Structural basis of stepwise G protein activation by the viral chemokine receptor US28.

G protein-coupled receptors (GPCRs) govern diverse cellular responses and are crucial drug targets. However, the sequence of structural events from G protein recognition to GDP release has remained elusive. Here, we leveraged the viral chemokine GPCR US28 to capture multiple activation states of the US28-G q complex. Using cryo-electron microscopy and an engineered chemokine superagonist, we determined three distinct complex structures, capturing a GDP-bound Encounter state, the nucleotide-free Canonical state, and a putative intermediate bridging the two states, the Transition-to-Canonical state. These structures, along with simulations and functional data, provide high-resolution snapshots of a plausible G protein activation trajectory and support a stepwise conformational model for G protein activation. This activation cascade closely parallels mechanisms proposed for human GPCRs, suggesting a conserved GPCR signaling mechanism.