Allosteric coupling from G protein to the agonist binding pocket in GPCRs

摘要

G protein-coupled receptors (GPCRs) remain the primary conduit by which cells detect environmental stimuli and communicate with each other. Upon activation by extracellular agonists, these seven transmembrane domain (7TM)-containing receptors interact with heterotrimeric G proteins to regulate downstream second messenger and/or protein kinase cascades. Crystallographic evidence from a prototypic GPCR, the β2-adrenergic receptor (β2AR), in complex with its cognate G protein, Gs, has provided a model for how agonist binding promotes conformational changes that propagate through the GPCR and into the nucleotide binding pocket of the G protein α-subunit to catalyze GDP release, the key step required for GTP binding and activation of G proteins. The structure also offers hints on how G protein binding may, in turn, allosterically influence ligand binding. Here we provide functional evidence that G protein coupling to β2AR stabilizes a ‘closed’ receptor conformation characterized by restricted access to and egress from the hormone binding site. Surprisingly, the effects of G protein on the hormone binding site can be observed in the absence of a bound agonist, where G protein coupling driven by basal receptor activity impedes the association of agonists, partial agonists, antagonists and inverse agonists. The ability of bound ligands to dissociate from the receptor is also hindered, providing a structural explanation for the G protein-mediated enhancement of agonist affinity, which has been observed for many GPCR-G protein pairs. Our studies also suggest that in contrast to agonist binding alone, coupling of a G protein in the absence of an agonist stabilizes large structural changes in a GPCR. The effects of nucleotide-free G protein on ligand binding kinetics are shared by other members of the superfamily of GPCRs, suggesting that a common mechanism may underlie G protein-mediated enhancement of agonist affinity.