The arginine finger of RasGAP helps Gln-61 align the nucleophilic water inGAP-stimulated hydrolysis of GTP

The Pas family of GTPases is a collection of molecular switches that link r eceptors on the plasma membrane to signaling pathways that regulate cell pr oliferation and differentiation. The accessory GTPase-activating proteins ( GAPs) negatively regulate the cell signaling by increasing the slow intrins ic CTP to GDP hydrolysis rate of Ras. Mutants of Ras are found in 25-30% of human tumors. The most dramatic: property of these mutants is their insens itivity to the negative regulatory action of GAPs. All known oncogenic muta nts of Pas map to a small subset of amino acids. Gln-61 is particularly imp ortant because virtually all mutations of this residue eliminate sensitivit y to GAPs. Despite its obvious importance for carcinogenesis, the role of G ln-61 in the GAP-stimulated GTPase activity of Ras has remained a mystery. Our molecular dynamics simulations of the p21ras-p120GAP-GTP complex sugges t that the local structure around the catalytic region can be different fro m that revealed by the x-ray crystal structure. We find that the carbonyl o xygen on the backbone of the arginine finger supplied in trans by p120GAP ( Arg-789) interacts with a water molecule in the active site that is forming a bridge between the NH2 group of the Gln-61 and the gamma -phosphate of C TP. Thus, Arg-789 may play a dual role in generating the nucleophile as wel l as stabilizing the transition state for P-O bond cleavage.