ABSTRACT
The lipid composition of the plasma membrane is a key parameter in controlling signal transduction through G protein-coupled receptors (GPCRs). Adenosine A2A receptor (A2AAR) is located in the lipid bilayers of cells, containing acyl chains derived from docosahexaenoic acid (DHA). For the NMR studies, we prepared A2AAR in lipid bilayers of nanodiscs, containing DHA chains and other acyl chains. The DHA chains in nanodiscs enhanced the activation of G proteins by A2AAR. Our NMR studies revealed that the DHA chains redistribute the multiple conformations of A2AAR toward those preferable for G protein binding. In these conformations, the rotational angle of transmembrane helix 6 is similar to that in the A2AAR-G protein complex, suggesting that the population shift of the equilibrium causes the enhanced activation of G protein by A2AAR. These findings provide insights into the control of neurotransmissions by A2AAR and the effects of lipids on various GPCR functions.
Subject(s)
Adenosine A2 Receptor Agonists/chemistry , Adenosine A2 Receptor Agonists/pharmacology , Docosahexaenoic Acids/chemistry , Docosahexaenoic Acids/pharmacology , Magnetic Resonance Spectroscopy , Receptor, Adenosine A2A/chemistry , Receptor, Adenosine A2A/metabolism , Cell Membrane/metabolism , Docosahexaenoic Acids/analogs & derivatives , Lipid Bilayers , Models, Molecular , Molecular Conformation , Protein Binding , Recombinant Proteins , Signal Transduction , Solvents , Structure-Activity RelationshipABSTRACT
G-protein-coupled receptor (GPCR) ligands impart differing degrees of signaling in the G-protein and arrestin pathways, in phenomena called "biased signaling". However, the mechanism underlying the biased signaling of GPCRs is still unclear, although crystal structures of GPCRs bound to the Gâ protein or arrestin are available. In this study, we observed the NMR signals from methionine residues of the µ-opioid receptor (µOR) in the balanced- and biased-ligand-bound states. We found that the intracellular cavity of µOR exists in an equilibrium between closed and multiple open conformations with coupled conformational changes on the transmembrane helices 3, 5, 6, and 7, and that the population of each open conformation determines the G-protein- and arrestin-mediated signaling levels in each ligand-bound state. These findings provide insight into the biased signaling of GPCRs and will be helpful for development of analgesics that stimulate µOR with reduced tolerance and dependence.
Subject(s)
Receptors, Opioid, mu/chemistry , Nuclear Magnetic Resonance, Biomolecular , Protein ConformationABSTRACT
G-protein-coupled receptors (GPCRs) exist in conformational equilibrium between active and inactive states, and the former population determines the efficacy of signaling. However, the conformational equilibrium of GPCRs in lipid bilayers is unknown owing to the low sensitivities of their NMR signals. To increase the signal intensities, a deuteration method was developed for GPCRs expressed in an insect cell/baculovirus expression system. The NMR sensitivities of the methionine methyl resonances from the ß2 -adrenergic receptor (ß2 AR) in lipid bilayers of reconstituted high-density lipoprotein (rHDL) increased by approximately 5-fold upon deuteration. NMR analyses revealed that the exchange rates for the conformational equilibrium of ß2 AR in rHDLs were remarkably different from those measured in detergents. The timescales of GPCR signaling, calculated from the exchange rates, are faster than those of receptor tyrosine kinases and thus enable rapid neurotransmission and sensory perception.
