Phosphorylation of RhoGDI by p21-activated kinase 1.

Rho GTPase activation is partially regulated at the level of guanine nucleotide dissociation inhibitors, or GDIs. The binding of Rho GTPases to GDIs has been shown to dramatically reduce the action of guanine nucleotide exchange factors (GEFs) to initiate Rho GTPase activation. The GDI-GTPase complex thus serves as a major point of regulation of Rho GTPase activity and function. It is likely that specific mechanisms exist to dissociate individual members of the Rho GTPase family from cytosolic Rho GDI complexes to facilitate the activation process. Such dissociation would likely be tightly coupled to GEF-mediated guanine nucleotide exchange and membrane association of the activated GTPase, resulting in effector binding and functional responses. Accumulating evidence suggests that the phosphorylation of either the Rho GTPases themselves and/or phosphorylation of GDIs might serve as a mechanism for regulating the formation and/or dissociation of Rho GTPase-GDI complexes. Indeed, the selective release of Rac1 from RhoGDI complexes induced by the p21-activated kinase-regulated phosphorylation of RhoGDI has been reported. We describe here methods for the analysis of RhoGDI phosphorylation and regulation by p21-activated kinase 1 (Pak1).