The nucleotide exchange rates of rho and rac small GTP-binding proteins are enhanced to different extents by their regulatory protein Smg GDS.

Rho p21 and rac p21 small GTP-binding proteins are regulated by the same inhibitory and stimulatory GDP/GTP exchange proteins termed rho GDI and smg GDS, respectively. RhoA p21 and rac1 p21 bind with similar affinities to rho GDI and both proteins also bind with similar affinities to smg GDS. RhoA p21 and rac1 p21 have similar GDP/GTP exchange rates in the absence of rho GDI and smg GDS. The velocity of the GDP/GTP exchange reaction for rhoA p21 was enhanced much more by smg GDS than was the velocity of nucleotide exchange for rac1 p21. In the presence and absence of smg GDS, rho GDI reduced the velocities of these nucleotide exchange reactions of rhoA p21 and rac1 p21 to similar extents. These results suggest that rhoA p21 is activated first followed by rac1 p21 activation in response to extracellular signals.