Positive allosteric modulation of a GPCR ternary complex

The activation of a G protein-coupled receptor (GPCR) leads to the formation of a ternary complex between agonist, receptor, and G protein that is characterised by high-affinity binding. Allosteric modulators bind to a distinct binding site from the orthosteric agonist and can modulate both the affinity and the efficacy of orthosteric agonists. The influence allosteric modulators have on the high-affinity active state of the GPCR-G protein ternary complex is unknown due to limitations on attempting to characterize this interaction in recombinant whole cell or membrane-based assays. Here, we use purified M ₂ muscarinic acetylcholine receptor (mAChR) reconstituted into nanodiscs to show that once the agonist-bound high-affinity state is promoted by the G protein, positive allosteric modulators stabilise the ternary complex that, in the presence of nucleotides leads to an enhanced initial rate of signalling. Our results enhance our understanding of how allosteric modulators influence orthosteric ligand signalling and will aid the design of allosteric therapeutics.