Involvement of protein kinase C in the phosphorylation of rhodopsin

The possibility that protein kinase C is involved in phototransduction by phosphorylating rhodopsin was explored in situ and in vitro. Pretreatment of intact retinas with phorbol myristate acetate markedly increased the light-dependent phosphorylation of rhodopsin, with the greatest effects observed at lower light levels. Phorbol myristate acetate treatment did not affect rhodopsin phosphorylation in retinas not exposed to light, suggesting that protein kinase C modulates the phosphorylation state of rhodopsin in a light-dependent manner. Limited proteolysis of rhodopsin phosphorylated in situ indicates that protein kinase C modifies rhodopsin on a domain distinct from that recognized by rhodopsin kinase. In vitro, protein kinase C purified from bovine retinas phosphorylated unbleached and bleached rhodopsin. Our results are consistent with protein kinase C phosphorylating unbleached rhodopsin in response to low light, suggesting that protein kinase C plays a role in light adaptation.