v-Src Induces Tyrosine Phosphorylation of Focal Adhesion Kinase Independently of Tyrosine 397 and Formation of a Complex with Src*

The non-receptor tyrosine kinase FAK plays a key role at sites of cellular adhesion. It is subject to regulatory tyrosine phosphorylation in response to a variety of stimuli, including integrin engagement after attachment to extracellular matrix, oncogene activation, and growth factor stimulation. Here we use an antibody that specifically recognizes the phosphorylated form of the putative FAK autophosphorylation site, Tyr³⁹⁷. We demonstrate that FAK phosphorylation induced by integrins during focal adhesion assembly differs from that induced by activation of a temperature-sensitive v-Src, which is associated with focal adhesion turnover and transformation. Specifically, although v-Src induces tyrosine phosphorylation of FAK, there is no detectable phosphorylation of Tyr³⁹⁷. Moreover, activation of v-Src results in a net decrease in fibronectin-stimulated phosphorylation of Tyr³⁹⁷, suggesting possible antagonism between v-Src and integrin-induced phosphorylation. Our mutational analysis further indicates that the binding of v-Src to Tyr³⁹⁷ of FAK in its phosphorylated form, which is normally mediated, at least in part, by the SH2 domain of Src, is not essential for v-Src-induced cell transformation. We conclude that different stimuli can induce phosphorylation of FAK on distinct tyrosine residues, linking specific phosphorylation events to ensuing biological responses.