β-arrestin1 directly engages Gαs to sustain endosomal GPCR signaling

{beta}-arrestins ({beta}arrs) are canonical terminators of G protein-coupled receptor (GPCR) signaling, yet internalized receptors paradoxically sustain {beta}arr-dependent Gs/cAMP signaling from endosomes. How a desensitizing scaffold can instead promote G protein activation has remained unresolved, because the receptor-{beta}arr-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 {beta}arr1 binds Gs directly, engages its nucleotide-free state, and does so far more strongly than it engages G{beta}{gamma}. A cryo-EM structure of the agonist-bound PTH1R-{beta}arr1-Gs complex at ~3.1 &Aring resolution reveals how: {beta}arr1 anchors to the phosphorylated receptor C-tail, leaving the transmembrane core free for Gs, and reorients to build a direct, three-node interface in which its finger, C- and back loops clamp the Switch I, Switch II and 3/{beta}5 regions of Gs. Disrupting either side of this interface abolishes complex assembly and selectively collapses the internalization-dependent phase of cAMP production, and the same interface governs {beta}arr1-Gs coupling at the glucagon-like peptide-1 receptor. {beta}-arrestin1 therefore acts as a positive regulator that holds Gs in a signaling-competent state, providing a unifying structural basis for sustained endosomal GPCR signaling.