title: Regulation of deactivation of photoreceptor G protein by its target enzyme and cGMP.
abstract: The photoreceptor G protein, transducin, is one of the class of heterotrimeric G proteins that mediates between membrane receptors and intracellular enzymes or ion channels. Light-activated rhodopsin catalyses the exchange of GDP for GTP on multiple transducin molecules. Activated transducin then stimulates cyclic GMP phosphodiesterase by releasing an inhibitory action of the phosphodiesterase gamma-subunits. This leads to a decrease in cGMP levels in the rod, and closure of plasma membrane cationic channels gated by cGMP. In this and other systems, turn-off of the response requires the GTP bound to G protein to be hydrolysed by an intrinsic GTPase activity. Here we report that the interaction of transducin with cGMP phosphodiesterase, specifically with its gamma-subunits, accelerates GTPase activity by several fold. Thus the gamma-subunits of the phosphodiesterase serve a function analogous to the GTPase-activating proteins that regulate the class of small GTP-binding proteins. The acceleration can be partially suppressed by cGMP, most probably through the non-catalytic cGMP-binding sites of phosphodiesterase alpha and beta-subunits. This cGMP regulation may function in light-adaptation of the photo-response as a negative feedback that decreases the lifetime of activated cGMP phosphodiesterase as light causes decreases in cytoplasmic cGMP.
authors.affiliation: Laboratory of Molecular Biology, University of Wisconsin, Madison 53706.
authors.lastname: Arshavsky VYu
authors.forename: M D
authors.initials: MD
authors.lastname: Bownds
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000818:Animals; D001001:Anura; D002417:Cattle; D006152:Cyclic GMP / Q000494:pharmacology*; D004789:Enzyme Activation; D005246:Feedback; D020558:GTP Phosphohydrolases / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D008027:Light; D010786:Photoreceptor Cells / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000378:metabolism; D015293:Transducin / Q000378:metabolism*
chemical_list: D011994:Recombinant Proteins; D006160:Guanosine Triphosphate; D012243:Rhodopsin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D020558:GTP Phosphohydrolases; D015293:Transducin; D006152:Cyclic GMP