Domains determining ligand specificity for Ca2+ receptors

The Ca2+ receptor is a G protein-coupled receptor that enables parathyroid cells and certain other cells in the body to respond to changes in the leve l of extracellular Ca2+. The Ca2+ receptor is a member of a family of G pro tein-coupled receptors that includes metabotropic glutamate receptors (mGlu Rs), gamma-aminobutyric acid, receptors, and putative pheromone receptors. As a family, these receptors are characterized by limited sequence homology and an unusually large putative extracellular domain (ECD). The ECD of the mGluRs is believed to determine agonist selectivity, but the functions of the structural domains of the Ca2+ receptor are not known. To identify stru ctural determinants for cation recognition and activation of the Ca2+ recep tor (and to further study the mGluRs), two chimeric receptors were construc ted in which the large ECD of the Ca2+ receptor and the mGluR1 were interch anged. When expressed in Xenopus laevis oocytes, one of these. chimeras, na med CaR/mGluR1 [ECD of the Ca2+ receptor and transmembrane domain (TMD) of the mGluR1], responded to cation agonists (Gd3+, Ca2+, neomycin) of the Ca2 + receptor at concentrations similar to those necessary for activation of t he native Ca2+ receptor. A reciprocal construct, named mGluR1/CaR (ECD of t he mGluR1 and TMD of the Ca2+ receptor), was responsive to mGluR agonists b ut was much less sensitive to two of three cation agonists known to activat e the Ca2+ receptor. A deletion construct of the Ca2+ receptor (Delta ntcaR ), which lacked virtually the entire ECD, was only activated by one of thre e agonists tested. These results suggest that the primary determinants for agonist activation of both the Ca2+ receptor and the mGluRs are found in th e large ECD and that the Ca2+ receptor is possibly distinguished from the m GluRs in that it may contain sites in the TMD that permit activation by cer tain cation agonists.