Single molecule analysis reveals positive allosteric modulators are required to stabilize the active state of a metabotropic glutamate receptor

Metabotropic glutamate receptors (mGlu) are G protein-coupled receptors that represent promising targets for brain diseases. Much hope in drug development come from the discovery of positive allosteric modulators that display subtype selectivity, and act by increasing agonist potency, as well as efficacy in most cases. How such compounds can influence agonist efficacy remains unclear. Here, we explore the structural dynamics of the full-length mGlu2 dimers at submillisecond timescales using single molecule FRET on diffusing receptors in optimized detergent micelles. We show that glutamate binding in the Venus flytrap extracellular domains does not stabilize fully the receptors in their active states. The full activation of all receptors can only be observed in the presence of either a positive allosteric modulator or the Gi protein. Our results provide important new insights on the fast kinetics and the action of the allosteric modulators on mGlu activation at the single molecule level.