RECOVERIN HAS S-MODULIN ACTIVITY IN FROG RODS

In vertebrate photoreceptors, light induces hydrolysis of cGMP by acti vating cGMP phosphodiesterase (PDE), which results in closure of the c GMP-activated cation channel. During light adaptation, the cytoplasmic Ca2+ concentration decreases, and this decrease is one of the underly ing mechanisms of light adaptation. Sensitivity-modulating protein (S- modulin) is a Ca2+-binding protein involved in light adaptation in fro g rods; it regulates both the light sensitivity of PDE and the lifetim e of activated PDE by controlling rhodopsin phosphorylation in a Ca2+- dependent manner. Recoverin has been reported as a Ca2+-dependent regu lator of guanylate cyclase in bovine rods (Dizhoor, A. M., Ray, S., Ku mar, S., Niemi, G., Spencer, M., Brolley, D., Walsh, K. A., Philipov, P. P., Hurley, J. B., and Stryer, L. (1991) Science 251, 915-918). Her e, we show that recoverin has similar activity as S-modulin, and the a mino acid sequences of both proteins are similar. The results strongly suggest that recoverin is bovine S-modulin and regulates PDE activati on.