Studies on the structure and mechanism of H-ras p21

Current knowledge of the structure of H-ras p21 is reviewed with particular emphasis on the interaction between guanine nucleotides and the active site of the protein. The nature of the conformational change induced by GTP hydrolysis is discussed. The major change is seen in the region known as the effector loop (loop 2), with significant but less well-defined changes occurring in loop 4, which is implicated in the GTPase reaction. Other evidence concerning the mechanism of GTP hydrolysis and its activation by GAP (GTPase-activating protein) is also discussed. Evidence regarding the rate limiting step in the p21 GTPase reaction, and the manner in which this and possibly other steps are accelerated by GAP, is inconclusive.