Evidence for distinct cation and calcimimetic compound (NPS 568) recognition domains in the transmembrane regions of the human Ca2+ receptor

The Ca2+ receptor, a member of the family 3 of G protein-coupled receptors (GPCR), responds not only to its primary physiological ligand Ca2+ but also to other di- and trivalent metals (Mg2+, Gd3+) and the organic polycations spermine and poly-L-Arginine. As has been found for other family 3 GPCRs, the large amino-terminal extracellular domain (ECD) of the Ca2+ receptor is the primary Ca2+ binding domain. To examine how the signal is propagated from the ECD to the seven-transmembrane core domain (7TM) we constructed a Ca2+ receptor mutant (T903-Rhoc) lacking the entire ECD but containing the 7TM. We have found that this structure initiates signaling in human embryonic kidney (HEK) 293 cells stably expressing the construct. One or more cation recognition sites are also located within the 7TM. Not only Ca2+, but also several other Ca2+ receptor-specific agonists, Mg2+, Gd3+, spermine, and poly-L-Arginine, can activate T903-Rhoc truncated receptor-initiated phosphoinositide hydrolysis in HEK 293 cells. The phenylalkylamine compound, N-PS 568, identified as a positive allosteric modulator of the Ca2+ receptor can selectively potentiate the actions of Ca2+ and other polycationic agonists on the T903-Rhoc receptor. Similarly, organic polycations synergistically activate T903-Rhoc with di- and trivalent metals. Alanine substitution of all the acidic residues in the second extracellular loop of the T903-Rhoc receptor significantly impairs activation by metal ions and organic polycations in the presence of NPS 568 but not the synergistic activation of Ca2+ with poly-L-Arginine. These data indicate that although the ECD has been thought to be the main determinant for Ca2+ recognition, the 7TM core of the Ca2+ receptor contains activating site(s) recognizing Ca2+ and Gd3+ as well as the allosteric modulators NPS 568 and organic polycations that may play important roles in the regulation of receptor activation.