Control of light-activated phosphorylation in frog photoreceptor membranes

Abstract

Some factors which regulate the efficiency of light in activating rhodopsin phosphorylation were examined. Phosphate incorporation was measured after illumination in suspensions of bullfrog rod outer segments incubated with [.gamma.-32P]ATP. Delaying ATP addition after illumination caused maximum phosphate incorporation to decrease 80% within 2 h. This decay occurred in urea-treated, extracted rod outer segment membranes. The decay of the light effect was not influenced by regeneration of opsin to rhodopsin or the presence of long-lived photoproducts. Regeneration of opsin increased the amount of phosphorylation initiated by a 2nd exposure to light. Further phosphorylation could also occur after phosphate groups were removed from the membranes by dephosphorylation. Small amounts of light (bleaching less than 5% of the rhodopsin present) were more effective, by 10-fold, in initiating phosphorylation than were larger amounts.