Structural and molecular determinants regulating mGluR5 surface expression

Trafficking of G protein-coupled receptors (GPCRs) to the plasma membrane is a pivotal process to fulfill their biological functions. Metabotropic glutamate receptors (mGluRs; mGluR1-8) are expressed throughout the CNS and are important for modulating synaptic transmission and plasticity. Group I mGluRs, including mGluR1 and mGluR5, have long intracellular C-terminal tails containing multiple protein binding domains and sites for phosphorylation and ER retention. We have now investigated some of the structural determinants for mGluR5 trafficking to the plasma membrane by studying a series of truncations and ligand binding mutants. We also take advantage of dimer formation between the extracellular domain (ECD) of mGluR5 and design an ECD based surface-binding assay to evaluate dimerization and surface expression of mGluR5 containing various truncations or point mutations. We found that the C terminus is not essential for mGluR5 surface expression. In contrast, the 7th transmembrane domain (TM7) plays a critical role in its surface expression in both heterologous cells and neurons. Furthermore, a ligand binding mutation within the ECD of mGluR5 (Y64A/T174A) that blocks ligand binding impairs both surface expression and dimerization of mGluR5 in neurons. The integrity of both the whole 7TM domain and the C- terminal tail of mGluR5 are also important for stabilizing dimerization with the ECD. Thus multiple domains regulate dimerization and trafficking of mGluR5. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.