Mechanistic insights into G protein association with a G protein-coupled receptor

Abstract

G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by promoting guanine nucleotide exchange. Here, we investigate the process of functional association between G proteins and GPCRs and describe the events that ultimately lead to the ejection of GDP from its binding pocket in the Gα subunit. In atomic detail, we reveal the temporal progression of structural rearrangements of GDP-bound heterotrimeric Gs protein (GsGDP) upon coupling to the β2-adrenergic receptor (β2AR) using molecular dynamics simulations. The binding of GsGDP to the β2AR is followed by long-range allosteric effects that significantly reduce the energy needed for GDP release, the rate-limiting step during G-protein activation. In particular, the opening of α1-αF helices, displacement of the αG helix, and the opening of the α-helical domain weaken the interaction between GDP and the G protein. Signal transduction to the G protein occurs via a novel receptor interface, confirmed by site-directed mutagenesis and functional assays. From this β2AR-GsGDP intermediate, the G protein must undergo an in-plane rotation along the receptor axis to reach the β2AR-Gsempty state. The simulations shed light on how the structural elements at the receptor-G-protein interface interact to transmit the signal over 30Å to the nucleotide-binding site. Our analysis extends the current limited view of nucleotide-free snapshots to include additional states and structural features responsible for signaling and G protein coupling specificity.