A metastable GαS/β-arrestin complex sustains endosomal PTH1R signaling

G protein-coupled receptors (GPCRs) initiate cellular signaling by activating heterotrimeric G proteins at the plasma membrane and are silenced when beta-arrestin binds the phosphorylated receptor, preventing G protein binding. However, many receptors continue to generate cAMP after internalization, even while barr is bound, a paradox that lacked a molecular mechanism in live cells. A direct barrestin1-Gas interface in the PTH1R-barr1-Gas complex has recently been defined structurally, but whether the complex operates in endosomes, and how it behaves there, has not been established. Using intact cells, we examine the assembly at three spatial resolutions. Confocal imaging with a nanobody selective for nucleotide-free Gas places activated Gas, barr, and agonist-occupied PTH1R on the same endosomes. Single-molecule localization microscopy, with illumination modes that separate the plasma membrane from the endosomal membrane in single cells, shows that the agonist recruits Gas and barr at both membranes. FRET, however, reveals a specific interaction only on endosomes, where a Gas inhibitor restricted to that compartment abolishes the sustained cAMP phase without affecting receptor internalization. Weighted-ensemble-enhanced molecular simulations show that the complex is metastable, releasing Gas through a large rigid-body rotation away from barr while barr itself tightens its interactions with the receptor. These findings provide a molecular basis for endosomal Gs/cAMP signaling and open a new avenue for drug development targeting this pathway.