The nonreceptor protein-tyrosine kinase CSK complexes directly with the GTPase-activating protein-associated p62 protein in cells expressing v-Src or activated c-Src

Roles of C-terminal Src kinase in the initiation and the termination of the high affinity IgE receptor-mediated signaling

As an attempt to analyze the roles of C-terminal Src kinase (Csk) in the high affinity IgE receptor (FcepsilonRI)-mediated signaling, we overexpressed Csk, a membrane-targeted form of Csk (mCsk), and a kinase-defective, membrane-targeted form of Csk (mCsk(-)) in rat basophil leukemia (RBL) 2H3 cells. Specific activity of Lyn at the basal state was decreased in Csk-expressing cells, and further decreased in mCsk-expressing cells. In mCsk(-)-expressing cells, basal specific activity of Lyn was increased, thereby indicating that mCsk(-) functioned as a dominant negative molecule. The onset of FcepsilonRI-mediated Lyn activation was delayed in Csk-expressing cells, and further delayed in mCsk-expressing cells. In mCsk(-)-expressing cells, Lyn activation was rapid and quite long lasting. These findings indicate (i) Csk negatively regulates rapid FcepsilonRI/Lyn coupling, and (ii) Csk activity is potentially required for its termination. The onsets of the series of events including tyrosyl phosphorylation of Syk, mitogen-activated protein (MAP) kinase activation, elevation of intracellular calcium concentration ([Ca2+]i), and histamine release were all stepwisely delayed in Csk-expressing cells and in mCsk-expressing cells. The durations of Syk phosphorylation and MAP kinase activation also closely correlated with those of Lyn activation, but [Ca2+]i elevation and histamine release followed different temporal patterns: the delayed responses in Csk-expressing cells and in mCsk-expressing cells led to sustained [Ca2+]i oscillation and histamine release, while the prompt responses in parent cells and mCsk(-)-expressing cells rapidly subsided. These findings provide further evidence that the initiations of the FcepsilonRI-mediated signals are upstreamly regulated by Src family protein tyrosine kinases and revealed that their terminations are regulated by Lyn-dependent (Syk and MAP kinase) and -independent ([Ca2+]i elevation and histamine release) mechanisms.

Inhibitory tyrosine protein kinase p50csk is associated with protein-tyrosine phosphatase PTP-PEST in hemopoietic and non-hemopoietic cells

p50(csk) is a cytosolic tyrosine protein kinase expressed in all cell types. Accumulating data show that it inhibits multiple cellular processes, as a consequence of its ability to repress the enzymatic activity of Src family tyrosine protein kinases. We previously demonstrated that, via its Src homology 3 (SH3) domain, Csk is tightly bound to PEP, a protein-tyrosine phosphatase (PTP) exclusively expressed in hemopoietic cells. In this report, we have tested the possibility that Csk also interacts with PTP-PEST, a ubiquitous PTP sharing structural homology with PEP. Our studies revealed that Csk was associated with PTP-PEST in a variety of cell types, including non-hemopoietic cells. This interaction involved the SH3 region of p50(csk) and a proline-rich region (PPPLPERTPESFVLADM) outside the catalytic region of PTP-PEST. Even though both PTP-PEST and PEP were associated with Csk, significant differences were noted between these two PTPs. PTP-PEST, but not PEP, was also complexed with Shc, an adaptor molecule implicated in the Ras pathway. Moreover, PTP-PEST and PEP were found to accumulate primarily in distinct intracellular compartments in cell fractionation studies. In combination, these findings indicated that, like PEP, PTP-PEST is probably involved in Csk-mediated functions in mammalian cells. Moreover, they suggested that the roles of Csk-PTP-PEST and Csk-PEP are likely to be different.

Differential effects of B cell receptor and B cell receptor-FcgammaRIIB1 engagement on docking of Csk to GTPase-activating protein (GAP)-associated p62

The stimulatory and inhibitory pathways initiated by engagement of stimulatory receptors such as the B cell receptor for antigen (BCR) and inhibitory receptors such as Fcgamma receptors of the IIB1 type (FcgammaRIIB1) intersect in ways that are poorly understood at the molecular level. Because the tyrosine kinase Csk is a potential negative regulator of lymphocyte activation, we examined the effects of BCR and FcgammaRIIB1 engagement on the binding of Csk to phosphotyrosine-containing proteins. Stimulation of a B lymphoma cell line, A20, with intact anti-IgG antibody induced a direct, SH2-mediated association between Csk and a 62-kD phosphotyrosine-containing protein that was identified as RasGTPase-activating protein-associated p62 (GAP-A.p62). In contrast, stimulation of A20 cells with anti-IgG F(ab')2 resulted in little increase in the association of Csk with GAP-A.p62. The effect of FcgammaRIIB1 engagement on this association was abolished by blockade of FcgammaRIIB1 with the monoclonal antibody 2.4G2. Furthermore, the increased association between Csk and GAP-A.p62 seen upon stimulation with intact anti-Ig was abrogated in the FcgammaRIIB1-deficient cell line IIA1.6 and recovered when FcgammaRIIB1 expression was restored by transfection. The differential effects of BCR and BCR-FcgammaRIIB1-mediated signaling on the phosphorylation of GAP-A.p62 and its association with Csk suggest that docking of Csk to GAP-A.p62 may function in the negative regulation of antigen receptor-mediated signals in B cells.

Juxtamembrane tyrosine residues couple the Eph family receptor EphB2/Nuk to specific SH2 domain proteins in neuronal cells

Eph-related receptor tyrosine kinases have been implicated in the control of axonal navigation and fasciculation. To investigate the biochemical mechanisms underlying such functions, we have expressed the EphB2 receptor (formerly Nuk/Cek5/Sek3) in neuronal NG108-15 cells, and have observed the tyrosine phosphorylation of multiple cellular proteins upon activation of EphB2 by its ligand, ephrin-B1 (formerly Elk-L/Lerk2). The activated EphB2 receptor induced the tyrosine phosphorylation of a 62-64 kDa protein (p62[dok]), which in turn formed a complex with the Ras GTPase-activating protein (RasGAP) and SH2/SH3 domain adaptor protein Nck. RasGAP also bound through its SH2 domains to tyrosine-phosphorylated EphB2 in vitro, and complexed with activated EphB2 in vivo. We have localized an in vitro RasGAP-binding site to conserved tyrosine residues Y604 and Y610 in the juxtamembrane region of EphB2, and demonstrated that substitution of these amino acids abolishes ephrin-B1-induced signalling events in EphB2-expressing NG108-15 cells. These tyrosine residues are followed by proline at the + 3 position, consistent with the binding specificity of RasGAP SH2 domains determined using a degenerate phosphopeptide library. These results identify an EphB2-activated signalling cascade involving proteins that potentially play a role in axonal guidance and control of cytoskeletal architecture.

Sequence specificity of C-terminal Src kinase (CSK)--a comparison with Src-related kinases c-Fgr and Lyn

An eicosapeptide encompassing the C-terminal tail of c-Src (Tyr527) which is conserved in most Src-related protein kinases, is phosphorylated by C-terminal Src kinase (CSK) and by the two Src-related protein kinases c-Fgr and Lyn, with similar kinetic constants. Two related peptides reproducing the C-terminal segments of c-Src mutants defective in CSK phosphorylation [MacAuley, A., Okada, M., Nada, S., Nakagawa, H. & Cooper, J. A. (1993) Oncogene 8, 117-124] AFLEDSCTGTEPLYQRGENL (mutant number 28) and AFLEDNFTGTKPQYHPGENL (mutant number 29), proved a better and a much worse substrates, respectively than the wild-type peptide, with either CSK or the two Src kinases. By changing individual residues in the best peptide substrate, it was shown that the main element responsible for its improved phosphorylation is leucine at position -1 (instead of glutamine), while lysine at position -3 (instead of glutamate) has a detrimental effect, possibly accounting for the negligible phosphorylation of peptide derived from mutant number 29. By contrast to most peptide substrates, including the Src C-terminal peptides, which exhibit relatively high K(m) values, a polyoma-virus-middle-T-antigen-(mT)-derived peptide with tyrosine embedded in a highly hydrophobic sequence (EEEPQFEEIPIYLELLP) exhibits with CSK a quite low K(m) value (63 microM). Consistent with this, the optimal sequence selected by CSK in an oriented peptide library is XXXIYMFFF. This is different from sequences selected by Lyn (DEEIYEELX) and c-Fgr (XEEIYGIFF), although they all share a high selection for a hydrophobic residue at n-1. In sharp contrast, TPKIIB/p38syk, related to the catalytic domain of p72syk, selects acidic residues at nearly all positions, n-1 included. These data support the notion that the features determining the specific phosphorylation of the C-terminal tyrosine residue of Src do not reside in the primary structure surrounding the target tyrosine. They also show that this site does not entirely fulfil the optimal consensus sequence recognized by CSK, disclosing the possibility that as yet unrecognized CSK targets structurally unrelated to the C-terminal tyrosine residue of Src kinases may exist.

Aberrant Ras regulation and reduced p190 tyrosine phosphorylation in cells lacking p120-Gap

The Ras guanine nucleotide-binding protein functions as a molecular switch in signalling downstream of protein-tyrosine kinases. Ras is activated by exchange of GDP for GTP and is turned off by hydrolysis of bound GTP to GDP. Ras itself has a low intrinsic GTPase activity that can be stimulated by GTPase-activating proteins (GAPs), including p120-Gap and neurofibromin. These GAPs possess a common catalytic domain but contain distinct regulatory elements that may couple different external signals to control of the Ras pathway. p120-Gap, for example, has two N-terminal SH2 domains that directly recognize phosphotyrosine motifs on activated growth factor receptors and cytoplasmic phosphoproteins. To analyze the role of p120-Gap in Ras regulation in vivo, we have used fibroblasts derived from mouse embryos with a null mutation in the gene for p120-Gap (Gap). Platelet-derived growth factor stimulation of Gap-/- cells led to an abnormally large increase in the level of Ras-GTP and in the duration of mitogen-activated protein (MAP) kinase activation compared with wild-type cells, suggesting that p120-Gap is specifically activated following growth factor stimulation. Induction of DNA synthesis in response to platelet-derived growth factor and morphological transformation by the v-src and EJ-ras oncogenes were not significantly affected by the absence of p120-Gap. However, we found that normal tyrosine phosphorylation of p190-rhoGap, a cytoplasmic protein that associates with the p120-Gap SH2 domains, was dependent on the presence of p120-Gap. Our results suggest that p120-Gap has specific functions in downregulating the Ras/MAP kinase pathway following growth factor stimulation, and in modulating the phosphorylation of p190-rhoGap, but is not required for mitogenic signalling.

A role for Sam68 in cell cycle progression antagonized by a spliced variant within the KH domain

Sam68 is the main tyrosine-phosphorylated and Src-associated protein in mitotic cells. Sam68 exhibits a conserved functional KH (hnRNPK homology) RNA binding domain and binds single strand nucleic acids. Tyrosine phosphorylation mediates the interaction of Sam68 with many SH3- and SH2-containing proteins and negatively regulates its nucleic acid binding properties. But the function and the impact of Sam68 on cell signaling and cell proliferation remains elusive. We report here the identification of a natural isoform of Sam68 with a deletion within the KH domain. This isoform, called Sam68DeltaKH, is specifically expressed at growth arrest upon confluency in normal cells. In cells that do not enter quiescence at confluency such as Src-transformed cells, no recruitment of Sam68DeltaKH is observed. Transfected Sam68DeltaKH inhibits serum-induced DNA synthesis and cyclin D1 expression. Sam68 overcomes these effects, suggesting that isoforms of Sam68 are involved, through KH domain signaling, in cell proliferation, and more precisely in G1/S transition.

p62(dok): a constitutively tyrosine-phosphorylated, GAP-associated protein in chronic myelogenous leukemia progenitor cells

Characteristic of chronic myelogenous leukemia (CML) is the presence of the chimeric p210(bcr-abl) protein possessing elevated protein tyrosine kinase activity relative to normal c-abl tyrosine kinase. Hematopoietic progenitors isolated from CML patients in the chronic phase contain a constitutively tyrosine-phosphorylated protein that migrates at 62 kDa by SDS-PAGE and associates with the p120 ras GTPase-activating protein (GAP). We have purified p62(dok) from a hematopoietic cell line expressing p210(bcr-abl). p62(dok) is a novel protein with features of a signaling molecule. Association of p62(dok) with GAP correlates with its tyrosine phosphorylation. p62(dok) is rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases.

Identification of the Abl- and rasGAP-associated 62 kDa protein as a docking protein, Dok

A 62 kDa protein is highly phosphorylated in many cells containing activated tyrosine kinases. This protein, characterized mainly by its avid association with rasGAP, has proved elusive. Anti-phosphotyrosine antibody was used to purify p62. From peptide sequence, molecular cloning revealed a cDNA encoding a novel protein, p62dok, with little homology to others but with a prominent set of tyrosines and nearby sequences suggestive of SH2 binding sites. In cells, v-Abl tyrosine kinase binds and strongly phosphorylates p62dok, which then binds rasGAP. A monoclonal antibody, 2C4, to the rasGAP-associated p62 reacts with p62dok. Thus, p62dok appears to be the long-sought major substrate of many tyrosine kinases.

Association of inhibitory tyrosine protein kinase p50csk with protein tyrosine phosphatase PEP in T cells and other hemopoietic cells

p50csk is a tyrosine protein kinase (TPK) that represses the activity of Src family TPKs. We previously showed that Csk is a potent negative regulator of antigen receptor signaling in T lymphocytes and that its Src homology (SH) 3 and SH2 domains are required to inhibit these signals. To test the idea that the Csk SH3 and SH2 domains mediate interactions with other cellular proteins, we attempted to identify Csk-associated polypeptides using the yeast two-hybrid system. The results of our experiments demonstrated that Csk physically associates with PEP, a protein tyrosine phosphatase (PTP) expressed in hemopoietic cells. Further analyses revealed that this interaction was mediated by the Csk SH3 domain and by a proline-rich region (PPPLPERTP) in the non-catalytic C-terminal portion of PEP. The association between Csk and PEP was documented in transiently transfected Cos-1 cells and in a variety of cells of hemopoietic lineages, including T cells. Additional analyses demonstrated that the association between Csk and PEP is highly specific. Together, these data indicated that PEP may be an effector and/or a regulator of p50csk in T cells and other hemopoietic cells. Moreover, they allowed the identification of PEP as the first known ligand for the Csk SH3 domain.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Csk is constitutively associated with a 60-kDa tyrosine-phosphorylated protein in human T cells

The protein-tyrosine kinase Csk is one of the main down-regulators of the Src family of kinases. Csk may be involved in the down-regulation of T cell receptor (TCR) signaling by C-terminal tyrosine phosphorylation of Lck and Fyn; however, it is not known how Csk activity is regulated or how it targets these Src family members. We used Jurkat T cells and normal human T cells to examine proteins that bind to the SH2 domain of Csk. In both Jurkat and normal T cells, the Src homology 2 (SH2) domain of Csk bound constitutively to a tyrosine-phosphorylated protein of 60 kDa (p60). The 60-kDa protein was detected in Csk immunoprecipitates from both unstimulated and CD3-stimulated cells. In addition to p60, a protein of 190 kDa coprecipitated with Csk, and both proteins were phosphorylated on tyrosine residues by the immunocomplex. Small amounts of GTPase-activating protein (GAP) were detected in anti-Csk immunoprecipitates, suggesting that p60 may be a GAP-associated protein. Our data demonstrate that the SH2 domain of Csk specifically associates with at least two tyrosine-phosphorylated proteins in normal human T cells, that this association is independent of TCR/CD3 activation, and that Csk may be a part of a multiprotein complex containing GAP.

Detection of a physical and functional interaction between Csk and Lck which involves the SH2 domain of Csk and is mediated by autophosphorylation of Lck on tyrosine 394

The COOH-terminal Src kinase (Csk) is responsible for the phosphorylation of the conserved, negative regulatory, carboxyl-terminal tyrosine of most of the Src family protein tyrosine kinases. Up to now, no stable binding of Csk to Src kinases has been detected. We therefore decided to analyze this interaction using two systems which allow detection of transient interaction. We produced and purified recombinant proteins in the glutathione S-transferase prokaryotic expression system. First, using real-time biospecific interaction analysis (BIAcore(TM)), we detected in vitro a specific interaction between Csk and one of its substrates Lck, a lymphocyte-specific member of the Src family. This interaction requires the autophosphorylation of Lck on tyrosine 394 (the phosphorylation of which is correlated with an increase of the kinase activity) and involves a functional Csk SH2 domain. Second, using the yeast two-hybrid system, we confirmed in vivo the physical interaction between Csk and Lck. Furthermore, in vitro we showed that autophosphorylation of Lck on tyrosine 394 enhances the phosphorylation of Lck by Csk on the negative regulatory site, tyrosine 505, suggesting that activated Lck serves preferentially as substrate for Csk. These findings might explain the mechanism(s) by which Csk interacts with most of Src kinases to down-regulate their kinase activity.