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H Sabe

Publications and source records attributed to H Sabe.

At least 37 records · Page 2Linked to original sources
PubMed1997

Mitosis specific serine phosphorylation and downregulation of one of the focal adhesion protein, paxillin.

Mitotic cells typically lack well-formed focal adhesions. As an approach to explore the dynamic process regulating the focal adhesion assembly, we examined states of focal adhesion proteins during mitosis of the cell cycle. We found that the amount of paxillin was significantly reduced during mitosis of the cell cycle, whereas other focal adhesion proteins including talin, vinculin and Focal Adhesion Kinase did not. Proteolytic degradation appeared to be involved in the mitotic reduction, but transcriptional and/or translational controls of the mRNA were not essential for this downregulation. Moreover, concurrent with the decreased protein level, phosphorylation status of paxillin altered during mitosis; mitotic paxillin was phosphorylated primarily on serine and dephosphorylated on tyrosine while interphase one was phosphorylated both on serine and tyrosine. We found that mitotic phosphorylation created an electrophoretically slow-migrating population of paxillin which was barely detected in interphase cells. This mitotic specific modification occurred with both alpha and beta isoforms of paxillin. We also examined the fate of paxillin protein by changing its protein amount. We found that majority of paxillin overexpressed was subjected to the specific modification but not to the downregulation in the mitotic arrested cells. On the other hand, paxillin exogenously expressed at a moderate level was subjected to both the mitotic modification and downregulation. Collectively, we concluded that paxillin's specific serine phosphorylation together with the proteolytic downregulation of a limited fraction of paxillin is taken place during the mitosis of the cell cycle.

Animals↗
PubMed1997

Cell to substratum adhesion is involved in v-Src-induced cellular protein tyrosine phosphorylation: implication for the adhesion-regulated protein tyrosine phosphatase activity.

Protein tyrosine phosphorylation accompanies the integrin-mediated cell to substratum adhesion, and is essential for the progression of G1/S phase of the cell-cycle in normal fibroblasts. To examine how cellular protein tyrosine phosphatase (PTPase) activity is involved in regulating the adhesion-dependent protein tyrosine phosphorylation, we employed fibroblast cells bearing an active form of a protein tyrosine kinase (PTK), v-Src. We found that the v-Src induced tyrosine phosphorylation in certain proteins such as tensin, talin, p120, p80/85 (cortactin) and paxillin was greatly reduced when the cell to substratum adhesion was lost. Readhesion of the cells onto fibronectin restored these phosphorylation events, while this was inhibited by the addition of RGD peptide. The kinase activity of the v-Src was unchanged by the loss of cell to substratum adhesion. On the other hand, treatment with a protein tyrosine phosphatase inhibitor vanadate caused much the same increase in the v-Src-mediated cellular tyrosine phosphorylation between cells adhered to the culture environments and cells kept in suspension. These data suggest that PTPase(s) appears to be more critical than the v-Src PTK in determining the cell adhesion-dependent protein tyrosine phosphorylation. Moreover, most of the protein tyrosine phosphorylations that are mediated by the v-Src but still dependent on the cell adhesion were indeed greatly reduced during an anchorage-independent growth of v-Src cells. Thus our data collectively indicate that the v-Src induced high level of tyrosine phosphorylation in certain types of proteins are still under the control of the integrin(s) or the cell adhesion to culture substratum, and most of these adhesion-regulated high levels of tyrosine phosphorylations are not essential for the transformed phenotype.

Actins↗
PubMed1997

Monocyte cells and cancer cells express novel paxillin isoforms with different binding properties to focal adhesion proteins.

The versatility of integrin functions is mediated by engagement of a number of proteins that assemble with integrins. Among them, paxillin is one of the important molecules interacting with a variety of signaling molecules and cytoskeletal building blocks. We report here that paxillin is not a single molecule with a unique physiological property. We identified two human paxillin isoforms, beta and gamma. These isoforms have distinct amino acid insertions; each consists of a distinct exon, at the same site of previously reported paxillin (paxillin alpha). Several proteins were co-precipitated with paxillin, and we found that beta bound to focal adhesion kinase but weakly to vinculin, and gamma bound to vinculin but only weakly to focal adhesion kinase, although both bound equally to talin. No additional proteins were found to bind to beta and gamma over those binding to alpha. Unlike the alpha isoform, beta and gamma mRNAs were not detected in normal tissues, but several cancer cells expressed both alpha and beta proteins simultaneously. All three isoform proteins were expressed in promonocytic cells with ratios comparable with each other, and the expression patterns were altered during differentiation of floating promonocytic cells into adherent macrophage-like cells. Therefore, each isoform of paxillin exhibits distinct expression and different biochemical as well as physiological properties and thereby appears to act as a distinct module involved in different functions of integrins.

Cell Adhesion Molecules↗
PubMed1996

Evidence of a novel redox-linked activation mechanism for the Src kinase which is independent of tyrosine 527-mediated regulation.

The kinase activity of p60c-src has been shown to be basically regulated through phosphorylation and dephosphorylation of Y527. We found that catalytic activity of the immunoprecipitated c-Src kinase from NIH3T3 cells was elevated several folds by exposure to 0.5-50 microM of sulfhydryl-reactive Hg2+. Vmax of the kinase was increased whereas Km was decreased. N-acetylcysteine neutralized this Hg2+ effect, suggesting a critical role of the Hg2+-mediated sulfhydryl modification of the kinase in the mechanism. Addition of protein tyrosine phosphatase inhibitor Na3VO4 into the reaction mixture did not inhibit the Hg2+-mediated activation. Further study revealed that Hg2+ was capable of activating the v-Src kinase lacking Y527 and the c-Src kinase from mutant cells defective of the Y527-phosphorylating Csk kinase. Cyanogen bromide cleavage maps of radiolabeled Src proteins showed that Hg2+ selectively promoted the autophosphorylation at Y416 and that the previously in vivo radiolabeled phosphorous on Y527 was not deleted during the promotion of Y416 autophosphorylation by Hg2+. Phosphoamino acid analysis demonstrated selective promotion of phosphorylation at tyrosine but not at serine/threonine. Not like bivalent Hg2+, monovalent p-chloromercuribenzenesulfonic acid was incapable of activating c-Src kinase. These results suggest a novel Y416 phosphorylation-linked activation pathway for Src kinases which is initially triggered independent of Y527-mediated or serine/threonine phosphorylation-linked regulation, possibly through sulfhydryl-based protein structural modification for functional alteration.

3T3 Cells↗
PubMed1996

Paxillin association in vitro with integrin cytoplasmic domain peptides.

Short cytoplasmic domains of integrin heterodimers are crucial for transduction of signals generated by adhesion of cells to the extracellular matrix. Here, we describe the use of peptides mimicking the intracellular tails of integrin alpha5beta1 to assay in vitro associations with cytoskeletal proteins. Our results suggest that the focal adhesion protein, paxillin, may interact directly with the intracellular region of the integrin beta1 subunit. Paxillin is known to form stable complexes with several signaling molecules, including focal adhesion kinase. Physical interaction between paxillin and the beta1 cytoplasmic domain suggests a model in which paxillin may function as a key intermediary in integrin-mediated signal transduction.

Amino Acid Sequence↗
PubMed1996

CD45 modulates phosphorylation of both autophosphorylation and negative regulatory tyrosines of Lyn in B cells.

CD45 is a tyrosine phosphatase that is required for normal B cell receptor (BCR)-mediated signaling. It has been shown that Src-family tyrosine kinases such as Lyn could be a potential substrate for CD45. In vitro studies indicate that activities of Src family tyrosine kinases are regulated by tyrosine phosphorylation; C-terminal phosphorylation is inhibitory, and autophosphorylation is stimulatory. We report here that both autophosphorylation and C-terminal negative regulatory tyrosines of Lyn were hyperphosphorylated in CD45-deficient DT40 B cells. In this mutant cell, BCR-induced protein-tyrosine phosphorylation and calcium mobilization were severely compromised, as seen in Lyn-deficient cells. Consistent with this observation, Lyn activation upon receptor ligation was profoundly decreased in CD45-deficient cells. Taken together, our results suggest that dephosphorylation of tyrosine residues at both autophosphorylation and negative regulatory sites is mediated by CD45 in vivo, and that dephosphorylation of C-terminal tyrosine is a prerequisite for participation of Lyn in BCR signaling.

Animals↗
PubMed1996

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.

Amino Acid Sequence↗
PubMed1996

Csk enhances insulin-stimulated dephosphorylation of focal adhesion proteins.

Insulin has pleiotropic effects on the regulation of cell physiology through binding to its receptor. The wide variety of tyrosine phosphorylation motifs of insulin receptor substrate 1 (IRS-1), a substrate for the activated insulin receptor tyrosine kinase, may account for the multiple functions of insulin. Recent studies have shown that activation of the insulin receptor leads to the regulation of focal adhesion proteins, such as a dephosphorylation of focal adhesion kinase (pp125FAK). We show here that C-terminal Src kinase (Csk), which phosphorylates C-terminal tyrosine residues of Src family protein tyrosine kinases and suppresses their kinase activities, is involved in this insulin-stimulated dephosphorylation of focal adhesion proteins. We demonstrated that the overexpression of Csk enhanced and prolonged the insulin-induced dephosphorylation of pp125FAK. Another focal adhesion protein, paxillin, was also dephosphorylated upon insulin stimulation, and a kinase-negative mutant of Csk was able to inhibit the insulin-induced dephosphorylation of pp125FAK and paxillin. Although we have shown that the Csk Src homology 2 domain can bind to several tyrosine-phosphorylated proteins, including pp125FAK and paxillin, a majority of protein which bound to Csk was IRS-1 when cells were stimulated by insulin. Our data also indicated that tyrosine phosphorylation levels of IRS-1 appear to be paralleled by the dephosphorylation of the focal adhesion proteins. We therefore propose that the kinase activity of Csk, through the insulin-induced complex formation of Csk with IRS-1, is involved in insulin's regulation of the phosphorylation levels of the focal adhesion proteins, possibly through inactivation of the kinase activity of c-Src family kinases.

Amino Acid Sequence↗
PubMed1995

Possible v-Crk-induced transformation through activation of Src kinases.

p47gag-crk (v-Crk) encoded by avian sarcoma virus CT10, causes an elevation of tyrosine phosphorylation of several cellular proteins. The lack of a protein-tyrosine kinase domain in v-Crk suggests its co-operation with cellular protein-tyrosine kinase activity. We have shown that suppression of a certain fraction of c-Src activity by Csk may require the binding of Csk to tyrosine-phosphorylated paxillin. In this study, we detected co-immunoprecipitation of tyrosine-phosphorylated paxillin with v-Crk in CT10-transformed chicken embryo fibroblasts (CEF), and demonstrated that v-Crk binding to paxillin can inhibit Csk binding to paxillin. A phosphotyrosine peptide, which can inhibit v-Crk binding to paxillin, did not inhibit Csk binding to paxillin, suggesting that v-Crk and Csk bind to different tyrosine-phosphorylated sites in paxillin. We also found that the kinase activity of the endogenous c-Src in CEF is elevated severalfold after CT10-transformation. We therefore suggest that the competitive binding of overexpressed v-Crk affects an efficient interaction of Csk with tyrosine-phosphorylated paxillin in CT10-transformed CEF. This would result in a failure in the suppression of the kinase activities of a population of c-Src and other Src family protein-tyrosine kinases as well, and these kinases may then contribute to the phosphorylation of cellular proteins in CT10-transformed CEF.

Amino Acid Sequence↗
PubMed1994

IL-2 can support growth of CD8+ T cells but not CD4+ T cells of human IL-2 receptor beta-chain transgenic mice.

We have generated transgenic mice expressing the human (h) IL-2R beta-chain on lymphoid cells under the control of the mouse H-2Kd promoter. Spleen cells and thymocytes of the transgenic mice were cultured in the presence of 5 nM hIL-2. After a 10-day culture, the expanded populations were analyzed by flow cytometry and shown to be composed of CD8+ T cells and gamma delta T cells. Surprisingly, CD4+ T cells of the transgenic mice did not proliferate in response to hIL-2, although the CD4+ T cells expressed the transgenic hIL-2R beta-chain as well as the endogenous gamma-chain on their surface and bound 125I-labeled IL-2. When CD4+ T cells of the transgenic mice were stimulated with anti-CD3 mAb, the CD4+ T cells proliferated in response to hIL-2. These findings suggest that CD4+ T cells may require another triggering signal to respond to IL-2 even when IL-2Rs are expressed. By contrast, CD8+ T cells and gamma delta T cells respond to IL-2 as long as IL-2Rs are expressed.

Animals↗
PubMed1994

Unphosphorylated and tyrosine-phosphorylated forms of a focal adhesion protein, paxillin, are substrates for calpain II in vitro: implications for the possible involvement of calpain II in mitosis-specific degradation of paxillin.

Cell-to-substratum adhesion becomes weakened during mitosis of the cell cycle in fibroblasts. The level of one focal adhesion protein, paxillin, is greatly reduced in mitotic-arrested cells. We show here the possible involvement of calpain II, known to be localized in focal adhesion plaques, in the degradation of paxillin. Paxillin is tyrosine-phosphorylated during interphase of the cell cycle by protein tyrosine kinases (PTK) such as c-Src and Csk, and becomes dephosphorylated during mitosis. Our data, however, indicate that tyrosine phosphorylation of paxillin does not affect the rate of paxillin degradation by calpain in vitro.

3T3 Cells↗
PubMed1994

Comparative study of three protein-tyrosine phosphatases. Chicken protein-tyrosine phosphatase lambda dephosphorylates c-Src tyrosine 527.

To examine the substrate preference of protein tyrosine phosphatases (PTPs), we compared the activity of three transmembrane PTPs on dephosphorylation and regulation of c-Src and v-Src: chicken PTP lambda (ChPTP lambda), chicken PTP alpha (ChPTP alpha), and human leukocyte common antigen-related molecule (HLAR). In vitro, all three PTPs dephosphorylated v-Src, but only ChPTP lambda dephosphorylated c-Src. Their activities were also compared in Cos cells coexpressing Src and the phosphatase domains of three PTPs. These domains were fused with peptides for myristylation, so they associated with the cellular membrane. When c-Src was coexpressed with myrPTP lambda, its kinase activity was elevated 3-4-folds. This activation was less obvious when c-Src was coexpressed with myrPTP alpha or myrLAR. Analysis by cyanogen bromide cleavage showed that ChPTP lambda and myrPTP lambda dephosphorylated Tyr-527 of c-Src. Our data demonstrated the different activities of three PTPs on phosphoproteins, suggesting that Src Tyr-527 may require more specific PTP(s) than Src Tyr-416 for dephosphorylation in vivo.

Amino Acid Sequence↗
PubMed1994

Analysis of the binding of the Src homology 2 domain of Csk to tyrosine-phosphorylated proteins in the suppression and mitotic activation of c-Src.

Csk (C-terminal Src kinase), a protein-tyrosine kinase, bearing the Src homology 2 and 3 (SH2 and SH3) domains, has been implicated in phosphorylation of c-Src Tyr-527, resulting in suppression of c-Src kinase activity. We found that mutations in the SH2 or SH3 domain of Csk, though they did not affect its kinase activity, resulted in a loss of suppression of c-Src activity in fibroblasts. In normal fibroblasts, tyrosine-phosphorylated paxillin and focal adhesion kinase pp125FAK, which colocalize at focal adhesion plaques, were the major proteins to which the Csk SH2 domain bound. Loss of binding to these proteins by the Csk SH2 mutants correlated with loss of the activity to suppress c-Src. Consistent with this observation, the levels of tyrosine phosphorylation of paxillin and pp125FAK were greatly reduced during mitosis, whereas the kinase activity of c-Src was elevated. We suggest that the SH2 domain is required for Csk to suppress c-Src, perhaps in combination with the SH3 domain, by anchoring Csk to a particular subcellular location where c-Src may exist. Our data also indicate that a certain fraction of the Csk and Src family kinases function at the focal adhesion plaques. The activity of the c-Src kinase localized at the focal adhesion plaques appears to be regulated by cell adhesion to the extracellular matrix.

Animals↗
PubMed1994

Tyrosine kinases Lyn and Syk regulate B cell receptor-coupled Ca2+ mobilization through distinct pathways.

Stimulation of B lymphocytes through their antigen receptor (BCR) results in rapid increases in tyrosine phosphorylation on a number of proteins and induces both an increase of phosphatidylinositol and mobilization of cytoplasmic free calcium. The BCR associates with two classes of tyrosine kinase: Src-family kinase (Lyn, Fyn, Blk or Lck) and Syk kinase. To dissect the functional roles of these two types of kinase in BCR signaling, lyn-negative and syk-negative B cell lines were established. Syk-deficient B cells abolished the tyrosine phosphorylation of phospholipase C-gamma 2, resulting in the loss of both inositol 1,4,5-trisphosphate (IP3) generation and calcium mobilization upon receptor stimulation. Crosslinking of BCR on Lyn-deficient cells evoked a delayed and slow Ca2+ mobilization, despite the normal kinetics of IP3 turnover. These results demonstrate that Syk mediates IP3 generation, whereas Lyn regulates Ca2+ mobilization through a process independent of IP3 generation.

Animals↗
PubMed1994

v-src transformation of rat embryo fibroblasts. Inefficient conversion to anchorage-independent growth involves heterogeneity of primary cultures.

To clarify whether a single oncogene can transform primary cells in culture, we compared the transforming effect of a recombinant retrovirus (ZSV) containing the v-src gene in rat embryo fibroblasts (REFs) to that in the rat cell line 3Y1. In the focus assay, REFs exhibited resistance to transformation as only six foci were observed in the primary cultures as opposed to 98 in 3Y1 cells. After G418 selection, efficiency of transformation was again somewhat lower with REFs compared to that with 3Y1 cells, but the number of G418-resistant REF colonies was much greater than the number of foci in REF cultures. Furthermore, while 98% of G418-resistant colonies of ZSV-infected REFs were morphologically transformed, only 25% were converted to anchorage-independent growth, as opposed to 100% conversion seen in ZSV-infected 3Y1 cells. The poor susceptibility of REFs to anchorage-independent transformation did not involve differences in expression and subcellular distribution of p60v-src, or its kinase activity in vitro and in vivo. It rather reflected a property of the primary cultures, as cloning of REFs before ZSV infection demonstrated that only 2 out of 6 REF clones tested were permissive for anchorage-independent growth. The nonpermissive phenotype was dominant over the permissive one in somatic hybrid cells, and associated with organized actin filament bundles and a lower growth rate, both before and after ZSV infection. These results indicate that the poor susceptibility of REFs to anchorage-independent transformation by p60v-src reflects the heterogeneity of the primary cultures. REFs can be morphologically transformed by p60v-src with high efficiency but only a small fraction is convertible to anchorage-independent growth. REF resistance seems to involve the presence of a suppressor factor which may emerge from REF differentiation during embryonic development.

Actins↗
PubMed1994

Functional analysis of Csk in signal transduction through the B-cell antigen receptor.

In B cells, two classes of protein tyrosine kinases (PTKs), the Src family of PTKs (Lyn, Fyn, Lck, and Blk) and non-Src family of PTKs (Syk), are known to be involved in signal transduction induced by the stimulation of the B-cell antigen receptor (BCR). Previous studies using Lyn-negative chicken B-cell clones revealed that Lyn is necessary for transduction of signals through the BCR. The kinase activity of the Src family of PTKs is negatively regulated by phosphorylation at the C-terminal tyrosine residue, and the PTK Csk has been demonstrated to phosphorylate this C-terminal residue of the Src family of PTKs. To investigate the role of Csk in BCR signaling, Csk-negative chicken B-cell clones were generated. In these Csk-negative cells, Lyn became constitutively active and highly phosphorylated at the autophosphorylation site, indicating that Csk is necessary to sustain Lyn in an inactive state. Since the C-terminal tyrosine phosphorylation of Lyn is barely detectable in the unstimulated, wild-type B cells, our data suggest that the activities of Csk and a certain protein tyrosine phosphatase(s) are balanced to maintain Lyn at a hypophosphorylated and inactive state. Moreover, we show that the kinase activity of Syk was also constitutively activated in Csk-negative cells. The degree of activation of both the Lyn and Syk kinases in Csk-negative cells was comparable to that observed in wild-type cells after BCR stimulation. However, BCR stimulation was still necessary in Csk-negative cells to elicit tyrosine phosphorylation of cellular proteins, as well as calcium mobilization and inositol 1,4,5-trisphosphate generation. These results suggest that not only activation of the Lyn and Syk kinases but also additional signals induced by the cross-linking of the BCR are required for full transduction of BCR signaling.

Animals↗
PubMed1994

Specific motifs recognized by the SH2 domains of Csk, 3BP2, fps/fes, GRB-2, HCP, SHC, Syk, and Vav.

Src homology 2 (SH2) domains provide specificity to intracellular signaling by binding to specific phosphotyrosine (phospho-Tyr)-containing sequences. We recently developed a technique using a degenerate phosphopeptide library to predict the specificity of individual SH2 domains (src family members, Abl, Nck, Sem5, phospholipase C-gamma, p85 subunit of phosphatidylinositol-3-kinase, and SHPTP2 (Z. Songyang, S. E. Shoelson, M. Chaudhuri, G. Gish, T. Pawson, W. G. Haser, F. King, T. Roberts, S. Ratnofsky, R. J. Lechleider, B. G. Neel, R. B. Birge, J. E. Fajardo, M. M. Chou, H. Hanafusa, B. Schaffhausen, and L. C. Cantley, Cell 72:767-778, 1993). We report here the optimal recognition motifs for SH2 domains from GRB-2, Drk, Csk, Vav, fps/fes, SHC, Syk (carboxy-terminal SH2), 3BP2, and HCP (amino-terminal SH2 domain, also called PTP1C and SHPTP1). As predicted, SH2 domains from proteins that fall into group I on the basis of a Phe or Tyr at the beta D5 position (GRB-2, 3BP2, Csk, fps/fes, Syk C-terminal SH2) select phosphopeptides with the general motif phospho-Tyr-hydrophilic (residue)-hydrophilic (residue)-hydrophobic (residue). The SH2 domains of SHC and HCP (group III proteins with Ile, Leu, of Cys at the beta D5 position) selected the general motif phospho-Tyr-hydrophobic-Xxx-hydrophobic, also as predicted. Vav, which has a Thr at the beta D5 position, selected phospho-Tyr-Met-Glu-Pro as the optimal motif. Each SH2 domain selected a unique optimal motif distinct from motifs previously determined for other SH2 domains. These motifs are used to predict potential sites in signaling proteins for interaction with specific SH2 domain-containing proteins. The Syk SH2 domain is predicted to bind to Tyr-hydrophilic-hydrophilic-Leu/Ile motifs like those repeated at 10-residue intervals in T- and B-cell receptor-associated proteins. SHC is predicted to bind to a subgroup og these same motifs. A structural basis for the association of Csk with Src family members is also suggested from these studies.

Amino Acid Sequence↗