Mechanisms of multimodal sensing by extracellular Ca(2+)-sensing receptors: a domain-based survey of requirements for binding and signalling

Abstract

In this article we consider the molecular basis of sensing and signalling by the extracellular calcium-sensing receptor. We consider the nature of its ligands and sensing modalities, the identities of its major protein domains and their roles in sensing, signalling and trafficking as well as the significance of receptor homo- and hetero-dimerization. Finally, we consider the current, incomplete, state of knowledge regarding the requirements for ligand-specific signalling.

Figure 1

Molecular model of a CaR protomeric VFT domain. A model of a single subunit based on the mGlu-1 crystal structure 1EWK (Kunishima et al., 2000). Putative Ca²⁺_o binding sites (1–5) were identified by aromatized terbium luminescence analysis of globular sub-domains (Huang et al., 2009). Site ‘1’ also corresponds to the conserved L-amino acid-binding site of class C GPCRs raising the possibility that Ca²⁺ and amino acid binding are closely associated. CaR, calcium-sensing receptor; mGlu, metabotropic glutamate receptor; GPCR, G-protein-coupled receptor; VFT domain, Venus Fly Trap domain.

Figure 2

Schematic representation of the CaR's Cysteine rich domain. The Cys-rich (CR) domain has nine conserved cysteine residues (black circles), all of which are predicted to participate in di-sulfide bonds (broken lines). In total there are four predicted intra-domain di-sulfides and one disulfide between CaR residues 561 in the CR domain and 236 in lobe 2 of the VFT domain. A 14 amino acid linker (grey circles) supports signal transmission from the VFT domain to the heptahelical domain. CaR, calcium-sensing receptor; VFT domain, Venus Fly Trap domain.

Figure 3

Schematic representation of the CaR's heptahelical domain. The seven transmembrane helices are shown together with the alternating intracellular loops (residues in dark grey) and extracellular loops (residues in light grey). Residues that interact with both calcimimetics and calcilytics are enclosed in double-lined circles. Residues that interact with calcilytics alone are enclosed in single, bold circles. Residues in iL-2 and iL-3 that support G_q/11-dependent activation of PI-PLC are highlighted with broken-lines. CaR, calcium-sensing receptor; PI-PLC, phosphatidylinositol-specific phospholipase-C.

Figure 4

Schematic representation of the CaR's intracellular C-terminal domain. The C-terminal domain supports receptor expression and activation of PI-PLC (residues 863–874; double circles), activation of PI-PLC alone (highlighted in grey), as well as cooperativity and resistance to desensitization (residues 868–886 indicated by broken line). Residues 960–990 (labelled in black) provide a high-affinity binding site for filamin-A. The major PK-C phosphorylation site at T-888 is labelled ‘▾’. CaR, calcium-sensing receptor; PI-PLC, phosphatidylinositol-specific phospholipase-C.