[Recent advances in the structural biology of the class C G protein-coupled receptors: The metabotropic Glutamate receptor 5].

Class C GPCRs, that include metabotropic glutamate receptors (mGlu), taste receptors, GABA receptor and Calcium-sensing receptor, are unusual in terms of their molecular architecture and allosteric regulation. They all form obligatory dimers, dimerization being fundamental for their function. More specifically, the mGlu are activated by the main excitatory neurotransmitter, L-glutamate. mGlu activation by glutamate binding in the venus flytrap domain (VFT) triggers conformational changes that are transmitted, through the Cystein-Rich Domain (CRD), to the conserved fold of 7 transmembrane helices (7TM), that couples to intracellular G protein. mGlu activity can also be allosterically modulated by positive (PAM) or negative (NAM) allosteric modulators binding to the 7TM. Recent progress in cryo-electron microscopy (cryoEM) has allowed unprecedented advances in deciphering the structural and molecular basis of their activation mechanism. The agonist induces a large movement between the subunits, bringing the 7TMs together and stabilizing a 7TM conformation structurally similar to the inactive state. The diversity of inactive conformations for the class C was unexpected but allows PAM stabilising a 7TM active conformation independent of the conformational changes induced by agonists, representing an alternative mode of mGlu activation. Here we present and discuss recent structural characterisation of mGlu receptors, highlighting findings that make the class C of GPCR unique. Understanding the structural basis of mGlu dimer signaling represents a landmark achievement and paves the way for structural investigation of GPCR dimer signaling in general. Structural information will open new avenues for structure-based drug design.