Domains Determining Ligand Specificity for Ca2+Receptors

• Published in: Molecular pharmacology 1999-04, Vol.55 (4), p.642-648
• Main Authors: Hammerland, L G, Krapcho, K J, Garrett, J E, Alasti, N, Hung, B C, Simin, R T, Levinthal, C, Nemeth, E F, Fuller, F H
• Format: Article
• Language: English
• Subjects: Animals; Binding Sites; Calcium-Binding Proteins - agonists; Calcium-Binding Proteins - chemistry; Calcium-Binding Proteins - genetics; Calcium-Binding Proteins - metabolism; Ligands; Oocytes - physiology; Protein Conformation; Receptors, Metabotropic Glutamate - agonists; Receptors, Metabotropic Glutamate - genetics; Receptors, Metabotropic Glutamate - metabolism; Recombinant Fusion Proteins - agonists; Recombinant Fusion Proteins - metabolism; Xenopus laevis
• ISSN: 0026-895X; 1521-0111
• DOI: 10.1016/S0026-895X(24)23025-X

The Ca 2+ receptor is a G protein-coupled receptor that enables parathyroid cells and certain other cells in the body to respond to changes in the level of extracellular Ca 2+ . The Ca 2+ receptor is a member of a family of G protein-coupled receptors that includes metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid B 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 Ca 2+ receptor are not known. To identify structural determinants for cation recognition and activation of the Ca 2+ receptor (and to further study the mGluRs), two chimeric receptors were constructed in which the large ECD of the Ca 2+ receptor and the mGluR1 were interchanged. When expressed in Xenopus laevis oocytes, one of these chimeras, named CaR/mGluR1 [ECD of the Ca 2+ receptor and transmembrane domain (TMD) of the mGluR1], responded to cation agonists (Gd 3+ , Ca 2+ , neomycin) of the Ca 2+ receptor at concentrations similar to those necessary for activation of the native Ca 2+ receptor. A reciprocal construct, named mGluR1/CaR (ECD of the mGluR1 and TMD of the Ca 2+ receptor), was responsive to mGluR agonists but was much less sensitive to two of three cation agonists known to activate the Ca 2+ receptor. A deletion construct of the Ca 2+ receptor (ΔntCaR), which lacked virtually the entire ECD, was only activated by one of three agonists tested. These results suggest that the primary determinants for agonist activation of both the Ca 2+ receptor and the mGluRs are found in the large ECD and that the Ca 2+ receptor is possibly distinguished from the mGluRs in that it may contain sites in the TMD that permit activation by certain cation agonists.