Mapping the conformational landscape of the stimulatory heterotrimeric G protein

Heterotrimeric G proteins serve as membrane-associated signaling hubs, in concert with their cognate G-protein-coupled receptors. Fluorine nuclear magnetic resonance spectroscopy was employed to monitor the conformational equilibria of the human stimulatory G-protein α subunit (Gα) alone, in the intact Gαβγ heterotrimer or in complex with membrane-embedded human adenosine A receptor (AR). The results reveal a concerted equilibrium that is strongly affected by nucleotide and interactions with the βγ subunit, the lipid bilayer and AR. The α1 helix of Gα exhibits significant intermediate timescale dynamics. The α4β6 loop and α5 helix undergo membrane/receptor interactions and order-disorder transitions respectively, associated with G-protein activation. The αN helix adopts a key functional state that serves as an allosteric conduit between the βγ subunit and receptor, while a significant fraction of the ensemble remains tethered to the membrane and receptor upon activation.