The opsins

Opsins are membrane proteins with molecular masses of 30-50 kDa that are related to the protein moiety of the photoreceptive molecule rhodopsin; they typically act as light sensors in animals [1-4]. Photoreceptive proteins similar to the animal opsins in three-dimensional structure but not in amino-acid sequence have been found in archaea, bacteria, fungi, and a green alga, Chlamydomonas reinhardtii [5,6]. These non-animal opsins function as light-driven ion pumps or light sensors but there is no evidence that they are structurally related to animal opsins, so they are not considered further here.Since the first sequence of an opsin, bovine rhodopsin, was determined by conventional protein sequencing in 1982 [7,8] and cDNA sequencing in 1983 [9], more than 1,000 opsins have been identified. The molecular phylogenetic tree shows three large clusters, and detailed analyses have revealed that the opsin family is divided into seven subfamilies; there is less than about 25% amino-acid similarity between subfamilies but more than about 40% among members of a single family (Figure 1). The division into subfamilies corresponds well to functional classification of opsins, which is based partly on the type of G protein coupled to each of these G-protein-coupled receptors (GPCRs). The seven subfamilies are as follows: the vertebrate visual (transducin-coupled) and non-visual opsin subfamily; the encephalopsin/tmt-opsin subfamily; the Gq-coupled opsin/melanopsin subfamily; the Go-coupled opsin subfamily; the peropsin subfamily; the retinal photoisomerase subfamily; and the neuropsin subfamily. Members of the Gq-coupled opsin/melanopsin, Go-coupled opsin, encephalopsin/tmt-opsin and retinal photoisomerase subfamilies are found in both deuterostomes (such as cephalochordates and vertebrates) and protostomes (such as molluscs and insects; Figure 1), suggesting that diversification of the subfamilies occurred much earlier in animal evolution than the deuterostome-protostome split [