New insights into how the Rho guanine nucleotide dissociation inhibitor regulates the interaction of Cdc42 with membranes.

The subcellular localization of the Rho family GTPases is of fundamental importance to their proper functioning in cells. The Rho guanine nucleotide dissociation inhibitor (RhoGDI) plays a key regulatory role by influencing the cellular localization of Rho GTPases and is essential for the transforming activity of oncogenic forms of Cdc42. However, the mechanism by which RhoGDI helps Cdc42 to undergo the transition between a membrane-associated protein and a soluble (cytosolic) species has been poorly understood. Here, we examine how RhoGDI influences the binding of Cdc42 to lipid bilayers. Despite having similar affinities for the signaling-inactive (GDP-bound) and signaling-active (GTP-bound) forms of Cdc42 in solution, we show that when RhoGDI interacts with Cdc42 along the membrane surface, it has a much higher affinity for GDP-bound Cdc42 compared with its GTP-bound counterpart. Interestingly, the rate for the dissociation of Cdc42.RhoGDI complexes from membranes is unaffected by the nucleotide-bound state of Cdc42. Moreover, the membrane release of Cdc42.RhoGDI complexes occurs at a similar rate as the release of Cdc42 alone, with the major effect of RhoGDI being to impede the re-association of Cdc42 with membranes. These findings lead us to propose a new model for how RhoGDI influences the ability of Cdc42 to move between membranes and the cytosol, which highlights the role of the membrane in helping RhoGDI to distinguish between the GDP- and GTP-bound forms of Cdc42 and holds important implications for how it functions as a key regulator of the cellular localization and signaling activities of this GTPase.