Apparent activation of the MgATP-dependent protein phosphatase by pp60v-src identification of an activity like that of glycogen synthase kinase 3 in immunoaffinity purified pp60v-src preparations

Immunoaffinity purified pp60^v-src was found to activate the MgATP-dependent protein phosphatase in the presence of MgATP. Although preliminary evidence suggested that phosphorylation of the inhibitor-2 subunit on tyrosine residues was responsible for the activation, preincubation of the pp60^v-src preparation at 41°C resulted in a rapid loss of its protein kinase activities towards both casein and inhibitor-2 while its ability to activate the protein phosphatase complex was relatively insensitive to this treatment. This result demonstrated that pp60^v-src was not responsible for activation of the MgATP-dependent protein phosphatase. A protein kinase activity which phosphorylated glycogen synthase on serine residues was detected in the pp60^v-src preparation. The protein kinase was active in the presence of inhibitors of phosphorylase kinase, glycogen synthase kinase 5/casein kinase II, and cAMP-dependent protein kinase. It is, therefore, likely that activation of the MgATP-dependent protein phosphatase resulted from the presence of a glycogen synthase kinase 3 like activity in the pp60^v-src preparation. Our results illustrate the importance of applying multiple criteria to link the phosphorylation of a protein with an observed change in its activity.     

Effects of phosphorylation of protein phosphatase 1 by pp60v-src on the interaction of the enzyme with substrates and inhibitor proteins

Phosphorylation of protein phosphatase 1 by pp60v-src decreased its activity towards phosphorylase kinase and glycogen synthase as well as towards phosphorylase a. Kinetic experiments indicated that the primary effect of phosphorylation was to increase the Km for each of the substrate proteins. There was little or no change in the Vmax for the reactions. The possibility that phosphorylation of protein phosphatase 1 altered its regulation by inhibitors-1 and -2 was also examined. Phosphorylation of protein phosphatase 1 did not prevent the reversible inhibition of the enzyme by inhibitor-1 or inhibitor-2 nor did it prevent the association of inhibitor-2 with protein phosphatase 1 to form the MgATP-dependent protein phosphatase. Protein phosphatase 1 is not a substrate for pp60v-src when it is complexed with inhibitor-2 to form the inactive MgATP-dependent protein phosphatase. Here we have shown that protein phosphatase 1 is also not phosphorylated by pp60v-src following activation of the MgATP-dependent protein phosphatase with glycogen synthase kinase-3 and MgATP. This indicates that the inability of pp60v-src to phosphorylate protein phosphatase 1 is not due to the change in protein phosphatase 1 conformation which accompanies the inactivation of the MgATP-dependent protein phosphatase. Rather, it appears to be the result of steric hindrance by inhibitor-2. This suggests that the pp60v-src phosphorylation site is closely associated with the inhibitor-2 binding site involved in the formation of the MgATP dependent protein phosphatase. The pp60v-src phosphorylation site was previously localized to a small (Mr less than or equal to 4000) domain which can be selectively degraded by chymotrypsin. Here we have shown that chymotryptic digestion increased the Km of unphosphorylated protein phosphatase 1 for each of the three phosphoprotein substrates used in this study. This effect was similar to that observed after phosphorylation of protein phosphatase 1. These results indicate that the pp60v-src phosphorylation site is in a region of protein phosphatase 1 which influences substrate binding and which may be near the active site.     

Increase in the phosphotransferase specific activity of purified Rous sarcoma virus pp60v-src protein after incubation with ATP plus Mg2+.

pp60v-src, the product of the Rous sarcoma virus src gene, was partially purified by immunoaffinity chromatography from extracts of Rous sarcoma virus-transformed field vole cells. Incubation of this preparation with ATP plus Mg2+ and subsequent repurification by chromatography on hexylamine-agarose resulted in a net increase in the specific activity of the src protein kinase. This increase in phosphotransferase activity was detected by using a variety of substrates including casein, tubulin, and a 34,000-dalton protein presumed to be an in vivo target substrate of pp60v-src. In all cases, the phosphorylation was at tyrosine residues, and the kinase activity was inhibited by preincubation of the enzyme with immunoglobulin G prepared from tumor-bearing rabbit sera. The implications of these results for the regulation and control of pp60v-src-associated kinase activity are discussed.     

Enzymatic characteristics of pp60v-src isolated from vanadium-treated transformed cells

The transforming protein of Rous sarcoma virus (RSV) typically appears as a single phosphorylated polypeptide designated pp60^v-src, pp60^v-src possesses a protein kinase activity specific for tyrosine residues on select protein substrates. Treatment of RSV-transformed cells with vanadium ions resulted in the appearance of an electrophoretic variant of pp60^v-src and was paralleled by a significant increase in the src kinase specific activity in purified enzyme preparations. Both the normal (standard) src kinase and the src kinase preparations obtained from vanadium-treated cells exhibited similar optimal activity profiles for MgCl₂, KCl, and pH. Furthermore, their site specificities of phosphorylation of the substrates casein and vinculin were the same. The reaction kinetic profile of the standard src kinase showed a nonlinear pattern, while the vanadium enzyme exhibited conventional linear Michaelis-Menten kinetics. These results are discussed with respect to the possible functional regulation of pp60^v-src activity by a vanadium-sensitive protein phosphatase activity.     

A mutation at the ATP-binding site of pp60v-src abolishes kinase activity, transformation, and tumorigenicity.

We constructed a mutant, called RSV-SF2, at the ATP-binding site of pp60v-src. In this mutant, lysine-295 is replaced with methionine. SF2 pp60v-src was found to have a half-life similar to that of wild-type pp60v-src and was localized in the membranous fraction of the cell. Rat cells expressing SF2 pp60v-src were morphologically untransformed and do not form tumors. The SF2 pp60v-src isolated from these cells lacked kinase activity with either specific immunoglobulin or other substrates, and expression of SF2 pp60v-src failed to cause an increase of total phosphotyrosine in the proteins of infected cells. Wild-type pp60v-src was phosphorylated on serine and tyrosine in infected cells, and the analogous phosphorylations could also be carried out in vitro. Phosphorylation of serine was catalyzed by a cyclic AMP-dependent protein kinase, and phosphorylation of tyrosine was perhaps catalyzed by pp60v-src itself. By contrast, SF2 pp60v-src could not be phosphorylated on serine or tyrosine either in infected cells or in vitro. These findings strengthen the belief that the phosphotransferase activity of pp60v-src is required for neoplastic transformation by the protein and suggest that the binding of ATP to pp60v-src elicits an allosteric change required for phosphorylation of serine in the protein.     

Forms of pp60v-src isolated from Rous sarcoma virus-transformed cells.

It has previously been shown that an electrophoretic variant form of the Rous sarcoma virus transforming protein, pp60v-src, exists in src-transformed cells. This variant, which was readily observed in vanadate-treated cells, was characterized as possessing extensive amino-terminal domain phosphotyrosine modification. Its appearance was further correlated with increased src-specific protein kinase activity. In this study, we used a src-specific monoclonal antibody (MAb) to resolve immunologic forms of pp60v-src. The MAb was able to distinguish between two populations of typical lower-band pp60v-src and was unreactive with the electrophoretic variant upper-band pp60v-src species. Using serial immunoprecipitations, we resolved four populations of pp60v-src: src protein either immunoreactive or unreactive with the MAb from both untreated and vanadate-treated transformed cells. The pp60v-src in each fraction displayed a distinct phosphoamino acid composition and tryptic phosphopeptide profile. However, analysis of their tyrosyl kinase specific activities showed that the immunologically resolved populations of pp60v-src from a given culture did not differ. Both pp60v-src fractions from vanadate-treated cells exhibited similar kinase specific activities, which were greatly enhanced over those of enzyme preparations from untreated cells. Since the MAb-reactive pp60v-src fraction from vanadate-treated cells lacked the electrophoretic variant upper-band pp60v-src species yet still possessed enhanced enzymatic specific activity, the initially stated correlation between the appearance of the electrophoretic variant src form and increased src kinase activity breaks down. These results suggest that yet to be defined modifications of the src protein may be involved in its functional regulation.     

Aberrant protein phosphorylation at tyrosine is responsible for the growth-inhibitory action of pp60v-src expressed in the yeast Saccharomyces cerevisiae.

Expression of pp60v-src, the transforming protein of Rous sarcoma virus, arrests the growth of the yeast Saccharomyces cerevisiae. To determine the basis of this growth arrest, yeast strains were constructed that expressed either wild-type v-src or various mutant v-src genes under the control of the galactose-inducible, glucose repressible GAL1 promoter. When shifted to galactose medium, cells expressing wild-type v-src ceased growth immediately and lost viability, whereas cells expressing a catalytically inactive mutant (K295M) continued to grow normally, indicating that the kinase activity of pp60v-src is required for its growth inhibitory effect. Mutants of v-src altered in the SH2/SH3 domain (XD4, XD6, SPX1, and SHX13) and a mutant lacking a functional N-terminal myristoylation signal (MM4) caused only a partial inhibition of growth, indicating that complete growth inhibition requires either targeting of the active kinase or binding of the kinase to phosphorylated substrates, or both. Cells arrested by v-src expression displayed aberrant microtubule structures, alterations in DNA content and elevated p34CDC28 kinase activity. Immunoblotting with antiphosphotyrosine antibody showed that many yeast proteins, including the p34CDC28 kinase, became phosphorylated at tyrosine in cells expressing v-src. Both the growth inhibition and the tyrosine-specific protein phosphorylation observed following v-src expression were reversed by co-expression of a mammalian phosphotyrosine-specific phosphoprotein phosphatase (PTP1B). However a v-src mutant with a small insertion in the catalytic domain (SRX5) had the same lethal effect as wild-type v-src, yet induced only very low levels of protein-tyrosine phosphorylation. These results indicate that inappropriate phosphorylation at tyrosine is the primary cause of the lethal effect of pp60v-src expression but suggest that only a limited subset of the phosphorylated proteins are involved in this effect.     

Inhibition of the tyrosine kinase activity of v-src, v-fgr, and v-yes gene products by a monoclonal antibody which binds both amino and carboxy peptide fragments of pp60v-src.

A monoclonal antibody, R2D2, raised to the src gene product of Rous sarcoma virus was found to inhibit the tyrosine protein kinase activity of pp60v-src in autophosphorylation reactions and in reactions involving exogenously added substrates, such as casein and histone. R2D2 also inhibited the enzymatic activity of two related viral transforming proteins, pp70gag-fgr and pp90gag-yes. The inhibitory ability of R2D2 was dependent upon immunoglobulin concentration and could be demonstrated in both immune complexes formed directly with R2D2 and preformed immune complexes to which R2D2 was added. Binding sites in both the amino-terminal 110 amino acid residues and the carboxy-terminal 240 amino acids of pp60v-src were identified for R2D2. These results indicate that at least part of the epitope recognized by R2D2 resides within a region of the src protein which is required for protein kinase activity. The localization of the R2D2 epitope to the amino- as well as to the carboxy-terminal portions of pp60v-src, together with results of studies analyzing the relative binding efficiencies of R2D2 to the intact protein and to V-8 proteolytic fragments of pp60v-src, are consistent with the view that the R2D2 epitope is conformational in nature and that it is assembled from residues contained within both N-terminal and C-terminal regions of the molecule.     

Myristoylation-regulated direct interaction between calcium-bound calmodulin and N-terminal region of pp60v-src

pp60v-src tyrosine protein kinase was suggested to interact with Ca2+-bound calmodulin (Ca2+/CaM) through the N-terminal region based on its structural similarities to CAP-23/NAP-22, a myristoylated neuron-specific protein, whose myristoyl group is essential for interaction with Ca2+/CaM; (1) the N terminus of pp60v-src is myristoylated like CAP-23/NAP-22; (2) both lysine residues are required for the myristoylation-dependent interaction and serine residues that are thought to regulate the interaction through the phosphorylations located in the N-terminal region of pp60v-src. To verify this possibility, we investigated the direct interaction between pp60v-src and Ca2+/CaM using a myristoylated peptide corresponding to the N-terminal region of pp60v-src. The binding assay indicated that only the myristoylated peptide binds to Ca2+/CaM, and the non-myristoylated peptide is not able to bind to Ca2+/CaM. Analyses of the binding kinetics revealed two independent reactions with the dissociation constants (KD) of 2.07 x 10(-9)M (KD1) and 3.93 x 10(-6)M (KD2), respectively. Two serine residues near the myristoyl moiety of the peptide (Ser2, Ser11) were phosphorylated by protein kinase C in vitro, and the phosphorylation drastically reduced the interaction. NMR experiments indicated that two molecules of the myristoylated peptide were bound around the hydrophobic clefts of a Ca2+/CaM molecule. The small-angle X-ray scattering analyses showed that the size of the peptide-Ca2+/CaM complex is 2-3A smaller than that of the known Ca2+/CaM-target molecule complexes. These results demonstrate clearly the direct interaction between pp60v-src and Ca2+/CaM in a novel manner different from that of known Ca2+/CaM, the target molecules, interactions.     

pp60v-src reactivation inhibits serum-induced accumulation of inositol phosphates and phosphatidylethanol in tsNRK

In tsRSV-infected NRK (tsNRK) cells, pp60(v-src) reactivation by temperature-shift from a nonpermissive temperature, 39 C, to a permissive one, 32 degrees C, induced the production of inositol phosphates (IPt) and phosphatidylethanol (PEt). This was accompanied by an increase in membrane-associated protein kinase C (PKC) activity in the absence of exogenous growth factors. However, with serum-stimulation, the amounts of IPt and PEt at 32 degrees C were less than those at 39 degrees C. Pretreatment with PKC inhibitors, Ro-31-8220 and staurosporine, enhanced the accumulation of IPt but not of PEt at 32 degrees C. The tyrosine phosphorylation of phospholipase Cgamma1 (PLCgamma1) was increased either by serum or by pp60(v-src) reactivation. These results suggest that serum transduces its signal through PLCgamma1 mediation, and that pp60(v-src), possibly through the PKC mediation, negatively affects serum-induced PLCgamma1 activation.     

Restriction of the in vitro and in vivo tyrosine protein kinase activities of pp60c-src relative to pp60v-src.

The tyrosine protein kinase activities of pp60c-src and pp60v-src were compared. The activities were qualitatively similar in vitro when the src proteins were bound in an immune complex with monoclonal antibody; both proteins utilized either ATP or GTP as phosphate donors, preferred Mn2+ to Mg2+, and had similar exogenous substrate specificities. The specific activity of pp60c-src was about 10-fold lower than that of pp60v-src for exogenous substrate phosphorylation but was only 1.1- to 2-fold lower than that of pp60v-src for autophosphorylation. Six glycolytic enzymes, including three not previously identified as substrates for pp60src phosphorylation, were phosphorylated by both pp60c-src and pp60v-src. Levels of pp60c-src fourfold higher than the amount of pp60v-src in src-plasmid-transformed cells did not detectably alter the level of phosphotyrosine in cellular proteins, but increasing the expression of pp60c-src another twofold (which induces cells to form foci in monolayer culture (P.J. Johnson, P.M. Coussens, A.V. Danko, and D. Shalloway, Mol. Cell. Biol. 5:1073-1083, 1985) resulted in a threefold increase in the level of cellular protein phosphotyrosine. Immunoprecipitation and analysis of the alkali-stable phosphoproteins by two-dimensional electrophoresis showed that, in contrast to pp60v-src-transformed cells, pp36 and enolase are only weakly phosphorylated in these high-level pp60c-src overexpresser cells. Even allowing for the in vitro differences in specific activities of phosphorylation, these results suggest that the pp60c-src tyrosine protein phosphorylating activity may be restricted relative to that of pp60v-src by additional in vivo mechanisms.     

Structurally and functionally modified forms of pp60v-src in Rous sarcoma virus-transformed cell lysates.

When analyzed from transformed cell lysates, pp60v-src, the product of the Rous sarcoma virus src gene, typically appears as a single polypeptide of 60,000 molecular weight, phosphorylated at two major sites, an amino-terminal region serine residue and carboxy-terminal region tyrosine residue. We describe here the identification of variant forms of pp60v-src present in transformed cell lysates that exhibited an altered electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gels. This change in migration appeared to be the result of some alteration in the amino-terminal portion of the molecule and paralleled the appearance of extensive amino-terminal region tyrosine phosphorylation on the pp60v-src molecule. These structural modifications were further correlated with a dramatic increase in the protein kinase-specific activity of pp60v-src. The detection of these variant forms of pp60v-src depended on the prior treatment of the transformed cell cultures with vanadium ions or the inclusion in the cell disruption buffer of Mg2+ or ATP-Mg2+. The implications is that modified, highly active forms of the pp60v-src protein exist in transformed cells, but are transient and rapidly converted to stable forms, possibly by specific dephosphorylation. We suggest that amino-terminal region tyrosine phosphorylation of pp60v-src, presumably the result of autophosphorylation, serves to greatly enhance src protein enzymatic activity, but that much of the regulation of this transforming protein's function may involve a phosphotyrosyl protein phosphatase.     

Specific desensitization of the epidermal growth factor receptor by pp60v-src

BALB/c 3T3 cells infected with a temperature-sensitive mutant (LA90) of RSV have been used to investigate possible heterologous interactions between the pp60v-src tyrosyl kinase and the epidermal growth factor (EGF) and bradykinin receptors. The LA90 pp60v-src exhibits a very rapid activation t1/2 (less than 5 min) of protein kinase activity on decreasing the temperature from 40 degrees C to 35 degrees C. This change in temperature was also found to induce a very rapid decrease in the affinity for 125I-EGF of receptors on the RSV-LA90-infected cells but not of those on control parental cells. However, no significant changes were detected in the binding of 3H-bradykinin to either cell line. Two separable processes control the desensitization of the EGF receptor by pp60v-src, both of which are independent of protein kinase C. The first is rapid and transient, while the second is sensitive to cycloheximide and persists long after inactivation of pp60v-src.     

Inter- and intramolecular interactions of highly purified Rous sarcoma virus-transforming protein, pp60v-src

We have purified intact pp60v-src, the product of the Rous sarcoma virus src gene, over 2400-fold, based on the phosphorylation of tumor-bearing rabbit IgG. The purification procedure involved detergent extraction of the particulate fraction of the cells and sequential chromatography on hydroxylapatite, butyl agarose, DEAE-Sephacel, ADP-agarose, and Sephacryl S-200. Analysis of the preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single silver-stained band with an apparent molecular weight of 60,000. Our results show that the activities of this preparation were qualitatively similar to those described previously for partially purified pp60v-src. Upon analysis by two-dimensional gel electrophoresis, the purified pp60v-src yielded one major species which migrated to the same position as the least acidic of the three major species detectable in cellular lysates, suggesting that the pp60v-src had been dephosphorylated during the purification procedure. We found that pp60v-src was very prone to aggregation; to maintain it as a monomer both Nonidet P-40 and KCl were required. Under conditions which maintained pp60v-src as a monomer, the rate of autophosphorylation was independent of its concentration and thus proceeded via an intramolecular process. Preincubation of pp60v-src with ATP or GTP as well as nonphosphorylating analogs of ATP or GTP preserved its phosphorylating activity toward alpha-casein whereas its activity was reduced 80% upon preincubation in the absence of nucleotides. We suggest that protection with nucleotides rather than autophosphorylation accounts for the apparent increase in the activity of pp60v-src after incubation of the enzyme with ATP.     

Reconstitution of the Rous sarcoma virus transforming protein pp60v-src into phospholipid vesicles.

An artificial membrane system was developed to study the molecular basis for interaction of pp60v-src, the Rous sarcoma virus transforming protein, with lipid bilayers. pp60v-src was extracted from cell membranes by detergent solubilization and reincorporated into phospholipid vesicles. Reconstituted pp60v-src retained tyrosine kinase activity and was integrally associated with the liposome through a 10-kilodalton (kDa) amino-terminal domain. The same 10-kDa domain was shown to anchor pp60v-src to the plasma membrane of transformed cells. Reconstitution experiments performed with nonmyristylated pp60v-src proteins revealed that these polypeptides did not interact with phospholipid vesicles. In contrast, myristylated, soluble pp60v-src molecules (including a highly purified pp60v-src preparation) could be reconstituted into liposomes, but their interaction with the liposomal bilayer was not mediated by the 10-kDa amino-terminal domain. When membrane proteins were included during reconstitution of purified pp60v-src, binding through the 10-kDa anchor was restored. A model is presented to accommodate the different types of interactions of pp60v-src with liposomes; the model postulates the existence of an additional membrane component that anchors the pp60v-src polypeptide to the phospholipid bilayer.     

Microinjection of pp60v-src into Xenopus oocytes increases phosphorylation of ribosomal protein S6 and accelerates the rate of progesterone-induced meiotic maturation.

Microinjection of purified pp60v-src into Xenopus oocytes caused the phosphorylation of ribosomal protein S6 on serine residues and also increased total protein phosphorylation, with almost a two-fold increase in the percentage of phosphotyrosine present. In addition, pp60v-src accelerated the time course of progesterone-induced oocyte maturation, suggesting that the biochemical pathway influenced by pp60v-src is related to that induced by progesterone.     

Protein kinase C phosphorylates pp60src at a novel site

The transforming protein of Rous sarcoma virus (pp60v-src) and its normal cellular homolog (pp60c-src) are demonstrated to be phosphorylated at serine 12 in vivo under certain conditions. We propose that protein kinase C is responsible for this modification based on the following evidence. First, the tumor promoters, 12-O-tetradecanoylphorbol-13-acetate and teleocidin, and synthetic diacylglycerol, known activators of protein kinase C in vivo, cause nearly complete phosphorylation of pp60src at serine 12. Second, among five purified serine/threonine-specific protein kinases tested, only protein kinase C phosphorylates pp60c-src and pp60v-src in vitro at serine 12. Third, purified protein kinase C phosphorylates a synthetic peptide corresponding to the N-terminal 20 amino acids of pp60c-src at serine 12. The physiological significance of this novel phosphorylation is discussed.     

Nonphosphorylatable substrate analogs selectively block autophosphorylation and activation of the insulin receptor, epidermal growth factor receptor, and pp60v-src kinases

The receptors for insulin and epidermal growth factor undergo tyrosine autophosphorylation in response to ligand stimulation, while pp60v-src is an unregulated tyrosine kinase. In this report we show that each of the kinases phosphorylates an exogenous peptide that corresponds to the insulin proreceptor sequence 1142-1153. When the kinases were pre-phosphorylated, saturable Michaelis-Menten kinetics were observed. However, when the kinases had not been pre-phosphorylated biphasic kinetics were observed; at progressively higher substrate concentrations (greater than Km) less substrate phosphorylation was seen. Furthermore, when the kinases had not been pre-phosphorylated kinase autophosphorylation was inhibited at high substrate concentrations. On this basis we postulated that the substrate inhibition of substrate phosphorylation resulted directly from substrate inhibition of kinase autophosphorylation. To test this we designed additional peptides to function specifically as inhibitors of the kinases. Each of the 3 tyrosine residues within the substrate sequence were replaced either by 4-methoxyphenylalanine or phenylalanine, residues structurally similar to tyrosine but unable to accept phosphoryl transfer. Both analogs inhibited insulin and epidermal growth factor receptor autophosphorylation, whereas only the Phe-substituted analog inhibited pp60v-src phosphorylation. These data suggest that autophosphorylation of tyrosine residues near the kinase active site is a generalized mechanism for tyrosine kinase activation and that activation can be selectively blocked by substrates and nonphosphorylatable analogs.