title: Functional homology between signal-coupling proteins. Cholera toxin inactivates the GTPase activity of transducin.
abstract: Both the light-stimulated cGMP phosphodiesterase of retinal rod outer segments (ROS) and hormone-stimulated adenylate cyclase are regulated by guanine nucleotide-binding regulatory proteins (N). Transducin serves as the signal-carrying regulatory protein in ROS, and the N protein (also called G or G/F) performs this role in the adenylate cyclase system. The GTP form of these regulatory proteins activates the corresponding enzyme, whereas the GDP form does not. Both transducin and the N protein possess a GTPase activity that restores the regulatory protein to the unstimulated state. Cholera enterotoxin catalyzes the transfer of ADP-ribose from NAD+ to the N protein, which inhibits its GTPase activity and activates adenylate cyclase. We report here that the toxin also catalyzes ADP-ribosylation of the alpha-subunit of transducin in ROS membranes. This modification of the guanine nucleotide-binding subunit of transducin is markedly enhanced by the bleaching of rhodopsin and by the addition of guanosine-5'-(beta, gamma-imino)triphosphate. In contrast, GDP, GTP, and guanosine-5'-(3-O)thiotriphosphate inhibit the reaction, while GMP and ATP have no effect. Under optimal conditions, toxin catalyzes labeling of 0.7 mol of the alpha-subunit of transducin/mol of bound [3H]guanosine-5'-(beta, gamma-imido)triphosphate and causes 70% inhibition of the light-dependent GTPase activity of transducin in ROS. These results indicate close functional homology between transducin of ROS and the N protein of adenylate cyclase.
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D002772:Cholera Toxin / Q000494:pharmacology*; D020558:GTP Phosphohydrolases / Q000037:antagonists & inhibitors*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D008565:Membrane Proteins / Q000037:antagonists & inhibitors*; D009243:NAD / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors*; D010786:Photoreceptor Cells / Q000201:enzymology*; D012374:Rod Cell Outer Segment / Q000201:enzymology*; D015293:Transducin
chemical_list: D008565:Membrane Proteins; D009243:NAD; D000246:Adenosine Diphosphate Ribose; D006165:Guanylyl Imidodiphosphate; D002772:Cholera Toxin; D010744:Phosphoric Monoester Hydrolases; D020558:GTP Phosphohydrolases; D015293:Transducin