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.

Vinculin: a cytoskeletal target of the transforming protein of Rous sarcoma virus

Vinculin, a protein associated with the cytoplasmic face of the focal adhesion plaques which anchor actin-containing microfilaments to the plasma membrane and attach a cell to the substratum, contains 8-fold more phosphotyrosine in cells transformed by Rous sarcoma virus than in uninfected cells. Because the transforming protein of RSV, p60src, is a protein kinase that modifies cellular proteins through the phosphorylation of tyrosine and because phosphotyrosine is a very rare modified amino acid, this result is a very rare modified amino acid, this result suggests that vinculin is a primary substrate of p60src. Only trace amounts of phosphotyrosine were detected in myosin heavy chains, alpha-actinin, filamin, and the intermediate filament protein vimentin. The modification of vinculin by p60src may be responsible in part for the disruption of the microfilament organization and for the changes in cell shape and adhesiveness which accompany transformation by Rous sarcoma virus.

Acid and base hydrolysis of phosphoproteins bound to immobilon facilitates analysis of phosphoamino acids in gel-fractionated proteins

Immobilon, a membrane of polyvinylidene difluoride to which gel-fractionated proteins can be transferred electrophoretically, was found to be an excellent matrix for the analysis of the phosphoamino acid content of phosphoproteins. Hydrolysis of 32P-labeled proteins bound to Immobilon with 5.7 N HCl resulted in the release of 90% of the 32P in the form of Pi, phosphoamino acids, and phosphopeptides. Two-dimensional electrophoretic analysis of the released phosphoamino acids yielded undistorted patterns. Because direct hydrolysis of proteins transferred to Immobilon eliminated the need for both preparative extraction of proteins from a gel and recovery by precipitation, analysis was rapid and yields of phosphoamino acids were extremely consistent. The yield of phosphoamino acids from proteins bound to Immobilon, unlike that from proteins eluted from gels, was independent of the size of the protein. The detection of 32P-labeled, phosphotyrosine-containing proteins in sodium dodecyl sulfate-polyacrylamide gels has been shown to be substantially improved by incubation of the gel in 1.0 N KOH for 2 h at 55 degrees C. Base hydrolysis of proteins bound to Immobilon proved to be faster and more sensitive than hydrolysis of proteins in gels. Less than 10% of bound protein was lost from Immobilon during the 2-h incubation at 55 degrees C in 1.0 N KOH. The autoradiographic image after alkaline hydrolysis of proteins on Immobilon was sharper than that obtained after hydrolysis of proteins in the gel. In addition, unlike base-treated gels, the dimensions of the Immobilon filter were unaffected by treatment with base.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of CD45 alters phosphorylation of the lck-encoded tyrosine protein kinase in murine lymphoma T-cell lines

CD45 is a family of high molecular weight leukocyte cell surface glycoproteins. Recently, two related subregions of the cytoplasmic domain of CD45 have been shown to have 30-40% amino acid identity with a human placental protein phosphotyrosine phosphatase, and CD45 isolated from human spleen was found to exhibit intrinsic protein phosphotyrosine phosphatase (EC 3.1.3.48) activity. In the present studies, we demonstrate that each of the known isoforms of murine CD45 has an equivalent basal level of protein phosphotyrosine phosphatase activity and establish that this enzymatic activity is associated with the cytoplasmic domain of the glycoprotein. Studies with three independent sets of well-characterized parental CD45+, mutant CD45-, and revertant CD45+ lymphoma cell lines indicate that loss of CD45 increases the phosphorylation of the src-related leukocyte-specific tyrosine protein kinase p56lck on tyrosine-505, a putative negative regulatory site. This suggests that CD45 may play a role in leukocyte growth regulation by altering the kinase activity of p56lck.

Direct evidence that oncogenic tyrosine kinases and cyclic AMP-dependent protein kinase have homologous ATP-binding sites

p60src, the transforming protein of Rous sarcoma virus (RSV), is a protein kinase that has a strict specificity for tyrosine. The phosphorylation of cellular proteins by p60src (ref. 4) results in transformation. Recently, Barker and Dayhoff discovered that residues 259-485 of p60src have 22% sequence identity with residues 33-258 of the catalytic subunit of cyclic AMP-dependent protein kinase, an enzyme that has a specificity for serine. Because it was necessary to introduce eight gaps to align the two proteins, the question remained as to whether this apparent homology reflected a common evolutionary origin. We demonstrate here that the ATP analogue p-fluorosulphonylbenzoyl 5'-adenosine (FSBA) inactivates the tyrosine protein kinase activity of p60src by reacting with lysine 295. When aligned for maximum sequence identity, lysine 295 of p60src and the lysine in the catalytic subunit which also reacts specifically with FSBA are superimposed precisely. This functional homology is strong evidence that the protein kinases, irrespective of amino acid substrate specificity, comprise a single divergent gene family.

The lck tyrosine protein kinase interacts with the cytoplasmic tail of the CD4 glycoprotein through its unique amino-terminal domain

The CD4 lymphocyte surface glycoprotein and the lck tyrosine protein kinase p56lck are found as a complex in T lymphocytes. We have defined the domains in both proteins that are responsible for this interaction by coexpressing hybrid and deleted forms of the two proteins in HeLa cells. We have found that the unique 32 amino-terminal residues of p56lck and the 38 carboxy-terminal residues of CD4 that comprise the cytoplasmic domain are both necessary and sufficient by themselves for the interaction of the two proteins. The interaction appears to be independent of other T cell-specific proteins and probably occurs before CD4 reaches the cell surface. Our findings suggest that the specialized amino-terminal domains of other members of the src family of intracellular tyrosine kinases may also mediate transmembrane signaling via coupling to the cytoplasmic domains of specific transmembrane proteins.

The transforming proteins of Rous sarcoma virus, Harvey sarcoma virus and Abelson virus contain tightly bound lipid

We have found that the transforming proteins of Rous sarcoma virus, Harvey sarcoma virus and Abelson virus all contain tightly bound lipid. This modification could play a role in the binding of these proteins to cellular membranes. The lipid associated with p60src, the transforming protein of Rous sarcoma virus, is located in the NH2-terminal domain of the polypeptide. This is the region of the protein that has been shown previously to participate in binding the protein to membranes. Two mature forms of p21, the transforming protein of Harvey sarcoma virus, contain lipid. Lipid is not, however, associated with newly synthesized p21. While mature p60src and p21 are bound to cellular membranes, the newly synthesized forms of these proteins are not. The posttranslational addition of lipid may therefore be the means by which these proteins acquire an affinity for membranes.

Analysis of the sequence of amino acids surrounding sites of tyrosine phosphorylation

We have identified the single phosphorylated tyrosine in p60src, the transforming protein of Rous sarcoma virus, as part of the sequence. NH2-Arg-Leu-Ile-Glu-Asp-Asn-Glu-Tyr(P)-Thr-Ala-Arg-COOH. Therefore, this is a sequence that is recognized efficiently by a tyrosine protein kinase in vivo. Phosphorylation of tyrosine in cellular proteins appears to play a role in malignant transformation by four classes of genetically distinct RNA tumor viruses. Phosphorylated tyrosines in several other proteins resemble of the tyrosine in p60src in that they are located 7 residues to the COOH-terminal side of a basic amino acid and either 4 residues to the COOH-terminal side of, or in close proximity to, a glutamic acid residue. Therefore it is possible that these features play a role in the selection of sites of phosphorylation by some tyrosine protein kinases. However, several clear exceptions to this rule exist.

Expression of a new tyrosine protein kinase is stimulated by retrovirus promoter insertion

Tyrosine protein kinases are important both in the normal regulation of cellular proliferation and in the oncogenic transformation of cells by several tumour viruses. The LSTRA Moloney murine leukaemia virus (M-MuLV)-induced thymoma cell line contains approximately 20-fold more phosphotyrosine in protein than do typical haematopoietic cell lines; this seems to result from the expression of an abnormally high level of a cellular tyrosine protein kinase termed p56tck (refs 3, 4). This kinase is normally expressed at low levels in most, but not all, murine T cells. The elevated levels of p56tck could contribute to the malignant properties of LSTRA cells. Therefore, we have isolated cloned complementary DNAs encoding the whole of p56tck. Sequence analysis shows it to be a novel cellular tyrosine protein kinase which is distinct from all others described to date. p56tck is encoded in LSTRA cells by a hybrid messenger RNA; approximately 200 nucleotides at the 5' end of the mRNA are identical to the 5' end of the genome of M-MuLV. The three- to ninefold transcriptional activation of the gene therefore results from retroviral promoter insertion.

Myristoylated alpha subunits of guanine nucleotide-binding regulatory proteins

Antisera directed against specific subunits of guanine nucleotide-binding regulatory proteins (G proteins) were used to immunoprecipitate these polypeptides from metabolically labeled cells. This technique detects, in extracts of a human astrocytoma cell line, the alpha subunits of Gs (stimulatory) (alpha 45 and alpha 52), a 41-kDa subunit of Gi (inhibitory) (alpha 41), a 40-kDa protein (alpha 40), and the 36-kDa beta subunit. No protein that comigrated with the alpha subunit of Go (unknown function) (alpha 39) was detected. In cells grown in the presence of [3H]myristic acid, alpha 41 and alpha 40 contained 3H label, while the beta subunit did not. Chemical analysis of lipids attached covalently to purified alpha 41 and alpha 39 from bovine brain also revealed myristic acid. Similar analysis of brain G protein beta and gamma subunits and of Gt (transducin) subunits (alpha, beta, and gamma) failed to reveal fatty acids. The fatty acid associated with alpha 41, alpha 40, and alpha 39 was stable to treatment with base, suggesting that the lipid is linked to the polypeptide via an amide bond. These GTP binding proteins are thus identified as members of a select group of proteins that contains myristic acid covalently attached to the peptide backbone. Myristate may play an important role in stabilizing interactions of G proteins with phospholipid or with membrane-bound proteins.

Comparison of the expression of the src gene of Rous sarcoma virus in vitro and in vivo

We have compared the polypeptide products of the src gene of several strains of Rous sarcoma virus produced by in vitro translation of heat-denatured 70S virion RNA in the nuclease-treated reticulocyte lysate with those present in chick cells transformed by these viruses. We have done this by immunoprecipitation, using sera from rabbits injected at birth with Schmidt-Ruppin Rous sarcoma virus. In vitro translation results in the synthesis of at least nine polypeptides which appear to be encoded by the src gene. These range in size from 17,000 to 60,000 daltons. The sera from tumor-bearing rabbits precipitated these polypeptides arising from the in vitro translation of RNA from Schmidt-Ruppin Rous sarcoma virus of both subgroup A and subgroup D and from one stock of Prague Rous sarcoma virus of subgroup C. In each case, all of this family of related polypeptides could be precipitated except the smallest, the 17,000-dalton polypeptide. No precipitation of analogous polypeptides resulting from the translation of RNA from other strains of Rous sarcoma virus was observed. Cells transformed by these three strains of Rous sarcoma virus contain easily detectable amounts of a polypeptide, p60src, essentially identical to the 60,000-dalton in vitro product. With one exception, they do not contain significant amounts of polypeptides analogous to the smaller in vitro products which can be precipitated by these sera. Cells transformed by one stock of Schmidt-Ruppin Rous sarcoma virus of subgroup A did contain a 39,000-dalton polypeptide, which was related, by peptide mapping, to the 60,000-dalton polypeptide and was similar in size to a precipitable in vitro product. The 60,000-dalton polypeptide present in transformed cells appeared to be phosphorylated 10 to 25 min after its synthesis, metabolically very stable, and not derived from a precursor polypeptide. All immunoprecipitates from transformed cells which contained p60src also contained an 80,000-dalton phosphoprotein. This polypeptide is unrelated to p60src, as determined by peptide mapping, and may well be a host cell polypeptide which is specifically associated with p60src.

Growth-related changes in phosphorylation of yeast RNA polymerase II

The largest subunit of RNA polymerase II contains a unique C-terminal domain (CTD) consisting of tandem repeats of the consensus heptapeptide sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Two forms of the largest subunit can be separated by SDS-polyacrylamide gel electrophoresis. The faster migrating form termed IIA contains little or no phosphate on the CTD, whereas the slower migrating II0 form is multiply phosphorylated. CTD kinases with different phosphoryl acceptor specificities are able to convert IIA to II0 in vitro, and different phosphoisomers have been identified in vivo. In this paper we report the binding specificities of a set of monoclonal antibodies that recognize different phosphoepitopes on the CTD. Monoclonal antibodies like H5 recognize phosphoserine in position 2, whereas monoclonal antibodies like H14 recognize phosphoserine in position 5. The relative abundance of these phosphoepitopes changes when growing yeast enter stationary phase or are heat-shocked. These results indicate that phosphorylation of different CTD phosphoacceptor sites are independently regulated in response to environmental signals.

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.

Rous sarcoma virus transforming protein lacking myristic acid phosphorylates known polypeptide substrates without inducing transformation

Mutagenesis of glycine 2 of p60src, the transforming protein of Rous sarcoma virus (RSV), yields a protein that is neither myristylated nor bound to cellular membranes. Although these mutant viruses retain full tyrosine protein kinase activity, they are transformation-defective. We examined in detail tyrosine phosphorylation of cellular polypeptides and the phenotype induced by infection with two such viruses. Infection failed to cause growth in agar, cytoskeletal reorganization, or changes in fibronectin synthesis and protease secretion. Strikingly, tyrosine phosphorylation of the known substrates of p60src was extensive, and differed from that found in wild-type transformed cells only quantitatively. There was no apparent correlation between the extent to which any of eight known protein substrates of p60src were phosphorylated and the phenotype of infected cells. We suggest that the phosphorylation of as yet unidentified proteins, which are probably found in cellular membranes, is essential for transformation by RSV.

The absence of myristic acid decreases membrane binding of p60src but does not affect tyrosine protein kinase activity

We have constructed two point mutants of Rous sarcoma virus in which the amino-terminal glycine residue of the transforming protein, p60src, was changed to an alanine or a glutamic acid residue. Both mutant proteins failed to become myristylated and, more importantly, no longer transformed cells. The lack of transformation could not be attributed to defects in the catalytic activity of the mutant p60src proteins. In vitro phosphorylation of the peptide angiotensin or of the cellular substrate proteins enolase and p36 revealed no significant differences in the Km or specific activity of the mutant and wild-type p60src proteins. However, when cellular fractions were prepared, less than 12% of the nonmyristylated p60src proteins was bound to membranes. In contrast, more than 82% of the wild-type protein was associated with membranes. Wild-type p60src was phosphorylated by protein kinase C, a protein kinase which associates with membranes when activated. The mutant proteins were not. This finding supports the idea that within the intact cell the nonmyristylated p60src proteins are cytoplasmic and suggests that this apparent solubility is not an artifact of the cell fractionation procedure. The myristyl groups of p60src apparently encourages a tight association between protein and membranes and, by determining the cellular location of the enzyme, allows transformation to occur.

Correlation between Src family member regulation by the protein-tyrosine-phosphatase CD45 and transmembrane signaling through the T-cell receptor

Stimulation of tyrosine phosphorylation is an early and important event in antigen-induced T-cell activation. T-cell clones deficient in expression of CD45, a transmembrane protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48), are impaired in their ability to respond to either antigen or T-cell receptor cross-linking. Analysis of the CD45-deficient CD8+ T-cell clone L3M-93 demonstrates that the Src family members p56lck and p59fyn show increased immunoreactivity with anti-phosphotyrosine antibody and exhibit decreased kinase activity. The site of increased tyrosine phosphorylation in Src family members was identified by comparison of cyanogen bromide peptide maps. Phosphorylation of the C-terminal phosphopeptide, containing the negative regulatory site of tyrosine phosphorylation, from the CD45-deficient cells was increased 8-fold for p56lck and 2-fold for p59fyn. These data suggest that CD45 dephosphorylates the negative regulatory site of multiple Src family members in the cytotoxic T-lymphocyte clone L3 and show a correlation between the ability to respond efficiently to antigen and the dephosphorylation of Src family members by CD45.

Activation of the Lck tyrosine protein kinase by hydrogen peroxide requires the phosphorylation of Tyr-394

Exposure of cells to H2O2 mimics many of the effects of treatment of cells with extracellular ligands. Among these is the stimulation of tyrosine phosphorylation. In this study, we show that exposure of cells to H2O2 increases the catalytic activity of the lymphocyte-specific tyrosine protein kinase p56lck (Lck) and induces tyrosine phosphorylation of Lck at Tyr-394, the autophosphorylation site. Using mutant forms of Lck, we found that Tyr-394 is required for H2O2-induced activation of Lck, suggesting that phosphorylation of this site may activate Lck. In addition, H2O2 treatment induced phosphorylation at Tyr-394 in a catalytically inactive mutant of Lck in cells that do not express endogenous Lck. This demonstrates that a kinase other than Lck itself is capable of phosphorylating Lck at the so-called autophosphorylation site and raises the possibility that this as yet unidentified tyrosine protein kinase functions as an activator of Lck. Such an activating enzyme could play an important role in signal transduction in T cells.

Stimulation of glucose transport in cultures of density-inhibited chick embryo cells

The rate of glucose transport in sparse, rapidly growing chick-embryo fibroblasts is much greater than that in density-inhibited cells. The addition of fresh chicken serum or trypsin to the medium of density-inhibited cells causes a large increase in the rate of glucose transport that is detectable 15 min after addition. The increase in glucose transport precedes the increase in DNA synthesis by 5-6 hr. Only small changes in rates of transport are seen with nucleosides or a nonmetabolizable amino acid. The increase in glucose transport requires protein synthesis but not RNA or DNA synthesis.

Protein tyrosine phosphorylation is induced in murine B lymphocytes in response to stimulation with anti-immunoglobulin

Activation of both T and B lymphocytes through their membrane receptors for antigen is known to induce breakdown of inositol phospholipids. In addition, T cell activation by antigen is accompanied by increased protein tyrosine phosphorylation of components of the T cell antigen receptor. We now provide evidence that B cell activation through membrane immunoglobulin is also coupled to stimulation of protein tyrosine kinase activity. One potential candidate for a B lymphocyte protein tyrosine kinase is an 80 kd molecule that is itself phosphorylated at tyrosine residues in response to stimulation with anti-immunoglobulin antibodies.

Ir-genes in H-2 regulate generation of anti-viral cytotoxic T cells. Mapping to K or D and dominance of unresponsiveness

H-2 dependent and virus-specific Ir genes regulate the generation of primary virus-specific K or D restricted cytotoxic T-cell responses in vivo. The following examples have been analyzed in some detail: first, Dk restricted responses to vaccinia in Sendai viruses are at least 30 times lower than the corresponding K-restricted responses irrespective of the H-2 haplotypes (k, b, d, dxs, dxq) of K and I regions; in contrast, LCMV infection generates high responses to Dk. These findings are consistent with but do not prove that this Ir gene maps to D. Second, Db restricted responses to vaccinia and Sendai viruses are high in strains possessing the Kq or KbIb, KbaIb haplotype, are very low in strains with Kk, and relatively low in mouse strains of the KdI-Ad haplotype; LCMV generates high Db restricted response in the presence of Kk. This Ir gene for the response to vaccinia and Sendai viruses maps to K since B10.BYR (KqIkdDb) is a responder and B10.A (2R) is a nonresponder (KkIkdDb). Third, virus and K or D allele specific nonresponsiveness is dominant with variable penetrance; in heterozygous mice the nonresponder Kk allele over-rides responsiveness normally found in KbDb or KqDb combinations. Fourth, when (responder X nonresponder)F1 lymphocytes are stimulated in an environment expressing vaccinia virus plus only a high responder Kb or Kq allelle and Db, response to vaccinia Db is high; in contrast when the same F1 cells are stimulated in an environment expressing the low responder allele Kk, response to vaccinia Db is low. Thus absence of Kk during immunization allows generation of high responsive Db restricted vaccinia specific cytotoxic T cells. The Dk dependent low response to vaccinia Dk can be explained by a preclusion rule or by failure of vaccinia to complex with Db; however the analysis of Kk dependent low response to vaccinia Db does not support these explanations or that self-tolerance is responsible for this Ir effect but is compatible with the interpretation that Kk vaccinia is immunodominant over Db vaccinia. These results are discussed with respect to (a) possible mechanisms of regulation by Ir genes and (b) H-2 polymorphism and HLA-disease association.