Allosteric interactions in the parathyroid hormone GPCR–arrestin complex formation

Peptide ligands of class B G protein-coupled receptors (GPCRs) act via a two-step binding process, but the essential mechanisms that link their extracellular binding to intracellular receptor–arrestin interactions are not fully understood. Using NMR, crosslinking coupled to mass spectrometry, signaling experiments, and computational approaches on the parathyroid hormone (PTH) type 1 receptor (PTHR), we show that initial binding of the PTH C-terminal part constrains the conformation of the flexible PTH N-terminal signaling epitope before a second binding event occurs. A “hot spot” PTH residue, His9, that inserts into the PTHR transmembrane domain at this second step allosterically engages receptor–arrestin coupling. A conformational change in PTHR intracellular loop 3 permits favorable interactions with β-arrestin’s finger loop. These results unveil structural determinants for PTHR–arrestin complex formation and reveal that the two-step binding mechanism proceeds via cooperative fluctuations between ligand and receptor, which extend to other class B GPCRs.