The role of Arg(78) in the metabotropic glutamate receptor mGlu(1) for agonist binding and selectivity

The metabotropic glutamate receptors belong to family C of the G-protein co upled receptor superfamily. These receptors all possess large extracellular amino terminal domains, where agonist binding takes place. We have previou sly constructed a molecular model of the amino terminal domain of the mGlu( 1) receptor based on a weak amino acid sequence similarity with a family of bacterial periplasmic binding proteins (PBPs). The residues Ser(165) and T hr(188) were demonstrated to be involved in agonist binding to the receptor . Here, we report that mutation of Arg(78) in the mGlu(1b) receptor to leuc ine or glutamate completely knocks out [H-3]quisqualic acid binding to the receptor. The constructed mutants, R78L and R78E, have also been characteri zed in a inositol phosphate assay. Here, the potency of (S)-glutamic acid a nd (S)-quisqualic acid was reduced 1000- and 100-fold, respectively, on R78 L compared to the wild type (WT) receptor. (S)-Quisqualic acid was as poten t on mutant R78E as it was on R78L, whereas (S)-glutamic acid was unable to activate R78E significantly at concentrations up to 10 mM. in conclusion, Arg(78) appears to be essential for agonist binding to the mGlu(1) receptor , most likely, through the formation of an ionic bond between its positivel y charged side chain and the distal acid group of the agonists. Furthermore , the different impact of the two mutations on (S)-glutamic acid and (S)-qu isqualic acid potencies strongly indicates that while Arg(78) appears to be a common site of interaction for the agonists, the Group I subtype selecti vity of (S)-quisqualic acid is probably determined by other residues in the amino terminal domain. (C) 2000 Elsevier Science B.V. All rights reserved.