The Conformational Equilibria of a Human GPCR Compared between Lipid Vesicles and Aqueous Solutions by Integrative F-NMR.

Endogenous phospholipids influence the conformational equilibria of G protein-coupled receptors, regulating their ability to bind drugs and form signaling complexes. However, most studies of GPCR-lipid interactions have been carried out in mixed micelles or lipid nanodiscs. Though useful, these membrane mimetics do not fully replicate the physical properties of native cellular membranes associated with large assemblies of lipids. We investigated the conformational equilibria of the human A adenosine receptor (AAR) in phospholipid vesicles using F solid-state magic angle spinning NMR (SSNMR). By applying an optimized sample preparation workflow and experimental conditions, we were able to obtain F-SSNMR spectra for both antagonist- and agonist-bound complexes with sensitivity and line widths closely comparable to those achieved using solution NMR. This facilitated a direct comparison of the AAR conformational equilibria across detergent micelle, lipid nanodisc, and lipid vesicle preparations. While antagonist-bound AAR showed similar conformational equilibria across all membrane and membrane mimetic systems, the conformational equilibria of agonist-bound AAR exhibited differences among different environments. This suggests that the conformational equilibria of GPCRs may be influenced not only by specific receptor-lipid interactions but also by the membrane properties found in larger lipid assemblies.