The G protein-coupling profile of metabotropic glutamate receptors, as determined with exogenous G proteins, is independent of their ligand recognition domain

Metabotropic glutamate (mGlu), Ca2+-sensing, gamma-aminobutyric acid(B), and a large number of pheromone receptors constitute a peculiar family of G protein-coupled receptors. They possess a large extracellular domain that has been proposed to constitute their ligand binding domain. The aim of the current study was to examine whether this large ligand binding domain had any influence on the G protein-coupling selectivity of the receptor, and vice versa. We chose mGlu receptors, which are classified into three groups according to their sequence homology and pharmacology, as representatives of this receptor family. To define a G protein-coupling profile for these receptors, we used a set of exogenous phospholipase C-activating G proteins in the same way that synthetic ligands are used to define agonist and antagonist pharmacological profiles. This set includes G(alpha 15), G(alpha 16), G(alpha q) and chimeric G(alpha q) proteins with the last few amino acids of either G(alpha i2), (G(alpha qi)), G(alpha o) (G(alpha qo)), or G(alpha z) (G(alpha qz)). Cotransfection of mGlu receptors with these G proteins and examination of their coupling to phospholipase C revealed that group I, II, and III receptors have distinct G protein-coupling profiles. By swapping the extracellular domains of the most distantly related mGlu receptors (the rat group I mGlu1a and the Drosophila melanogaster group II DmGluA receptors), we show that the extracellular domain determines the agonist pharmacological profile and that this domain does not modify the G protein-coupling profile determined by the seven-transmembrane-domain region of mGlu receptors.