The amino acid sequence has been determined for the carboxyl-terminal 41 amino acids of chicken red-sensitive cone pigment, iodopsin. This sequence is distinct from but structurally homologous to that of other visual pigments. It contains a region rich in the hydroxy amino acids serine and threonine. In the related rod cell visual pigment, rhodopsin, such serines and threonines have previously been identified as sites for phosphorylation by rhodopsin kinase. Phosphorylation of photolyzed rhodopsin serves to terminate its ability to function in visual transduction as an activator of G-protein. We have purified and reconstituted both chicken rhodopsin and chicken iodopsin and shown them to be phosphorylated by bovine rhodopsin kinase. Chicken iodopsin has a Km and Vmax similar to but distinguishably different from that for bovine rhodopsin. These results, in conjunction with other data, suggest that visual pigments in cone cells, upon absorption of light, undergo functional processes similar to those of the visual pigments in rod cells.
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