RPE-retinal G protein-coupled receptor also known asRGR-opsin is aprotein that in humans is encoded by theRGRgene.[5][6] RGR-opsin is a member of therhodopsin-like receptor subfamily ofGPCR. Like otheropsins which bind retinaldehyde, it contains a conserved lysine residue in the seventh transmembrane domain.[7][8] RGR-opsin comes in different isoforms produced by alternative splicing.[6]
RGR-opsin preferentially binds all-trans-retinal,[8] which is the dominant form in the dark adapted retina, upon light exposure it is isomerized to 11-cis-retinal.[9] Therefore, RGR-opsin presumably acts as a photoisomerase to convert all-trans-retinal to 11-cis-retinal, similar toretinochrome ininvertebrates. 11-cis-retinal is isomerized back within rhodopsin and the iodopsins in therods andcones of the retina. RGR-opsin is exclusively expressed in tissue close to the rods and cones, theretinal pigment epithelium (RPE) andMüller cells.[7]
The RGR-opsins are restricted to theechinoderms, thehemichordates the craniates.[10] Thecraniates are thetaxon that containsmammals and with them humans. The RGR-opsins are one of the seven subgroups of the chromopsins. The other groups are theperopsins, theretinochromes, the nemopsins, the astropsins, the varropsins, and the gluopsins.[10] 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).[10][11] However, the bilaterian clades constitute aparaphyletic taxon without the opsins from thecnidarians.[10][11][12][13]
The phylogenetic relationship of the RGR-opsins 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 RGR-opsins.
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.[10]
^"Human PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^Chen XN, Korenberg JR, Jiang M, Shen D, Fong HK (June 1996). "Localization of the human RGR opsin gene to chromosome 10q23".Human Genetics.97 (6):720–722.doi:10.1007/BF02346179.PMID8641686.S2CID6086858.
^abJiang M, Pandey S, Fong HK (December 1993). "An opsin homologue in the retina and pigment epithelium".Investigative Ophthalmology & Visual Science.34 (13):3669–3678.PMID8258527.
^abShen D, Jiang M, Hao W, Tao L, Salazar M, Fong HK (November 1994). "A human opsin-related gene that encodes a retinaldehyde-binding protein".Biochemistry.33 (44):13117–13125.doi:10.1021/bi00248a022.PMID7947717.
^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.
Jiang M, Shen D, Tao L, Pandey S, Heller K, Fong HK (April 1995). "Alternative splicing in human retinal mRNA transcripts of an opsin-related protein".Experimental Eye Research.60 (4):401–406.doi:10.1016/S0014-4835(05)80096-X.PMID7789419.
Shen D, Jiang M, Hao W, Tao L, Salazar M, Fong HK (November 1994). "A human opsin-related gene that encodes a retinaldehyde-binding protein".Biochemistry.33 (44):13117–13125.doi:10.1021/bi00248a022.PMID7947717.
Tarttelin EE, Bellingham J, Bibb LC, Foster RG, Hankins MW, Gregory-Evans K, et al. (March 2003). "Expression of opsin genes early in ocular development of humans and mice".Experimental Eye Research.76 (3):393–396.doi:10.1016/S0014-4835(02)00300-7.PMID12573668.
Lin MY, Kochounian H, Moore RE, Lee TD, Rao N, Fong HK (July 2007). "Deposition of exon-skipping splice isoform of human retinal G protein-coupled receptor from retinal pigment epithelium into Bruch's membrane".Molecular Vision.13:1203–1214.PMID17679941.
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