Transformation by Rous sarcoma virus: a cellular substrate for transformation-specific protein phosphorylation contains phosphotyrosine

Isolation and characterization of three forms of 36-kDa Ca2+-dependent actin- and phospholipid-binding proteins from human placenta membrane

We purified three forms of 36-kDa proteins, two monomeric 36-kDa proteins, which had pIs of 7.5 (36K-I) and 6.4 (36K-II), and one 36-kDa complex (36K-C) consisting of two subunits, 36-kDa (pI 7.5) and 12-kDa (pI 5.8), from human placenta membrane. The 36-kDa subunit of 36K-C was identical to 36K-I as judged by pI, cyanogen bromide peptide mapping and immunological cross-reactivity. The three proteins showed F-actin- and phosphatidylserine-binding abilities dependent on Ca2+ concentrations at millimolar and micromolar levels, respectively. They all had phospholipase A2 inhibitory activity. Only 36K-II was phosphorylated extensively at tyrosine residue in Ca2+- and EGF- dependent manners in the membrane fraction of A431 cells. 36K-I was the best substrate for src kinase, whereas 36K-II was the best for fps kinase. However, 36K-C was not phosphorylated by any kinases used here.

The amino acid sequence of protein II and its phosphorylation site for protein kinase C; the domain structure Ca2+-modulated lipid binding proteins

Protein II isolated from porcine intestinal epithelium is a Ca2+-modulated lipid-binding protein. The amino acid sequence of porcine protein II reported here sheds new light on the properties of a multigene protein family which includes the tyrosine kinase substrates of the sarc gene (p36) and of the EGF-receptor (p35). The sequence consolidates the structural principle in which an amino-terminal tailpiece of variable length is followed by a core built from four internally homologous segments for those proteins in the 35-40 kd range. Sequence data also show that the core can now be described as two domains each containing one low and one high homology segment. This view accounts for two Ca2+ sites, lipid aggregation and F-actin bundling--when present--and suggests that properties of the cores in which protein II differs from p36 and p35 arise primarily from segments 1 and 2. The protease-sensitive tailpiece of protein II is very short and lacks the phosphorylatable tyrosine present in the larger tail domains of p36 and p35. It harbors, however, like the p36 domain, the major site for in vitro phosphorylation by the Ca2+- and lipid-activated protein kinase C. In protein II this site is most likely threonine 6. The sequence alignment also explains why protein II does not interact with a unique p11, a property probably specific for p36. Our results further suggest that liver endonexin may reflect two protein species both closely related to protein II.

In vivo effect of sodium orthovanadate on pp60c-src kinase

We have compared the tyrosine kinase activity of pp60c-src isolated from intact chicken embryo fibroblasts treated with micromolar sodium orthovanadate for 4 h and from untreated cells. We found an approximate 50% reduction in both autophosphorylation of pp60c-src and phosphorylation of casein when examined in the immune complex kinase assay. The reduction of in vitro enzymatic activity correlated with a vanadate-induced increase in in vivo phosphorylation of pp60c-src at the major site of tyrosine phosphorylation in the carboxyl-terminal half of the molecule and at serine in the amino-terminal half of the molecule. Our observations in vivo and those of Courtneidge in vitro (EMBO J. 4:1471-1477, 1985) suggest that vanadate may enhance a cellular regulatory mechanism that inhibits the activity of pp60c-src in normal cells. A likely candidate for this mechanism is phosphorylation at a tyrosine residue distinct from tyrosine 416, probably tyrosine 527 in the carboxyl-terminal sequence of amino acids unique to pp60c-src. The regulatory role, if any, of serine phosphorylation in pp60c-src remains unclear. The 36-kilodalton phosphoprotein, a substrate of pp60v-src, showed a significant phosphorylation at tyrosine after treatment of normal chicken embryo fibroblasts with vanadate. Assuming that pp60c-src is inhibited intracellularly by vanadate, either another tyrosine kinase is stimulated by vanadate (e.g., a growth factor receptor) or the 36-kilodalton phosphoprotein in normal cells is no longer rapidly dephosphorylated by a tyrosine phosphatase in the presence of vanadate.

Detection of phosphotyrosine-containing proteins in polyomavirus middle tumor antigen-transformed cells after treatment with a phosphotyrosine phosphatase inhibitor

Cells transformed with the middle tumor antigen (mT) of polyomavirus were treated with sodium orthovanadate (Na3VO4), an inhibitor of phosphotyrosine phosphatases, to enhance for the detection of cellular proteins which are phosphorylated on tyrosine. Na3VO4 treatment of mT-transformed rat F1-11 cells resulted in a 16-fold elevation in the level of phosphotyrosine associated with total cellular proteins. Parental F1-11 cells displayed only a twofold increase in phosphotyrosine following Na3VO4 treatment. The abundance of phosphotyrosine in Na3VO4-treated mT-transformed F1-11 cells was twofold higher than in untreated Rous sarcoma virus (RSV)-transformed F1-11 cells and 3.5-fold lower than in Na3VO4-treated RSV-transformed F1-11 cells. Tyrosine phosphorylation of many cellular proteins, including p36, the major substrate of the RSV pp60v-src protein, was detected in Na3VO4-treated mT-transformed F1-11 cells at levels comparable to those observed in RSV-transformed cells. Some of the major protein species recognized by antiphosphotyrosine antibodies in Na3VO4-treated mT-transformed cells displayed electrophoretic mobilities similar to those detected in RSV-transformed F1-11 cells. Tyrosine phosphorylation of p36 was also detected in fibroblasts infected with polyomavirus. There was no detectable difference in the kinase activity of pp60c-src:mT extracted from untreated and Na3VO4-treated mT-transformed cells; however, Na3VO4 treatment of F1-11 and mT-transformed F1-11 cells was shown to inhibit the activity of phosphotyrosine phosphatases in a crude assay of total cellular activity with pp60v-src as the substrate. Thus, Na3VO4 treatment may allow the detection of phosphotyrosine-containing proteins in mT-transformed cells by preventing the turnover of phosphate on substrates phosphorylated by activated cellular protein-tyrosine kinases associated with mT. These results suggest that tyrosine phosphorylation of cellular proteins may be involved in the events that are responsible for mT-induced cellular transformation.

Association of three chicken proteins with the 34 kD target of Rous sarcoma virus tyrosine kinase

Transformation of chicken embryo fibroblasts by avian retroviruses induces the tyrosine phosphorylation of a 34-39 kD cellular protein (p34). In vitro, p34 isolated from intestine interacts with F-actin in a Ca2+-dependent manner. We report here that, in the absence of Ca2+ chelators, three proteins co-purified with p34 extracted from a cytosolic or membrane fraction of chicken embryo fibroblasts; these two fractions account respectively for 10-20% and 50% of the total cellular p34. Isolated from the cytosoluble fraction of fibroblasts by sucrose gradient centrifugation and hydrophobic chromatography, p34 and the other proteins behaved as a homogeneous species upon non-denaturing gel electrophoresis, gel filtration, and CsCl density gradient centrifugation, thus indicating a strong association. Moreover, an analysis by electron microscopy following uranyl acetate staining revealed particles with a raspberry-like shape. This association was always disrupted by the calcium-chelating agent, EGTA.

In vivo effect of sodium orthovanadate on pp60c-src kinase

We have compared the tyrosine kinase activity of pp60c-src isolated from intact chicken embryo fibroblasts treated with micromolar sodium orthovanadate for 4 h and from untreated cells. We found an approximate 50% reduction in both autophosphorylation of pp60c-src and phosphorylation of casein when examined in the immune complex kinase assay. The reduction of in vitro enzymatic activity correlated with a vanadate-induced increase in in vivo phosphorylation of pp60c-src at the major site of tyrosine phosphorylation in the carboxyl-terminal half of the molecule and at serine in the amino-terminal half of the molecule. Our observations in vivo and those of Courtneidge in vitro (EMBO J. 4:1471-1477, 1985) suggest that vanadate may enhance a cellular regulatory mechanism that inhibits the activity of pp60c-src in normal cells. A likely candidate for this mechanism is phosphorylation at a tyrosine residue distinct from tyrosine 416, probably tyrosine 527 in the carboxyl-terminal sequence of amino acids unique to pp60c-src. The regulatory role, if any, of serine phosphorylation in pp60c-src remains unclear. The 36-kilodalton phosphoprotein, a substrate of pp60v-src, showed a significant phosphorylation at tyrosine after treatment of normal chicken embryo fibroblasts with vanadate. Assuming that pp60c-src is inhibited intracellularly by vanadate, either another tyrosine kinase is stimulated by vanadate (e.g., a growth factor receptor) or the 36-kilodalton phosphoprotein in normal cells is no longer rapidly dephosphorylated by a tyrosine phosphatase in the presence of vanadate.

Identification of the EDTA-extractable protein in lens as calpactin I

The EDTA-extractable protein (EEP) is a major extrinsic protein of lens membrane. The 35 kilodalton (kDa) polypeptide of the EEP cross-reacted to antibody prepared against calpactin I, a substrate for the src protein and an inhibitor of phospholipase A2. Calpactin I is also thought to play a structural role in linking cytoskeleton to membrane. The 35 kDa protein in bovine lens contained phosphotyrosine residues that can be detected by affinity purified antibody to this moiety. Although there is some microheterogeneity of EEP using two dimensional gel electrophoresis, at least one of the chick polypeptides, immunoreactive for calpactin I, can be phosphorylated in whole lens culture. These results suggest a regulatory function for the EEP in lens.

Detection of phosphotyrosine-containing proteins in polyomavirus middle tumor antigen-transformed cells after treatment with a phosphotyrosine phosphatase inhibitor

Cells transformed with the middle tumor antigen (mT) of polyomavirus were treated with sodium orthovanadate (Na3VO4), an inhibitor of phosphotyrosine phosphatases, to enhance for the detection of cellular proteins which are phosphorylated on tyrosine. Na3VO4 treatment of mT-transformed rat F1-11 cells resulted in a 16-fold elevation in the level of phosphotyrosine associated with total cellular proteins. Parental F1-11 cells displayed only a twofold increase in phosphotyrosine following Na3VO4 treatment. The abundance of phosphotyrosine in Na3VO4-treated mT-transformed F1-11 cells was twofold higher than in untreated Rous sarcoma virus (RSV)-transformed F1-11 cells and 3.5-fold lower than in Na3VO4-treated RSV-transformed F1-11 cells. Tyrosine phosphorylation of many cellular proteins, including p36, the major substrate of the RSV pp60v-src protein, was detected in Na3VO4-treated mT-transformed F1-11 cells at levels comparable to those observed in RSV-transformed cells. Some of the major protein species recognized by antiphosphotyrosine antibodies in Na3VO4-treated mT-transformed cells displayed electrophoretic mobilities similar to those detected in RSV-transformed F1-11 cells. Tyrosine phosphorylation of p36 was also detected in fibroblasts infected with polyomavirus. There was no detectable difference in the kinase activity of pp60c-src:mT extracted from untreated and Na3VO4-treated mT-transformed cells; however, Na3VO4 treatment of F1-11 and mT-transformed F1-11 cells was shown to inhibit the activity of phosphotyrosine phosphatases in a crude assay of total cellular activity with pp60v-src as the substrate. Thus, Na3VO4 treatment may allow the detection of phosphotyrosine-containing proteins in mT-transformed cells by preventing the turnover of phosphate on substrates phosphorylated by activated cellular protein-tyrosine kinases associated with mT. These results suggest that tyrosine phosphorylation of cellular proteins may be involved in the events that are responsible for mT-induced cellular transformation.

The protein-tyrosine kinase substrate p36 is also a substrate for protein kinase C in vitro and in vivo

p36, a major in vivo substrate of protein-tyrosine kinases, is shown to be phosphorylated at serine 25, a site very close to the major site of tyrosine phosphorylation by pp60v-src, tyrosine 23 (J. R. Glenney, Jr., and B. F. Tack, Proc. Natl. Acad. Sci. USA 82:7884-7888, 1985). We present evidence suggesting that protein kinase C mediates phosphorylation of serine 25.

Phosphotyrosine antibodies identify the p210c-abl tyrosine kinase and proteins phosphorylated on tyrosine in human chronic myelogenous leukemia cells

Antibodies against phosphotyrosine are a powerful tool with which to identify proteins phosphorylated on tyrosine residues, such as viral oncogene-encoded transforming proteins and their cellular protein substrates. Probed on human leukemia cell lines, phosphotyrosine antibodies recognized a 210,000-molecular-weight protein (p210) in K562 cells, a cell line derived from a Philadelphia (Ph)'-positive chronic myelogenous leukemia (CML), but recognized no protein in control Ph'-negative non-CML leukemia cells. The p210 protein was also recognized by antisera against v-abl-encoded polypeptides and displayed kinase activity, phosphorylating itself on tyrosine, in an immunocomplex kinase assay. These data are consistent with reported findings of the expression of a recombined bcr-abl gene in Ph'-positive CML cells, leading to the synthesis of an altered p210c-abl protein endowed with tyrosine kinase activity. Phosphotyrosine antibodies also detected the expression of the p210c-abl protein in fresh bone marrow cells harvested from CML patients in blast crisis. Besides the p210c-abl protein kinase, phosphotyrosine antibodies recognized other proteins with molecular weights of 110,000, 68,000, and 36,000 (p110, p68, and p36) in K562 cells. When [gamma-32P]ATP was added to nonionic detergent-extracted cells, these proteins became phosphorylated on tyrosine, as confirmed by phosphoamino acid analysis. A comparison with fibroblasts transformed by the v-abl, v-src, and v-fps oncogenes suggested the identity of the p36 protein with the common 36-kilodalton protein substrate of viral oncogene-encoded tyrosine kinases. Enhanced tyrosine phosphorylation of cellular proteins is thus a feature shared by cells transformed by v-abl and cells expressing a rearranged bcr-abl gene.

Modulation of p36 phosphorylation in human cells: studies using anti-p36 monoclonal antibodies

We have characterized two monoclonal antibodies which recognize human p36. These have been used to examine the sites and extent of serine and tyrosine phosphorylation of p36 in human cells treated with epidermal growth factor and platelet-derived growth factor and in human cells transformed with viruses whose oncogenes encode protein-tyrosine kinases.

Functionally distinct serine phosphorylation sites of p36, the cellular substrate of retroviral protein kinase; differential inhibition of reassociation with p11

P36 was originally defined as the major cytoplasmic target of retrovirally coded tyrosine-kinases. While recently much has been learned about its biochemistry, the functional importance of its tyrosine and serine phosphorylation has not been approached. As p36 is now understood as a multi-ligand protein its in vitro phosphorylation by three different serine/threonine kinases was followed. Monomeric p36 is a much better substrate than the complex containing two copies each of p36 and p11 (protein I). All p36 phosphorylation sites occur within the amino-terminal 29 residues specifically released by mild proteolysis. As this region harbors an important interaction site for p11 the reduced phosphorylation of p36 in the protein I complex results most likely from a lowered accessibility. Phosphorylation of p36 is serine specific. Reconstitution experiments define at least two functionally distinct sites. One product of protein kinase C reconstitutes with p11 to protein I, while this complex formation normal for p36 is observed neither for the second phosphorylation product nor for the derivatives resulting from phosphorylation by calmodulin or cAMP dependent kinases. The results lend direct support to the hypothesis that phosphorylation of p36 can modulate one of its molecular functions. Obvious implications for other Ca2+-dependent lipid binding proteins are discussed.

Five enzymes of the glycolytic pathway serve as substrates for purified epidermal-growth-factor-receptor kinase

Several enzymes of the glycolytic pathway are phosphorylated in vitro and in vivo by retroviral transforming protein kinases. These substrates include the enzymes phosphoglycerate mutase (PGM), enolase and lactate dehydrogenase (LDH). Here we show that purified EGF (epidermal growth factor)-receptor kinase phosphorylates the enzymes PGM and enolase and also the key regulatory enzymes of the glycolytic pathway, phosphofructokinase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in an EGF-dependent manner. Stoichiometry of phosphate incorporation into GAPDH (calculated from native Mr) is the highest, reaching approximately 1. LDH and other enzymes of the glycolytic pathway are not phosphorylated by the purified EGF-receptor kinase. These enzymes are phosphorylated under native conditions, and the Km values of EGF-receptor kinase for their phosphorylation are close to the physiological concentrations of these enzymes in the cell. EGF stimulates the reaction by 2-5-fold by increasing the Vmax. without affecting the Km of this process. Phosphorylation is rapid at 22 degrees C and at higher temperatures. However, unlike the self-phosphorylation of EGF-receptor, which occurs at 4 degrees C, the glycolytic enzymes are poorly phosphorylated at this temperature. Some enzymes, in particular enolase, increase the receptor Km for ATP in the autophosphorylation process and thus may act as competitive inhibitors of EGF-receptor self-phosphorylation. On the basis of the Km values of EGF receptor for the substrate enzymes and for ATP in the phosphorylation reaction, these enzymes may also be substrates in vivo for the EGF-receptor kinase.

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.

A p60v-src-related tyrosine kinase in the acetylcholine receptor-rich membranes of Narke japonica: association and dissociation of phosphatidylinositol kinase activity

We have isolated a tyrosine-specific protein kinase from the acetylcholine receptor (AChR)-rich membranes of the electric ray Narke japonica. The enzyme is immunologically related to p60v-src, the product of the transforming gene of Rous sarcoma virus. A substantial phosphatidylinositol (PI) kinase activity was associated with this enzyme when it was purified through tyrosine-agarose affinity chromatography used previously for the purification of p60v-src. However, by subsequent chromatography on casein-agarose, most of the associated PI kinase activity was separated from the tyrosine kinase activity. The results suggest that the tyrosine-specific protein kinase in the AChR-rich membranes of N. japonica has no intrinsic PI kinase activity.

Primary structure of bovine calpactin I heavy chain (p36), a major cellular substrate for retroviral protein-tyrosine kinases: homology with the human phospholipase A2 inhibitor lipocortin

An amplified Okayama-Berg plasmid cDNA library was constructed from total poly(A)+ RNA isolated from the Madin-Darby bovine kidney cell line MDBK. This library was screened with a partial murine calpactin I heavy chain (p36) cDNA clone, the identification of which was based on bovine p36 tryptic peptide sequences generated during the course of these studies. The largest p36 cDNA insert (p36/6 of 1.6 kilobase pairs) was fully sequenced by the dideoxy method. The DNA sequence of this insert had an open reading frame of 1014 base pairs and coded for a protein with a molecular weight of 38 481. The deduced protein sequence of 338 residues was concordant with 173 residue positions of p36 determined at the protein level. The 5'- and 3'-ends of p36/6 contained 54 and 307 base pairs of untranslated sequence, respectively. Examination of poly(A)+ RNA prepared from the Madin-Darby cell line indicated a p36 mRNA species of about 1.6 kilobases. Four regions of internal homology, each about 70 amino acid residues in length, were observed in the deduced protein sequence for p36. Thirty-three of the 70 residue positions were conserved in at least three of the four repeating units. A comparison of derived amino acid sequence for bovine p36 with that previously determined for human lipocortin [Wallner, B. P., Mattaliano, R. J., Hession, C., Cate, R. L., Tizard, R., Sinclair, L. K., Foeller, C., Chow, E. P., Browning, J. L., Ramachandran, K. L., & Pepinsky, R. B. (1986) Nature (London) 320, 77-81] revealed extensive homology (66% overall) and the presence of four repetitive regions in the lipocortin structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Two human 35 kd inhibitors of phospholipase A2 are related to substrates of pp60v-src and of the epidermal growth factor receptor/kinase

We have purified two 35 kd phospholipase A2 inhibitors from human placenta, which we refer to as lipocortin I and II. Both proteins exhibit similar biochemical properties and occur in placenta at about 0.2% of the total protein. By peptide mapping, sequence, and immunological analyses, we show that lipocortin I and the 35 kd substrate for the EGF-receptor/kinase from A431 cells are the same protein. By similar criteria, we determine that lipocortin II is the human analogue of pp36, a major substrate for pp60src, which has been characterized in chicken embryo fibroblasts and in bovine brush border preparations. The amino acid sequences of lipocortin I and II that we deduced from cDNA clones share 50% homology, indicating that they probably evolved from a common gene.

The cDNA sequence for the protein-tyrosine kinase substrate p36 (calpactin I heavy chain) reveals a multidomain protein with internal repeats

We have isolated and sequenced a full-length cDNA clone for the protein-tyrosine kinase substrate p36 (calpactin I heavy chain). This sequence predicts a 339 amino acid (Mr 38,493) protein containing an N-terminal region of 20 amino acids, known to interact with a 10 kd protein (light chain), and a C-terminal region, found to contain two Ca2+/phospholipid-binding sites, that can be aligned as four 70 amino acid repeats. A single p36 gene was detected in the mouse genome, and a major p36 mRNA of 1.6 kb was found to be expressed in different mouse tissues. Unexpectedly, p36 and the phospholipase A2 inhibitor lipocortin I were found to be 50% identical in sequence over the C-terminal 300 residues. The function of p36 and its relation to other proteins are discussed.

The use of Rous sarcoma virus transformation mutants with differing tyrosine kinase activities to study the relationships between vinculin phosphorylation, pp60v-src location and adhesion plaque integrity

Tyrosine-specific phosphorylation of cellular proteins has been implicated in the neoplastic transformation of cells by Rous sarcoma virus (RSV). One of the putative substrates for the src gene product (pp60v-src) of RSV is the cytoskeletal protein vinculin, giving rise to the hypothesis that tyrosine-specific phosphorylation of vinculin disrupts adhesion plaque integrity, leading to the characteristic rounded morphology of RSV-transformed cells. We have investigated this hypothesis by analysing the properties of fibroblasts transformed by conditional and non-conditional mutants of RSV which confer different morphologies on infected cells, with respect to formation of microfilament bundles, formation of vinculin-containing adhesion plaques, the deposition of a fibronectin-containing extracellular matrix, the localization of pp60v-src and the tyrosine-specific phosphorylation of vinculin. Cells transformed by the temperature-sensitive (ts) RSV mutant LA32 cultured at 41 degrees C were morphologically normal, and contained prominent microfilament bundles and well-developed adhesion plaques. However, these cells had a fully active pp60v-src kinase, had pp60v-src concentrated in their adhesion plaques and contained vinculin which was heavily phosphorylated on tyrosine residues. Cells transformed by a recovered avian sarcoma virus, rASV 2234.3 exhibited a markedly fusiform morphology with pp60v-src concentrated in well-developed adhesion plaques and an elevation of the phosphotyrosine content of vinculin. Cells transformed by LA32 at restrictive temperature comprise morphologically normal cells, indistinguishable from untransformed CEF, yet which contain tyrosine-phosphorylated vinculin and suggest that neither tyrosine-specific phosphorylation of vinculin nor pp60v-src concentration in adhesion plaques is sufficient for the rounded morphology of RSV-transformed cells.

Modulation of p36 phosphorylation in human cells: studies using anti-p36 monoclonal antibodies

We have characterized two monoclonal antibodies which recognize human p36. These have been used to examine the sites and extent of serine and tyrosine phosphorylation of p36 in human cells treated with epidermal growth factor and platelet-derived growth factor and in human cells transformed with viruses whose oncogenes encode protein-tyrosine kinases.

The protein-tyrosine kinase substrate p36 is also a substrate for protein kinase C in vitro and in vivo

p36, a major in vivo substrate of protein-tyrosine kinases, is shown to be phosphorylated at serine 25, a site very close to the major site of tyrosine phosphorylation by pp60v-src, tyrosine 23 (J. R. Glenney, Jr., and B. F. Tack, Proc. Natl. Acad. Sci. USA 82:7884-7888, 1985). We present evidence suggesting that protein kinase C mediates phosphorylation of serine 25.

Intracellular localization and processing of pp60v-src proteins expressed by two distinct temperature-sensitive mutants of Rous sarcoma virus

The transforming protein of Rous sarcoma virus, pp60v-src, is known to be a tyrosine protein kinase, but the mechanism of cell transformation remains unclear. In further investigating pp60v-src structure and function, we have analyzed two temperature-sensitive (ts) Rous sarcoma virus src gene mutants, tsLA29 and tsLA32. The mutations in tsLA29 and tsLA32 map in the carboxy-terminal region and the amino-terminal half of pp60v-src, respectively, and encode mutant proteins with either temperature-labile (tsLA29) or -stable (tsLA32) kinase activities. Here we examined the intracellular processing and localization of these pp60v-src mutants and extended our characterization of transformation parameters expressed by cells infected by the Rous sarcoma virus variants. No obvious defects in functional integrity of the tsLA32 pp60v-src could yet be demonstrated, whereas the tsLA29 pp60v-src was perturbed not only in kinase activity, but also in aspects of protein processing and localization. Analysis of transformation parameters expressed by infected cells demonstrated the complete temperature lability of both mutants.

Phosphorylation of tyrosine residues of calmodulin in Rous sarcoma virus-transformed cells

Calmodulin, a wide-spread eukaryotic Ca2+-binding protein, was phosphorylated at its tyrosine residues in Rous sarcoma virus (RSV)-transformed chicken and rat cells but not in normal chicken embryo fibroblasts. In contrast, serine and threonine phosphorylation of calmodulin was found to occur in both normal and virus-transformed cells. In an in vitro system containing purified src kinase from RSV-transformed cells, tyrosine phosphorylation of calmodulin by the src kinase was inhibited by Ca2+. Furthermore, the tyrosine-phosphorylated calmodulin showed slower mobility than that of nonphosphorylated calmodulin in NaDodSO4/polyacrylamide gel electrophoresis when Ca2+ was present. These results suggest that the structure of calmodulin Ca2+ complex may be altered by tyrosine phosphorylation. It is thus inferred that Ca2+ may regulate the level of tyrosine phosphorylation of calmodulin in RSV-transformed cells, and phosphorylation in turn may attenuate the function of this protein in vivo.

Two related but distinct forms of the Mr 36,000 tyrosine kinase substrate (calpactin) that interact with phospholipid and actin in a Ca2+-dependent manner

A method was devised that allows the identification of proteins related to the Mr 36,000 tyrosine kinase substrate calpactin based on their ability to interact with actin and phospholipid in a calcium-dependent manner. Two distinct proteins, detected in human A431 cells and fibroblasts, were resolved by two-dimensional gel electrophoresis. One of these proteins (calpactin I) appears identical to the Mr 34,000-39,000 substrate of the pp60src tyrosine kinase and the second (calpactin II) reacts with antibodies to the Mr 35,000 substrate of the epidermal growth factor receptor. Both proteins interact with phospholipid and actin, are rather basic, and share structural and antigenic determinants. A major difference between the two proteins is noted in their state of association with the Mr 10,000 light chain; i.e., calpactin I is associated with the light chain while calpactin II is not.

Phosphotyrosine antibodies identify the p210c-abl tyrosine kinase and proteins phosphorylated on tyrosine in human chronic myelogenous leukemia cells

Antibodies against phosphotyrosine are a powerful tool with which to identify proteins phosphorylated on tyrosine residues, such as viral oncogene-encoded transforming proteins and their cellular protein substrates. Probed on human leukemia cell lines, phosphotyrosine antibodies recognized a 210,000-molecular-weight protein (p210) in K562 cells, a cell line derived from a Philadelphia (Ph)'-positive chronic myelogenous leukemia (CML), but recognized no protein in control Ph'-negative non-CML leukemia cells. The p210 protein was also recognized by antisera against v-abl-encoded polypeptides and displayed kinase activity, phosphorylating itself on tyrosine, in an immunocomplex kinase assay. These data are consistent with reported findings of the expression of a recombined bcr-abl gene in Ph'-positive CML cells, leading to the synthesis of an altered p210c-abl protein endowed with tyrosine kinase activity. Phosphotyrosine antibodies also detected the expression of the p210c-abl protein in fresh bone marrow cells harvested from CML patients in blast crisis. Besides the p210c-abl protein kinase, phosphotyrosine antibodies recognized other proteins with molecular weights of 110,000, 68,000, and 36,000 (p110, p68, and p36) in K562 cells. When [gamma-32P]ATP was added to nonionic detergent-extracted cells, these proteins became phosphorylated on tyrosine, as confirmed by phosphoamino acid analysis. A comparison with fibroblasts transformed by the v-abl, v-src, and v-fps oncogenes suggested the identity of the p36 protein with the common 36-kilodalton protein substrate of viral oncogene-encoded tyrosine kinases. Enhanced tyrosine phosphorylation of cellular proteins is thus a feature shared by cells transformed by v-abl and cells expressing a rearranged bcr-abl gene.

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.

Protein phosphorylation at tyrosine residues in v-abl transformed mouse lymphocytes and fibroblasts

Phosphotyrosine antibodies were employed to immunodecorate and immunoprecipitate proteins phosphorylated at tyrosine residues in cells transformed by Abelson murine leukemia virus (A-MuLV). In pre-B and pre-T lymphoma cells transformed by A-MuLV, the major phosphotyrosine-containing protein has an MW of 160 kDa and shares immunologically detectable sequences with the v-abl oncogene product. Moreover, two different proteins of approximately 100 and 68 kDa, heavily phosphorylated at tyrosine, were identified. Lack of immunological cross-reactivity with viral products and phosphopeptide mapping showed that the 100 and 68 kDa proteins are coded by cellular genes. Phosphoproteins were undetectable in control resting lymphocytes. The 68 and the 100 kDa proteins were phosphorylated to different extents in proliferating lymphocytes, either stimulated by the growth factor IL-2, or transformed by M-MuLV (lacking the oncogene coded kinase). In fibroblasts transformed by A-MuLV, phosphotyrosine antibodies identified 2 proteins of 120 and 70 kDa. By immunological cross-reaction and by phosphopeptide mapping, the first was identified as a 120 kDa form of the v-abl coded kinase. The 70 kDa protein is coded by a cellular gene, is not structurally related to the 120 kDa v-abl kinase, and is different from any phosphotyrosine-containing protein detected in A-MuLV-transformed lymphocytes. These data show that, upon v-abl-induced transformation, phosphorylation at tyrosine takes place also on proteins other than the 160 or 120-kDa oncogene products. In lymphocytes and fibroblasts these proteins are different, suggesting that the cascade of events triggered by the v-abl gene in different cell types involves tyrosine phosphorylation of different specific proteins.

Different localization of the product of the v-rel oncogene in chicken fibroblasts and spleen cells correlates with transformation by REV-T

Reticuloendotheliosis virus strain T (REV-T) is a highly oncogenic avian retrovirus that transforms early lymphoid cells in vivo and in vitro, but REV-T does not transform chicken embryo fibroblasts (CEF). Using antisera to p59v-rel, the v-rel oncogene product of REV-T, we show that p59v-rel is expressed at equal levels and is a phosphoprotein in REV-T infected spleen cells and CEF. Biochemical fractionation and immunofluorescence of REV-T infected nontransformed CEF show that p59v-rel is loosely associated with the nucleus. However, in REV-T transformed spleen cells p59v-rel is primarily a cytoplasmic protein. MSB-1 cells, a Marek's disease virus transformed T cell leukemic line, and E26 virus transformed myeloid cells show nuclear staining of p59v-rel when they are infected by REV-T. Our results indicate that there is a correlation between a cytoplasmic localization of p59v-rel and transformation by REV-T, and they suggest that p59v-rel cannot transform cells in which it assumes solely a nuclear location.

The protein-tyrosine kinase substrate, p81, is homologous to a chicken microvillar core protein

p81, a protein-tyrosine kinase substrate previously identified in epidermal growth factor-treated A431 cells, is demonstrated to be homologous to ezrin, an 80-kD component of microvillar core proteins. p81 has been characterized using antiserum raised against purified chicken intestinal ezrin. p81, located by indirect immunofluorescent staining, is concentrated in surface projections of A431 cells such as microvilli and retraction fibers. None of the conditions of biochemical cell fractionation tested completely solubilizes p81; the insoluble p81 partitions as if associated with the cytoskeleton. The soluble form of p81 behaves as a monomer in all extraction procedures studied. EGF-stimulated phosphorylation of p81 does not appear to change its intracellular location. p81 exhibits a wide tissue distribution with highest levels of expression in small intestine, kidney, thymus, and lung. Intermediate levels are found in spleen, thymus, lymph nodes, and bone marrow, with low levels in brain, heart, and testes. p81 is undetectable in muscle and liver. In A431 cells, p81 is phosphorylated on serine and threonine residues. Upon EGF treatment, approximately 10% of p81 becomes phosphorylated on tyrosine, and the phosphorylation of threonine residues increases.

Protein-tyrosine kinase activity in Saccharomyces cerevisiae

Saccharomyces cerevisiae was examined for tyrosine kinase activity in vitro because this organism offers molecular and genetic approaches for analyzing the role of tyrosine phosphorylation in cellular growth control that are unavailable in higher eukaryotes. Yeast extracts phosphorylated a random copolymer (glutamic acid:tyrosine, 80:20) at tyrosine in a reaction that was linear with respect to time and protein concentration. In the absence of added copolymer, phosphotyrosine was 0.1 percent of the total phosphoamino acids labeled with [gamma-32P]adenosine triphosphate in endogenous yeast proteins. However, specific activities of these reactions were low (approximately 1 percent of those in extracts of chick embryo fibroblasts). Lack of significant incorporation of label from [alpha-32P]adenosine triphosphate into the copolymer or endogenous yeast proteins demonstrated that nucleotide interconversion, adenylylation, and subsequent hydrolysis could not account for the generation of phosphotyrosine observed.

Homology between the subunits of S100 and a 10kDa polypeptide associated with p36 of pig lymphocytes

The polypeptide of about 10,000-Mr which is associated with the p36 polypeptide of pig mesenteric lymph node lymphocytes has a N-terminal 51 amino acid sequence that is 56 and 42% homologous with the N-terminii of the alpha and beta subunits respectively of S100.

Co-precipitation of intestinal p36 with a 73-K protein and a high molecular weight factor

p36 is a major substrate of tyrosine kinases that co-localizes with spectrin in nonerythroid cells. Recent studies by Gerke & Weber [14] have shown that p36 can be isolated from intestine by selective extraction with the Ca2+-chelating agent EGTA. We now show that p36 can be re-precipitated by adding free Ca2+ to 1 mM with the co-precipitation of a high molecular weight (MW) factor and a polypeptide of 73K. The 73K protein was purified to apparent homogeneity, rabbit antibodies were raised to it and used in Western blots and immunofluorescence microscopy. The 73K protein is found in a wide range of tissues and is particularly concentrated in fibroblasts, where its distribution partially overlaps that of non-erythroid spectrin.

Histone phosphorylation in explants of mouse mammary glands and tumors

Rates of histone phosphorylation were measured in explants of mammary glands from mouse strains with high and low tumor incidence. Explants of hormone dependent and independent mouse mammary tumors were also investigated. All mouse strains studied showed predominant phosphorylation of H2A histone at serine and threonine residues. No differences in rates of H2A phosphorylation in glands were found between strains having different mammary tumor susceptibility. Hormone-dependent GR mouse mammary tumors also showed high H2A phosphorylation, but in some tumors also H1 and H3 were phosphorylated. Hormone-dependent GR tumors had 2-5 times higher histone phosphorylation at serine and threonine than hormone-independent tumors.

Amino-terminal sequence of p36 and associated p10: identification of the site of tyrosine phosphorylation and homology with S-100

p36 is a major substrate of both viral and growth factor-receptor-associated tyrosine protein kinases. p36 can be isolated as a complex consisting of a subunit of Mr 36,000 (p36) and a subunit of Mr 10,000 (p10), and it represents an abundant cellular protein. We have isolated the p36-p10 complex from bovine intestinal epithelium and analyzed the amino terminus of both subunits. Sequence analysis of the first 56 amino acids of p10 demonstrates a striking sequence homology (48% identically placed residues) with the Mr 10,000 calcium-binding proteins from bovine brain, termed S-100. Intestinal p36 could be effectively labeled on a single tyrosine in vitro with immunoprecipitated pp60v-src and [gamma-32P]ATP. Mild proteolysis of p36 with chymotrypsin resulted in the cleavage into large (Mr, 33,000) and small domains (Mr, 3000), with the latter representing the phosphorylated amino terminus. Although the amino terminus is apparently blocked, sequence analysis of a secondary tryptic peptide of the Mr 3000 fragment as well as the amino-terminal sequence of the Mr 33,000 domain and overlapping peptides clearly established the site of tyrosine phosphorylation.

Phosphorylation of p36 in vitro with pp60src. Regulation by Ca2+ and phospholipid

P36 is a major substrate of the tyrosine protein kinases. P36 isolated from bovine intestine was used in phosphorylation reactions with pp60src. Phosphorylation was stimulated 3-5-fold by Ca2+, however the Km was the same (2.5 microM) at high or low Ca2+. Although the level of free Ca2+ needed for this enhanced phosphorylation was 10(-4)-10(-3) M, phosphatidylserine shifted the Ca2+ sensitivity to the 10(-6)-10(-5) M range. Independent evidence suggested that p36 interacts directly with liposomes containing phosphatidylserine. This raises the possibility that p36, like c-kinase, is a Ca2+-activated, phospholipid-dependent protein.

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.

Rous sarcoma virus variants that encode src proteins with an altered carboxy terminus are defective for cellular transformation

The src gene of Rous sarcoma virus (v-src) and its cellular homolog, the c-src gene, share extensive sequence homology. The most notable differences between these genes reside in the region encoding the carboxy terminus of the src proteins. We constructed mutations within the 3' end of the v-src gene to determine the significance of this region to the transforming potential of the v-src protein, pp60v-src. The mutants CHdl300 and CHis1511 contain mutations that alter the last 23 amino acids of pp60v-src, whereas the mutant CHis1545-C contains a linker insertion that alters the last 11 amino acids of pp60v-src, and the mutant CHis1545-H contains a linker insertion that results in a 9-amino-acid insertion at position 415. Plasmids bearing each of these mutations were unable to transform chicken cells when introduced into these cells by DNA transfection. In addition, the structurally altered src proteins encoded by the mutants had much-reduced levels of tyrosine protein kinase activity in vivo, as measured by autophosphorylation and phosphorylation of the 34,000-Mr cellular protein, and in vitro, as determined by measuring the level of pp60src autophosphorylation. These data indicate that the carboxy-terminal amino acid sequences play an important role in maintaining the structure of the catalytic domain of pp60v-src. In contrast, the transfection of chicken cells with plasmid DNA containing a chimeric v-c-src gene resulted in morphological cell transformation and the synthesis of an enzymatically active hybrid protein. Therefore, the carboxy-terminal sequence alterations observed in the c-src protein do not alone serve to alter the functional activity of a hybrid v-c-src protein appreciably.

Phytohemagglutinin-induced changes in tyrosine protein kinase and its endogenous substrates in human lymphocytes

We have measured changes in tyrosine protein kinase activity in the particulate and soluble fractions of phytohemagglutinin (PHA)-stimulated human lymphocytes using a synthetic tyrosine-containing peptide as substrate. Following PHA stimulation, the activity of this enzyme remained stable for 24 h and then fell sharply in both fractions. Phosphorylation of endogenous substrates was also studied by gel electrophoresis of the same subcellular preparations which had been incubated with [gamma-32P]ATP. Gels were treated with alkali to hydrolyse phosphoserine, dried and autoradiographed. In the particulate fraction from resting lymphocytes, two prominent bands were labelled, one of 55-60 kD and another of 38 kD. Direct analysis showed that these proteins were labelled largely on tyrosine residues. The 38 kD band became less intensely labelled 24 h following PHA addition, and had disappeared by 72 h. Two new bands appeared at this time: a 42 kD band which was phosphorylated mostly on threonine and a 32 kD band phosphorylated on tyrosine. The 55-60 kD band remained unchanged up to 96 h following PHA addition. The final band pattern in stimulated lymphocytes resembled that of the malignant T lymphoblastic cell line JM1. Our data suggest that tyrosine protein kinases in resting lymphocytes are part of a mechanism which transduces external growth signals to the cell interior, and that this mechanism is inactivated once the signal has been transmitted. The pattern of endogenous substrates which are phosphorylated on tyrosine and threonine residues in the malignant T lymphoblastic cell lines JM1 is likely to be characteristic of proliferating T lymphoid cells rather than of the malignant state, since similar bands appear in normal lymphocytes upon PHA stimulation.