Class B1 GPCR activation by an intracellular agonist
G protein-coupled receptors (GPCRs) typically bind specific ligands within their orthosteric-binding pockets. This ligand binding initiates an allosteric conformational change in the receptor, leading to the activation of intracellular transducers like G proteins and β-arrestins. Since these signals often cause unwanted side effects, it’s crucial to understand how each transducer is selectively activated. To this end, many orthosteric-biased agonists have been developed, and there is growing interest in intracellular-biased agonists. These agonists bind within the receptor’s intracellular cavity, selectively modulating specific signaling pathways without causing allosteric rearrangements from the extracellular side. However, current structural data is limited to antagonist-bound receptors, with no direct evidence supporting biased agonist binding within the intracellular cavity. This gap in knowledge hampers the understanding of intracellular-biased agonism and the development of potential drugs.
In this study, we present the cryogenic electron microscopy structure of a complex between the human parathyroid hormone type 1 receptor (PTH1R) and the Gs protein, bound to the PTH1R agonist PCO371. PCO371 binds within an intracellular pocket of PTH1R and directly interacts with Gs, rearranging the intracellular region towards an active conformation without triggering extracellular allosteric signal propagation. PCO371 stabilizes a significantly outward-bent conformation of transmembrane helix 6, which favors G protein binding over β-arrestins. Additionally, PCO371 binds within a highly conserved intracellular pocket, activating 7 out of 15 class B1 GPCRs. Our findings reveal a novel and conserved intracellular agonist-binding pocket and provide evidence for a biased signaling mechanism that targets the receptor-transducer interface.