Deletions in the SH2 domain of p60v-src prevent association with the detergent-insoluble cellular matrix

Signaling Enzymes and Ion Channels Being Modulated by the Actin Cytoskeleton at the Plasma Membrane

A cell should deal with the changing external environment or the neighboring cells. Inevitably, the cell surface receives and transduces a number of signals to produce apt responses. Typically, cell surface receptors are activated, and during this process, the subplasmalemmal actin cytoskeleton is often rearranged. An intriguing point is that some signaling enzymes and ion channels are physically associated with the actin cytoskeleton, raising the possibility that the subtle changes of the local actin cytoskeleton can, in turn, modulate the activities of these proteins. In this study, we reviewed the early and new experimental evidence supporting the notion of actin-regulated enzyme and ion channel activities in various cell types including the cells of immune response, neurons, oocytes, hepatocytes, and epithelial cells, with a special emphasis on the Ca²⁺ signaling pathway that depends on the synthesis of inositol 1,4,5-trisphosphate. Some of the features that are commonly found in diverse cells from a wide spectrum of the animal species suggest that fine-tuning of the activities of the enzymes and ion channels by the actin cytoskeleton may be an important strategy to inhibit or enhance the function of these signaling proteins.

A novel nuclear Src and p300 signaling axis controls migratory and invasive behavior in pancreatic cancer

The presence of Src in the nuclear compartment has been previously reported, although its significance has remained largely unknown. We sought to delineate the functions of the nuclear pool of Src within the context of malignant progression. Active Src is localized within the nuclei of human pancreatic cancer cells and mouse fibroblasts over-expressing c-Src where it is associated with p300. Nuclear Src additionally promotes the tyrosine phosphorylation of p300 in pancreatic cancer Panc-1 cells. Src, together with p300, is associated with the high-mobility group AT-hook (HMGA)2 and SET and MYND domain-containing protein (SMYD)3 gene promoters and regulates their expression in a Src-dependent manner. These nuclear Src-dependent events correlate with anchorage-independent soft-agar growth and the migratory properties in both pancreatic Panc-1 cells and mouse fibroblasts over-expressing Src. Moreover, analyses of human pancreatic ductal adenocarcinoma (PDAC) tumor tissues detected the association of nuclear Src with the HMGA2 and SMYD3 gene promoters. Our findings for the first time show the critical importance of nuclear Src and p300 function in the migratory properties of pancreatic cancer cells. Further, data together identify a previously unknown role of nuclear Src in the regulation of gene expression in association with p300 within the context of cells harboring activated or over-expressing Src. This novel mechanism of nuclear Src-p300 axis in PDAC invasiveness and metastasis may provide an opportunity for developing more effective early clinical interventions for this lethal disease.

Active Src is complexed with and phosphorylates p300 in the nucleus, and the complex is bound to HMGA2 and SMYD3 genes, thereby regulating their expression to promote pancreatic tumor cell migration and invasiveness.

Evidence for secretion-like coupling involving pp60src in the activation and maintenance of store-mediated Ca2+ entry in mouse pancreatic acinar cells

Store-mediated Ca2+ entry (SMCE) is one of the main pathways for Ca2+ influx in non-excitable cells. Recent studies favour a secretion-like coupling mechanism to explain SMCE, where Ca2+ entry is mediated by an interaction of the endoplasmic reticulum (ER) with the plasma membrane (PM) and is modulated by the actin cytoskeleton. To explore this possibility further we have now investigated the role of the actin cytoskeleton in the activation and maintenance of SMCE in pancreatic acinar cells, a more specialized secretory cell type which might be an ideal cellular model to investigate further the properties of the secretion-like coupling model. In these cells, the cytoskeletal disrupters cytochalasin D and latrunculin A inhibited both the activation and maintenance of SMCE. In addition, stabilization of a cortical actin barrier by jasplakinolide prevented the activation, but not the maintenance, of SMCE, suggesting that, as for secretion, the actin cytoskeleton plays a double role in SMCE as a negative modulator of the interaction between the ER and PM, but is also required for this mechanism, since the cytoskeleton disrupters impaired Ca2+ entry. Finally, depletion of the intracellular Ca2+ stores induces cytoskeletal association and activation of pp60(src), which is independent on Ca2+ entry. pp60(src) activation requires the integrity of the actin cytoskeleton and participates in the initial phase of the activation of SMCE in pancreatic acinar cells.

The actin filament-associated protein AFAP-110 is an adaptor protein that modulates changes in actin filament integrity

The actin filament-associated protein of 110 kDa (AFAP-110) was first identified as an SH3/SH2 binding partner for the nonreceptor tyrosine kinase, Src. Subsequent data have demonstrated that AFAP-110 can interact with other Src family members. AFAP-110 contains additional protein binding modules including two pleckstrin homology domains, a leucine zipper motif and a target sequence for serine/threonine phosphorylation. AFAP-110 interacts with actin filaments directly via a carboxy terminal actin-binding domain. Thus AFAP-110 may function as an adaptor protein by linking Src family members and/or other signaling proteins to actin filaments. AFAP-110 also has an intrinsic capability to alter actin filament integrity that can be revealed upon conformational changes associated with phosphorylation or mutagenesis. Recent data has indicated that AFAP-110 may also serve to activate cSrc in response to this conformational change as well. Thus, AFAP-110 may function in several ways by (1) acting as an adaptor protein that links signaling molecules to actin filaments, (2) serving as a platform for the construction of larger signaling complexes, (3) serving as an activator of Src family kinases in response to cellular signals that alter its conformation and (4) directly effecting actin filament organization as an actin filament cross-linking protein. Here, we will review the structure and function of AFAP-110 as well as potential binding partners and effectors of AFAP-110's ability to alter actin filament integrity.

UCS15A, a non-kinase inhibitor of Src signal transduction

Src tyrosine kinase plays key roles in signal transduction following growth factor stimulation and integrin-mediated cell-substrate adhesion. Since src-signal transduction defects are implicated in a multitude of human diseases, we have sought to develop new ways to identify small molecule inhibitors using a yeast-based, activated-src over-expression system. In the present study, we describe the identification of a unique src-signal transduction inhibitor, UCS15A. UCS15A was found to inhibit the src specific tyrosine phosphorylation of numerous proteins in v-src-transformed cells. Two of these phosphoproteins were identified as bona-fide src substrates, cortactin and Sam68. UCS15A differed from conventional src-inhibitors in that it did not inhibit the tyrosine kinase activity of src. In addition, UCS15A appeared to differ from src-destabilizing agents such as herbimycin and radicicol that destabilize src by interfering with Hsp90. Our studies suggest that UCS15A exerted its src-inhibitory effects by a novel mechanism that involved disruption of protein-protein interactions mediated by src. One of the biological consequences of src-inhibition by UCS15A was its ability to inhibit the bone resorption activity of osteoclasts in vitro. These data suggest that UCS15A may inhibit the bone resorption activity of osteoclasts, not by inhibiting src tyrosine kinase activity, but by disrupting the interaction of proteins associated with src, thereby modulating downstream events in the src signal transduction pathway.

Substrate recognition by osteoclast precursors induces C-src/microtubule association

The osteoclast is distinguished from other macrophage polykaryons by its polarization, a feature induced by substrate recognition. The most striking component of the polarized osteoclast is its ruffled membrane, probably reflecting insertion of intracellular vesicles into the bone apposed plasmalemma. The failure of osteoclasts in c-src-/- osteopetrotic mice to form ruffled membranes indicates pp60(c-src) (c-src) is essential to osteoclast polarization. Interestingly, c-src itself is a vesicular protein that targets the ruffled membrane. This being the case, we hypothesized that matrix recognition by osteoclasts, and their precursors, induces c-src to associate with microtubules that traffic proteins to the cell surface. We find abundant c-src associates with tubulin immunoprecipitated from avian marrow macrophages (osteoclast precursors) maintained in the adherent, but not nonadherent, state. Since the two proteins colocalize only within adherent avian osteoclast-like cells examined by double antibody immunoconfocal microscopy, c-src/tubulin association reflects an authentic intracellular event. C-src/tubulin association is evident within 90 min of cell-substrate recognition, and the event does not reflect increased expression of either protein. In vitro kinase assay demonstrates tubulin-associated c-src is enzymatically active, phosphorylating itself as well as exogenous substrate. The increase in microtubule-associated kinase activity attending adhesion mirrors tubulin-bound c-src and does not reflect enhanced specific activity. The fact that microtubule-dissociating drugs, as well as cold, prevent adherence-induced c-src/tubulin association indicates the protooncogene complexes primarily, if not exclusively, with polymerized tubulin. Association of the two proteins does not depend upon protein tyrosine phosphorylation and is substrate specific, as it is induced by vitronectin and fibronectin but not type 1 collagen. Finally, consistent with cotransport of c-src and the osteoclast vacuolar proton pump to the polarized plasmalemma, the H+-ATPase decorates microtubules in a manner similar to the protooncogene, specifically coimmunoprecipitates with c-src from the osteoclast light Golgi membrane fraction, and is present, with c-src, in preparations enriched with acidifying vesicles reconstituted from the osteoclast ruffled membrane.

CDC37 is required for p60v-src activity in yeast

Mutations in genes encoding the molecular chaperones Hsp90 and Ydj1p suppress the toxicity of the protein tyrosine kinase p60v-src in yeast by reducing its levels or its kinase activity. We describe isolation and characterization of novel p60v-src-resistant, temperature-sensitive cdc37 mutants, cdc37-34 and cdc37-17, which produce less p60v-src than the parental wild-type strain at 23 degrees C. However, p60v-src levels are not low enough to account for the resistance of these strains. Asynchronously growing cdc37-34 and cdc37-17 mutants arrest in G1 and G2/M when shifted from permissive temperatures (23 degrees C) to the restrictive temperature (37 degrees C), but hydroxyurea-synchronized cdc37-34 and cdc37-17 mutants arrest in G2/M when released from the hydroxyurea block and shifted from 23 to 37 degrees C. The previously described temperature-sensitive cdc37-1 mutant is p60v-src-sensitive and produces wild-type amounts of p60v-src at permissive temperatures but becomes p60v-src-resistant at its restrictive temperature, 38 degrees C. In all three cdc37 mutants, inactivation of Cdc37p by incubation at 38 degrees C reduces p60v-src-dependent tyrosine phosphorylation of yeast proteins to low or undetectable levels. Also, p60v-src levels are enriched in urea-solubilized extracts and depleted in detergent-solubilized extracts of all three cdc37 mutants prepared from cells incubated at the restrictive temperature. These results suggest that Cdc37p is required for maintenance of p60v-src in a soluble, biologically active form.

Disease specificity of kinase domains: the src-encoded catalytic domain converts erbB into a sarcoma oncogene

src and erbB are two tyrosine kinase-encoding oncogenes carried by retroviruses, which have distinct disease specificities. The former induces predominantly sarcomas, and the latter, leukemias. Src and ErbB have similar catalytic domains but have very different regulatory domains. A wealth of information exists concerning how different regulatory domains [Src homology 2 (SH2) and SH3 domains and autophosphorylation sites] control substrate and disease specificities. Whether the catalytic domain helps determine these specificities remains to be explored. Here we show that the Src catalytic domain is enzymatically active when substituted into the ErbB backbone and interacts with the ErbB regulatory domain. This ErbB/Src chimera displays autophosphorylation and substrate phosphorylation patterns different from those of both Src and ErbB. Neither SH2 and SH3 nor autophosphorylation sites are required for the Src catalytic domain to exert its fibroblast transforming ability. Most significantly, the catalytic domain can convert erbB from a leukemogenic oncogene into a sarcomagenic oncogene, suggesting that the leukemogenic determinants in part reside within the ErbB catalytic domain.

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.

Inhibition of heat shock protein HSP90-pp60v-src heteroprotein complex formation by benzoquinone ansamycins: essential role for stress proteins in oncogenic transformation

The molecular mechanisms by which oncogenic tyrosine kinases induce cellular transformation are unclear. Herbimycin A, geldanamycin, and certain other benzoquinone ansamycins display an unusual capacity to revert tyrosine kinase-induced oncogenic transformation. As an approach to the study of v-src-mediated transformation, we examined ansamycin action in transformed cells and found that drug-induced reversion could be achieved without direct inhibition of src phosphorylating activity. To identify mechanisms other than kinase inhibition for drug-mediated reversion, we prepared a solid phase-immobilized geldanamycin derivative and affinity precipitated the molecular targets with which the drug interacted. In a range of cell lines, immobilized geldanamycin bound elements of a major class of heat shock protein (HSP90) in a stable and pharmacologically specific manner. Consistent with these binding data, we found that soluble geldanamycin and herbimycin A inhibited specifically the formation of a previously described src-HSP90 heteroprotein complex. A related benzoquinone ansamycin that failed to revert transformed cells did not inhibit the formation of this complex. These results demonstrate that HSP participation in multimolecular complex formation is required for src-mediated transformation and can provide a target for drug modulation.

Csk suppression of Src involves movement of Csk to sites of Src activity

Csk phosphorylates Src family members at a key regulatory tyrosine in the C-terminal tail and suppresses their activities. It is not known whether Csk activity is regulated. To examine the features of Csk required for Src suppression, we expressed Csk mutants in a cell line with a disrupted csk gene. Expression of wild-type Csk suppressed Src, but Csk with mutations in the SH2, SH3, and catalytic domains did not suppress Src. An SH3 deletion mutant of Csk was fully active against in vitro substrates, but two SH2 domain mutants were essentially inactive. Whereas Src repressed by Csk was predominantly perinuclear, the activated Src in cells lacking Csk was localized to structures resembling podosomes. Activated mutant Src was also in podosomes, even in the presence of Csk. When Src was not active, Csk was diffusely located in the cytosol, but when Src was active, Csk colocalized with activated Src to podosomes. Csk also localizes to podosomes of cells transformed by an activated Src that lacks the major tyrosine autophosphorylation site, suggesting that the relocalization of Csk is not a consequence of the binding of the Csk SH2 domain to phosphorylated Src. A catalytically inactive Csk mutant also localized with Src to podosomes, but SH3 and SH2 domain mutants did not, suggesting that the SH3 and SH2 domains are both necessary to target Csk to places where Src is active. The failure of the catalytically active SH3 mutant of Csk to regulate Src may be due to its inability to colocalize with active Src.

Csk suppression of Src involves movement of Csk to sites of Src activity

Csk phosphorylates Src family members at a key regulatory tyrosine in the C-terminal tail and suppresses their activities. It is not known whether Csk activity is regulated. To examine the features of Csk required for Src suppression, we expressed Csk mutants in a cell line with a disrupted csk gene. Expression of wild-type Csk suppressed Src, but Csk with mutations in the SH2, SH3, and catalytic domains did not suppress Src. An SH3 deletion mutant of Csk was fully active against in vitro substrates, but two SH2 domain mutants were essentially inactive. Whereas Src repressed by Csk was predominantly perinuclear, the activated Src in cells lacking Csk was localized to structures resembling podosomes. Activated mutant Src was also in podosomes, even in the presence of Csk. When Src was not active, Csk was diffusely located in the cytosol, but when Src was active, Csk colocalized with activated Src to podosomes. Csk also localizes to podosomes of cells transformed by an activated Src that lacks the major tyrosine autophosphorylation site, suggesting that the relocalization of Csk is not a consequence of the binding of the Csk SH2 domain to phosphorylated Src. A catalytically inactive Csk mutant also localized with Src to podosomes, but SH3 and SH2 domain mutants did not, suggesting that the SH3 and SH2 domains are both necessary to target Csk to places where Src is active. The failure of the catalytically active SH3 mutant of Csk to regulate Src may be due to its inability to colocalize with active Src.

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

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

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

Suppression of c-Src activity by C-terminal Src kinase involves the c-Src SH2 and SH3 domains: analysis with Saccharomyces cerevisiae

The kinase activity of c-Src is normally repressed in vertebrate cells by extensive phosphorylation of Y-527. C-terminal Src kinase (CSK) is a candidate for the enzyme that catalyzes this phosphorylation. We have used budding yeast to study the regulation of c-Src activity by CSK in intact cells. Expression of c-Src in Saccharomyces cerevisiae, which lacks endogenous c-Src and Y-527 kinases, induces a kinase-dependent growth inhibition. Coexpression of CSK in these cells results in phosphorylation of c-Src on Y-527 and suppression of the c-Src phenotype. CSK does not fully suppress the activity of c-Src mutants lacking portions of the SH2 or SH3 domains, even though these mutant proteins are phosphorylated on Y-527 by CSK both in vivo and in vitro. These results suggest that both the SH2 and SH3 domains of c-Src are required for the suppression of c-Src activity by Y-527 phosphorylation.

Suppression of c-Src activity by C-terminal Src kinase involves the c-Src SH2 and SH3 domains: analysis with Saccharomyces cerevisiae

The kinase activity of c-Src is normally repressed in vertebrate cells by extensive phosphorylation of Y-527. C-terminal Src kinase (CSK) is a candidate for the enzyme that catalyzes this phosphorylation. We have used budding yeast to study the regulation of c-Src activity by CSK in intact cells. Expression of c-Src in Saccharomyces cerevisiae, which lacks endogenous c-Src and Y-527 kinases, induces a kinase-dependent growth inhibition. Coexpression of CSK in these cells results in phosphorylation of c-Src on Y-527 and suppression of the c-Src phenotype. CSK does not fully suppress the activity of c-Src mutants lacking portions of the SH2 or SH3 domains, even though these mutant proteins are phosphorylated on Y-527 by CSK both in vivo and in vitro. These results suggest that both the SH2 and SH3 domains of c-Src are required for the suppression of c-Src activity by Y-527 phosphorylation.

Polyamines are essential for cell transformation by pp60v-src: delineation of molecular events relevant for the transformed phenotype

Ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, becomes upregulated during cell proliferation and transformation. Here we show that intact ODC activity is needed for the acquisition of a transformed phenotype in rat 2R cells infected with a temperature-sensitive mutant of Rous sarcoma virus. Addition of the ODC inhibitor alpha-difluoromethyl ornithine (DFMO) to the cells (in polyamine-free medium) before shift to permissive temperature prevented the depolymerization of filamentous actin and morphological transformation. Polyamine supplementation restored the transforming potential of pp60v-src. DFMO did not interfere with the expression of pp60v-src or its in vitro tyrosine kinase activity. The tyrosine phosphorylation of most cellular proteins, including ras GAP, did not either display clear temperature- or DFMO-sensitive changes. A marked increase was, however, observed in the tyrosine phosphorylation of phosphatidylinositol 3-kinase and proteins of 33 and 36 kD upon the temperature shift, and these hyperphosphorylations were partially inhibited by DFMO. A DFMO-sensitive increase was also found in the total phosphorylation of calpactins I and II. The well-documented association of GAP with the phosphotyrosine-containing proteins p190 and p62 did not correlate with transformation, but a novel 42-kD tyrosine phosphorylated protein was complexed with GAP in a polyamine- and transformation-dependent manner. Further, tyrosine phosphorylated proteins of 130, 80/85, and 36 kD were found to coimmunoprecipitate with pp60v-src in a transformation-related manner. Altogether, this model offers a tool for sorting out the protein phosphorylations and associations critical for the transformed phenotype triggered by pp60v-src, and implicates a pivotal role for polyamines in cell transformation.

Redistribution of activated pp60c-src to integrin-dependent cytoskeletal complexes in thrombin-stimulated platelets

Thrombin stimulation of platelets induces a transient increase in the specific activity of pp60c-src followed by a redistribution of pp60c-src to the Triton X-100-insoluble, cytoskeleton-rich fraction. Concomitant with the observed increase in pp60c-src activity was a rapid dephosphorylation of tyrosine 527 in 10 to 15% of pp60c-src molecules. In addition, we found that pp60c-src from the Triton-insoluble fraction was phosphorylated on tyrosine 416, the autophosphorylation site which is phosphorylated in activated oncogenic variants of pp60src. Furthermore, in platelets from patients with Glanzmann's thrombasthenia (which are deficient in the integrin receptor GPIIb-IIIa), pp60c-src was not translocated to the Triton-insoluble fraction, and there was a sustained increase in pp60c-src activity following thrombin treatment. These results suggest that pp60c-src is rapidly activated in thrombin-stimulated platelets, potentially by a protein tyrosine phosphatase, before it translocates to a cytoskeletal fraction, where many of its potential substrates are found. The evidence that the cytoskeletal association of pp60c-src is dependent upon engagement of the integrin receptor GPIIb-IIIa suggests that integrin-cytoskeletal complexes may serve to compartmentalize and anchor activated enzymes involved in signal transduction.

Redistribution of activated pp60c-src to integrin-dependent cytoskeletal complexes in thrombin-stimulated platelets

Thrombin stimulation of platelets induces a transient increase in the specific activity of pp60c-src followed by a redistribution of pp60c-src to the Triton X-100-insoluble, cytoskeleton-rich fraction. Concomitant with the observed increase in pp60c-src activity was a rapid dephosphorylation of tyrosine 527 in 10 to 15% of pp60c-src molecules. In addition, we found that pp60c-src from the Triton-insoluble fraction was phosphorylated on tyrosine 416, the autophosphorylation site which is phosphorylated in activated oncogenic variants of pp60src. Furthermore, in platelets from patients with Glanzmann's thrombasthenia (which are deficient in the integrin receptor GPIIb-IIIa), pp60c-src was not translocated to the Triton-insoluble fraction, and there was a sustained increase in pp60c-src activity following thrombin treatment. These results suggest that pp60c-src is rapidly activated in thrombin-stimulated platelets, potentially by a protein tyrosine phosphatase, before it translocates to a cytoskeletal fraction, where many of its potential substrates are found. The evidence that the cytoskeletal association of pp60c-src is dependent upon engagement of the integrin receptor GPIIb-IIIa suggests that integrin-cytoskeletal complexes may serve to compartmentalize and anchor activated enzymes involved in signal transduction.

Shc proteins are phosphorylated and regulated by the v-Src and v-Fps protein-tyrosine kinases

The mammalian shc gene encodes two overlapping proteins of 46 and 52 kDa, each with a C-terminal Src homology 2 (SH2) domain and an N-terminal glycine/proline-rich sequence, that induce malignant transformation when overexpressed in mouse fibroblasts. p46shc, p52shc, and an additional 66-kDa shc gene product become highly tyrosine phosphorylated in Rat-2 cells transformed by the v-src or v-fps oncogene. Experiments using temperature-sensitive v-src and v-fps mutants indicate that Shc tyrosine phosphorylation is rapidly induced upon activation of the v-Src or v-Fps tyrosine kinases. These results suggest that Shc proteins may be directly phosphorylated by the v-Src and v-Fps oncoproteins in vivo. In cells transformed by v-src or v-fps, or in normal cells stimulated with epidermal growth factor, Shc proteins complex with a poorly phosphorylated 23-kDa polypeptide (p23). Activated tyrosine kinases therefore regulate the association of Shc proteins with p23 and may thereby control the stimulation of an Shc-mediated signal transduction pathway. The efficient phosphorylation of Shc proteins and the apparent induction of their p23-binding activity in v-src- and v-fps-transformed cells are consistent with the proposition that the SH2-containing Shc polypeptides are biologically relevant substrates of the oncogenic v-Src and v-Fps tyrosine kinases.

Effects of SH2 and SH3 deletions on the functional activities of wild-type and transforming variants of c-Src

The amino-termina, noncatalytic half of Src contains two domains, designated the Src homology 2 (SH2) and Src homology 3 (SH3) domains, that are highly conserved among members of the Src family of tyrosine kinases. The SH2 domain (which can be further divided into the B and C homology boxes) and the SH3 domain (also referred to as the A box) are also found in several proteins otherwise unrelated to protein tyrosine kinases. It is believed that these domains are important for directing specific protein-protein interactions necessary for the proper functioning of Src. To determine the importance of the SH2 and SH3 domains in regulating the functions of c-Src, we evaluated mutants of c-Src lacking the A box (residues 88 to 137), the B box (residues 148 to 187) or the C box (residues 220 to 231). Each of these deletions caused a 14- to 30-fold increase in the in vitro level of kinase activity of c-Src. Chicken embryo fibroblasts expressing the deletion mutants displayed a transformed cell morphology, formed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Src substrates p36, p85, p120, p125, the GTPase-activating protein (GAP), and several GAP-associated proteins were phosphorylated on tyrosine in cells expressing the A, B, or C box deletion mutant. p110 was highly phosphorylated in cells expressing the C box mutant, was weakly phosphorylated in cells expressing the B box mutant, and was not phosphorylated in cells expressing the A box mutant. Expression of the mutant proteins caused a reorganization of the actin cytoskeleton similar to that seen in v-Src-transformed cells. In addition, deletion of the A, B, or C box did not diminish the transforming or enzymatic activity of an activated variant of c-Src, E378G. These data indicate that deletion of the A, B, or C homology box causes an activation of the catalytic and transforming potential of c-Src and that while these mutations caused subtle differences in substrate phosphorylation, the homology boxes are not required for many of the phenotypic changes associated with transformation by Src.

Multiple SH2-mediated interactions in v-src-transformed cells

The Src homology 2 (SH2) domain is a noncatalytic region which is conserved among a number of signaling and transforming proteins, including cytoplasmic protein-tyrosine kinases and Ras GTPase-activating protein (GAP). Genetic and biochemical data indicate that the SH2 domain of the p60v-src (v-Src) protein-tyrosine kinase is required for full v-src transforming activity and may direct the association of v-Src with specific tyrosine-phosphorylated proteins. To test the ability of the v-Src SH2 domain to mediate protein-protein interactions, v-Src polypeptides were expressed as fusion proteins in Escherichia coli. The bacterial v-Src SH2 domain bound a series of tyrosine-phosphorylated proteins in a lysate of v-src-transformed Rat-2 cells, including prominent species of 130 and 62 kDa (p130 and p62). The p130 and p62 tyrosine-phosphorylated proteins that complexed v-Src SH2 in vitro also associated with v-Src in v-src-transformed Rat-2 cells; this in vivo binding was dependent on the v-Src SH2 domain. In addition to binding soluble p62 and p130, the SH2 domains of v-Src, GAP, and v-Crk directly recognized these phosphotyrosine-containing proteins which had been previously denatured and immobilized on a filter. In addition, the SH2 domains of GAP and v-Crk bound to the GAP-associated protein p190 immobilized on a nitrocellulose membrane. These results show that SH2 domains bind directly to tyrosine-phosphorylated proteins and that the Src SH2 domain can bind phosphorylated targets of the v-Src kinase domain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of SH2 and SH3 deletions on the functional activities of wild-type and transforming variants of c-Src

The amino-termina, noncatalytic half of Src contains two domains, designated the Src homology 2 (SH2) and Src homology 3 (SH3) domains, that are highly conserved among members of the Src family of tyrosine kinases. The SH2 domain (which can be further divided into the B and C homology boxes) and the SH3 domain (also referred to as the A box) are also found in several proteins otherwise unrelated to protein tyrosine kinases. It is believed that these domains are important for directing specific protein-protein interactions necessary for the proper functioning of Src. To determine the importance of the SH2 and SH3 domains in regulating the functions of c-Src, we evaluated mutants of c-Src lacking the A box (residues 88 to 137), the B box (residues 148 to 187) or the C box (residues 220 to 231). Each of these deletions caused a 14- to 30-fold increase in the in vitro level of kinase activity of c-Src. Chicken embryo fibroblasts expressing the deletion mutants displayed a transformed cell morphology, formed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Src substrates p36, p85, p120, p125, the GTPase-activating protein (GAP), and several GAP-associated proteins were phosphorylated on tyrosine in cells expressing the A, B, or C box deletion mutant. p110 was highly phosphorylated in cells expressing the C box mutant, was weakly phosphorylated in cells expressing the B box mutant, and was not phosphorylated in cells expressing the A box mutant. Expression of the mutant proteins caused a reorganization of the actin cytoskeleton similar to that seen in v-Src-transformed cells. In addition, deletion of the A, B, or C box did not diminish the transforming or enzymatic activity of an activated variant of c-Src, E378G. These data indicate that deletion of the A, B, or C homology box causes an activation of the catalytic and transforming potential of c-Src and that while these mutations caused subtle differences in substrate phosphorylation, the homology boxes are not required for many of the phenotypic changes associated with transformation by Src.

Multiple SH2-mediated interactions in v-src-transformed cells

The Src homology 2 (SH2) domain is a noncatalytic region which is conserved among a number of signaling and transforming proteins, including cytoplasmic protein-tyrosine kinases and Ras GTPase-activating protein (GAP). Genetic and biochemical data indicate that the SH2 domain of the p60v-src (v-Src) protein-tyrosine kinase is required for full v-src transforming activity and may direct the association of v-Src with specific tyrosine-phosphorylated proteins. To test the ability of the v-Src SH2 domain to mediate protein-protein interactions, v-Src polypeptides were expressed as fusion proteins in Escherichia coli. The bacterial v-Src SH2 domain bound a series of tyrosine-phosphorylated proteins in a lysate of v-src-transformed Rat-2 cells, including prominent species of 130 and 62 kDa (p130 and p62). The p130 and p62 tyrosine-phosphorylated proteins that complexed v-Src SH2 in vitro also associated with v-Src in v-src-transformed Rat-2 cells; this in vivo binding was dependent on the v-Src SH2 domain. In addition to binding soluble p62 and p130, the SH2 domains of v-Src, GAP, and v-Crk directly recognized these phosphotyrosine-containing proteins which had been previously denatured and immobilized on a filter. In addition, the SH2 domains of GAP and v-Crk bound to the GAP-associated protein p190 immobilized on a nitrocellulose membrane. These results show that SH2 domains bind directly to tyrosine-phosphorylated proteins and that the Src SH2 domain can bind phosphorylated targets of the v-Src kinase domain.(ABSTRACT TRUNCATED AT 250 WORDS)

Point mutations in the abl SH2 domain coordinately impair phosphotyrosine binding in vitro and transforming activity in vivo

We have constructed a series of point mutations in the highly conserved FLVRES motif of the src homology 2 (SH2) domain of the abl tyrosine kinase. Mutant SH2 domains were expressed in bacteria, and their ability to bind to tyrosine-phosphorylated proteins was examined in vitro. Three mutants were greatly reduced in their ability to bind both phosphotyrosine itself and tyrosine-phosphorylated cellular proteins. All of the mutants that retained activity bound to the same set of tyrosine-phosphorylated proteins as did the wild type, suggesting that binding specificity was unaffected. These results implicate the FLVRES motif in direct binding to phosphotyrosine. When the mutant SH2 domains were inserted into an activated abl kinase and expressed in murine fibroblasts, decreased in vitro phosphotyrosine binding correlated with decreased transforming ability. This finding implies that SH2-phosphotyrosine interactions are involved in transmission of positive growth signals by the nonreceptor tyrosine kinases, most likely via the assembly of multiprotein complexes with other tyrosine-phosphorylated proteins.

Point mutations in the abl SH2 domain coordinately impair phosphotyrosine binding in vitro and transforming activity in vivo

We have constructed a series of point mutations in the highly conserved FLVRES motif of the src homology 2 (SH2) domain of the abl tyrosine kinase. Mutant SH2 domains were expressed in bacteria, and their ability to bind to tyrosine-phosphorylated proteins was examined in vitro. Three mutants were greatly reduced in their ability to bind both phosphotyrosine itself and tyrosine-phosphorylated cellular proteins. All of the mutants that retained activity bound to the same set of tyrosine-phosphorylated proteins as did the wild type, suggesting that binding specificity was unaffected. These results implicate the FLVRES motif in direct binding to phosphotyrosine. When the mutant SH2 domains were inserted into an activated abl kinase and expressed in murine fibroblasts, decreased in vitro phosphotyrosine binding correlated with decreased transforming ability. This finding implies that SH2-phosphotyrosine interactions are involved in transmission of positive growth signals by the nonreceptor tyrosine kinases, most likely via the assembly of multiprotein complexes with other tyrosine-phosphorylated proteins.

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

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

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.

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.

Requirement of phosphatidylinositol-3 kinase modification for its association with p60src

When purified p60v-src was mixed with lysates of chicken embryo fibroblasts and immunoprecipitated with anti-Src antibody, phosphatidylinositol (PI)-3 kinase activity was found to be present in the Src protein immunoprecipitates. The level of bound PI-3 kinase activity was 5 to 10 times higher in lysates obtained from cells transformed by the src, fps, or yes oncogene than in lysates of uninfected cells. This increase in associated PI-3 kinase activity appears to be due to increased binding of this enzyme to p60v-src. This change most likely resulted from tyrosine phosphorylation of PI-3 kinase or an associated protein, since the PI-3 kinase activity that can bind to p60v-src was depleted by antiphosphotyrosine antibody. Binding of PI-3 kinase did not require either p60src protein kinase activity or autophosphorylation of p60v-src tyrosine residues. Furthermore, binding was markedly decreased by deletions in the N-terminal SH2 region but unchanged by deletion of the C-terminal half of p60v-src containing the catalytic domain. Taking these data together, it appears that PI-3 kinase or its associated protein is phosphorylated on tyrosine and that the phosphorylated form can bind to the N-terminal half of p60v-src, which contains the SH2 domain.

Requirement of phosphatidylinositol-3 kinase modification for its association with p60src

When purified p60v-src was mixed with lysates of chicken embryo fibroblasts and immunoprecipitated with anti-Src antibody, phosphatidylinositol (PI)-3 kinase activity was found to be present in the Src protein immunoprecipitates. The level of bound PI-3 kinase activity was 5 to 10 times higher in lysates obtained from cells transformed by the src, fps, or yes oncogene than in lysates of uninfected cells. This increase in associated PI-3 kinase activity appears to be due to increased binding of this enzyme to p60v-src. This change most likely resulted from tyrosine phosphorylation of PI-3 kinase or an associated protein, since the PI-3 kinase activity that can bind to p60v-src was depleted by antiphosphotyrosine antibody. Binding of PI-3 kinase did not require either p60src protein kinase activity or autophosphorylation of p60v-src tyrosine residues. Furthermore, binding was markedly decreased by deletions in the N-terminal SH2 region but unchanged by deletion of the C-terminal half of p60v-src containing the catalytic domain. Taking these data together, it appears that PI-3 kinase or its associated protein is phosphorylated on tyrosine and that the phosphorylated form can bind to the N-terminal half of p60v-src, which contains the SH2 domain.