Oral Presentation ESA-SRB-NZSE-CaSR 2026 in conjunction with ENSA

Mechanisms of physiological activation and modulation of the calcium-sensing receptor: molecular origins of promiscuous G protein signaling (145967)

Hao Zuo 1 , Jinseo Park 1 , Aurel Frangaj 1 , Joachim Frank 1 , Oliver B Clarke 1 , Jonathan A Javitch 1 , Arthur D Conigrave 2 , Qing R Fan 1
  1. Columbia University, New York, NEW YORK, United States
  2. University of Sydney, Sydney, Australia

Background

The calcium-sensing receptor (CaSR) maintains extracellular Ca2+ homeostasis by sensing circulating Ca2+ levels and regulating parathyroid hormone secretion (1,2). Beyond calcium regulation, CaSR mediates diverse physiological processes through activation of multiple signaling pathways (3-5). This versatility arises from its ability to respond to diverse ligands and engage multiple G-protein subtypes (2,6-8).

Aims

To elucidate the molecular mechanisms underlying CaSR activation, modulation, and promiscuous G-protein coupling.

Methods

We combine x-ray crystallography, cryo-electron microscopy (cryo-EM), and cell-based signaling assays to investigate CaSR structure and function.

Results

Our extracellular-domain structures revealed distinct binding sites for Ca2+, L-amino acids, and phosphate (9). L-amino acids and Ca2+ act as co-agonists to activate CaSR, while phosphate inhibits receptor activity (9). We also captured near-full-length structures of CaSR in multiple functional states and found that activation involves the formation of a novel transmembrane interface enabled by a helix-breaking conformational transition (10). Allosteric modulators regulate this conformational landscape by stabilizing distinct receptor states (10).

To understand how CaSR translates these conformational changes into diverse intracellular responses, we determined structures of CaSR complexed with heterotrimeric G proteins from three subfamilies: Gq, Gi, and Gs (11). The CaSR engages each G protein through a conserved binding mode, in which the C-terminal helix of Gα binds a shallow intracellular pocket formed by the receptor intracellular loops, transmembrane helix 3, and C-terminal region (11). G protein binding further expands and stabilizes the transmembrane dimer interface through phospholipid interactions (11). The conformational flexibility of intracellular loop 2 enables CaSR to accommodate diverse Gα subtypes (11). We also identified a key Gα residue that determines Gq and Gs versus Gi selectivity (11).

Conclusions

These studies reveal how CaSR integrates extracellular cues, undergoes ligand- and modulator-dependent activation, and achieves promiscuous G protein coupling.

 

  1. 1 Brown, E. M. et al. Cloning and characterization of an extracellular Ca(2+)-sensing receptor from bovine parathyroid. Nature 366, 575-580 (1993).
  2. 2 Hofer, A. M. & Brown, E. M. Extracellular calcium sensing and signalling. Nat. Rev. Mol. Cell Biol. 4, 530-538 (2003).
  3. 3 Gray, E. et al. Activation of the extracellular calcium-sensing receptor initiates insulin secretion from human islets of Langerhans: involvement of protein kinases. J. Endocrinol. 190, 703-710 (2006).
  4. 4 Mace, O. J., Schindler, M. & Patel, S. The regulation of K- and L-cell activity by GLUT2 and the calcium-sensing receptor CasR in rat small intestine. J. Physiol. 590, 2917-2936 (2012).
  5. 5 Ruat, M. & Traiffort, E. Roles of the calcium sensing receptor in the central nervous system. Best Pract. Res. Clin. Endocrinol. Metab. 27, 429-442 (2013).
  6. 6 Conigrave, A. D. & Ward, D. T. Calcium-sensing receptor (CaSR): pharmacological properties and signaling pathways. Best Pract. Res. Clin. Endocrinol. Metab. 27, 315-331 (2013).
  7. 7 Magno A L, Ward B K, and Ratajczak T. The calcium-sensing receptor: a molecular perspective. Endocr Rev 32: 3-30 (2011).
  8. 8 Abid, H. A., Ifnoue, A. & Gorvin, C. M. Heterogeneity of G protein activation by the calcium-sensing receptor. J. Mol. Endocrinol. 67, 41-53 (2021).
  9. 9 Geng, Y. et al. Structural mechanism of ligand activation in human calcium-sensing receptor. Elife 5, e13662 (2016).
  10. 10 Park, J. et al. Symmetric activation and modulation of the human calcium-sensing receptor. Proc. Natl. Acad. Sci. U. S. A. 118, e2115849118 (2021).
  11. 11 Zuo, H. et al. Promiscuous G-protein activation by the calcium-sensing receptor. Nature 629, 481-488 (2024).