RECOVERIN INHIBITS THE PHOSPHORYLATION OF DARK-ADAPTED RHODOPSIN MORETHAN IT DOES THAT OF BLEACHED RHODOPSIN - A POSSIBLE MECHANISM THROUGH WHICH RHODOPSIN KINASE IS PREVENTED FROM PARTICIPATION IN A SIDE REACTION

In its resting state rhodopsin kinase is present in an inactive form a nd is activated after interaction with light-activated rhodopsin (Rho ). The activated rhodopsin kinase then phosphorylates Rho but is also able to catalyse the phosphorylation of dark-adapted rhodopsin. A con sequence of the latter behaviour of the activated kinase is that at lo w levels of bleach a large number of phosphoryl groups are incorporate d per mol of Rho. Recoverin- and Ca2+-dependent inhibition of rhodops in kinase was found to be inversely related to the extent of bleaching ; the lower the fraction of rhodopsin bleached, the greater the inhibi tion. The IC50 of recoverin is approx. 1 mu M at a 0.2 % level of blea ch and about 5 mu M in a fully bleached sample. The inhibitory effect of recoverin was studied separately on the phosphorylation of rhodopsi n and Rho. The formation of phosphorylated rhodopsin was inhibited 4. 5-fold more strongly than that of phosphorylated Rho. These results a re interpreted to suggest that one of the roles of the recoverin-depen dent regulation of the activity of rhodopsin kinase is to prevent the enzyme from participating in the unwanted phosphorylation of dark-adap ted rhodopsin, directing it to fulfil its 'correct' function of quench ing the transduction activity of Rho.