Protein Domain : Retinal pigment epithelium GPCR IPR001793

Type	Family
Description	G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups [ ]. The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence [ ]. The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) [, , , , ]. GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice []. Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs [].The rhodopsin-like GPCRs (GPCRA) represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices [ , , ].Retinal pigment epithelium (RPE) hosts a putative GPCR. The RPE-retinal GPCR (RGR) covalently binds all-trans- and 11-cis-retinal after reductionby sodium borohydride [ ]. All-trans-retinal is bound preferentially overthe 11-cis isomer. The human sequence is 86% identical to that of bovine RGR [, ], and a lysine residue, analogous to the retinaldehyde attachmentsite of rhodopsin, is conserved in TM domain 7 [ ]. The human gene, whosestructure is distinct from that of the visual pigment genes, spans 14.8 kb and is split into 7 exons []. This suggests that the rgr gene representsthe earliest independent branch of the vertebrate opsin gene family [ ].Since the RGR gene product preferentially binds all-trans-retinal, it is thought that one of its functions may be to catalyse isomerisation of thechromophore by a retinochrome-like mechanism [ ].
Short Name	RPE_GPCR