Structural Coupling between the Rho-Insert Domain of Cdc42 and the Geranylgeranyl Binding Site of RhoGDI

The small GTPase proteins are components of the intracellular signaling system, alternating between active (membrane-bound and GTP-loaded) and inactive (GDP-loaded and cytosolic) states. In the inactive state, the proteins are soluble in the cytoplasm. To compensate for the energetic penalty of extraction of the hydrophobic moiety from the membrane phase, the inactive state is stabilized via formation of a complex with the RhoGDI proteins that provide a hydrophobic pocket for the binding of the hydrophobic moieties. The signals delivered by the Rho subfamily involve a specific, short, highly exposed α-helix (Rho-insert), located close to the GDP binding site. Upon simulating the complex in solution, we observed that the Rho-insert domain of Cdc42 can assume two basic orientations. One is the canonical one, as detected in both crystals and NMR spectra of concentrated protein solutions. The second orientation appears only in the RhoGDI–Cdc42 complex where the GER moiety of Cdc42 is properly inserted into the specific binding site of RhoGDI. Any impairment of the GER–RhoGDI interactions, such as insertion of specific mutations in the hydrophobic binding site, abolished the coupling between the proteins and the Rho-insert domain, preserving its canonical orientation as in the crystalline structure. The noncanonical conformation of the Rho-insert domain is not a simulation artifact, as it appears in crystals of plant Rho proteins (ROP4, ROP5, and ROP7). In accord with the notion that the Rho-insert domain participates in downstream signaling, we propose that the deformation of the Rho-insert is part of the signal transmissions.