Structural Basis of Signal Instigation Through Family C GPCRs

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Abstract The γ-aminobutyric acid B receptor (GABABR) and the metabotropic glutamate receptors (mGluRs) belong to the Family C of G protein coupled receptors (GPCRs) and critically regulate neuronal excitability, synaptic transmission and plasticity. Many disorders of the CNS have been linked to alterations in neuronal excitability via the glutamatergic and GABAergic system. Accordingly, mGluRs and GABABR have been the subject of an enormous drug discovery effort as they represent major therapeutic targets for treating numerous physiological dysfunctions and for neurodegenerative and neuropsychiatric conditions. Apart from the prototypical seven transmembrane helix (7TM) domain, Family C GPCRs also include a large extracellular ‘venus fly trap’ (VFT) domain that constitutes the orthosteric ligand binding site. Binding of ligand to the extracellular VFT domain triggers a large conformational change in the VFT domains from an open to a closed conformation. This clam- shell like closure of the extracellular domain results in receptor engagement and activation of G proteins on the intracellular side of the transmembrane domain with a mechanism that remains unclear. Receptor activated G proteins then act to either enhance or repress secondary messenger signaling cascades. We recently showed cryoEM structures of near-full length mGluR5 and GABABR in inactive and active conformations, revealing extensive transitions in the organization of the 7TM dimer upon ligand binding to the VFT. Notwithstanding this progress, several key questions remain regarding the allosteric communication across the cell membrane by Family C GPCRs, and particularly the mechanism of G protein coupling and activation. To address these questions, we propose to obtain the structures of mGluR2, mGluR5 and GABABR in complex with their cognate G proteins and probe the structural insights using molecular dynamics simulations and mutagenesis coupled to functional assays. The similarities and differences amongst these receptor-G protein complexes will allow us to contrast and compare our findings and examine aspects of G protein coupling and selectivity. Collectively, these studies will enable us to create a detailed mechanistic framework to understand Family C GPCR signaling and will form the basis for the design of novel therapeutic strategies targeting these receptors.