Heptahelical domain of metabotropic glutamate receptor 5 behaves like rhodopsin-like receptors

doi:10.1073/pnas.0304699101

Heptahelical domain of metabotropic glutamate receptor 5 behaves like rhodopsin-like receptors

^†Department of Molecular Pharmacology, Laboratory of Functional Genomics, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2580, CCIPE, 34094 Montpellier Cedex 5, France; ^‡Center for Structural Biology, Centre National de la Recherche Scientifique–Institut National de la Santé et de la Recherche Médicale, University Montpellier-I, 34060 Montpellier, France; and ^§Laboratory of Pharmacological and Toxicological Chemistry and Biochemistry, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 8601, University Paris-V, 75270 Paris Cedex 6, France

Edited by Shigetada Nakanishi, Kyoto University, Kyoto, Japan (received for review July 25, 2003)

Abstract

Although agonists bind directly in the heptahelical domain (HD) of most class-I rhodopsin-like G protein coupled receptors (GPCRs), class-III agonists bind in the extracellular domain of their receptors. Indeed, the latter possess a large extracellular domain composed of a cysteine-rich domain and a Venus flytrap module. Both the low sequence homology and the structural organization of class-III GPCRs raised the question of whether or not the HD of these receptors functions the same way as rhodopsin-like GPCRs. Here, we show that the HD of metabotropic glutamate receptor 5 (mGlu₅) displays the same agonist-independent constitutive activity as the wild-type receptor. Moreover, we show that the noncompetitive antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine hydrochloride] and the positive allosteric modulator DFB (3,3′-difluorobenzaldazine) act as inverse agonist and full agonist, respectively, on the mGlu₅ HD in the absence of the extracellular domain. This finding illustrates that, like rhodopsin-like receptors, the HD of mGluRs can constitutively couple to G proteins and be negatively and positively regulated by ligands. These data show that the HD of mGluRs behave like any other class-I GPCRs in terms of G protein coupling and regulation by various types of ligands.

Footnotes

↵ ¶ To whom correspondence should be addressed. E-mail: jppin{at}ccipe.cnrs.fr.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: DFB, 3,3′-difluorobenzaldazine; GPCR, G protein-coupled receptor; HA, hemagglutinin; HD, heptahelical domain; CRD, cysteine-rich domain; IP, inositol phosphate; MPEP, 2-methyl-6-(phenylethynyl)-pyridine hydrochloride; VFTM, Venus flytrap module; GABA, γ-aminobutyric acid; mGluR, metabotropic glutamate receptor.