Suppression of GTP/Tα-dependent activation of cGMP phosphodiesterase by ADP-ribosylation by its γ subunit in amphibian rod photoreceptor membranes

Our previous study has shown that P gamma, the regulatory subunit of cGMP phosphodiesterase (PDE), is ADP-ribosylated by endogenous ADP-ribosyltransferase when P gamma is free or complexed with the catalytic subunits of PDE in amphibian rod photoreceptor membranes. The P gamma domain containing ADP-ribosylated arginines was shown to be involved in its interaction with T alpha, a key interaction for PDE activation, In this study, we describe a possible function of the P gamma ADP-ribosylation in the GTP/T alpha-dependent PDE activation. When rod membranes were preincubated with or without NAD and washed with a buffer containing GTP, the PDE activity of NAD-preincubated membranes was increased by the GTP-washing only to similar to 50% of that of membranes preincubated without NAD. The P gamma release by the GTP-washing from these NAD-preincubated membranes was also suppressed to similar to 50% of that preincubated without NAD. Taking into consideration that similar to 50% of P gamma is ADP-ribosylated under these conditions, these observations suggest that the ADP-ribosylated P gamma, cannot interact with GTP/T alpha. We have also shown that a soluble fraction of ROS contains an enzyme(s) to release the radioactivity of [P-32] ADP-ribosylated P gamma in concentration- and time-dependent manners, suggesting that the P gamma ADP-ribosylation is reversible. Rod ADP-ribosyltransferase solubilized from membranes by phosphatidylinositol-specific phospholipase C was separated into two fractions by ion-exchange columns. Biochemical characterization of these two fractions, including measurement of the K-m for NAD and P gamma, estimation of their molecular masses, ADP-ribosylation of P gamma arginine mutants, effects of ADP-ribosyltransferase inhibitors on the P gamma ADP-ribosylation, and effects of salts and pH on the P gamma ADP-ribosylation, indicates that rod ADP-ribosyltransferase contains two isozymes, and that these two isozymes have similar properties for the P gamma ADP-ribosylation. Our observations strongly suggest that the negative regulation of PDE through the reversible P gamma ADP-ribosylation may function in the phototransduction mechanism.