Activation of rhodopsin kinase.

The present study confirms our original assertion that peptides corresponding to the C-terminal sequence of rhodopsin are phosphorylated by rhodopsin kinase (RK), but only in the presence of photo-activated rhodopsin [Rho*, which is functionally equivalent to metarhodopsin II (Meta II)]. Under optimized conditions, the extent of peptide phosphorylation reached up to 60% that of Rho*. Rho* phosphorylation began to plateau within 15 min of the initiation of photolysis, whereas the peptide phosphorylation continued linearly for >60 min. This lack of co-ordination in the phosphorylation of the physiological (Rho*) and synthetic (peptide) substrates necessitated the present study, which showed that RK is activated for peptide phosphorylation not only by Meta II but also by Meta III, as well as by the phosphorylated derivatives of these species, but not by opsin. These results led to the conclusion that all the derivatives of opsin, which contain the Schiff base linkage with the all-trans-retinylidene moiety, retain the ability to activate RK.