Neuropsin is a protein that in humans is encoded by the OPN5gene. 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.
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.
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