β-arrestin1 directly engages Gαs to sustain endosomal GPCR signaling

β-arrestins (βarrs) are canonical terminators of G protein–coupled receptor (GPCR) signaling, yet internalized receptors paradoxically sustain βarr-dependent G _s /cAMP signaling from endosomes. How a desensitizing scaffold can instead promote G protein activation has remained unresolved, because the receptor–βarr–G protein assemblies described so far position the two transducers on opposite faces of the receptor, with no contact between them. Using the parathyroid hormone type 1 receptor (PTH1R), we show that βarr1 binds Gα _s directly, engages its nucleotide-free state, and does so far more strongly than it engages Gβγ. A cryo-EM structure of the agonist-bound PTH1R– βarr1–Gα _s complex at ∼3.1 Å resolution reveals how: βarr1 anchors to the phosphorylated receptor C-tail, leaving the transmembrane core free for Gα _s , and reorients to build a direct, three-node interface in which its finger, C- and back loops clamp the Switch I, Switch II and α3/β5 regions of Gα _s . Disrupting either side of this interface abolishes complex assembly and selectively collapses the internalization-dependent phase of cAMP production, and the same interface governs βarr1–Gα _s coupling at the glucagon-like peptide-1 receptor. β-arrestin1 therefore acts as a positive regulator that holds Gα _s in a signaling-competent state, providing a unifying structural basis for sustained endosomal GPCR signaling.