PHOTORECEPTOR PROTEIN S26, A CONE HOMOLOG OF S-MODULIN IN FROG RETINA

A frog retinal protein named s26 is a 26-kDa protein found during puri fication of S-modulin in frog retina (Kawamura, S. (1992) Photochem. P hotobiol. 56, 1173-1180). To identify its role in frog retina, first s 26 was purified to nearly homogeneity with three chromatographical ste ps, Based on the partial amino acid sequences of the proteolysed fragm ents of s26, we isolated cDNAs that encode s26. The analysis of its am ino acid sequence revealed that s26 is an S-modulin-like protein, whil e it shows higher homology to visinin. Visinin is a Ca2+-binding prote in reported to be present in chicken cones, but its localization in th e retina had been a subject in dispute, The present study showed that s26 is present in cone photoreceptors, The study also showed that s26 inhibits phosphorylation of rhodopsin after a light flash at high Ca2 concentrations as S-modulin does. From these results, we concluded th at s26 is a cone homologue of S-modulin, The result is consistent with the idea that each type of photoreceptors expresses each cell-type sp ecific version of phototransduction proteins.