Critical Extracellular Ca2+ 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 Ca2+. 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 Ca2+ to describe how calcium ions allosterically engage receptor–G-protein coupling. We find that the binding of Ca2+ 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 Ca2+ 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 Ca2+-coupling site to the intracellular G-protein binding site. These results unveil structural determinants for Ca2+ allosterism in the PTH1R–PTH–spep complex and give insights into pluridimensional GPCR signaling regulated by calcium ions.