👤 P Krogsgaard-Larsen

Recently three orphan G-protein coupled receptors, RAIG1, GPRC5B and GPRC5C, with homology to members of family C (metabotropic glutamate receptor-like) have been identified. Using the protein sequences of these receptors as queries we identified overlapping expressed sequence tags which were predicted to encode an additional subtype. The full length coding regions of mouse mGprc5d and human GPRC5D were cloned and shown to contain predicted open reading frames of 300 and 345 amino acids, respectively. GPRC5D has seven putative transmembrane segments and is expressed in the cell membrane. The four human receptor subtypes, which we assign to group 5 of family C GPCRs, show 31-42% amino acid sequence identity to each other and 20-25% sequence identity to the transmembrane domains of metabotropic glutamate receptor subtypes 2 and 3 and other family C members. In contrast to the remaining family C members, the group 5 receptors have short amino terminal domains of some 30-50 amino acids. GPRC5D was shown to be clustered with RAIG1 on chromosome 12p13.3 and like RAIG1 and GPRC5B to consist of three exons, the first exon being the largest containing all seven transmembrane segments. GPRC5D mRNA is widely expressed in the peripheral system but all four receptors show distinct expression patterns. Interestingly, mRNA levels of all four group 5 receptors were found in medium to high levels in the kidney, pancreas and prostate and in low to medium levels in the colon and the small intestine, whereas other organs only express a subset of the genes. In an attempt to delineate the signal transduction pathway(s) of the orphan receptors, a series of chimeric receptors containing the amino terminal domain of the calcium sensing receptor or metabotropic glutamate receptor subtype 1, and the seven transmembrane domain of the orphan receptors were constructed and tested in binding and functional assays. Show less

Query of GenBank with the amino acid sequence of human metabotropic glutamate receptor subtype 2 (mGluR2) identified a predicted gene product of unknown function on BAC clone CIT987SK-A-69G12 (located on chromosome band 16p12) as a homologous protein. The transcript, entitled GPRC5B, was cloned from an expressed sequence tag clone that contained the entire open reading frame of the transcript encoding a protein of 395 amino acids. Analysis of the protein sequence reveal that GPRC5B contains a signal peptide and seven transmembrane alpha-helices, which is a hallmark of G-protein-coupled receptors (GPCRs). GPRC5B displays homology to retinoic acid-inducible gene 1 (RAIG1, 33% sequence identity) and to several family C (mGluR-like) GPCRs (20-25% sequence identity). Both RAIG1 and GPRC5B have short extracellular amino-terminal domains (ATDs) that contrast the very long ATDs characterizing the receptors currently assigned to family C. However, our results strongly indicate that RAIG1 and GPRC5B form a new subgroup of family C characterized by short ATDs. GPRC5B mRNA is widely expressed in peripheral and central tissues with highest abundance in kidney, pancreas, and testis. This mRNA expression pattern is markedly different from that of RAIG1, which shows a slightly more restricted expression pattern with highest abundance in lung tissue. Show less