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

Heterogeneity of signal transduction by Na-K-ATPase α-isoforms: role of Src interaction

Of the four Na-K-ATPase α-isoforms, the ubiquitous α1 Na-K-ATPase possesses both ion transport and Src-dependent signaling functions. Mechanistically, we have identified two putative pairs of domain interactions between α1 Na-K-ATPase and Src that are critical for α1 signaling function. Our subsequent report that α2 Na-K-ATPase lacks these putative Src-binding sites and fails to carry on Src-dependent signaling further supported our proposed model of direct interaction between α1 Na-K-ATPase and Src but fell short of providing evidence for a causative role. This hypothesis was specifically tested here by introducing key residues of the two putative Src-interacting domains present on α1 but not α2 sequence into the α2 polypeptide, generating stable cell lines expressing this mutant, and comparing its signaling properties to those of α2-expressing cells. The mutant α2 was fully functional as a Na-K-ATPase. In contrast to wild-type α2, the mutant gained α1-like signaling function, capable of Src interaction and regulation. Consistently, the expression of mutant α2 redistributed Src into caveolin-1-enriched fractions and allowed ouabain to activate Src-mediated signaling cascades, unlike wild-type α2 cells. Finally, mutant α2 cells exhibited a growth phenotype similar to that of the α1 cells and proliferated much faster than wild-type α2 cells. These findings reveal the structural requirements for the Na-K-ATPase to function as a Src-dependent receptor and provide strong evidence of isoform-specific Src interaction involving the identified key amino acids. The sequences surrounding the putative Src-binding sites in α2 are highly conserved across species, suggesting that the lack of Src binding may play a physiologically important and isoform-specific role.

Protein Interaction and Na/K-ATPase-Mediated Signal Transduction

The Na/K-ATPase (NKA), or Na pump, is a member of the P-type ATPase superfamily. In addition to pumping ions across cell membrane, it is engaged in assembly of multiple protein complexes in the plasma membrane. This assembly allows NKA to perform many non-pumping functions including signal transduction that are important for animal physiology and disease progression. This article will focus on the role of protein interaction in NKA-mediated signal transduction, and its potential utility as target for developing new therapeutics.

c-Src and mitosis

The transforming potential and by inference the physiological function of the proto-oncoprotein pp60c-src closely correlate with the level of its protein tyrosine kinase activity. We have investigated the cell cycle-dependent regulation of this activity using mouse fibroblasts overexpressing chicken or mouse pp60c-src as a model system. During mitosis pp60c-src becomes phosphorylated at specific serine and threonine residues by p34cdc2. At the same time its tyrosine kinase activity, assayed in vitro, is increased approximately twofold and accessibility of its SH2 domain for binding relevant phosphotyrosine-containing ligands increases by about 15-fold. A kinase-defective mutant of pp60c-src exhibits a substantial (50-70%) decrease in phosphorylation at Tyr527 during mitosis. Phosphorylation of this residue negatively regulates kinase activity. Indirect evidence indicates a lesser decrease in wild-type pp60c-src Tyr527 phosphorylation during mitosis. Coordinate mutation of the mitosis-specific phosphorylation (MSP) sites in kinase-defective pp60c-src greatly reduces, though does not abolish, its mitosis-specific tyrosine dephosphorylation. Similarly, coordinate mutation of the three MSP sites in chicken pp60c-src or the corresponding two sites in mouse pp60c-src does not completely block mitotic stimulation of kinase activity. Thus, additional events beyond p34cdc2-mediated phosphorylation are involved in cell-cycle dependent regulation of pp60c-src activity. This is also suggested by the stimulation of pp60c-src kinase activity and decrease in phosphorylation of Tyr527 observed following treatment of fibroblasts with okadaic acid, a potent inhibitor of types 1 and 2A serine/threonine phosphatases. The potential role of cell cycle-dependent regulation of phosphatases and kinases acting on the regulatory tyrosine residue of pp60c-src is discussed.

Three-dimensional matrix induces sustained activation of ERK1/2 via Src/Ras/Raf signaling pathway

Research in cell signaling often depends on tissue culture, but the artificial substrates used to grow cells in vitro are likely to distort the conclusions, particularly when adhesion-mediated signaling events are investigated. Studies of signal transduction pathways operating in cells grown in three-dimensional (3D) matrices provide a better system, giving a closer insight of the cell signaling in vivo. We compared the steady-state levels of ERK1/2 activity in primary human fibroblasts, induced by cell-derived 3D fibronectin matrix or fibronectin, coated on flat surfaces. 3D environment caused ERK1/2 stimulation concomitant with a 2.5-fold increase in Ras GTP loading and Src activation. Under these conditions FAK autophosphorylation was suppressed. Treatment with Src inhibitor PP2 abolished these effects indicating that 3D fibronectin matrix activated ERK1/2 through Src/Ras/Raf pathway, bypassing FAK. These observations suggest that within in vivo-like conditions Src may have a leading role in the induction of sustained ERK1/2 activation.

Transient cerebral ischemia increases tyrosine phosphorylation of the synaptic RAS-GTPase activating protein, SynGAP

Cerebral ischemia results in activation of the mitogen-activated protein kinase pathway and increased tyrosine phosphorylation of proteins associated with postsynaptic densities (PSDs). The authors investigated the possible relation between these events by determining the effect of ischemia on tyrosine phosphorylation of the brain-specific, PSD-enriched, Ras-GTPase activating protein, SynGAP. Transient (15 minutes) global ischemia was produced in rats by 4-vessel occlusion and PSDs prepared from forebrains immediately after ischemia or at 20 minutes, 1 hour, or 24 hours of reperfusion. Tyrosine phosphorylation of SynGAP was elevated relative to sham-operated controls by 20 minutes of reperfusion and remained elevated for at least 24 hours. Tyrosine phosphorylation of SynGAP also increased in CA1 and CA3/DG subfields of the hippocampus. Enhanced tyrosine phosphorylation of SynGAP was not accompanied by a change in PSD RasGAP activity. SynGAP bound to the SH2 domains of Src and Fyn in a tyrosine phosphorylation-dependent fashion, and this interaction increased after ischemia. SynGAP binds to the PDZ domains of PSD-95/SAP90 and coimmunoprecipitated with PSD-95. The coimmunoprecipitation of SynGAP with PSD-95 decreased after ischemia. The results indicate that changes in the properties and interactions of SynGAP may be involved in the neuropathology of ischemia.

Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta receptor (betaPDGFR) is not the only tyrosine kinase able to associate with the GTPase-activating protein of Ras (RasGAP). The interaction of non-betaPDGFR kinase(s) with RasGAP was dependent on stimulation with platelet-derived growth factor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Because the tyrosine phosphorylation site of RasGAP is in a sequence context that is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasGAP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immunodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP results in the formation of a complex between RasGAP and Src. To begin to address the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that did not activate Src was unable to efficiently mediate the tyrosine phosphorylation of phospholipase Cgamma (PLCgamma). Taken together, these observations support the following hypothesis. When RasGAP is recruited to the betaPDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLCgamma. This hypothesis offers a mechanistic explanation for our previously published findings that the recruitment of RasGAP to the betaPDGFR attenuates the tyrosine phosphorylation of PLCgamma. Finally, these findings suggest a novel way in which RasGAP negatively regulates signal relay by the betaPDGFR.

Enhancement of protein kinase C-dependent O2 production in Epstein-Barr virus-transformed B lymphocytes by p120Ras-GAP antisense oligonucleotide

The mammalian Ras GTPase-activating protein (p120Ras-GAP) interacts with activated members of the Ras superfamily of GTP-binding proteins to accelerate their deactivation by sharply increasing their rates of GTP hydrolysis. Among the Ras-family proteins interacting with p120Ras-GAP is Rap1A/Krev1, whose activity is not affected by p120Ras-GAP but which competes with Ras for p120Ras-GAP. A second protein that interacts with p120Ras-GAP is P190Rac-GAP, which activates the GTPase of guanine nucleotide-binding proteins of the Rho family (including Rac1 and Rac2). Both these p120Ras-GAP-binding proteins are of interest in connection with the regulation of the respiratory burst oxidase, Rap1A/Krev1 because it copurifies with cytochrome b558 and p190Ras-GAP because it inhibits the Rac2-dependent activation of the respiratory burst oxidase in a cell-free system. Using an 18-mer antisense oligonucleotide, we were able to decrease the expression of p120Ras-GAP in Epstein-Barr virus-transformed B lymphocytes. Under conditions where p120Ras-GAP expression was significantly depressed by antisense oligonucleotides, we observed a 40% increase in protein kinase C-dependent but not receptor-dependent O2 production. In contrast, sense and scrambled oligonucleotides had no effect on either p120Ras-GAP expression or O2 production. Our results suggest a role for p120Ras-GAP as a negative regulator in the protein kinase C-mediated activation of the respiratory burst oxidase.

N-terminal sequences contained in the Src homology 2 and 3 domains of p120 GTPase-activating protein are required for full catalytic activity toward Ras

The p120 GTPase-activating protein (GAP) is a negative regulator of Ras, which has a central role in signal transduction pathways that control cell proliferation. p120 GAP accelerates the conversion of activated Ras-GTP to its inactive form, Ras-GDP, thereby inhibiting mitogenic signaling. To examine potential contributions of p120 N-terminal sequences to regulation of its C-terminal catalytic domain, we constructed deletion mutants lacking defined regions, including the variable hydrophobic region as well as the Src homology 2 (SH2) and 3 (SH3) domains. These mutant proteins were expressed in infected Sf9 insect cells from recombinant baculoviruses and assayed in vitro for their ability to stimulate the intrinsic GTPase activity of purified Ras. While deletion of the variable hydrophobic region had no effect on p120 GAP activity, deletion of the entire SH2/SH3/SH2 region severely impaired catalytic activity toward Ras. Deletion of individual SH2 and SH3 domains within this region partially inhibited p120 GAP activity. Moreover, p120 N-terminal sequences enhanced the Ras GTPase-stimulating activity of the neurofibromin GAP-related domain. These results demonstrate that sequences in the SH2/SH3/SH2 region of p120 GAP are required for full catalytic activity toward Ras. Together with earlier findings that the p120 GAP SH2 domains mediate interactions with several GAP-associated proteins, our results suggest multiple roles for the N-terminal sequences in regulating p120 GAP catalytic activity and mitogenic signaling pathways. In addition, our results raise the possibility that SH2 domain point mutations in p120 GAP detected in some basal cell carcinomas reduce catalytic activity toward Ras and thereby contribute to oncogenesis.

The Ras GTPase-activating protein (GAP) is an SH3 domain-binding protein and substrate for the Src-related tyrosine kinase, Hck

The Ras GTPase-activating protein (GAP) is a target for protein tyrosine kinases of both the receptor and cytoplasmic classes and may serve to integrate tyrosine kinase and Ras signaling pathways. In this report, we provide evidence that GAP is an SH3 domain-binding protein and substrate for the Src-related tyrosine kinase Hck, which has been implicated in the regulation of myeloid cell growth, differentiation, and function. Wild-type (WT) or kinase-inactive (K269E) mutant Hck proteins were co-expressed with bovine GAP using the baculovirus/Sf-9 cell system. GAP was readily phosphorylated on tyrosine by WT but not K269E Hck. GAP was present in WT Hck immunoprecipitates from the co-infected cells, indicative of Hck.GAP complex formation. Unexpectedly, GAP also associated with the kinase-inactive mutant of Hck, suggesting that tyrosine autophosphorylation of Hck is not required for complex formation. The WT and K269E forms of Hck also associated with GAP mutants lacking either the C-terminal catalytic domain (delta CAT) or the Src homology region (delta SH), indicating that these GAP domains are dispensable for complex formation. Recombinant GST fusion proteins containing the Hck, Src, Fyn, or Lck SH3 domains associated with full-length GAP, delta CAT, and delta SH, all of which share an N-terminal proline-rich region resembling an SH3-binding motif (PPLPPPPPQLP). Deletion of the highly conserved YXY sequence from the Hck SH3 domain abolished binding. GAP-SH3 interaction was also inhibited by the proline-rich peptide GFPPLPPPPPQLPTLG, which corresponds to N-terminal amino acids 129-144 of bovine GAP. An N-terminal deletion mutant of GAP lacking this proline-rich region did not bind to the Hck SH3 domain. These data implicate the Hck SH3 domain in GAP interaction, and suggest a general function for the SH3 domains of Src family kinases in recognition of GAP via its proline-rich N-terminal domain.

pp60src-mediated phosphorylation of connexin 43, a gap junction protein

Several laboratories have demonstrated a decrease in gap junctional communication in cells transformed by the src oncogene of the Rous sarcoma virus. The decrease in gap junctional communication was associated with tyrosine phosphorylation of the gap junction protein, connexin 43 (Cx43). This study was initiated to determine if the phosphorylation of Cx43 is the result of a direct kinase-substrate interaction between the highly active tyrosine kinase, pp60v-src, and Cx43. Previous biochemical studies have been limited by the low levels of Cx43 protein in fibroblast cell lines. To obtain larger quantities of Cx43, we constructed a recombinant baculovirus expressing Cx43 in Spodoptera frugiperda (Sf-9) cells and subsequently purified the expressed Cx43 by immunoaffinity chromatography. We observed that this partially purified Cx43 was phosphorylated on tyrosine in vitro in the presence of kinase-active pp60src. Phosphotryptic peptide mapping indicated that the in vitro phosphorylated Cx43 contained phosphopeptides which comigrated with a subset of tryptic peptides prepared from Cx43 phosphorylated in vivo. Furthermore, coinfection of Sf-9 cells with recombinant baculoviruses encoding pp60v-src and Cx43 resulted in the accumulation of phosphotyrosine in Cx43. Taken together, the evidence presented in this paper demonstrates that kinase active pp60c-src is capable of phosphorylating Cx43 in a direct manner. Since the presence of phosphotyrosine on Cx43 is correlated with the down-regulation of gap-junctional communication, these results suggest that pp60v-src regulates gap junctional gating activity via tyrosine phosphorylation of Cx43.

Transcriptional suppression of the human T-cell leukemia virus type I long terminal repeat occurs by an unconventional interaction of a CREB factor with the R region

To analyze regulation of the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR), cell lines were generated from LTR-tax x LTR-beta-galactosidase (beta-Gal) doubly transgenic mouse fibroblastic tumors. The HTLV-I LTR directs expression of both the tax and lacZ genes, and Tax up-modulates both promoters in primary cells. However, once cells were transformed by tax, beta-Gal but not tax expression was suppressed. Supertransformation of these cells with v-src suppressed both beta-Gal and tax expression. This suppression was reversed by treatment with the tyrosine kinase inhibitor herbimycin A or protein kinase A inhibitor H8. Electrophoretic mobility shift assays demonstrated augmented binding in the R but not U3 region. This binding was competitively inhibited by a high-affinity CREB oligodeoxynucleotide and super-shifted with a specific CREB antibody. Treatment of cells with the cyclic AMP analog dibutyryl cyclic AMP also transiently increased the R region binding dramatically. In vitro DNase I footprint analysis identified a protein-binding sequence in the R region which corresponded with suppression. However, this target sequence lacked a conventional CREB-binding site. A 70.5-kDa DNA-binding protein was partially purified by affinity chromatography, along with a 49-kDa protein which reacted with CREB-specific sera. These data demonstrate that HTLV-I LTR suppression is associated with CREB factor binding in the R region, probably by direct interaction with a 70.5-kDa protein, and provide a novel mechanism for maintenance of viral latency.

Intracellular signalling mediated by protein-tyrosine kinases: networking through phospholipid metabolism

In recent years, it has become apparent that receptor-mediated intracellular signals are not linear cascades beginning at the plasma membrane and terminating with the production of a needed metabolite or the induction of gene expression. Instead, complex networks of interactive intracellular signals are activated in response to extracellular stimuli. Many responses to extracellular stimuli are mediated by protein-tyrosine kinases (PTKs). Activating PTKs leads to the recruitment of a variety of intracellular signalling molecules that execute a complex set of instructions. The response to PTK activity is dependent upon which PTK is activated and the cellular context in which the PTK exists. Several signalling molecules recruited by PTKs are involved in the metabolism of phospholipids. In this Mini Review, intracellular signalling networks activated by PTKs are discussed with an emphasis on the potential for generating highly specific and sophisticated responses to PTK activity through phospholipid metabolism.

Modulation of pp60v-src and pp60c-src expression in Rous sarcoma virus-transformed hamster fibroblasts transfected with activated N-ras

Three phenotypically different hamster cell lines transformed with Rous sarcoma virus (RSV) were transfected with plasmid DNA containing an activated N-ras oncogene, and nine clones expressing various levels of p21N-ras were characterized. We examined the effects of p21N-ras on expression and kinase activity of resident src proteins by using a variety of assays that allowed us to discriminate between viral and cellular src proteins. In eight clones with a 10- to 20-fold increase in p21N-ras levels relative to the endogenous protein, we observed a marked reduction in the synthesis and kinase activity of p60v-src. This decrease correlated with transcriptional downregulation of RSV genomic and v-src subgenomic mRNAs. In the same cells, we found a concomitant accumulation of p60c-src and, accordingly, an increase in its protein kinase activity without an apparent increase in c-src mRNA levels. Therefore, modulation of viral and cellular src proteins in cells overexpressing p21N-ras appeared to result from two distinct effects: a downregulation of long terminal repeat-driven transcription and a more complex interaction with cellular effectors that control the stability of p60c-src. Such modulation also seemed to depend on the levels of p21N-ras and, possibly, on host-cell factors, since it was not observed in the third cell line, in which the relative increase in p21N-ras was only 2.5-fold to fivefold.

Substrate-specific modulation of Src-mediated phosphorylation of Ras and caseins by sphingosines and other substrate modulators

It is important for the understanding of protein kinase action to differentiate between regulation at the enzyme and at the substrate levels. For example, the inhibitors dinitrophenol-tyrosine and tyrphostins act at the enzyme level to inhibit phosphorylation of all substrates by c-Src and v-Src kinases. In contrast, polylysine acts at the substrate level to stimulate Src-mediated phosphorylation of beta-casein but to inhibit phosphorylation of alpha-casein. Here we demonstrate novel enzyme-specific and substrate-specific modulations of Src kinase activity of potential physiological significance. At the enzyme level, we observed that c-Src kinase preferentially phosphorylates alpha-casein, while the v-Src kinase prefers beta-casein. At the substrate level we observed substrate-specific modulation by physiological factors including sphingosine, sphingosine derivatives and the ganglioside GM3. Galactosyl-sphingosine (psychosine) was more effective in stimulating phosphorylation of beta-casein and poly(E1A1Y1) than sphingosine. Glucosyl- and lactosyl-sphingosine were ineffective. Rat was extensively phosphorylated by c-Src in the presence of polylysine, and to a lesser extent in the sphingosine and galactosyl-sphingosine. These unexpected differences point out another potential mechanism for regulation of c-Src and v-Src kinase activities and may help to explain some of the pleotyptic manifestations of protein tyrosine kinase actions.

Non–Neutralizing Monoclonal Antibodies Against RAS GTPase–Activating Protein: Production, Characterization and Use in an Enzyme Immunometric Assay

We studied several monoclonal antibodies (mAbs) raised against the 100 kD Ras GTPase activating protein (p100-GAP), which was purified from human placenta. These antibodies recognized p120-GAP and p100-GAP in native and in denatured forms. The most reactive, GP15 and GP200, both recognized distinct epitopes and did not neutralize GTPase stimulatory activity. These two mAbs were selected for a two-site enzyme immunoassay, using covalent conjugates of the antibodies coupled to the tetrameric form of acetylcholinesterase as tracer. This assay was used to quantify Ras-GAP in both normal and tumor tissues and cell extracts.