Opsin-5, also known asG-protein coupled receptor 136 orneuropsin is aprotein that in humans is encoded by theOPN5gene. Opsin-5 is a member of theopsin subfamily of theG protein-coupled receptors.[5][6][7] It is aphotoreceptor protein sensitive toultraviolet (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, heterotrimericG protein Gi-mediated pathway in mammalian and avian tissues.[8][9]
Human neuropsin is expressed in the eye, brain, testes, and spinal cord. Neuropsin belongs to the seven-exon subfamily of mammalian opsin genes that includesperopsin (RRH) andretinal G protein coupled receptor (RGR). Neuropsin has differentisoforms created by alternative splicing.[7]
When reconstituted with11-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).[8]
The neuropsins are one of three subgroups of the tetraopsins (also known as RGR/Go or Group 4 opsins). The other groups are the chromopsins and the Go-opsins. The tetraopsins are one of the five major groups of the animalopsins, also known as type 2 opsins). The other groups are the ciliary opsins (c-opsins, cilopsins), the rhabdomeric opsins (r-opsins, rhabopsins), the xenopsins, and the nessopsins. Four of these subclades occur inBilateria (all but the nessopsins).[11][16] However, the bilaterian clades constitute aparaphyletic taxon without the opsins from thecnidarians.[11][16][17][18]
The phylogenetic relationship of the neuropsins to the other opsins
Phylogenetic reconstruction of the opsins. The outgroup contains otherG protein-coupled receptors. The frame highlights the tetraopsins, which are expanded in the next image.
Phylogenetic reconstruction of the tetraopsins. The outgroup contains other G protein-coupled receptors including the other opsins. The frame highlights the neuropsins, which are expanded in the next image.
In thephylogeny above, Eachclade contains sequences from opsins and other G protein-coupled receptors. The number of sequences and two pie charts are shown next to the clade. The first pie chart shows the percentage of a certainamino acid at the position in the sequences corresponding to position 296 in cattle rhodopsin. The amino acids are color-coded. The colors are red forlysine (K), purple forglutamic acid (E), dark and mid-gray for other amino acids, and light gray for sequences that have no data at that position. The second pie chart gives the taxon composition for each clade, green stands forcraniates, dark green forcephalochordates, mid green forechinoderms, pale pink forannelids, dark blue forarthropods, light blue formollusks, and purple forcnidarians. The branches branches to the clades have pie charts, which give support values for the branches. The values are from right to left SH-aLRT/aBayes/UFBoot. The branches are considered supported when SH-aLRT ≥ 80%, aBayes ≥ 0.95, and UFBoot ≥ 95%. If a support value is above its threshold the pie chart is black otherwise gray.[11]
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