Identification of putative transmembrane receptor sequences homologous to the calcium-sensing G-protein-coupled receptor

Abstract

The sensing of extracellular calcium is a general paradigm for regulating diverse cellular functions in many tissues. A calcium-sensing receptor (Casr) belonging to the metabotropic glutamate family of G-protein-coupled receptors (GPCR) that transduces the effects of extracellular calcium in the parathyroid gland as well as other tissues has been identified. The diversity of GPCR families and the recent finding of calcium sensing in cells lacking the known Casr suggest the existence of additional receptors related to Casr. By polymerase chain reaction (PCR) amplification and screening of genomic libraries, we have identified multiple Casr-related sequences (Casr-rs) in the mouse. Using primers designed to regions of the first and third intracellular loops of Casr, we initially PCR amplified a 497-bp Casr-related sequence (Casr-rs1) with high homology to Casr. The deduced protein sequence of Casr-rs1 is 63% similar and 40% identical to Casr over the available transmembrane region. We screened a mouse genomic library with a Casr-rs1 probe and identified two additional Casr-related sequences (Casr-rs2 and Casr-rs3). In the predicted transmembrane domain, Casr-rs2 and Casr-rs3 are 95% identical to Casr-rs1. We mapped Casr-rs1 to mouse Chromosome (Chr) 7 by interspecific backcross analysis, whereas the known Casr localizes to mouse Chr 16. By fluorescence in situ hybridization, Casr-rs2 also localized to mouse Chr 7 and Casr-rs3 mapped to mouse Chr 4. We were able to distinquish Casr-rs1 from Casr-rs2 by PCR using specific primers, suggesting that they are distinct genes clustered on Chr 7. By RT-PCR, we identified additional Casr-rs transcripts in mouse kidney, brain, testis, embryo, and MC3T3-E1 osteoblasts, but not in lung or liver. The homologous sequence in mouse kidney, embryo, and MC3T3-E1 osteoblasts, designated Casr-rs4, has a deduced amino acid sequence that is 100% similar and 97% identical to that of Casr-rs1. The sequence amplified from mouse brain, Casr-rs5, has a deduced protein sequence that is 96% similar and 92% identical to that of Casr-rs1. Our findings establish the existence of a novel multimembered family of Casr-related sequences in the mouse which may encode receptors that transduce responses to diverse extracellular cations.