title: Computational analysis of the extracellular domain of the Ca²⁺-sensing receptor: an alternate model for the Ca²⁺ sensing region.
abstract: The extracellular Ca(2+) sensing receptor (CaSR) belongs to Class C G-protein-coupled receptors (GPCRs) which include receptors for amino acids, γ-aminobutyric acid and glutamate neurotransmitters. CaSR has been described as having an extended sequence containing a Ca(2+) binding pocket within an extracellular amino (N)-terminal domain, called a Venus Fly Trap (VFT) module. CaSR is thought to consist of three domains: 1) a Ca(2+-)sensory domain, 2) a region containing 7 transmembrane (TM) helices, and 3) a carboxy (C)-terminal tail. We find that SPOCTOPUS (a combination of hidden Markov models and artificial neural networks) predicts that Homo sapiens CaSR contains two additional TM helices ((190)D - G(210); (262)S-E(282)), with the second TM helix containing a pore-lining region ((265)K - I(280)). This predicts that the putative Ca(2+) sensory domain is within an extracellular loop, N-terminal to the highly conserved heptahelical bundle. This loop contains both the cysteine-rich domain ((537)V - C(598)) and a 14 residue "linker" sequence ((599)I - F(612)) thought to support signal transmission to the heptahelical bundle. Thus domain 1 may contain a 189 residue N-terminal extracellular region followed successively by TM-1, a short intracellular loop, TM-2 and a 329 residue extracellular loop; rather than the proposed 620 residue VFT module based on crystallography of the N-terminal region of mGluR1. Since the topologies of the two proteins differ, the published CaSR VFT model is questionable. CaSR also contains multiple caveolin-binding motifs and cholesterol-binding (CRAC/CARC) domains, facilitating localization to plasma membrane lipid rafts. Ion sensing may involve combination of pore-lining regions from CaSR dimers and CaSR-bound caveolins to form ion channels capable of monitoring ionized Ca(2+) levels.
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461 USA. Electronic address: gene.morrill@einstein.yu.edu.
authors.forename: Gene A
authors.initials: GA
authors.lastname: Morrill
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461 USA. Electronic address: kostellow@aol.com.
authors.forename: Adele B
authors.initials: AB
authors.lastname: Kostellow
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461 USA. Electronic address: raj.gupta@einstein.yu.edu.
authors.forename: Raj K
authors.initials: RK
authors.lastname: Gupta
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001665:Binding Sites; D022461:Caveolins / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008969:Molecular Sequence Data; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D044169:Receptors, Calcium-Sensing / Q000737:chemistry* / Q000378:metabolism*; D018094:Receptors, Metabotropic Glutamate / Q000737:chemistry / Q000378:metabolism
chemical_list: C095550:CASR protein, human; D022461:Caveolins; D044169:Receptors, Calcium-Sensing; D018094:Receptors, Metabotropic Glutamate; C104077:metabotropic glutamate receptor type 1; D002784:Cholesterol
keywords: Ca-sensing receptor; Caveolin; Cholesterol; Glutamate receptors; Pore-lining regions; Transmembrane helices