Peropsin, a visual pigment-like receptor, is aprotein that in humans is encoded by theRRHgene.[5][6] It belongs like other animalopsins to theG protein-coupled receptors.[6] Even so, the first peropsins were already discovered inmice andhumans in 1997,[5] not much is known about them.[7]
Inamphioxus, acephalochordate, a peropsin binds in the dark-state all-trans-retinal instead of 11-cis-retinal,[9] as it is in cattle rhodopsin.[10][11][12][13][14] Therefore, peropsins have been suggested to be photoisomerases.[9]
The human peropsin gene lies onchromosome 4 band 4q25 and has sixintrons[6][18] like RGR-opsins. However only two of these introns are inserted at the same place, which still indicates that peropsins and RGR-opsins are more closely related to each other than to the ciliary and rhabdomeric opsins.[18] This shared gene structure is also reflected in opsin phylogenies, where peropsins and RGR-opsins are in the same group: The chromopsins.[18][7][19][20]
The peropsins are restricted to thecraniates and the cephalochordates.[7] Thecraniates are thetaxon that containsmammals and with them humans. The peropsins are one of the seven subgroups of the chromopsins. The other groups are theRGR-opsins, theretinochromes, the nemopsins, the astropsins, the varropsins, and the gluopsins.[7] The chromopsins are one of three subgroups of the tetraopsins (also known as RGR/Go or Group 4 opsins). The other groups are theneuropsins 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).[7][19] However, the bilaterian clades constitute aparaphyletic taxon without the opsins from thecnidarians.[7][19][20][21]
The phylogenetic relationship of the peropsins 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 chromopsins, which are expanded in the next image.
Phylogenetic reconstruction of the chromopsins. The outgroup contains other G protein-coupled receptors including the other opsins. The frame highlights the peropsins.
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), orange forarginine (R), 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, brown fornematodes, pale pink forannelids, dark blue forarthropods, light blue formollusks, and purple forcnidarians. The 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.[7]
SinceRGR-opsin may be associated withretinitis pigmentosa,[22] which is like peropsin also expressed in the retinal pigment epithelium, peropsin was screened for a link with retinitis pigmentosa.[23] However, no link could be established.[23][24]
^Oroshnik W (June 1956). "The Synthesis and Configuration of Neo-B Vitamin A and Neoretinine b".Journal of the American Chemical Society.78 (11):2651–2652.doi:10.1021/ja01592a095.
^Bailey MJ, Cassone VM (March 2004). "Opsin photoisomerases in the chick retina and pineal gland: characterization, localization, and circadian regulation".Investigative Ophthalmology & Visual Science.45 (3):769–775.doi:10.1167/iovs.03-1125.PMID14985289.
^Morimura H, Saindelle-Ribeaudeau F, Berson EL, Dryja TP (December 1999). "Mutations in RGR, encoding a light-sensitive opsin homologue, in patients with retinitis pigmentosa".Nature Genetics.23 (4):393–394.doi:10.1038/70496.PMID10581022.S2CID35176366.
^Rivolta C, Berson EL, Dryja TP (December 2006). "Mutation screening of the peropsin gene, a retinal pigment epithelium specific rhodopsin homolog, in patients with retinitis pigmentosa and allied diseases".Molecular Vision.12:1511–1515.PMID17167409.
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