title: Evidence for in vivo phosphorylation of the Grb2 SH2-domain binding site on focal adhesion kinase by Src-family protein-tyrosine kinases.
abstract: Focal adhesion kinase (FAK) is a nonreceptor protein-tyrosine kinase (PTK) that associates with integrin receptors and participates in extracellular matrix-mediated signal transduction events. We showed previously that the c-Src nonreceptor PTK and the Grb2 SH2/SH3 adaptor protein bound directly to FAK after fibronectin stimulation (D. D. Schlaepfer, S.K. Hanks, T. Hunter, and P. van der Geer, Nature [London] 372:786-791, 1994). Here, we present evidence that c-Src association with FAK is required for Grb2 binding to FAK. Using a tryptic phosphopeptide mapping approach, the in vivo phosphorylation of the Grb2 binding site on FAK (Tyr-925) was detected after fibronectin stimulation of NIH 3T3 cells and was constitutively phosphorylated in v-Src-transformed NIH 3T3 cells. In vitro, c-Src phosphorylated FAK Tyr-925 in a glutathione S-transferase-FAK C-terminal domain fusion protein, whereas FAK did not. Using epitope-tagged FAK constructs, transiently expressed in human 293 cells, we determined the effect of site-directed mutations on c-Src and Grb2 binding to FAK. Mutation of FAK Tyr-925 disrupted Grb2 binding, whereas mutation of the c-Src binding site on FAK (Tyr-397) disrupted both c-Src and Grb2 binding to FAK in vivo. These results support a model whereby Src-family PTKs are recruited to FAK and focal adhesions following integrin-induced autophosphorylation and exposure of FAK Tyr-397. Src-family binding and phosphorylation of FAK at Tyr-925 creates a Grb2 SH2-domain binding site and provides a link to the activation of the Ras signal transduction pathway. In Src-transformed cells, this pathway may be constitutively activated as a result of FAK Tyr-925 phosphorylation in the absence of integrin stimulation.
authors.affiliation: Molecular Biology and Virology Laboratory, Salk Institute for Biological Studies, La Jolla, California 92037, USA.
authors.forename: D D
authors.initials: DD
authors.lastname: Schlaepfer
authors.forename: T
authors.initials: T
authors.lastname: Hunter
mesh_terms: D016475:3T3 Cells; D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002448:Cell Adhesion; D015815:Cell Adhesion Molecules / Q000378:metabolism*; D002460:Cell Line; D066246:ErbB Receptors / Q000378:metabolism; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D051380:GRB2 Adaptor Protein; D005982:Glutathione Transferase; D006801:Humans; D016023:Integrins / Q000502:physiology; D051379:Mice; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D010748:Phosphopeptides / Q000737:chemistry / Q000302:isolation & purification; D010766:Phosphorylation; D011107:Polylysine; D016133:Polymerase Chain Reaction; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014443:Tyrosine; D018909:src Homology Domains; D019061:src-Family Kinases / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D015815:Cell Adhesion Molecules; D051380:GRB2 Adaptor Protein; C495948:GRB2 protein, human; C495949:Grb2 protein, mouse; D016023:Integrins; D010748:Phosphopeptides; D011506:Proteins; D011993:Recombinant Fusion Proteins; D011107:Polylysine; D014443:Tyrosine; D005982:Glutathione Transferase; D066246:ErbB Receptors; D011505:Protein-Tyrosine Kinases; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; C496050:PTK2 protein, human; C496051:Ptk2 protein, mouse; D019061:src-Family Kinases