THE RAS-RASGAP COMPLEX - STRUCTURAL BASIS FOR GTPASE ACTIVATION AND ITS LOSS IN ONCOGENIC RAS MUTANTS

The three-dimensional structure of the complex between human H-Ras bou nd to guanosine diphosphate and the guanosine triphosphatase (GTPase)- activating domain of the human GTPase-activating protein p120(GAP) (GA P-334) in the presence of aluminum fluoride was solved at a resolution of 2.5 angstroms, The structure shows the partly hydrophilic and part ly hydrophobic nature of the communication between the two molecules, which explains the sensitivity of the interaction toward both sails an d lipids, An arginine side chain (arginine-789) of GAP-334 is supplied into the active site of Pas to neutralize developing charges in the t ransition state, The switch II region of Pas is stabilized by GAP-334, thus allowing glutamine-61 of Pas, mutation of which activates the on cogenic potential, to participate in catalysis, The structural arrange ment in the active site is consistent with a mostly associative mechan ism of phosphoryl transfer and provides an explanation for the activat ion of Pas by glycine-12 and glutamine-61 mutations, Glycine-12 in the transition state mimic is within van der Waals distance of both argin ine-789 of GAP-334 and glutamine-61 of Pas, and even its mutation to a lanine would disturb the arrangements of residues in the transition st ate.