The CASR gene: Alternative splicing and transcriptional control, and calcium-sensing receptor (CaSR) protein: Structure and ligand binding sites

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor encoded by a single copy gene. The human CASR gene spans ∼103-kb and has eight exons. Promoters P1 and P2 drive transcription of exons 1A and 1B, respectively, encoding alternative 5′-UTRs that splice to exon 2 encoding the common part of the 5′-UTR. Exons 2–7 encode the CaSR protein of 1078 amino acids. Functional elements responsive to 1,25-dihydroxyvitamin D, proinflammatory cytokines, and glial cells missing-2 are present in the CASR promoters. Evolutionarily, the exon structure, first seen in aquatic vertebrates, is well-conserved with a single linkage disequilibrium haplotype block for protein coding exons 2–7. Structural features of the human CaSR protein are: an N-terminal signal peptide (19 amino acids (aa)); an extracellular domain (∼600 aa) having a bi-lobed Venus Flytrap (VFT) domain with several Ca2+-binding sites; and a nine-cysteines domain that transduces the activation signal to the 7-transmembrane domain (250 aa) and the C-terminal tail (216 aa).