title: Mechanism of RGS4, a GTPase-activating protein for G protein alpha subunits.
abstract: GTP hydrolysis by guanine nucleotide-binding proteins, an essential step in many biological processes, is stimulated by GTPase-activating proteins (GAPs). The mechanisms whereby GAPs stimulate GTP hydrolysis are unknown. We have used mutational, biochemical, and structural data to investigate how RGS4, a GAP for heterotrimeric G protein alpha subunits, stimulates GTP hydrolysis. Many of the residues of RGS4 that interact with Gi alpha 1 are important for GAP activity. Furthermore, optimal GAP activity appears to require the additive effects of interactions along the RGS4-G alpha interface. GAP-defective RGS4 mutants invariably were defective in binding G alpha subunits in their transition state; furthermore, the apparent strengths of GAP and binding defects were correlated. Thus, none of these residues of RGS4, including asparagine 128, the only residue positioned at the active site of Gi alpha 1, is required exclusively for catalyzing GTP hydrolysis. These results and structural data (Tesmer, J. G. G., Berman, D. M., Gilman, A. G., and Sprang, S. R. (1997) Cell 89, 251-261) indicate that RGS4 stimulates GTP hydrolysis primarily by stabilizing the transition state conformation of the switch regions of the G protein, favoring the transition state of the reactants. Therefore, although monomeric and heterotrimeric G proteins are related, their GAPs have evolved distinct mechanisms of action.
authors.affiliation: Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: S P
authors.initials: SP
authors.lastname: Srinivasa
authors.forename: N
authors.initials: N
authors.lastname: Watson
authors.forename: M C
authors.initials: MC
authors.lastname: Overton
authors.forename: K J
authors.initials: KJ
authors.lastname: Blumer
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001216:Asparagine / Q000378:metabolism; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors*; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011485:Protein Binding; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D020710:RGS Proteins; D051381:Rats; D012441:Saccharomyces cerevisiae; D013329:Structure-Activity Relationship
chemical_list: D011506:Proteins; D020710:RGS Proteins; C098799:RGS4 protein; D001216:Asparagine; D006160:Guanosine Triphosphate; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D020558:GTP Phosphohydrolases; D019206:GTP-Binding Protein alpha Subunits, Gi-Go