PHOSPHATIDYLINOSITOL-STIMULATED PHOSPHORYLATION OF AN INHIBITORY SUBUNIT OF CGMP PHOSPHODIESTERASE IN VERTEBRATE ROD PHOTORECEPTORS

An inhibitory subunit (P-gamma) of cGMP phosphodiesterase from vertebrate rod photoreceptors (frog, toad, and bovine) was phosphorylated by cytosolic protein kinase(s) derived from intact frog rod outer segments. The phosphorylation of frog P-gamma was stimulated by phosphatidylinositol but not by cAMP or cGMP. One- and two-dimensional gel electrophoresis revealed that 70-80% of P-gamma was phosphorylated with 1 mol of phosphate per frog P-gamma under optimal conditions. A peptide that derived from an active domain of bovine P-gamma was also phosphorylated. Phosphorylation of frog P-gamma was inhibited by addition of the peptide to the reaction mixture. Phosphorylation of frog P-gamma was also inhibited by addition of transducin subunits or active (P-gamma-less) cGMP phosphodiesterase. Okadaic acid, on the other hand, enhanced P-gamma phosphorylation, suggesting the presence of protein phosphatase(s) in the cytosolic fraction. These data suggest another mechanism for the regulation of cGMP phosphodiesterase in vertebrate rod photoreceptors.