| We report the discovery and molecular characterization of a putative novel G-protein-coupled receptor, GPR48, that resembles proteins in the glycoprotein hormone G-protein-coupled receptor family. We initially identified GPR48 by searching the EST database with amino acid sequences conserved among the transmembrane segments of G-protein-coupled receptors. The EST comprised the putative c-terminal portion of a receptor that we assigned the moniker GPR48. GPR48 is an orphan receptor with signature sequences of the G-protein-coupled receptors and an unidentified cognate ligand.; Northern blot analysis demonstrated high expression of GPR48 in the adult human pancreas, moderate levels in placenta, kidney, brain, and heart. Additionally, this receptor is expressed 7 days post-coitus in the mouse, indicating its potential involvement in development. 5' RACE was performed from a human pancreas cDNA library to derive the remainder of the clone. Human GPR48 cDNA is 951 amino acids in length. The large extracellular amino terminus is composed of seventeen leucine-rich repeats. Evaluation of GPR48 DNA sequence and protein indicates that the structural motifs common among all G-protein-coupled receptors are also extant in GPR48.; The genomic structure of GPR48 has several features in common with genes in the glycoprotein hormone receptor family. Analogous to these receptors, most of the leucine-rich repeats are encoded on single small exons, and the last exon encodes the seven transmembrane segments, and their introns are homologous. The gene spans approximately 60 kb with 18 exons and 17 introns. We localized GPR48 to human chromosome 11p14--p13 and to mouse chromosome 2 (65 cM) using several mapping techniques: monochromosomal somatic cell hybrid panel, radiation hybrid panel, mouse backcross panel, and in silico analysis. Furthermore, GPR48 may be implicated in a disease pathology called Wilms' Tumor, Aniridia, Genitourinary, and Mental Retardation that results from deletions in the regions of 11p13.; To measure constitutive receptor activity, we overexpressed the receptor and mutated the receptor in two regions, IL3 (Q742G) and TMS3 (L636R) separately. No ligand independent GPR48 and GPR48 mutant cAMP or IP accumulation was measured. The lack of cellular signaling may be attributed to the observation that GPR48-GFP fluorescence was predominantly detected cytoplasmically in HEK293 cells. |
