot procedure with antibodies to GAP or Src used as probes. Results suggest that p60c-src is associated with a complex containing GAP in immunoprecipitates from lysates of normal rat and chicken cells. However, GAP is not phosphorylated in p60c-src immunoprecipitates subjected to in vitro kinase reactions. By contrast, GAP undergoes tyrosyl phosphorylation in vitro when immunoprecipitates of p60v-src prepared from transformed cell lysates are incubated with ATP. Our findings suggest that p60v-src and p60c-src associate with complexes containing GAP and provide a biochemical link between both kinases and GAP/Ras signal transduction pathways. These results are consistent with the hypothesis that GAP has a role in mediating normal functions of p60c-src as well as oncogenic activities of p60v-src.     

The product of the protooncogene c-src is modified during the cellular response to platelet-derived growth factor.

We have observed a modification of the cellular protein kinase pp60c-src, elicited in murine 3T3 fibroblasts by platelet-derived growth factor (PDGF). The modification occurred rapidly after addition of PDGF to the culture medium and was first detected as a reduction in the electrophoretic mobility of a portion of the pp60c-src molecules. A similarly modified form of the viral homologue pp60v-src occurs in vivo in the absence of stimulation by PDGF. The occurrence of modified forms of both pp60c-src and pp60v-src was associated with a novel phosphorylation at tyrosine in the amino-terminal domains of the proteins. The time-course and dose-response for this modification of pp60c-src paralleled PDGF-induced increases in phosphorylation of pp36, a major cellular substrate for several tyrosine-specific protein kinases. In parallel experiments, treatment of cells with PDGF increased the kinase activity of pp60c-src in an immunocomplex assay. These results suggest pp60c-src may play a role in the mitogenic response to PDGF.     

Activation of the transforming potential of p60c-src by a single amino acid change.

Previous work showed that overexpression of the cellular src (c-src) gene does not cause transformation of chicken cells in culture. However, viral stocks isolated from cells transfected with Rous sarcoma virus DNA containing the c-src gene in place of the viral src gene did occasionally produce foci. Virus obtained from these foci were highly transforming and appeared to arise via spontaneous mutation in the c-src-containing viral populations. The p60 proteins of the transforming mutant src viruses were found to have higher levels of in vitro tyrosine kinase activity than the levels observed with the parental viruses. In this study, we have molecularly cloned the src DNA sequences of two transforming mutant src viruses. When compared to the DNA sequence of the parental c-src viruses, the mutant viruses contain single point mutations that result in single amino acid changes in the src gene products (p60 proteins). Both amino acid changes reside in the tyrosine kinase domain of the protein. The mutation detected in one virus involves replacement of the normal Glu-378 in p60c-src by Gly, whereas the p60 of the other transforming virus has Phe instead of the normal Ile-441. Our data indicate that when p60c-src is expressed at elevated levels in a retroviral context, a single amino acid change in its primary sequence can activate the kinase activity of this protein and cause cellular 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.     

Characterization of sites for tyrosine phosphorylation in the transforming protein of Rous sarcoma virus (pp60v-src) and its normal cellular homologue (pp60c-src).

The transforming protein of Rous sarcoma virus (pp60v-src) and its normal cellular homologue (pp60c-src) appear to be protein kinases that phosphorylate tyrosine in a variety of protein substrates. In addition, pp60v-src and pp60-c-src are themselves phosphorylated on serine and tyrosine. It is likely that these phosphorylations serve to regulate the function(s) of pp60v-src and pp60c-src. We have therefore characterized the sites of tyrosine phosphorylation in the two proteins. Tyrosine phosphorylation of pp60v-src in infected cells occurs mainly (if not entirely) at residue 419 in the deduced amino acid sequence of the protein. Surrounding this residue is the sequence Leu-Ile-Glu-Asp-Asn-Glu-Tyr(P)-Thr-Ala-Arg. This peptide is distinguished by the fact that three out of the four amino acids that precede the phosphorylated tyrosine are acidic in nature. These results define what may prove to be a widely used site for tyrosine phosphorylation in the regulation of cellular function. The same site was phosphorylated when partially purified pp60v-src was used in a phosphotransfer reaction in vitro. The results with pp60c-src were more complex. The site of tyrosine phosphorylation in vitro appeared to be the same as that found in pp60v-src. By contrast, phosphorylation of pp60c-src in vivo apparently occurred at a different, and currently unidentified, tyrosine residue. It is therefore possible that pp60v-src and pp60c-src respond differently to regulatory influences in the intact cell.     

Rous sarcoma virus variants that carry the cellular src gene instead of the viral src gene cannot transform chicken embryo fibroblasts.

The transforming activity of the cellular src (c-src) gene as well as of hybrid genes between viral and cellular src was tested by constructing derivatives of Rous sarcoma virus DNA in which all or part of the viral src gene (v-src) was replaced by the corresponding portion of the c-src gene. After these derivatives were introduced into chicken embryo fibroblasts by transfection, replication-competent virus was recovered, which induced the expression of p60src at a level equivalent to p60v-src expression in cells infected with Rous sarcoma virus wild type. Replacement of the portion of the v-src gene, either upstream or downstream of the Bgl I site, with the homologous portion of the c-src gene resulted in fully transforming viruses. On the other hand, the virus stock obtained from cells transfected with Rous sarcoma virus DNA containing the entire c-src gene had a very low titer of focus-forming virus, while it contained a high titer of infectious virus. We present evidence that the rare small foci are formed by mutant viruses generated from the original c-src-containing virus. These results indicate that overproduction of the c-src gene product does not cause cell transformation, and that this proto-oncogene is subject to a relatively high rate of mutation when incorporated in a retrovirus genome, resulting in the acquisition of transforming capacity.     

Role of p34cdc2-mediated phosphorylations in two-step activation of pp60c-src during mitosis.

Phosphorylation of pp60c-src by p34cdc2 at three amino-proximal serine/threonine residues is temporally correlated with, but insufficient for, mitotic activation of c-Src kinase. The direct cause of activation during mitosis appears to be temporally correlated partial dephosphorylation of Tyr-527, a residue whose phosphorylation strongly suppresses pp60c-src activity. Site-directed mutagenesis of the serine/threonine phosphorylation sites blocks half the mitosis-specific decrease in Tyr-527 phosphorylation and half the increase in pp60c-src kinase activity. We conclude that p34cdc2 partially activates pp60c-src by a two-step process in which its serine/threonine phosphorylations either sensitize pp60c-src to a Tyr-527 phosphatase or desensitize it to a Tyr-527 kinase. Furthermore, additional events, independent of these p34cdc2-mediated phosphorylations, participate in mitotic activation of pp60c-src.     

Endogenous c-src as a determinant of the tumorigenicity of src oncogenes.

We have compared the tumorigenicity of two src oncogenes, v-src and c-src(527), whose respective protein products pp60v-src and pp60c-src(527) show a different spectrum of amino acid substitutions vis-à-vis the c-src protooncogene-encoded product pp60c-src. Whereas the extent of primary tumor growth induced by c-src(527) was quite similar in the two chicken lines tested, the extent of v-src-induced tumor growth showed a marked line dependence. As examined with a line of chickens that shows immune-mediated regression of v-src-induced tumors, a weaker tumor immunity, as correlated with a greater level of primary tumor growth, resulted from inoculation of c-src(527) DNA than of v-src DNA. These observations indicated that the v-src-specific amino acid substitutions define a major tumor antigenicity. That a separate src-associated antigenicity is also targetable by the tumor immune response followed from the finding that the level of protective immunity against the growth of c-src(527) DNA-induced tumors was augmented under conditions of the prior regression of v-src DNA-induced tumors. As this latter antigenicity may include one or more c-src(527)-encoded peptides that are equivalent to c-src-encoded self peptides, these observations suggest that a host tolerance to pp60c-src can be broken so as to permit a tumor immune response based on recognition of self peptides of pp60c-src(527).     

Transforming gene product of Rous sarcoma virus phosphorylates tyrosine.

The protein kinase activity associated with pp60src, the transforming protein of Rous sarcoma virus, was found to phosphorylate tyrosine when assayed in an immunoprecipitate. Despite the fact that a protein kinase with this activity has not been described before, several observations suggest that pp60src also phosphorylates tyrosine in vivo. First, chicken cells transformed by Rous sarcoma virus contain as much as 8-fold more phosphotyrosine than do uninfected cells. Second, phosphotyrosine is present in pp60src itself, at one of the two sites of phosphorylation. Third, phosphotyrosine is present in the 50,000-dalton phosphoprotein that coprecipitates with pp60src extracted from transformed chicken cells. We infer from these observations that pp60src is a novel protein kinase and that the modification of proteins via the phosphorylation of tyrosine is essential to the malignant transformation of cells by Rous sarcoma virus. pp60sarc, the closely related cellular homologue of viral pp60src, is present in all vertebrate cells. This normal cellular protein, obtained from both chicken and human cells, also phosphorylated tyrosine when assayed in an immunoprecipitate. This is additional evidence of the functional similarity of these structurally related proteins and demonstrates that all uninfected vertebrate cells contain at least one protein kinase that phosphorylates tyrosine.     

Blood platelets express high levels of the pp60c-src-specific tyrosine kinase activity.

We have examined human and rabbit blood platelets for expression of pp60c-src, the normal cellular homolog of the transforming protein of Rous sarcoma virus. pp60c-src kinase activity was determined by an immune-complex kinase assay that uses enolase as the substrate, and pp60c-src protein levels were determined by an immunoblot assay. Lysates from platelets expressed high levels of pp60c-src-specific kinase activity and pp60c-src protein compared to the levels found in other tissues. pp60c-src was also found to be one of the major proteins phosphorylated in vitro in membranes isolated from platelets. Multiple protein species other than pp60c-src were also phosphorylated on tyrosine in the membrane phosphorylation reactions, and phosphotyrosine represented approximately equal to 80% of the total phosphoamino acid residues phosphorylated in the membranes. These results indicate that tyrosine kinases represent the major protein phosphorylating enzymes detected in isolated platelet membranes. Although the association of tyrosine kinase activity with many viral oncogene products and cellular growth hormone receptors has suggested a role for these enzymes in the regulation of cell proliferation, these results indicate that the expression of high levels of tyrosine kinase activity is not exclusively associated with proliferating cells.     

Mitosis-specific phosphorylation of polyomavirus middle-sized tumor antigen and its role during cell transformation.

Transformation of cells in culture by polyomavirus is mediated by one of its early gene products, middle-sized tumor antigen (MTAg). This protein forms multiple complexes with cellular enzymes such as tyrosine kinases (pp60c-src), a phosphatidylinositol 3-kinase, and phosphatase 2A. Association with MTAg leads to the activation of pp60c-src through interference with phosphorylation at Tyr-527, a site negatively regulating src kinase activity. MTAg abrogates mitosis-specific activation of pp60c-src, resulting in constitutive high kinase activity of the enzyme throughout all phases of the cell cycle. Here we report that MTAg is transiently modified during mitosis, resulting in an increase in its apparent molecular size on SDS/acrylamide gels. Similarly, MTAg isolated from interphase cells and phosphorylated by the cell cycle-regulated serine/threonine kinase p34cdc2 in vitro has increased molecular mass. The large molecular mass form of the protein can be converted to the authentic 56-kDa form upon dephosphorylation by potato acid phosphatase. Two putative phosphorylation sites for a cdc2-like kinase were identified as Thr-160 and -291, respectively. Conversion of Thr-160 to Ala resulted in a transformation-defective mutant protein that was still capable of associating with pp60c-src, phosphatidylinositol 3-kinase, and phosphatase 2A, while the corresponding mutant in position 291 was wild type with respect to all parameters measured so far. These data suggest that phosphorylation by p34cdc2 or a related cell cycle-regulated kinase modulates the interaction of MTAg with cellular targets that are crucial for cell transformation.     

Association of p60src with Triton X-100-resistant cellular structure correlates with morphological transformation.

More than 70% of wild-type Rous sarcoma virus p60v-src was found to be associated with a cellular structure resistant to nonionic detergent extraction that consists primarily of cytoskeletal proteins. On the other hand, nontransforming src proteins, including cellular p60c-src, nonmyristoylated forms, and those inactive in protein kinase, were found in the fraction solubilized by the detergent extraction. p60c-src was detergent-soluble even in transformed cells, suggesting that the association of p60v-src is not a result of cell transformation. Analyses with a variety of Rous sarcoma virus mutants showed a good correlation between the degree of association with the detergent-resistant structure and the extent of cell transformation caused by mutant src proteins, suggesting that this association may be significant for the process of cell transformation by Rous sarcoma virus.     

Overexpression of the c-src protein does not induce transformation of NIH 3T3 cells.

NIH 3T3 mouse cells were transfected with plasmids that induce efficient expression of either (i) the Rous sarcoma virus v-src gene, (ii) the chicken c-src gene, or (iii) a recombinant gene combining the 5' portion of c-src with the 3' end of v-src. Focus formation in tissue culture and formation of large colonies in soft agar did not occur in cells transfected with c-src. Cells transfected with c-src expression plasmids did not form foci but were isolated using a coselectable biological marker. They display morphological and substrate-independent growth characteristics intermediate between those of normal and v-src-transformed mouse cells, and lysates from these cells have enhanced in vitro tyrosine kinase activity. Transfection with the c-src-v-src recombinant induced focus formation with an efficiency similar to that obtained with a v-src expression plasmid. These results imply that v-src-induced transformation does not result just from overexpression of an essentially normal cellular protein but, at least in part, depends on the mutations distinguishing the cellular and viral proteins.     

pp60v-src tyrosine kinase is expressed and active in sarcoma-free avian embryos microinjected with Rous sarcoma virus.

Early embryonic avian tissue is resistant to transformation by Rous sarcoma virus. To determine the nature of this resistance, we examined the expression and properties of the Rous sarcoma virus transforming protein pp60v-src, in infected embryonic chicken limbs in ovo. Lysates from Rous sarcoma virus-infected limbs contained the viral structural protein p19gag, as detected by immunoblot analysis, and showed pp60v-src kinase activity in vitro. Immunoblot analysis of lysates with anti-phosphotyrosine antibodies revealed a number of phosphotyrosine-containing proteins present in lysates of Rous sarcoma virus-infected embryos but not in lysates of control, uninfected embryos. Anti-phosphotyrosine immunoreactivity was observed in frozen sections in the same cell types that expressed pp60v-src and p19gag. These studies demonstrate that pp60v-src is co-expressed with viral structural determinants in infected embryonic avian tissue. Furthermore, pp60v-src is active in ovo as a tyrosine-specific phosphotransferase, despite the apparent lack of sarcoma induction. The localization pattern of the major src gene substrate p36 (calpactin I) was compared with that of p19gag by double-label immunofluorescence and found to be generally nonoverlapping. These observations are consistent with the concept that the induction of tumors in ovo requires complementation between viral determinants and host factors. These host factors, which may be critical substrates of pp60v-src, are subject to developmental regulation in the avian embryo.     

Heat-shock protein hsp90 governs the activity of pp60v-src kinase.

During or immediately after synthesis in vertebrate cells, the oncogenic protein-tyrosine kinase pp60v-src associates with the approximately 90-kDa heat-shock protein (hsp90). In this complex, pp60v-src is not functional as a kinase. When pp60v-src is subsequently found inserted into the plasma membrane, it is active as a kinase and is no longer associated with hsp90. We have taken advantage of genetic manipulations possible in Saccharomyces cerevisiae to investigate the function and specificity of the association between hsp90 and pp60v-src. Expression of pp60v-src is known to be toxic to S. cerevisiae cells. We find that this toxicity is due to a very specific effect on growth, arrest at a particular point in the cell cycle. In cells expressing v-src, a mutation that lowers the level of hsp90 expression (i) relieves cell cycle arrest and rescues growth, (ii) reduces the level of tyrosine phosphorylation mediated by pp60v-src, (iii) changes the pattern of tyrosine phosphorylation, and (iv) reduces the concentration of pp60v-src. We conclude that hsp90 does not simply suppress pp60v-src kinase activity during transit to the plasma membrane, as previously suggested, but also stabilizes the protein and affects both its activity and specificity. This function of hsp90 is highly selective for pp60v-src: the same hsp90 mutation has no effect on the activity or specificity of the exogenous pp160v-abl tyrosine kinase; similarly, it does not affect the specificity and has only a very small effect on the activity of the exogenous pp60c-src kinase.     

Reduced tyrosine kinase specific activity is associated with hypophosphorylation of pp60c-src in cells infected with avian erythroblastosis virus.

Avian erythroblastosis virus (AEV) is a replication-defective retrovirus that causes erythroblastosis and sarcomas in chickens and transforms immature erythroid cells and fibroblasts in culture. AEV encodes two oncogenes, v-erbA and v-erbB, whose products are closely related to the thyroxine receptor and the epidermal growth factor receptor, respectively. Since tyrosine protein kinases have been implicated in the process of normal growth signal transduction, we wished to study the possible consequences of the expression of these mutated, growth-regulating receptor genes on the activity of the cellular tyrosine kinase pp60c-src. A continuous cell line from AEV-infected quail embryo fibroblasts was derived that exhibited a typical transformed phenotype and expressed the viral oncogene products, p75gag-erbA and gp66-68erbB. Using an immune-complex kinase assay, we found that the specific activity of pp60c-src in AEV-transformed quail cells was decreased by a factor of 6-30 relative to that found in uninfected quail cells. A concomitant 50-80% reduction of 32Pi incorporation into the pp60c-src protein from radiolabeled, transformed cells was also observed, indicating a relationship between hypophosphorylation and diminished enzyme activity. Partial proteolytic phosphopeptide analysis revealed a decrease in phosphorylation of both serine- and tyrosine-containing peptides, suggesting an activation of specific phosphatases or inhibition of specific kinases in the AEV-transformed quail cells. Similar results were found in pp60c-src precipitated from AEV-transformed chicken and rat cells.     

Coinfection of insect cells with recombinant baculovirus expressing pp60v-src results in the activation of a serine-specific protein kinase pp90rsk.

A recombinant baculovirus was constructed for the production of the serine-specific protein kinase, pp90rsk (where rsk is ribosomal S6 kinase), in insect cells. The Xenopus pp90rsk expressed in the infected cells had nearly undetectable enzyme activity in contrast to the same enzyme coproduced with the v-src oncogene product pp60v-src. The transforming gene product pp60v-src very effectively activated pp90rsk, whereas the products of c-src and the myristoylation-minus nontransforming virus NY315 were markedly less effective. Only a fraction of the total pp90rsk population was activated, and it could be partially separated from unactivated protein by ion-exchange chromatography. When compared to the unactivated form, the activated enzyme displayed about a 4000-fold increase in the capacity to phosphorylate the ribosomal protein S6. The enhanced enzymatic activity appeared to be due to phosphorylation of pp90rsk.