Rhodopsin is the major in situ substrate of protein kinase C in rod outer segments of photoreceptors.

Phorbol ester treatment of 32P-labeled retinas results in a light-dependent alteration in the phosphorylation state of rhodopsin. Previously we reported that phorbol myristate acetate causes an increase in the phosphorylation state of rhodopsin in retinas exposed to a brief flash of light, with the greatest increase in phosphorylation observed at lower (< or = 10%) bleach levels (Newton, A. C., and Williams, D. S. (1991) J. Biol. Chem. 266, 17725-17728). Here we show that phorbol myristate acetate causes a decrease in the phosphorylation of rhodopsin after exposure to levels of illumination that result in maximal bleaching of the visual receptor. In contrast, no rhodopsin phosphorylation is detected in control or phorbol ester-treated retinas in the dark. Partial proteolysis revealed that phorbol esters alter the phosphorylation of the carboxyl-terminal domain of rhodopsin. Rhodopsin is the major protein whose phosphorylation state is affected significantly by phorbol esters in situ, although a number of rod outer segment cytosolic and membrane proteins are phosphorylated by protein kinase C in vitro. Our data indicate that a major target of protein kinase C in rod outer segments is rhodopsin.