Amino acid residues of S-modulin responsible for interaction with rhodopsin kinase.

S-modulin in frog or its bovine homologue, recoverin, is a 23-kDa EF-hand Ca(2+)-binding protein found in rod photoreceptors. The Ca(2+)-bound form of S-modulin binds to rhodopsin kinase (Rk) and inhibits its activity. Through this regulation, S-modulin is thought to modulate the light sensitivity of a rod. In the present study, we tried to identify the interaction site of the Ca(2+)-bound form of S-modulin to Rk. First, we mapped roughly the interaction regions by using partial peptides of S-modulin. The result suggested that a specific region near the amino terminus is the interaction site of S-modulin. We then identified the essential amino acid residues in this region by using S-modulin mutant proteins: four amino acid residues (Phe(22), Glu(26), Phe(55), and Thr(92)) were suggested to interact with Rk. These residues are located in a small closed pocket in the Ca(2+)-free, inactive form of S-modulin, but exposed to the surface of the molecule in the Ca(2+)-bound, active form of S-modulin. Two additional amino acid residues (Tyr(108) and Arg(150)) were found to be crucial for the Ca(2+)-dependent conformational changes of S-modulin.