Critical Extracellular Ca 2+ Dependence of the Binding between PTH1R and a G-Protein Peptide Revealed by MD Simulations.

The parathyroid hormone type 1 receptor (PTH1R), a canonical class B GPCR, is regulated by a positive allosteric modulator, extracellular Ca 2+ . Calcium ions prolong the residence time of PTH on the PTH1R, leading to increased receptor activation and duration of cAMP signaling. But the essential mechanism of the allosteric behavior of PTH1R is not fully understood. Here, extensive molecular dynamics (MD) simulations are performed for the PTH1R-G-protein combinations with and without Ca 2+ to describe how calcium ions allosterically engage receptor-G-protein coupling. We find that the binding of Ca 2+ stabilizes the conformation of the PTH1R-PTH-spep (the α5 helix of Gs protein) complex, especially the extracellular loop 1 (ECL1). Moreover, the MM-GBSA result indicates that Ca 2+ allosterically promotes the interaction between PTH1R and spep, consistent with the observation of steered molecular dynamics (SMD) simulations. We further illuminate the possible allosteric signaling pathway from the stable Ca 2+ -coupling site to the intracellular G-protein binding site. These results unveil structural determinants for Ca 2+ allosterism in the PTH1R-PTH-spep complex and give insights into pluridimensional GPCR signaling regulated by calcium ions.