Molecular insights into peptide agonist engagement with the PTH1 receptor

The parathyroid hormone (PTH) 1 receptor (PTH1R) is a class B1 G protein-coupled receptor (GPCR) that critically regulates skeletal development and calcium homeostasis. Despite extensive study, the molecular underpinnings of PTH1R stimulation by its cognate hormones, as well as by therapeutic agents, remain unclear. Here, we describe cryo-EM structures of the PTH1R in complex with active fragments of the two hormones, PTH and parathyroid hormone related protein (PTHrP), the peptidic drug abaloparatide, as well as the engineered tool compounds, long-acting PTH (LA-PTH) and the truncated peptide, M-PTH(1-14). We found that the N-terminus of each agonist that is critical for activity, engages the transmembrane bundle in a topologically similar fashion, which reflects similarities in measures of Gs activation. The full-length peptides bind the extracellular domain (ECD) using a shared interface but induce subtly different ECD orientations relative to the transmembrane domain (TMD). In the structure bound to M-PTH, an agonist which only binds the TMD, the ECD is completely unresolved, demonstrating that the ECD is highly dynamic when unconstrained by a peptide. High resolutions enabled identification of water molecules near the peptide and G protein binding sites, some of which are structurally conserved with other class B1 GPCRs. Our results shed light on the action of orthosteric agonists of the PTH1R and provide a foundation for structure based-drug design.