Delineating the conformational landscape of the adenosine A2A receptor during G protein coupling

G-protein-coupled receptors (GPCRs) represent a ubiquitous membrane protein family and are important drug targets. Their diverse signaling pathways are driven by complex pharmacology arising from a conformational ensemble rarely captured by structural methods. Here, fluorine nuclear magnetic resonance spectroscopy (F-19 NMR) is used to delineate key functional states of the adenosine A(2A) receptor (A(2A)R) complexed with heterotrimeric G protein (G alpha(s)beta(1)gamma(2)) in a phospholipid membrane milieu. Analysis of A(2A)R spectra as a function of ligand, G protein, and nucleotide identifies an ensemble represented by inactive states, a G-protein-bound activation intermediate, and distinct nucleotide-free states associated with either partial- or fullagonist-driven activation. The G beta gamma subunit is found to be critical in facilitating ligand-dependent allosteric transmission, as shown by F-19 NMR, biochemical, and computational studies. The results provide a mechanistic basis for understanding basal signaling, efficacy, precoupling, and allostery in GPCRs.