Complex formation with focal adhesion kinase: A mechanism to regulate activity and subcellular localization of Src kinases

Tyrosine phosphorylation of focal adhesion kinase (FAK) creates a high-affi nity binding site for the src homology 2 domain of the Src family of tyrosi ne kinases. Assembly of a complex between FAK and Src kinases may serve to regulate the subcellular localization and the enzymatic activity of members of the Src family of kinases. We show that simultaneous overexpression of FAK and pp60(c-src) or p59(fyn) results in the enhancement of the tyrosine phosphorylation of a limited number of cellular substrates, including paxil lin. Under these conditions, tyrosine phosphorylation of paxillin is largel y cell adhesion dependent. FAK mutants defective for Src binding or focal a dhesion targeting fail to cooperate with pp60(c-src) or p59(fyn) to induce paxillin phosphorylation, whereas catalytically defective FAK mutants can d irect paxillin phosphorylation. The negative regulatory site of pp60(c-src) is hypophosphorylated when in complex with FAK, and coexpression with FAK leads to a redistribution of pp60(c-src) from a diffuse cellular location t o focal adhesions. A FAK mutant defective for Src binding does not effectiv ely induce the translocation of pp60(c-src) to focal adhesions. These resul ts suggest that association with FAK can alter the localization of Src kina ses and that FAK functions to direct phosphorylation of cellular substrates by recruitment of Src kinases.