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

Dissecting the activating mutations in v-erbB of avian erythroblastosis virus strain R

The v-erbB oncogene isolated from the R (or ES4) strain of avian erythroblastosis virus is capable of inducing erythroleukemia and fibrosarcomas. This oncogene differs from the proto-oncogene c-erbB, the avian homolog of the epidermal growth factor receptor, by its lack of an intact ligand-binding domain as well as additional alterations in its cytoplasmic coding sequences. By contrast, the insertionally activated c-erbB, a variant oncogene, which encodes a product that also lacks the ligand-binding domain but is otherwise unaltered in its cytoplasmic coding sequences, is capable of inducing leukemia but cannot induce sarcomas. In this report, we show that the critical changes for activating the sarcomagenic potential displayed by v-erbB R are two point mutations within the tyrosine kinase domain and an internal deletion of 21 amino acids in the carboxyl-terminal regulatory domain. The removal of the carboxyl-terminal autophosphorylation sites is not obligatory. These activating mutations (Arg-263 to His, Ile-384 to Ser, and the deletion of residues 494 to 514), when introduced singly into the insertionally activated c-erbB, all dramatically increase fibroblast-transforming potential. Arg-263 resides near the highly conserved HRD motif of the kinase domain, and its mutation to His increases the autophosphorylation activity. The other two mutations do not alter the intrinsic kinase activity and presumably affect other aspects of the receptor involved in growth signaling. Therefore, the high transforming potential of v-erbB R is a consequence of synergism among multiple activating mutations.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Molecular features of the viral and cellular Src kinases involved in interactions with the GTPase-activating protein

GTPase-activating protein (GAP) enhances the rate of GTP hydrolysis by cellular Ras proteins and is implicated in mitogenic signal transduction. GAP is phosphorylated on tyrosine in cells transformed by Rous sarcoma virus and serves as an in vitro substrate of the viral Src (v-Src) kinase. Our previous studies showed that GAP complexes stably with normal cellular Src (c-Src), although its association with v-Src is less stable. To further investigate the molecular basis for interactions between GAP and the Src kinases, we examined GAP association with and phosphorylation by a series of c-Src and v-Src mutants. Analysis of GAP association with c-Src/v-Src chimeric proteins demonstrates that GAP associates stably with Src proteins possessing low kinase activity and poorly with activated Src kinases, especially those that lack the carboxy-terminal segment of c-Src containing the regulatory amino acid Tyr-527. Phosphorylated Tyr-527 is a major determinant of c-Src association with GAP, as demonstrated by c-Src point mutants in which Tyr-527 is changed to Phe. While the isolated amino-terminal half of the c-Src protein is insufficient for stable GAP association, analysis of point substitutions of highly conserved amino acid residues in the c-Src SH2 region indicate that this region also influences Src-GAP complex formation. Therefore, our results suggest that both Tyr-527 phosphorylation and the SH2 region contribute to stable association of c-Src with GAP. Analysis of in vivo phosphorylation of GAP by v-Src mutants containing deletions encompassing the SH2, SH3, and unique regions suggests that the kinase domain of v-Src contains sufficient substrate specificity for GAP phosphorylation. Even though tyrosine phosphorylation of GAP correlates to certain extent with the transforming ability of various c-Src and v-Src mutants, our data suggest that other GAP-associated proteins may also have roles in Src-mediated oncogenic transformation. These findings provide additional evidence for the specificity of Src interactions with GAP and support the hypothesis that these interactions contribute to the biological functions of the Scr kinases.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

Phosphorylation of c-Src on tyrosine 527 by another protein tyrosine kinase

The protein tyrosine kinase activity of the cellular Src protein is negatively regulated by phosphorylation at tyrosine residue 527 (Tyr527). It has not been established whether this regulatory modification of Src is mediated by autophosphorylation or by another cellular protein kinase. The phosphorylation of a modified form of c-Src that lacks kinase activity was examined in mouse cells that do not express endogenous Src (because of the targeted disruption of both src alleles). Phosphorylation of the inactive form of Src on Tyr527 occurred to a similar extent in cells lacking endogenous Src as it did in cells expressing Src. Therefore, Tyr527 phosphorylation, and thus negative control of Src kinase activity, is mediated by another cellular protein tyrosine kinase.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Molecular features of the viral and cellular Src kinases involved in interactions with the GTPase-activating protein

GTPase-activating protein (GAP) enhances the rate of GTP hydrolysis by cellular Ras proteins and is implicated in mitogenic signal transduction. GAP is phosphorylated on tyrosine in cells transformed by Rous sarcoma virus and serves as an in vitro substrate of the viral Src (v-Src) kinase. Our previous studies showed that GAP complexes stably with normal cellular Src (c-Src), although its association with v-Src is less stable. To further investigate the molecular basis for interactions between GAP and the Src kinases, we examined GAP association with and phosphorylation by a series of c-Src and v-Src mutants. Analysis of GAP association with c-Src/v-Src chimeric proteins demonstrates that GAP associates stably with Src proteins possessing low kinase activity and poorly with activated Src kinases, especially those that lack the carboxy-terminal segment of c-Src containing the regulatory amino acid Tyr-527. Phosphorylated Tyr-527 is a major determinant of c-Src association with GAP, as demonstrated by c-Src point mutants in which Tyr-527 is changed to Phe. While the isolated amino-terminal half of the c-Src protein is insufficient for stable GAP association, analysis of point substitutions of highly conserved amino acid residues in the c-Src SH2 region indicate that this region also influences Src-GAP complex formation. Therefore, our results suggest that both Tyr-527 phosphorylation and the SH2 region contribute to stable association of c-Src with GAP. Analysis of in vivo phosphorylation of GAP by v-Src mutants containing deletions encompassing the SH2, SH3, and unique regions suggests that the kinase domain of v-Src contains sufficient substrate specificity for GAP phosphorylation. Even though tyrosine phosphorylation of GAP correlates to certain extent with the transforming ability of various c-Src and v-Src mutants, our data suggest that other GAP-associated proteins may also have roles in Src-mediated oncogenic transformation. These findings provide additional evidence for the specificity of Src interactions with GAP and support the hypothesis that these interactions contribute to the biological functions of the Scr kinases.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

c-src structure in human cancers with elevated pp60c-src activity

We used RNAase protection and restriction fragment length polymorphism assays to detect activating mutations of c-src in a spectrum of human tumours. No mutations were detected at codons 98, 381, 444, and 530. We conclude that mutational activation is not the mechanism of enhancement of pp60c-src-specific kinase activity found in a number of human cancer types.

Differential expression of two lck transcripts directed from the distinct promoters in HTLV-I+ and HTLV-I- T-cells

The lck gene encodes a lymphocyte-specific tyrosine protein kinase, p56lck, the expression of which is almost exclusive in T-cells. The expression of lck in human T-cell leukemia virus type I (HTLV-I)-transformed T-cell lines is closely associated with interleukin-2 (IL-2) dependence for their growth. That is, IL-2-dependent HTLV-I-transformed cell lines contain the lck message abundantly as HTLV-I-negative T-cell lines, whereas IL-2-independent HTLV-I-transformed cell lines express either no or little lck mRNA, although they are derived from T-cells. The lck gene contains 2 distinct promoters which direct 2 types of lck transcript with different 5' untranslated regions. In this study, we show that HTLV-I-transformed IL-2-dependent T-cell lines contain the upstream promoter-initiated lck transcript exclusively, in contrast to HTLV-I-negative transformed T-cell lines which express the down-stream promoter- as well as the upstream promoter-initiated lck transcript. In addition, lck mRNA disappears transiently in IL-2-dependent HTLV-I-transformed T-cell lines after stimulation for T-cell activation, which is also observed in peripheral blood T lymphocytes. These results indicate that the disappearance of lck mRNA in HTLV-I-transformed, IL-2-independent cell lines is caused by a mechanism which down-regulates the upstream promoter-initiated lck transcript and this IL-2-independent state may represent a further "activated" condition of the IL-2-dependent state by the stimulation which mediates T-cell activation.

An Epstein-Barr virus protein associated with cell growth transformation interacts with a tyrosine kinase

Epstein-Barr virus (EBV) encodes two integral membrane proteins in latently infected growth-transformed cells. One of these, LMP1, can transform rodent fibroblasts and induce markers of B-lymphocyte activation. The second, LMP2, colocalizes with LMP1 in a constitutive patch in the EBV-transformed B-lymphocyte plasma membrane. The experiments reported here demonstrate that LMP2 may biochemically interact with LMP1 and that LMP2 closely associates with and is an important substrate for a B-lymphocyte tyrosine kinase in EBV-transformed B lymphocytes or in B-lymphoma cells in which LMP2 is expressed by gene transfer. LMP2 is also serine and threonine phosphorylated. LMP2 localizes to a peripheral membrane (presumably plasma membrane) patch in transfected B-lymphoma cells and colocalizes with much of the cellular tyrosine-phosphorylated proteins. LMP2 undergoes tyrosine phosphorylation in anti-LMP2 or antiphosphotyrosine immunoprecipitates from transfected B-lymphoma cells or EBV-transformed B lymphocytes. The first 167 of the 497 amino acids of LMP2 retain full ability to associate with and act as a substrate for a tyrosine kinase. A 70-kDa phosphotyrosine cell protein associates with LMP2 in transfected cells or in EBV-transformed B lymphocytes and could be a mediator of the effects of LMP2.

Cloning of a complementary DNA for a protein-tyrosine kinase that specifically phosphorylates a negative regulatory site of p60c-src

The protein-tyrosine kinase activity of the proto-oncogene product p60c-src is negatively regulated by the phosphorylation of a tyrosine residue close to the C terminus, tyrosine 527. The phosphorylation might be catalysed by a so-far-unidentified tyrosine kinase, distinct from p60c-src. Recently we purified a protein-tyrosine kinase that specifically phosphorylates tyrosine 527 of p60c-src from neonatal rat brain. We have now confirmed the specificity of this enzyme by using a mutant p60c-src that has a phenylalanine instead of tyrosine 527, and cloned a complementary DNA that encodes the enzyme. The enzyme is similar to kinases of the src family in that it has two conserved regions, Src-homology regions 2 and 3, upstream of a tyrosine kinase domain. The amino-acid identity of each region is no more than 47%, however, and the enzyme lacks phosphorylation sites corresponding to tyrosines 416 and 527 of p60c-src and has no myristylation signal. These results suggest that this protein-tyrosine kinase, which might negatively regulate p60c-src, represents a new type of tyrosine kinase.

The tyrosine kinase encoded by the MET proto-oncogene is activated by autophosphorylation

Protein tyrosine kinases are crucially involved in the control of cell proliferation. Therefore, the regulation of their activity in both normal and neoplastic cells has been under intense scrutiny. The product of the MET oncogene is a transmembrane receptorlike tyrosine kinase with a unique disulfide-linked heterodimeric structure. Here we show that the tyrosine kinase activity of the MET-encoded protein is powerfully activated by tyrosine autophosphorylation. The enhancement of activity was quantitated with a phosphorylation assay of exogenous substrates. It involved an increase in the Vmax of the enzyme-catalyzed phosphotransfer reaction. No change was observed in the Km (substrate). A causal relationship between tyrosine autophosphorylation and activation of the kinase activity was proved by (i) the kinetic agreement between autophosphorylation and kinase activation, (ii) the overlapping dose-response relationship for ATP, (iii) the specificity for ATP of the activation process, (iv) the phosphorylation of tyrosine residues only, in the Met protein, in the activation step, (v) the linear dependence of the activation from the input of enzyme assayed, and (vi) the reversal of the active state by phosphatase treatment. Autophosphorylation occurred predominantly on a single tryptic peptide, most likely via an intermolecular reaction. The structural features responsible for this positive modulation of kinase activity were all contained in the 45-kDa intracellular moiety of the Met protein.

Association of p60c-src with polyoma virus middle-T antigen abrogating mitosis-specific activation

Polyoma middle-T antigen is required for tumorigenesis in animals and for viral transformation of a variety of cells in culture (reviewed in ref. 1). Middle-T associates with and thereby activates p60c-src, a cellular tyrosine kinase homologous to the oncogene product of Rous sarcoma virus. Activation of p60c-src by middle-T is accompanied both by dephosphorylation of tyrosine 527, a site which negatively regulates src kinase src kinase activity (reviewed in refs 4-6) and by autophosphorylation on tyrosine 416 (refs 7-10). Phosphoprotein p60c-src is subject to cell cycle-specific regulation. It is most active during mitosis and repressed in interphase. Here we report that mitotic p60c-src is dephosphorylated at tyrosine 527. We also show that in cells expressing middle-T, src kinase activity is high both in mitosis and during interphase. An oncogenic mutant src protein, p60c-src(527F), where tyrosine 527 is substituted by phenylalanine, is also highly active in all phases of the cell cycle.

The tyrosine kinase encoded by the MET proto-oncogene is activated by autophosphorylation

Protein tyrosine kinases are crucially involved in the control of cell proliferation. Therefore, the regulation of their activity in both normal and neoplastic cells has been under intense scrutiny. The product of the MET oncogene is a transmembrane receptorlike tyrosine kinase with a unique disulfide-linked heterodimeric structure. Here we show that the tyrosine kinase activity of the MET-encoded protein is powerfully activated by tyrosine autophosphorylation. The enhancement of activity was quantitated with a phosphorylation assay of exogenous substrates. It involved an increase in the Vmax of the enzyme-catalyzed phosphotransfer reaction. No change was observed in the Km (substrate). A causal relationship between tyrosine autophosphorylation and activation of the kinase activity was proved by (i) the kinetic agreement between autophosphorylation and kinase activation, (ii) the overlapping dose-response relationship for ATP, (iii) the specificity for ATP of the activation process, (iv) the phosphorylation of tyrosine residues only, in the Met protein, in the activation step, (v) the linear dependence of the activation from the input of enzyme assayed, and (vi) the reversal of the active state by phosphatase treatment. Autophosphorylation occurred predominantly on a single tryptic peptide, most likely via an intermolecular reaction. The structural features responsible for this positive modulation of kinase activity were all contained in the 45-kDa intracellular moiety of the Met protein.

Tyrosine phosphorylation of a 120-kilodalton pp60src substrate upon epidermal growth factor and platelet-derived growth factor receptor stimulation and in polyomavirus middle-T-antigen-transformed cells

The monoclonal antibody 2B12 is directed toward p120, a 120-kDa cellular protein originally identified as a protein tyrosine kinase substrate in cells expressing membrane-associated oncogenic variants of pp60src. In this report, we show that p120 was tyrosine phosphorylated in avian cells expressing membrane-associated, enzymatically activated variants of c-src, including variants having structural alterations in the src homology regions 2 and 3. In contrast, p120 was not tyrosine phosphorylated in cells expressing enzymatically activated, nonmyristylated pp60src. Furthermore, p120 was tyrosine phosphorylated in avian cells expressing middle T antigen, the transforming protein of polyomavirus, as well as in rodent cells stimulated with either epidermal growth factor (EGF) or platelet-derived growth factor. Analysis of the time course of p120 tyrosine phosphorylation in EGF-stimulated cells revealed a rapid onset of tyrosine phosphorylation. In addition, both the extent and duration of p120 phosphorylation increased when cells overexpressing the EGF receptor were stimulated with EGF. Biochemical analysis showed that p120 (in both normal and src-transformed cells) was membrane associated, was myristylated, and was phosphorylated on serine and threonine residues. Hence, p120 appears to be a substrate of both nonreceptor- and ligand-activated transmembrane receptor tyrosine kinases and of serine/threonine kinases and is perhaps a component of both mitogen-stimulated and tyrosine kinase oncogene-induced signaling pathways.

Transformation by pp60src or stimulation of cells with epidermal growth factor induces the stable association of tyrosine-phosphorylated cellular proteins with GTPase-activating protein

GTPase-activating protein (GAP) is a cytosolic protein that stimulates the rate of hydrolysis of GTP (GTP to GDP) bound to normal p21ras, but does not catalyze the hydrolysis of GTP bound to oncogenic, activated forms of the ras protein. Transformation of cells with v-src or activated transforming variants of c-src or stimulation of cells with epidermal growth factor resulted in the stable association of GAP with two tyrosine-phosphorylated cellular proteins of 64 kDa (p64) and 190 kDa (p190). Analysis of GAP immune complexes isolated from extracts of metabolically labeled src-transformed cells and epidermal growth factor-stimulated cells indicated that tyrosine phosphorylation of p64 and p190 appeared to be coincident with the stable association of these proteins with GAP. Quantitation of the amount of p64 associated with GAP in v-src-transformed cells, however, indicated that only 15 to 25% of tyrosine-phosphorylated p64 was found in complex with GAP. Mutations within the SH2 region of pp60src that render activated pp60src defective for transformation inhibited the efficient formation of complexes between GAP and the tyrosine-phosphorylated forms of p64 and p190. From these data, we suggest that tyrosine phosphorylation and stable association of p64 with GAP is an important step in mediating cellular signaling through the p21ras-GAP pathway.

Tyrosine phosphorylation of a 120-kilodalton pp60src substrate upon epidermal growth factor and platelet-derived growth factor receptor stimulation and in polyomavirus middle-T-antigen-transformed cells

The monoclonal antibody 2B12 is directed toward p120, a 120-kDa cellular protein originally identified as a protein tyrosine kinase substrate in cells expressing membrane-associated oncogenic variants of pp60src. In this report, we show that p120 was tyrosine phosphorylated in avian cells expressing membrane-associated, enzymatically activated variants of c-src, including variants having structural alterations in the src homology regions 2 and 3. In contrast, p120 was not tyrosine phosphorylated in cells expressing enzymatically activated, nonmyristylated pp60src. Furthermore, p120 was tyrosine phosphorylated in avian cells expressing middle T antigen, the transforming protein of polyomavirus, as well as in rodent cells stimulated with either epidermal growth factor (EGF) or platelet-derived growth factor. Analysis of the time course of p120 tyrosine phosphorylation in EGF-stimulated cells revealed a rapid onset of tyrosine phosphorylation. In addition, both the extent and duration of p120 phosphorylation increased when cells overexpressing the EGF receptor were stimulated with EGF. Biochemical analysis showed that p120 (in both normal and src-transformed cells) was membrane associated, was myristylated, and was phosphorylated on serine and threonine residues. Hence, p120 appears to be a substrate of both nonreceptor- and ligand-activated transmembrane receptor tyrosine kinases and of serine/threonine kinases and is perhaps a component of both mitogen-stimulated and tyrosine kinase oncogene-induced signaling pathways.

Transformation by pp60src or stimulation of cells with epidermal growth factor induces the stable association of tyrosine-phosphorylated cellular proteins with GTPase-activating protein

GTPase-activating protein (GAP) is a cytosolic protein that stimulates the rate of hydrolysis of GTP (GTP to GDP) bound to normal p21ras, but does not catalyze the hydrolysis of GTP bound to oncogenic, activated forms of the ras protein. Transformation of cells with v-src or activated transforming variants of c-src or stimulation of cells with epidermal growth factor resulted in the stable association of GAP with two tyrosine-phosphorylated cellular proteins of 64 kDa (p64) and 190 kDa (p190). Analysis of GAP immune complexes isolated from extracts of metabolically labeled src-transformed cells and epidermal growth factor-stimulated cells indicated that tyrosine phosphorylation of p64 and p190 appeared to be coincident with the stable association of these proteins with GAP. Quantitation of the amount of p64 associated with GAP in v-src-transformed cells, however, indicated that only 15 to 25% of tyrosine-phosphorylated p64 was found in complex with GAP. Mutations within the SH2 region of pp60src that render activated pp60src defective for transformation inhibited the efficient formation of complexes between GAP and the tyrosine-phosphorylated forms of p64 and p190. From these data, we suggest that tyrosine phosphorylation and stable association of p64 with GAP is an important step in mediating cellular signaling through the p21ras-GAP pathway.

Altered tyrosine 527 phosphorylation and mitotic activation of p60c-src

The tyrosine kinasee activity of p60c-src, the protein product of the c-src gene, increases during mitosis; this may be important in initiating at least some of the cellular changes that occur during this phase of the cell cycle. Although there is evidence that p60c-src is phosphorylated at several sites during mitosis, phosphorylation in vitro does not increase its kinase activity. We now report that the kinase activity of a p60c-src mutant with residue tyrosine 527 changed to phenylanine does not change during the cell cycle, suggesting that changes in the phosphorylation state of this residue may be responsible for the activation of p60c-src at mitosis. Although changes in phosphorylation at Tyr 527 cannot be detected with the wild-type protein we find that phosphorylation at Tyr 527 of a mutant with reduced kinase activity decreases threefold during mitosis. On the basis of these results we suggest that activation of p60c-src at mitosis results from decreased phosphorylation on Tyr 527, and that p60c-src may be or may activate the kinase that phosphorylates Tyr 527.

Structural elements that regulate pp59c-fyn catalytic activity, transforming potential, and ability to associate with polyomavirus middle-T antigen

Except for its unique amino-terminal region (residues 1 through 83), which possibly dictates substrate recognition, pp59c-fyn bears a high degree of homology with other members of the src family of tyrosine kinases. Here we show that the carboxy terminus of pp59c-fyn is necessary for stable middle-T-antigen association, that pp59c-fyn is normally phosphorylated on both serine and tyrosine residues, and that Tyr-531 and Tyr-420 are phosphorylation sites in vivo and in vitro, respectively. Analysis of a spontaneously generated mutant encoding a truncated form of pp59c-fyn and of variants specifically mutated at the Tyr-531 and Tyr-420 phosphorylation sites indicates that pp59c-fyn has regulatory elements analogous to those that have already been identified for other src-like tyrosine kinases. However, further examination of the pp59c-fyn variants suggests the likelihood of additional means by which its activities might be regulated. Although alteration of Tyr-531 to phenylalanine (531F) in pp59c-fyn results in a protein which is more active enzymatically that the wild type, the enhancement is much less than that for the analogous variant of pp60c-src. Furthermore, contrary to results of similar experiments on other src-like proto-oncogene products, 531F did not induce transformation of NIH 3T3 cells. Studies involving pp59c-fyn-pp60c-src chimeras in which the unique amino-terminal sequences (residues 1 through 83) of the two kinases were precisely interchanged implied that the inability of 531F to induce transformation is probably not caused by the absence of substrates for pp59c-fyn in NIH 3T3 cells but rather by the insufficient enhancement of pp59c-fyn kinase activity. It is therefore probable that the kinase and transforming activities of pp59c-fyn are repressed by additional regulatory elements possibly located in the amino-terminal half of the molecule.

Tyrosine phosphorylation of three cellular proteins correlates with transformation of rat 1 cells by pp60src

The analysis of phosphotyrosine-containing proteins in Rat 1 cells overexpressing either the tyrosine kinase pp60c-src or genetic variants containing alterations in functional and structural domains has led to the identification of three proteins whose tyrosine phosphorylation correlated with pp60src-induced cellular transformation. The tyrosine phosphorylation of one of these proteins, p120, has been previously shown by us and others to coincide with the presence of kinase-activated, membrane-associated pp60src in chicken embryo cells. The second protein was identified as the ras-associated GTPase-activating protein (GAP). The third protein whose tyrosine phosphorylation was markedly elevated in Rat 1 cells expressing activated, membrane-bound forms of pp60src had an apparent molecular mass of 64-67 kDa. The electrophoretic mobility of this protein varied in cells expressing different pp60src variants. The tyrosine-phosphorylated form of p64-67 was present in immune complexes containing GAP, suggesting a stable interaction between these two cellular proteins.

A retroviral promoter is sufficient to convert proto-src to a transforming gene that is distinct from the src gene of Rous sarcoma virus

The src genes of four natural isolates of avian sarcoma viruses differ from cellular proto-src in two genetic substitutions: the promoter of the cellular gene is replaced by a retroviral counterpart, and at least six codons from the 3' terminus are replaced by retroviral or heterologous cell-derived elements. Since virus constructs with a complete proto-src coding region failed to transform avian cells but acquired transforming function by point mutations of various codons, it has been proposed that point mutation is sufficient to convert proto-src to a transforming gene. However, promoter substitution is sufficient to convert two other proto-onc genes, proto-ras and proto-myc, to retroviral transforming genes. In view of this, we have reexamined whether promoter substitution, point mutation, or both are necessary to convert proto-src into a retroviral transforming gene. It was found that a recombinant virus (RpSV), in which the src gene of Rous sarcoma virus (RSV) was replaced by the complete coding region of proto-src, transformed quail and chicken embryo cells. The oncogene of RpSV differs from the src gene of RSV in three genetic properties: (i) it is weaker--e.g., transformed cells are flatter; (ii) it is slower--e.g., focus formation takes 9 to 12 days compared to 4 days for RSV; and (iii) its host range is narrower than that of RSV--e.g., only subsets of heterogeneous embryo cells are transformed by RpSV even after weeks or months. Replacement of the proto-src 3' terminus of RpSV by that of src from RSV generates a recombinant virus (RpvSV) that equals RSV in transforming function. It is concluded that a retroviral promoter, naturally substituted via illegitimate recombination with retroviruses, is sufficient to convert at least three proto-onc genes, src, myc, and ras, to retroviral transforming genes.

Activation of p56lck through mutation of a regulatory carboxy-terminal tyrosine residue requires intact sites of autophosphorylation and myristylation

Mutation of the major site of in vivo tyrosine phosphorylation of p56lck (tyrosine 505) to a phenylalanine constitutively enhances the p56lck-associated tyrosine-specific protein kinase activity. The mutant polypeptide is extensively phosphorylated in vivo at the site of in vitro Lck autophosphorylation (tyrosine 394) and is capable of oncogenic transformation of rodent fibroblasts. These observations have suggested that phosphorylation at Tyr-505 down regulates the tyrosine protein kinase activity of p56lck. Herein we have attempted to examine whether other posttranslational modifications may be involved in regulation of the enzymatic function of p56lck. The results indicated that activation of p56lck by mutation of Tyr-505 was prevented by a tyrosine-to-phenylalanine substitution at position 394. Furthermore, activation of p56lck by mutation of the carboxy-terminal tyrosine residue was rendered less efficient by substituting an alanine residue for the amino-terminal glycine. This second mutation prevented p56lck myristylation and stable membrane association and was associated with decreased in vivo phosphorylation at Tyr-394. Taken together, these findings imply that lack of phosphorylation at Tyr-505 may be insufficient for enhancement of the p56lck-associated tyrosine protein kinase activity. Our data suggest that activation of p56lck may be dependent on phosphorylation at Tyr-394 and that this process may be facilitated by myristylation, membrane association, or both.

Stoichiometry of cellular and viral components in the polyomavirus middle-T antigen-tyrosine kinase complex

Our results indicate that only one type of tyrosine kinase is present within each middle-T antigen-tyrosine kinase complex, suggesting that middle-T antigen forms separate complexes with different tyrosine kinases. Furthermore, we determined that there is only one molecule of middle-T antigen within any one of these complexes. We interpret this to mean that in any given cell, polyomavirus transformation involves, at least in part, the simultaneous deregulation of a number of separate pathways controlling cellular proliferation. Finally, we also demonstrate that the separate middle-T:pp60c-src and middle-T:pp59c-fyn complexes are each able to interact with the same cellular p81/85-kDa phosphoprotein, a possible component of the phosphatidylinositol kinase.

Stoichiometry of cellular and viral components in the polyomavirus middle-T antigen-tyrosine kinase complex

Our results indicate that only one type of tyrosine kinase is present within each middle-T antigen-tyrosine kinase complex, suggesting that middle-T antigen forms separate complexes with different tyrosine kinases. Furthermore, we determined that there is only one molecule of middle-T antigen within any one of these complexes. We interpret this to mean that in any given cell, polyomavirus transformation involves, at least in part, the simultaneous deregulation of a number of separate pathways controlling cellular proliferation. Finally, we also demonstrate that the separate middle-T:pp60c-src and middle-T:pp59c-fyn complexes are each able to interact with the same cellular p81/85-kDa phosphoprotein, a possible component of the phosphatidylinositol kinase.

Activation of p56lck through mutation of a regulatory carboxy-terminal tyrosine residue requires intact sites of autophosphorylation and myristylation

Mutation of the major site of in vivo tyrosine phosphorylation of p56lck (tyrosine 505) to a phenylalanine constitutively enhances the p56lck-associated tyrosine-specific protein kinase activity. The mutant polypeptide is extensively phosphorylated in vivo at the site of in vitro Lck autophosphorylation (tyrosine 394) and is capable of oncogenic transformation of rodent fibroblasts. These observations have suggested that phosphorylation at Tyr-505 down regulates the tyrosine protein kinase activity of p56lck. Herein we have attempted to examine whether other posttranslational modifications may be involved in regulation of the enzymatic function of p56lck. The results indicated that activation of p56lck by mutation of Tyr-505 was prevented by a tyrosine-to-phenylalanine substitution at position 394. Furthermore, activation of p56lck by mutation of the carboxy-terminal tyrosine residue was rendered less efficient by substituting an alanine residue for the amino-terminal glycine. This second mutation prevented p56lck myristylation and stable membrane association and was associated with decreased in vivo phosphorylation at Tyr-394. Taken together, these findings imply that lack of phosphorylation at Tyr-505 may be insufficient for enhancement of the p56lck-associated tyrosine protein kinase activity. Our data suggest that activation of p56lck may be dependent on phosphorylation at Tyr-394 and that this process may be facilitated by myristylation, membrane association, or both.

Functional analysis and nucleotide sequence of the promoter region of the murine hck gene

The structure and function of the promoter region and exon 1 of the murine hck gene have been characterized in detail. RNase protection analysis has established that hck transcripts initiate from heterogeneous start sites located within the hck gene. Fusion gene constructs containing hck 5'-flanking sequences and the bacterial Neor gene have been introduced into the hematopoietic cell lines FDC-P1 and WEHI-265 by using a self-inactivating retroviral vector. The transcriptional start sites of the fusion gene are essentially identical to those of the endogenous hck gene. Analysis of infected WEHI-265 cell lines treated with bacterial lipopolysaccharide (LPS) reveals a 3- to 5-fold elevation in the levels of endogenous hck mRNA and a 1.4- to 2.6-fold increase in the level of Neor fusion gene transcripts, indicating that hck 5'-flanking sequences are capable of conferring LPS responsiveness on the Neor gene. The 5'-flanking region of the hck gene contains sequences similar to an element which is thought to be involved in the LPS responsiveness of the class II major histocompatibility gene A alpha k. A subset of these sequences are also found in the 5'-flanking regions of other LPS-responsive genes. Moreover, this motif is related to the consensus binding sequence of NF-kappa B, a transcription factor which is known to be regulated by LPS.

Functional analysis and nucleotide sequence of the promoter region of the murine hck gene

The structure and function of the promoter region and exon 1 of the murine hck gene have been characterized in detail. RNase protection analysis has established that hck transcripts initiate from heterogeneous start sites located within the hck gene. Fusion gene constructs containing hck 5'-flanking sequences and the bacterial Neor gene have been introduced into the hematopoietic cell lines FDC-P1 and WEHI-265 by using a self-inactivating retroviral vector. The transcriptional start sites of the fusion gene are essentially identical to those of the endogenous hck gene. Analysis of infected WEHI-265 cell lines treated with bacterial lipopolysaccharide (LPS) reveals a 3- to 5-fold elevation in the levels of endogenous hck mRNA and a 1.4- to 2.6-fold increase in the level of Neor fusion gene transcripts, indicating that hck 5'-flanking sequences are capable of conferring LPS responsiveness on the Neor gene. The 5'-flanking region of the hck gene contains sequences similar to an element which is thought to be involved in the LPS responsiveness of the class II major histocompatibility gene A alpha k. A subset of these sequences are also found in the 5'-flanking regions of other LPS-responsive genes. Moreover, this motif is related to the consensus binding sequence of NF-kappa B, a transcription factor which is known to be regulated by LPS.

Serine/threonine-specific protein kinase activity associated with viral pp60src protein

Three different types of experiments are presented in this paper, the results of which converge to indicate that the viral src protein associates with and modulates the activity and/or the specificity of a serine/threonine protein kinase. Firstly, a 60-kDa protein from extracts of FR3T3 rat fibroblasts transformed by wild-type Rous sarcoma virus (SRD-FR3T3) is shown to be immunoprecipitated with a monoclonal antibody (mAb) raised against bacterially produced pp60v-src, the mAb327 [Lipsich, L. A., Lewis, A. J. & Brugge, J. S. (1983) J. Virol. 48, 352-360] and to be phosphorylated in vitro at serine/threonine/tyrosine residues, in the ratio 25:53:22. Under the same experimental conditions, the pp60c-src protein immunoprecipitated with mAb327 from extracts of NIH c-src overexpresser cells is phosphorylated exclusively on tyrosine residues. Secondly, the results of immunoprecipitation experiments using a tumor-bearing rabbit (TBR) serum and reported in an earlier work [David-Pfeuty, T. & Hovanessian, A. (1984) Eur. J. Biochem. 140, 325-342], together with those reported here, suggest that the TBR-immunoprecipitated pp60v-src coprecipitates with a cellular protein related to the 60-kDa subunit of the Ca2+/calmodulin protein kinase II from brain. Finally, partially purified preparations of pp60v-src, but not of pp60c-src, are shown to contain a Ca2+/calmodulin-dependent protein kinase activity that phosphorylates a 52-kDa protein substrate.

Evidence for epidermal growth factor (EGF)-induced intermolecular autophosphorylation of the EGF receptors in living cells

In response to epidermal growth factor (EGF) stimulation, the intrinsic protein tyrosine kinase of EGF receptor is activated, leading to tyrosine phosphorylation of several cellular substrate proteins, including the EGF receptor molecule itself. To test the mechanism of EGF receptor autophosphorylation in living cells, we established transfected cell lines coexpressing a kinase-negative point mutant of EGF receptor (K721A) with an active EGF receptor mutant lacking 63 amino acids from its carboxy terminus. The addition of EGF to these cells caused tyrosine phosphorylation of the kinase-negative mutant by the active receptor molecule, demonstrating EGF receptor cross-phosphorylation in living cells. After internalization the kinase-negative mutant and CD63 have separate trafficking pathways. This limits their association and the extent of cross-phosphorylation of K721A by CD63. The coexpression of the kinase-negative mutant together with active EGF receptors in the same cells suppressed the mitogenic response toward EGF as compared with that in cells that express active receptors alone. The presence of the kinase-negative mutant functions as a negative dominant mutation suppressing the response of active EGF receptors, probably by interfering with EGF-induced signal transduction. It appears, therefore, that crucial events of signal transduction occur before K721A and active EGF receptors are separated by their different endocytic itineraries.

Evidence for epidermal growth factor (EGF)-induced intermolecular autophosphorylation of the EGF receptors in living cells

In response to epidermal growth factor (EGF) stimulation, the intrinsic protein tyrosine kinase of EGF receptor is activated, leading to tyrosine phosphorylation of several cellular substrate proteins, including the EGF receptor molecule itself. To test the mechanism of EGF receptor autophosphorylation in living cells, we established transfected cell lines coexpressing a kinase-negative point mutant of EGF receptor (K721A) with an active EGF receptor mutant lacking 63 amino acids from its carboxy terminus. The addition of EGF to these cells caused tyrosine phosphorylation of the kinase-negative mutant by the active receptor molecule, demonstrating EGF receptor cross-phosphorylation in living cells. After internalization the kinase-negative mutant and CD63 have separate trafficking pathways. This limits their association and the extent of cross-phosphorylation of K721A by CD63. The coexpression of the kinase-negative mutant together with active EGF receptors in the same cells suppressed the mitogenic response toward EGF as compared with that in cells that express active receptors alone. The presence of the kinase-negative mutant functions as a negative dominant mutation suppressing the response of active EGF receptors, probably by interfering with EGF-induced signal transduction. It appears, therefore, that crucial events of signal transduction occur before K721A and active EGF receptors are separated by their different endocytic itineraries.

Identification of tyrosine 706 in the kinase insert as the major colony-stimulating factor 1 (CSF-1)-stimulated autophosphorylation site in the CSF-1 receptor in a murine macrophage cell line

The receptor for colony-stimulating factor 1 (CSF-1) is a ligand-activated protein-tyrosine kinase. It has been shown previously that the CSF-1 receptor is phosphorylated on serine in vivo and that phosphorylation on tyrosine can be induced by stimulation with CSF-1. We studied the phosphorylation of the CSF-1 receptor by using the BAC1.2F5 murine macrophage cell line, which naturally expresses CSF-1 receptors. Two-dimensional tryptic phosphopeptide mapping showed that the CSF-1 receptor is phosphorylated on several different serine residues in vivo. Stimulation with CSF-1 at 37 degrees C resulted in rapid phosphorylation on tyrosine at one major site and one or two minor sites. We identified the major site as Tyr-706. The identity of Tyr-706 was confirmed by mutagenesis. This residue is located within the kinase insert domain. There was no evidence that Tyr-973 (equivalent to Tyr-969 in the human CSF-1 receptor) was phosphorylated following CSF-1 stimulation. When cells were stimulated with CSF-1 at 4 degrees C, additional phosphotyrosine-containing phosphopeptides were detected and the level of phosphorylation of the individual phosphotyrosine-containing phosphopeptides was substantially increased. In addition, we show that CSF-1 receptors are capable of autophosphorylation at six to eight major sites in vitro.

Transformation of chicken embryo fibroblast cells by avian retroviruses containing the human Fyn gene and its mutated genes

The transforming activity of the human fyn protein, p59fyn, which is a kinase of the src family, was investigated by testing the effect of recombinant avian retrovirus (Fyn virus) expressing p59fyn on chickens or cultured chicken embryo fibroblast (CEF) cells. The Fyn virus did not induce transformed foci. After several passages of the virus stock on CEF cells, however, a few foci were detected in the presence of dimethyl sulfoxide. Chickens inoculated with Fyn virus at the stage of 12-day-old embryos developed fibrosarcomas 3 to 6 weeks after hatching. The viruses obtained from these foci and from one of the tumor tissues showed high transforming activity in the presence of dimethyl sulfoxide, suggesting that these viruses carry spontaneous mutations of the fyn gene. Four fyn genes from CEF DNAs infected with transforming viruses were molecularly cloned, and their products were confirmed to possess transforming activity. DNA sequence analysis of the fyn genes showed that two of the four mutants have Thr instead of Ile at position 338 in the kinase domain. The other two mutants carry deletions of 78 and 108 base pairs, respectively, which result in complete loss of region C of SH2. The overall level of proteins containing phosphotyrosine was significantly higher in transformed cells than in normal CEF cells. Our data indicate that when expressed at high levels in a retrovirus, normal p59fyn cannot cause cellular transformation, but that mutant p59fyn with either a single amino acid substitution in the kinase domain or a deletion including region C produces a transforming protein, perhaps due to enhanced tyrosine kinase activity. This is the first observation that deletion of region C can unmask the potential transforming activity of a src family kinase.

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

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

Characterization of purified pp60c-src protein tyrosine kinase from human platelets

Intact pp60c-src, the cellular homologue of the transforming protein of Rous sarcoma virus, was purified from human platelets. The purified fractions also contained small amounts of a 54-kDa proteolytic degradation product of pp60c-src. We investigated some of the biochemical and kinetic properties of pp60c-src protein tyrosine kinase. Maximum kinase activity occurred at pH 6.5 and required a mixture of 2 mM Mn2+/Mg2+ as divalent cations. The enzyme most strongly phosphorylated casein, followed by enolase and alcohol dehydrogenase. The Km value for ATP was 4 microM for substrate phosphorylation and for autophosphorylation. Using casein, we determined a Vmax for substrate phosphorylation by pp60c-src in the range of 1.9-3.4 nmol.min-1.mg-1. Since the Vmax value for the purified 54-kDa fragment of pp60c-src was also included in this value, we conclude that proteolytic degradation of a 6-kDa fragment from the N-terminus of pp60c-src did not affect its kinase activity. Tryptic phosphopeptide analysis identified Tyr-416 as the major autophosphorylation site. Preincubation of purified pp60c-src with ATP increased the amount of autophosphorylation accompanied by an increase in Vmax, whereas the Km values were not altered. Our data directly demonstrate that autophosphorylation at Tyr-416 exerts, in contrast to phosphorylation at Tyr-527, a positive regulatory effect on the pp60c-src kinase activity.

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

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

Transformation of chicken embryo fibroblast cells by avian retroviruses containing the human Fyn gene and its mutated genes

The transforming activity of the human fyn protein, p59fyn, which is a kinase of the src family, was investigated by testing the effect of recombinant avian retrovirus (Fyn virus) expressing p59fyn on chickens or cultured chicken embryo fibroblast (CEF) cells. The Fyn virus did not induce transformed foci. After several passages of the virus stock on CEF cells, however, a few foci were detected in the presence of dimethyl sulfoxide. Chickens inoculated with Fyn virus at the stage of 12-day-old embryos developed fibrosarcomas 3 to 6 weeks after hatching. The viruses obtained from these foci and from one of the tumor tissues showed high transforming activity in the presence of dimethyl sulfoxide, suggesting that these viruses carry spontaneous mutations of the fyn gene. Four fyn genes from CEF DNAs infected with transforming viruses were molecularly cloned, and their products were confirmed to possess transforming activity. DNA sequence analysis of the fyn genes showed that two of the four mutants have Thr instead of Ile at position 338 in the kinase domain. The other two mutants carry deletions of 78 and 108 base pairs, respectively, which result in complete loss of region C of SH2. The overall level of proteins containing phosphotyrosine was significantly higher in transformed cells than in normal CEF cells. Our data indicate that when expressed at high levels in a retrovirus, normal p59fyn cannot cause cellular transformation, but that mutant p59fyn with either a single amino acid substitution in the kinase domain or a deletion including region C produces a transforming protein, perhaps due to enhanced tyrosine kinase activity. This is the first observation that deletion of region C can unmask the potential transforming activity of a src family kinase.

Identification of tyrosine 706 in the kinase insert as the major colony-stimulating factor 1 (CSF-1)-stimulated autophosphorylation site in the CSF-1 receptor in a murine macrophage cell line

The receptor for colony-stimulating factor 1 (CSF-1) is a ligand-activated protein-tyrosine kinase. It has been shown previously that the CSF-1 receptor is phosphorylated on serine in vivo and that phosphorylation on tyrosine can be induced by stimulation with CSF-1. We studied the phosphorylation of the CSF-1 receptor by using the BAC1.2F5 murine macrophage cell line, which naturally expresses CSF-1 receptors. Two-dimensional tryptic phosphopeptide mapping showed that the CSF-1 receptor is phosphorylated on several different serine residues in vivo. Stimulation with CSF-1 at 37 degrees C resulted in rapid phosphorylation on tyrosine at one major site and one or two minor sites. We identified the major site as Tyr-706. The identity of Tyr-706 was confirmed by mutagenesis. This residue is located within the kinase insert domain. There was no evidence that Tyr-973 (equivalent to Tyr-969 in the human CSF-1 receptor) was phosphorylated following CSF-1 stimulation. When cells were stimulated with CSF-1 at 4 degrees C, additional phosphotyrosine-containing phosphopeptides were detected and the level of phosphorylation of the individual phosphotyrosine-containing phosphopeptides was substantially increased. In addition, we show that CSF-1 receptors are capable of autophosphorylation at six to eight major sites in vitro.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

The amino terminus of polyomavirus middle T antigen is required for transformation

In polyomavirus-transformed cells, pp60c-src is activated by association with polyomavirus middle T antigen. These complexes have a higher tyrosine kinase activity compared with that of unassociated pp60c-src. Genetic analyses have revealed that the carboxy-terminal 15 amino acids of pp60c-src and the amino-terminal half of middle T antigen are required for this association and consequent activation of the tyrosine kinase. To define in greater detail the borders of the domain in middle T antigen required for activation of pp60c-src, we constructed a set of unidirectional amino-terminal deletion mutants of middle T antigen. Analysis of these mutants revealed that the first six amino acids of middle T antigen are required for it to activate the kinase activity of pp60c-src and to transform Rat-1 fibroblasts. Analysis of a series of insertion and substitution mutants confirmed these observations and further revealed that mutations affecting the first four amino acids of middle T antigen reduced or abolished its capacity to activate the kinase activity of pp60c-src and to transform Rat-1 cells in culture. Our results suggest that the first four amino acids of middle T antigen constitute part of a domain required for activation of the pp60c-src tyrosyl kinase activity and for consequent cellular transformation.

Monoclonal antibodies to individual tyrosine-phosphorylated protein substrates of oncogene-encoded tyrosine kinases

Cellular transformation by oncogenic retroviruses encoding protein tyrosine kinases coincides with the tyrosine-specific phosphorylation of multiple protein substrates. Previous studies have shown that tyrosine phosphorylation of a protein of 120 kDa, p120, correlated with src transformation in chicken embryo fibroblasts. Additionally, we previously identified two phosphotyrosine-containing cellular proteins, p130 and p110, that formed stable complexes with activated variants of pp60src, the src-encoded tyrosine kinase. To study transformation-relevant tyrosine kinase substrates, we have generated monoclonal antibodies to individual tyrosine phosphoproteins, including p130, p120, p110, and five additional phosphoproteins (p210, p125, p118, p85, and p185/p64). These antibodies detected several of the same tyrosine phosphoproteins in chicken embryo fibroblasts transformed by avian retroviruses Y73 and CT10, encoding the yes and crk oncogenes, respectively. Protein substrates in mouse, rat, hamster, and human cells overexpressing activated variants of chicken pp60src were also detected by several of the monoclonal antibodies.

The retarded electrophoretic migration of p56lck induced by vanadate in lymphoma cells correlates with modified kinase activity

p56lck is a src related lymphocyte specific tyrosine protein kinase which undergoes specific changes during T-cell activation, particularly the appearance of slow migrating forms. To analyze these forms, LSTRA cells were treated with vanadate. This resulted in increased phosphorylation of p56lck with the appearance of slow migrating forms. Renaturation of the p56lck bands after gel migration showed that vanadate mostly increased the activity of the lower band of p56lck. The upper bands had a reduced specific activity. In addition, the upper bands from vanadate treated cells displayed additional phosphorylated sites.

Mutation of the serine 312 phosphorylation site does not alter the ability of mouse p53 to inhibit simian virus 40 DNA replication in vivo

Two mutations were introduced into the wild-type mouse p53 gene by oligonucleotide-directed mutagenesis. These mutations substituted alanine or aspartic acid for serine at position 312, which is constitutively phosphorylated. Phosphopeptide mapping of the mutant proteins, expressed in COS cells, confirmed the loss of phosphorylation at position 312. There were no changes in the ability of the mutant p53s to express the conformation-dependent epitope for monoclonal antibody PAb246 or to participate in complexes with the simian virus 40 (SV40) large T antigen. Replication of a plasmid containing the SV40 origin of replication was inhibited in COS cells by wild-type p53 and both of the phosphorylation site mutants with equal efficiency. A transforming mutant of p53, encoding valine at position 135, did not inhibit SV40 DNA replication in COS cells.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

Association of protein phosphatase 2A with polyoma virus medium tumor antigen

The polyoma virus medium and small tumor antigens, as well as simian virus 40 small tumor antigen, form specific complexes with two cellular proteins designated 61- and 37-kDa proteins. In this report, we demonstrate that the 61- and 37-kDa proteins correspond to the A and C subunits, respectively, of the serine- and threonine-specific protein phosphatase 2A (PP2A). On the one hand, antibodies raised against the 61-kDa protein reacted specifically with the purified A subunit of PP2A. Furthermore, the amino acid sequences of seven tryptic peptides from the A subunit were almost identical to sequences of the 61-kDa protein as deduced from the corresponding cDNA sequence. On the other hand, antibodies against the purified C subunit (catalytic subunit) of PP2A reacted specifically with the medium tumor antigen-associated 37-kDa protein. These data suggest a role of PP2A in cell transformation by polyoma virus and simian virus 40.

Tyrosine phosphorylation of a c-Src-like protein is increased in membranes of CD4- CD8- T lymphocytes from lpr/lpr mice

Mice homozygous for the autosomal recessive lpr gene have a disorder that results in autoimmunity and massive accumulation of T lymphocytes lacking CD4 and CD8 surface markers. These abnormal T cells exhibit constitutive tyrosine phosphorylation of a component of the CD3-T-cell receptor complex. We compared membrane tyrosine phosphorylation in lpr/lpr CD4- CD8- T cells and control T cells, lpr membranes exhibited a 7.3-fold increase (n = 16) in tyrosine phosphorylation of a 60-kilodalton protein. The increase was correlated with the Lpr but not the CD4- CD8- phenotype in that p60 phosphorylation was not increased in membranes from normal CD4- CD8- thymocytes. To identify the p60 in lpr cells, we examined the activity of several T-cell tyrosine-specific protein kinases. p56lck phosphorylation was only slightly increased in lpr membranes (2.2-fold; n = 16). Phorbol ester treatment of intact T cells before membrane isolation caused p56lck to migrate as pp60lck; however, pp60lck could be clearly distinguished from the pp60 in lpr cells by two-dimensional gel electrophoresis. The pp60 from lpr cells exhibited several isoforms at pH approximately 6.3 to 6.5. Although on two-dimensional gels pp60c-src had a pI (6.4 to 6.8) within a similar region, p60c-src mRNA, protein, and kinase activities were not increased in lpr cells. In addition, staphylococcal V8 proteolytic cleavage of the lpr pp60 isolated on two-dimensional gels yielded two major fragments, a pattern distinct from that of pp60c-src. However, by using an antiserum against the C-terminal sequence of c-Src and other related kinases, including p59fyn, the pp60 could be immunoprecipitated in greater amounts from lpr than from control T cells. When pp59(fyn) was selectively immunoprecipitated from T-cell membranes with specific antisera, its molecular weight, proteolytic cleavage pattern, and behavior on two-dimensional gels were identical to those of the pp60 from lpr cells. We conclude that p59(fyn) phosphorylation is increased in membranes from lpr/lpr CD4(-) CD8(-) T cells and that the increase is correlated with constitutive tyrosine phosphorylation and perhaps with the expansion of this unusual T-cell population.