The SH3 domain directs acto-myosin-dependent targeting of v-Src to focal adhesions via phosphatidylinositol 3-kinase

The v-Src oncoprotein is translocated to integrin-linked focal adhesions, w here its tyrosine kinase activity induces adhesion disruption and cell tran sformation. We previously demonstrated that the intracellular targeting of Src is dependent on the actin cytoskeleton, under the control of the Rho fa mily of small G proteins. However, the assembly of v-Src into focal adhesio ns does not require its catalytic activity or myristylation-dependent membr ane association. Here, we report that the SH3 domain is essential for the a ssembly of focal adhesions containing the oncoprotein by mediating a switch from a microtubule-dependent, perinuclear localization to actin-associated focal adhesions; furthermore, v-Src translocation to focal adhesions requi res myosin activity, at least under normal conditions when the actin cytosk eleton is being dynamically regulated. Although the SH3 domain of v-Src is also necessary for its association with focal adhesion kinase (FAK), which is often considered a likely candidate mediator of focal adhesion targeting via its carboxy-terminal targeting sequence, we show here that binding to FAK is not essential for the targeting of v-Src to focal adhesions, The p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase also associates w ith v-Src in an SH3-dependent manner, but in this case inhibition of PI 3-k inase activity suppressed assembly of focal adhesions containing the oncopr otein. Thus, the Src SH3 domain, which binds PI 3-kinase and which is neces sary for activation of Akt downstream, is required for the actin-dependent targeting of v-Src to focal adhesions.