Showing papers by "Joan S. Brugge published in 1992"

Association of the Shc and Grb2/Sem5 SH2-containing proteins is implicated in activation of the Ras pathway by tyrosine kinases.

Abstract: THE mammalian shc gene encodes two overlapping, widely expressed proteins of 46 and 52K, with a carboxy-terminal SH2 domain that binds activated growth factor receptors, and a more amino-terminal glycine/proline-rich region1. These shc gene products (Shc) are transforming when overexpressed in fibroblasts1. Shc proteins become phosphorylated on tyrosine in cells stimulated with a variety of growth factors1, and in cells transformed by v-src (ref. 2), suggesting that they are tyrosine kinase targets that control a mitogenic signalling pathway. Here we report that tyrosine-phosphorylated She proteins form a specific complex with a non-phosphorylated 23K polypeptide encoded by the grb2/sem-5 gene3,4. The grb2/sem-5 gene product itself contains an SH2 domain, which mediates binding to Shc, and is implicated in activation of the Ras guanine nucleotide-binding protein by tyrosine kinases in both Caenorhabditis elegans and mammalian cells3,4. Consistent with a role in signalling through Ras, shc overexpression induced Ras-dependent neurite outgrowth in PC 12 cells. These results suggest that She tyrosine phosphorylation can couple tyrosine kinases to Grb2/Sem-5, through formation of a Shc-Grb2/Sem-5 complex, and thereby regulate the mammalian Ras signalling pathway.

Integrin-dependent phosphorylation and activation of the protein tyrosine kinase pp125FAK in platelets.

Abstract: We have investigated mechanisms involved in integrin-mediated signal transduction in platelets by examining integrin-dependent phosphorylation and activation of a newly identified protein tyrosine kinase, pp125FAK (FAK, focal adhesion kinase). This kinase was previously shown to be localized in focal adhesions in fibroblasts, and to be phosphorylated on tyrosine in normal and Src-transformed fibroblasts. We show that thrombin and collagen activation of platelets causes an induction of tyrosine phosphorylation of pp125FAK and that pp125FAK molecules isolated from activated platelets display enhanced levels of phosphorylation in immune-complex kinase assays. pp125FAK was not phosphorylated on tyrosine after thrombin or collagen treatment of Glanzmann's thrombasthenic platelets deficient in the fibrinogen receptor GPIIb-IIIa, or of platelets pretreated with an inhibitory monoclonal antibody to GP IIb-IIIa. Fibrinogen binding to GP IIb-IIIa was not sufficient to induce pp125FAK phosphorylation because pp125FAK was not phosphorylated on tyrosine in thrombin-treated platelets that were not allowed to aggregate. These results indicate that tyrosine phosphorylation of pp125FAK is dependent on platelet aggregation mediated by fibrinogen binding to the integrin receptor GP IIb-IIIa. The induction of tyrosine phosphorylation of pp125FAK was inhibited in thrombin- and collagen-treated platelets preincubated with cytochalasin D, which prevents actin polymerization following activation. Under all of these conditions, there was a strong correlation between the induction of tyrosine phosphorylation of pp125FAK in vivo and stimulation of the phosphorylation of pp125FAK in vitro in immune-complex kinase assays. This study provides the first genetic evidence that tyrosine phosphorylation of pp125FAK is dependent on integrin-mediated events, and demonstrates that there is a strong correlation between tyrosine phosphorylation of pp125FAK in platelets, and the activation of pp125FAK-associated phosphorylating activity in vitro.

Solution structure of the SH3 domain of Src and identification of its ligand-binding site

Abstract: The Src homology 3 (SH3) region is a protein domain of 55 to 75 amino acids found in many cytoplasmic proteins, including those that participate in signal transduction pathways. The solution structure of the SH3 domain of the tyrosine kinase Src was determined by multidimensional nuclear magnetic resonance methods. The molecule is composed of two short three-stranded anti-parallel beta sheets packed together at approximately right angles. Studies of the SH3 domain bound to proline-rich peptide ligands revealed a hydrophobic binding site on the surface of the protein that is lined with the side chains of conserved aromatic amino acids.

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

Abstract: 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.

Cloning and characterization of a thermolabile v-src gene for use in reversible transformation of mammalian cells.

Abstract: The use of temperature-sensitive (ts) src mutants for studies of cell transformation and differentiation has been limited by the availability of cloned ts-src genes that are inactivated at temperatures compatible with growth of mammalian cells. In this report, we describe the cloning and characterization of the tsLA90src gene, which displays tight thermal sensitivity at 39.5 degrees C. Nucleotide sequence comparison of tsLA90 and wild-type src genes from the Schmidt-Ruppin subgroup A and D strains of Rous sarcoma virus (RSV) revealed four amino acid differences in tsLA90src. Substitution of one of these residues (Lys-280) from tsLA90src with its wild-type homolog (Glu-280) caused a reversion to a wild-type src phenotype. The cloned tsLA90 gene, designated tsUP1, was introduced into avian and mammalian retroviral vectors. Chicken embryo fibroblasts and immortalized mouse 3T3 cells infected with these viral vectors displayed a temperature-dependent transformed phenotype as assessed by cell morphology, secretion of plasminogen activator, transcriptional activation of the primary response genes, Egr-1 and TIS 10, and stimulation of tyrosine phosphorylation. In addition, chicken myoblasts (infected with RSVtsUP1) showed a temperature-dependent differentiation into myotubes. Thus, this cloned src gene should be ideally suited for inducing reversible transformation and differentiation of mammalian cells in culture.

pp60src tyrosine kinase modulates P19 embryonal carcinoma cell fate by inhibiting neuronal but not epithelial differentiation.

Abstract: P19 embryonal carcinoma cells provide an in vitro model system to analyze the events involved in neural differentiation. These multipotential stem cells can be induced by retinoic acid (RA) to differentiate into neural cells. We have investigated the ability of several variant forms of the protein-tyrosine kinase (PTK) pp60src to modulate cell fate determination in this system. Normally, P19 cells are induced to differentiate along a neural lineage when allowed to form extensive cell-cell contacts in large multicellular aggregates during exposure to RA. Through analysis of markers of epithelial (keratin and desmosomal proteins) and neuronal (neurofilament) cells we have found that RA-induced P19 cells transiently express epithelial markers before neuronal differentiation. Under these inductive conditions, expression of pp60v-src or expression of the neuronal variant pp60c-src+ inhibited neuronal differentiation, and resulted in maintained expression of an epithelial phenotype. Morphological analysis showed that expression of pp60src PTKs results in decreased cell-cell adhesion during the critical cell aggregation stage of the neural differentiation procedure. The effects of pp60v-src on cell fate and cell-cell adhesion could be mimicked by direct modulation of Ca+(+)-dependent cell-cell contact during RA induction of normal P19 cells. We conclude that the neural lineage of P19 cells includes an early epithelial intermediate and suggest that tyrosine phosphorylation can modulate cell fate determination during an early cell-cell adhesion-dependent event in neurogenesis.