Evolution and diversity of opsins

Abstract Many animals capture light information via opsin‐based pigments. Several thousands of opsins have been identified thus far and the opsin family is divided into eight groups. Members belonging to four out of the eight groups have been elucidated to couple to transducin, Go, Gs, and Gq, respectively, in photoreceptor cells. Accumulated evidence suggests a novel classification of the animal phototransductions, cyclic nucleotide signaling mediated by transducin, Go or Gs in ciliary photoreceptor cells and phosphoinositol signaling mediated by Gq in rhabdomeric photoreceptor cells. Varied opsin‐based pigments are spectroscopically classified into two types, bleaching and bistable pigments; that is, the photoproduct of vertebrate visual pigments dissociates its chromophore retinal over time and bleaches, whereas most other opsin‐based pigments convert to a stable photoproduct, which can revert to original dark state by subsequent light absorption. Mutational analyses of the both types of pigments implied that during molecular evolution of the vertebrae visual pigments, displacement of the counterion, important amino acid residue for visible light absorption of opsin‐based pigment, resulted in not only unique bleaching property but also acquisition of red‐sensitive visual pigment and higher G‐protein activation ability generated by larger light‐induced conformational change of the pigment. Interestingly, a bleaching pigment rhodopsin and parapinopsin, which closely relates to the vertebrate visual pigment but has a bistable nature, couple to visual arrestin and β arrestin, respectively, in the lamprey pineal organ, suggesting the bleaching property also might facilitate the evolution of visual arrestin which is specialized for vertebrate visual function. WIREs Membr Transp Signal 2012, 1:104–111. doi: 10.1002/wmts.6 For further resources related to this article, please visit the WIREs website .