Molecular mechanism of S-modulin action: binding target and effect of ATP

S-Modulin is suggested to increase the light sensitivity of rods by inhibiting phosphorylation of light-activated rhodopsin (Rh*) at high Ca2+ concentrations. The inhibition of rhodopsin phosphorylation was almost constant over a wide range of the Rh*/S-modulin ratio (10(-4)-approximately 10[1]). A 125I-labeled cross-linker that had been conjugated with S-modulin interacted with a protein of 60 kDa, a molecular mass close to that of frog rhodopsin kinase. These results suggested that the target molecule of S-modulin is rhodopsin kinase. To investigate the mechanism of the S-modulin action, we measured rhodopsin phosphorylation in the presence and absence of inhibition by S-modulin at various timings of ATP addition. The results suggested the following in situ mechanism of S-modulin action. After light-activation of rhodopsin kinase, the S-modulin/Ca2+ complex binds to the activated kinase and inhibits the phosphorylation of rhodopsin. The complex, however, does not affect the overall kinetics of the phosphorylation. The inhibition of the kinase by S-modulin is reversible in terms of the Ca2+ concentration. On the other hand, the kinase activity decreases as a function of time, probably via autophosphorylation.