Opsin

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.

Opsins can be classified 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 by some algae (as a component of channelrhodopsins) and fungi, whereas animals use type II opsins. No opsins have been found outside these groups (for instance in plants, 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, which however does not imply that the last common ancestor of type I and II opsins was itself an opsin, a light sensitive receptor. 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.

Like type II opsins, type I opsins have a seven transmembrane domain structure similar to that found in eukaryotic G-protein coupled receptors.

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