INTEGRIN-MEDIATED SIGNAL-TRANSDUCTION LINKED TO RAS PATHWAY BY GRB2 BINDING TO FOCAL ADHESION KINASE

THE cytoplasmic focal adhesion protein-tyrosine kinase (FAK) localizes with surface integrin receptors at sites where cells attach to the ex tracellular matrix. Increased FAK tyrosine phosphorylation occurs upon integrin engagement with fibronectin. Here we show that adhesion of m urine NIH3T3 fibroblasts to fibronectin promotes SH2-domain-mediated a ssociation of the GRB2 adaptor protein and the c-Src protein-tyrosine kinase (PTK) with FAK in vivo, and also results in activation of mitog en-activated protein kinase (MAPK). In v-Src-transformed NIH3T3, the a ssociation of v-Src, GRB2 and Sos with FAK is independent of cell adhe sion to fibronectin. The GRB2 SH2 domain binds directly to tyrosine-ph osphorylated FAK. Mutation of tyrosine residue 925 of FAK (YENV motif) to phenylalanine blocks GRB2 SH2-domain binding to PAK in vitro. Our results show that fibronectin binding to integrins on NIH3T3 fibroblas ts promotes c-Src and FAK association and formation of an integrin-act ivated signalling complex. Phosphorylation of FAK at Tyr 925 upon fibr onectin stimulation creates an SH2-binding site for GRB2 which may lin k integrin engagement to the activation of the Ras/MAPK signal transdu ction pathway.