Neuropsin is a protein that in humans is encoded by the OPN5 gene.   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. 
Opsins are members of the G protein-coupled receptor superfamily. 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.
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).
The neuropsins constitute one of the four subclades of the Go/RGR group of opsins, also known as RGR/Go or Group 4. Go/RGR is one of the four major subclades of type-II opsins, also known as metazoan or animal opsins. Go/RGR comprises Go-coupled,
RGR, peropsins, and neuropsins. Type-II opsins comprise four subclades: C-opsins (ciliary), R-opsins (rhabdomeric), Cnidops (cnidarian), and Go/RGR. Three of these subclades occur only in Bilateria (all but Cnidops). However, the bilaterian clades constitute a parphyletic taxon without Cnidops. 
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  (related to starfish), Amphiura filiformis 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 ( ) (a species of sea snail) and Lottia gigantea Pacific oyster ( Crassostrea gigas), in the water flea ( ) (an Daphnia pulex arthropod), and in the annelid worm . Capitella teleta
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