Stimulation of phospholipase A2 activity in bovine rod outer segments by the beta gamma subunits of transducin and its inhibition by the alpha subunit.

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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 trnasducin-mediated increase in cGMP phosphodiesterase activity, where activation occurs by action of the .alpha. subunit. The .alpha. subunits, which itself stimulated phospholipase A2 activity, inhibited the .beta..gamma.-induced stimulation of phospholipase A2. This inhibition appears to be the result 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 subunits is, therefore, a mode of action for these subunits in signal transduction.