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Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming. == Opsin classification == Opsins can be classified in any of several ways, including function (vision, phototaxis, photoperiodism, etc.), type of chromophore (retinal, flavine, bilin), molecular structure (tertiary, quaternary), signal output (phosphorylation, reduction, oxidation), etc. There are two groups of protein termed opsins.〔 type I opsins are employed by prokaryotes and - as the protein component of channelrhodopsins - by some algae, whereas animals use type II opsins. No opsins have been found outside these groups (for instance in plants, fungi, or placozoans).〔 At one time it was thought that type I and type II were related because of structural and functional similarities. With the advent of genetic sequencing it became apparent that sequence identity was no greater than could be accounted for by random chance. However, in recent years new methods have been developed specific to ''deep phylogeny''. As a result, several studies have found evidence of a possible phylogenetic relationship. According to one hypothesis, both type-I and type-II opsins belong to the ''transporter-opsin-G protein-coupled receptor (TOG) superfamily'', a proposed clade that includes G protein-coupled receptor (GPCR), Ion-translocating microbial rhodopsin (MR), and seven others. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Opsin」の詳細全文を読む スポンサード リンク
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