OPN5
Opsin-5, also known as G-protein coupled receptor 136 or neuropsin is a protein that in humans is encoded by the "OPN5" gene. Opsin-5 is a member of the opsin subfamily of the G protein-coupled receptors. It is a photoreceptor protein sensitive to ultraviolet (UV) light. The OPN5 gene was discovered in mouse and human genomes and its mRNA expression was also found in neural tissues. Neuropsin is bistable at 0 °C and activates a UV-sensitive, heterotrimeric G protein Gi-mediated pathway in mammalian and avian tissues. Function. Human neuropsin is expressed in the eye, brain , testes, and spinal cord. Neuropsin belongs to the seven-exon subfamily of mammalian opsin genes that includes peropsin (RRH) and retinal G protein coupled receptor (RGR). Neuropsin has different isoforms created by alternative splicing. Photochemistry. When reconstituted with 11-cis-retinal, mouse and human neuropsins absorb maximally at 380 nm. When illuminated these neuropsins are converted into blue-absorbing photoproducts (470 nm), which are stable in the dark. The photoproducts are converted back to the UV-absorbing form, when they are illuminated with orange light (> 520 nm). Phylogeny. The neuropsins form one of the three subclades of the tetraopsins . They are also known as Go/RGR, or Group 4 opsins. The tetraopsins contain three subclades: The Go-opsins, the neuropsins, and a clade formed by the RGR-opsins and the peropsins. The tetraopsins are one of the main clades of animal opsins, also known as metazoan or type-II opsins. Animal opsins contain four main clades: The c-opsins (ciliary), the r-opsins (rhabdomeric), the xenopsins, and the tetraopsins. Only some of these subclades occur in cnidarians. However, without the cnidarian opsins the remaining bilaterian opsins would be a parphyletic taxon. Species distribution. Neuropsin and its orthologs have been  found experimentally in a small number of animals, among them human, house mouse ("Mus musculus"), chicken ("Gallus gallus domesticus"), the Japanese quail ("Coturnix japonica"), the European brittle star "Amphiura filiformis" (related to starfish), the tardigrade water bear ("Hypsibius dujardini"), and the tadpole of "Xenopus laevis". Searches of publicly available databases of genetic sequences have found putative neuropsin orthologs in both major branches of Bilateria: protostomes and deuterostomes. Among protostomes, putative neuropsins have been found in the molluscs owl limpet ("Lottia gigantea") (a species of sea snail) and Pacific oyster ("Crassostrea gigas"), in the water flea ("Daphnia pulex") (an arthropod), and  in the annelid worm "Capitella teleta".