G protein-coupled receptors (GPCRs), the largest family of membrane proteins, enable cells to respond to a wide array of extracellular stimuli and control diverse intracellular signaling pathways. The Class C GPCRs, which contain the metabotropic glutamate receptors (mGluRs) and GABAB receptors, are expressed throughout the nervous system where they control excitability and synaptic strength and serve as drug targets for many neurological disorders including schizophrenia, fragile x syndrome, Alzheimer’s disease and depression. Understanding how Class C GPCRs are activated may aid the development of drugs for the treatment of these neurological disorders.
GPCRs change their shape when an activating ligand binds. Specific conformational changes activate the receptor and trigger intracellular signaling. To understand this dynamic process, Reza Vafabakhsh and Joshua Levitz from Ehud Isacoff’s Lab developed a novel single-molecule fluorescence resonance energy transfer (FRET) assay. They focused on two Class C GPCRs, mGluR2 and mGluR3. Each mGluR is composed of two identical protein subunits. Vafabakhsh and Levitz introduced a FRET donor or acceptor fluorophore near the ligand-binding domain of each mGluR subunit. The efficiency of energy transfer between the donor and acceptor fluorophore is dependent on the distance between them: if the FRET efficiency goes down, it indicates that the ligand-binding domains of the mGluR subunits are moving apart. Using this approach, they found that the ligand-binding domains of mGluRs rapidly interconvert between three distinct conformations: resting, intermediate, and active. mGluR3 was found to have a minimal level of intracellular signaling even when resting, while mGluR2 did not. Basal signaling of mGluR3 was mediated in part by calcium binding. This work supports a three-state model of receptor activation that reconciles a body of functional and structural studies.
Read the research article, “Conformational dynamics of a class C G-protein-coupled receptor,” by Reza Vafabakhsh*, Joshua Levitz*, & Ehud Y. Isacoff, in Nature.