Kinetic analysis of the binding of guanine nucleotides to bovine brain rhoB p20, a ras p21-like GTP-binding protein

We have investigated the kinetics of the binding of guanine nucleotides to bovine brain rhoB p20, a ras p21-like GTP-binding protein with GTPase activity. The initial velocities of the binding of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) to GDP-bound rhoB p20 and the dissociation of GDP from this protein were markedly increased by decreasing Mg2+ concentrations. The initial velocity of the binding of GTP gamma S to GDP-free rhoB p20 was not affected by changing Mg2+ concentrations. These results indicate that the dissociation of GDP from rhoB p20 limits the binding of GTP to this protein, and suggest that there is a factor stimulating the dissociation of GDP from rhoB p20 and thereby stimulating the binding of GTP to this protein in mammalian tissues. Consistently, the factor stimulating the dissociation of GDP, but not of GTP gamma S, from rhoB p20 was detected in bovine brain cytosol.