RRH

Like most opsins, peropsins have in its seventh transmembrane domain a lysine corresponding to amino acid position 296 in cattle rhodopsin, In amphioxus, a cephalochordate, a peropsin binds in the dark-state all-trans-retinal instead of 11-cis-retinal, as it is in cattle rhodopsin. Therefore, peropsins have been suggested to be photoisomerases. == Tissue localization ==

In mice, a peropsin is localized to the apical microvilli of the retinal pigment epithelium (RPE). A peropsin is also expressed in keratinocytes of the human skin. In keratinocyte cell culture, it reacts to UV light if retinal is supplied. In chicken, a peropsin is expressed with an RGR-opsin in the pineal gland and the retina. == Gene localization and structure ==

The human peropsin gene lies on chromosome 4 band 4q25 and has six introns 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. == Phylogeny ==

The peropsins are restricted to the craniates and the cephalochordates. In the phylogeny above, Each clade 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 certain amino acid at the position in the sequences corresponding to position 296 in cattle rhodopsin. The amino acids are color-coded. The colors are red for lysine (K), purple for glutamic acid (E), orange for arginine (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 for craniates, dark green for cephalochordates, mid green for echinoderms, brown for nematodes, pale pink for annelids, dark blue for arthropods, light blue for mollusks, and purple for cnidarians. 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. == Clinical significance ==

Since RGR-opsin may be associated with retinitis pigmentosa, which is like peropsin also expressed in the retinal pigment epithelium, peropsin was screened for a link with retinitis pigmentosa. However, no link could be established. == References ==