Coordination of cell polarization and migration by the Rho family GTPases requires Src tyrosine kinase activity.

The ability of a cell to polarize and move is governed by remodeling of the cellular adhesion/cytoskeletal network that is in turn controlled by the Rho family of small GTPases. However, it is not known what signals lie downstream of Rac1 and Cdc42 during peripheral actin and adhesion remodeling that is required for directional migration. We show here that individual members of the Rho family, RhoA, Rac1, and Cdc42, direct the specific intracellular targeting of c-Src tyrosine kinase to focal adhesions, lamellipodia, or filopodia, respectively, and that the adaptor function of c-Src (the combined SH3/SH2 domains coupled to green fluorescent protein) is sufficient for targeting. Furthermore, Src's catalytic activity is absolutely required at these peripheral cell-matrix attachment sites for remodeling that converts RhoA-dependent focal adhesions into smaller focal complexes along Rac1-induced lamellipodia (or Cdc42-induced filopodia). Consequently, cells in which kinase-deficient c-Src occupies peripheral adhesion sites exhibit impaired polarization toward migratory stimuli and reduced motility. Furthermore, phosphorylation of FAK, an Src adhesion substrate, is suppressed under these conditions. Our findings demonstrate that individual Rho GTPases specify Src's exact peripheral localization and that Rac1- and Cdc42-induced adhesion remodeling and directed cell migration require Src activity at peripheral adhesion sites.