title: Stimulation of phospholipase A2 activity in bovine rod outer segments by the beta gamma subunits of transducin and its inhibition by the alpha subunit.
abstract: In the rod outer segments (ROS) of bovine retina, light activation of phospholipase A2 has been shown to occur by a transducin-dependent mechanism. In this report, the transducin-mediated stimulation of phospholipase A2 is shown to require dissociation of the alpha beta gamma heterotrimer. Addition of transducin to dark-adapted transducin-poor ROS stimulated phospholipase A2 activity only with coincident exposure to white light or, in the dark, with addition of the hydrolysis-resistant GTP analog, guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S]). Both light and GTP[gamma-S] induced dissociation of the transducin subunits and led to severalfold increases in the phospholipase A2 activity of transducin-rich, but not transducin-poor, ROS. In contrast, pertussis toxin treatment of transducin, which stabilizes the associated state of this G protein, prevented the stimulation of phospholipase A2 by exogenous transducin in the presence of light. Addition of purified transducin subunits to dark-adapted transducin-poor ROS revealed that phospholipase A2 stimulation occurred by action of the beta gamma subunits. This is in contrast to the transducin-mediated increase in cGMP phosphodiesterase activity, where activation occurs by action of the alpha subunit. The alpha subunit, which itself slightly stimulated phospholipase A2 activity, inhibited the beta gamma-induced stimulation of phospholipase A2. This inhibition appears to be the result of subunit reassociation since addition of GTP[gamma-S] abolished the inhibitory effect of the alpha subunit on the beta gamma-induced increase in phospholipase A2, while pertussis toxin treatment of the subunits further inhibited phospholipase A2 activity. Modulation of phospholipase A2 activity by the transducin subunit is, therefore, a mode of action for these subunits in signal transduction.
mesh_terms: D000818:Animals; D002417:Cattle; D004789:Enzyme Activation; D019204:GTP-Binding Proteins / Q000502:physiology*; D007700:Kinetics; D008027:Light; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000302:isolation & purification / Q000502:physiology*; D010740:Phospholipases / Q000378:metabolism*; D010741:Phospholipases A / Q000037:antagonists & inhibitors / Q000378:metabolism*; D054467:Phospholipases A2; D010786:Photoreceptor Cells / Q000201:enzymology*; D012374:Rod Cell Outer Segment / Q000201:enzymology*; D015293:Transducin
chemical_list: D046911:Macromolecular Substances; D008565:Membrane Proteins; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; D019204:GTP-Binding Proteins; D015293:Transducin