G-ALPHA(12) AND G-ALPHA(13) STIMULATE RHO-DEPENDENT STRESS FIBER FORMATION AND FOCAL ADHESION ASSEMBLY

Rho, a member of the Ras superfamily of GTP-binding proteins, regulates actin polymerization resulting in the formation of stress fibers and the assembly of focal adhesions. In Swiss 3T3 cells, heterotrimeric G protein-coupled receptors for lysophosphatidic acid and gastrin releasing peptide stimulate Rho-dependent stress fiber and focal adhesion formation. The specific heterotrimeric G protein subunits mediating Rho-dependent stress fiber and focal adhesion formation have not been defined previously. We have expressed GTPase-deficient, constitutively activated G protein alpha subunits and mixtures of beta and gamma subunits in Swiss 3T3 cells. Measurement of actin polymerization and focal adhesion formation indicated that GTPase-deficient alpha(12) and alpha(13), but not the activated forms of a(i2) or a(q) stimulated stress fiber and focal adhesion assembly, Combinations of beta and gamma subunits were unable to stimulate stress fiber or focal adhesion formation. G alpha(12)- and alpha(13)-mediated stress fiber and focal adhesion assembly was inhibited by botulinum C3 exoenzyme, which ADP-ribosylates and inactivates Rho, indicating that a(12) and alpha(13), but not other G protein alpha subunits or beta gamma complexes, regulate Rho-dependent responses. The results define the integration of G(12) and G(13) with the regulation of the actin cytoskeleton.