SUNY at Albany
June 19-23, 2001
Modulation and Control of GTPase activity of heterotrimeric G(alpha) subunits
Heterotrimeric G(alpha) subunits are heptahelical receptor-regulated modulators of a variety of intracellular enzymes and ion channels that are collectively referred to as effectors. Like other members of the Ras superfamily, G(alpha) are guanine nucleotide binding proteins. G(alpha) isoforms are typically effector-specific: thus Gs(alpha) stimulates all isoforms of adenylyl cyclase, whereas Gi(alpha) inhibits Gs(alpha)-stimulated adenylyl cyclase activity; G13(alpha) activates a Rho guanine nucleotide exchange factor, thus serving to couple heterotrimeric and Rho stumilated signal transduction. G(alpha):GTP binds more tightly to effectors than doesG(alpha):GDP, whereas the latter binds stongly to G(beta):G(gamma) heterdimers. The kinetics of G(alpha)-mediated signaling is governed by two events: the rate at which G(alpha) hydrolyzes GTP, which leads to G(beta):G(gamma) inhibition of effector modulation, and the rate at which GDP is released from the active site of G(alpha) and replaced with GTP. The latter is stimulated by heptahelical receptors upon ligand stimulation; the former is governed by the inherent catalytic activity of G(alpha) and, for some isoforms of G(alpha), by GTPase activating proteins known as Regulators of G protein Signalling, or RGS. The intrinsic rate of G(alpha)-catalyzed GTP hydrolysis is only about 2-5 min(-1), allowing a substantial period of effector control. RGS activation increases this rate one-hundred-fold, and rapidly terminates signaling through G(alpha). Here we describe structural and mutagenic approaches to reveal the structural origin of the high kinetic barrier to GTP hydrolysis, and the mechanism by which it is relieved by RGS proteins. The recently-determined structure of the RGS domain of p115-rhoGEF leads to the inference that there are variations in the basic mechanism used by different members of the RGS family to recognize and stimulate the GTPase activity of their G(alpha) substrates.
J. Chen (1), Y. Wang (2), D. Fancy (1), X. Du (2) and S.R. Sprang (1,2)
Department of Biochemistry (1) , Howard Hughes Medical Institute (2), University of Texas Southwestern Medical Center at Dallas 5323 Harry Hines Blvd. Dallas, TX 75390-9050.