Evidence for a single heptahelical domain being turned on upon activation of a dimeric GPCR.
G-protein-coupled receptors (GPCRs) have been shown to form dimers, but the relevance of this phenomenon in G-protein activation is not known. Among the large GPCR family, metabotropic glutamate (mGlu) receptors are constitutive dimers. Here we examined whether both heptahelical domains (HDs) are turned on upon full receptor activation. To that aim, we measured G-protein coupling efficacy of dimeric mGlu receptors in which one subunit bears specific mutations. We show that a mutation in the third intracellular loop (i3 loop) known to prevent G-protein activation in a single subunit decreases coupling efficacy. However, when a single HD is blocked in its inactive state using an inverse agonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), no decrease in receptor activity is observed. Interestingly, in a receptor dimer in which the subunit that binds MPEP is mutated in its i3 loop, MPEP enhances agonist-induced activity, reflecting a 'better' activation of the adjacent HD. These data are consistent with a model in which a single HD is turned on upon activation of such homodimeric receptors and raise important issues in deciphering the functional role of GPCR dimer formation for G-protein activation.
Hlavackova
Veronika
V
Department of Molecular Pharmacology, Institute of Experimental Medicine, Czech Academy of Science, Prague, Czech Republic.
Goudet
Cyril
C
Kniazeff
Julie
J
Zikova
Alice
A
Maurel
Damien
D
Vol
Claire
C
Trojanova
Johana
J
Prézeau
Laurent
L
Pin
Jean-Philippe
JP
Blahos
Jaroslav
J
D002460:Cell Line; D019281:Dimerization; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D011725:Pyridines / Q000494:pharmacology; D018094:Receptors, Metabotropic Glutamate / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D018691:Excitatory Amino Acid Antagonists; D021122:Protein Subunits; D011725:Pyridines; D018094:Receptors, Metabotropic Glutamate; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; C104077:metabotropic glutamate receptor type 1; C121465:6-methyl-2-(phenylethynyl)pyridine