Light-regulated biochemical events in invertebrate photoreceptors. 2. Light-regulated phosphorylation of rhodopsin and phosphoinositides in squid photoreceptor membranes.

Phosphorylation of squid photoreceptor membrane components by Mg-[gamma-32P]ATP is regulated by light. Illumination of squid photoreceptors (Loligo opalescens or Loligo pealei) resulted in phosphorylation of rhodopsin and a 55 000-dalton protein. Rhodopsin phosphorylation was increased 15-20-fold by light, to an average of 0.9-1.8 phosphates/metarhodopsin. The linear dependence of rhodopsin phosphorylation on photoconversion of rhodopsin to metarhodopsin suggests that metarhodopsin is a light-activated substrate for phosphorylation. Phospholipids also were phosphorylated by [gamma-32P]ATP. In the dark, 32P was incorporated into phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidic acid. Illuminated membranes showed increased 32P incorporation into phosphatidic acid and decreased incorporation into the phosphorylated phosphoinositides. These results suggest, for the first time, the participation of a light-activated phospholipase C in squid photoreceptors.