Temperature and Pressure Based NMR Studies of Detergent Micelle Phase Equilibria

Allosteric modulation of the adenosine A2A receptor by cholesterol

Cholesterol is a major component of the cell membrane and commonly regulates membrane protein function. Here, we investigate how cholesterol modulates the conformational equilibria and signaling of the adenosine A2A receptor (A2AR) in reconstituted phospholipid nanodiscs. This model system conveniently excludes possible effects arising from cholesterol-induced phase separation or receptor oligomerization and focuses on the question of allostery. GTP hydrolysis assays show that cholesterol weakly enhances the basal signaling of A2AR while decreasing the agonist EC50. Fluorine nuclear magnetic resonance (19F NMR) spectroscopy shows that this enhancement arises from an increase in the receptor's active state population and a G-protein-bound precoupled state. 19F NMR of fluorinated cholesterol analogs reveals transient interactions with A2AR, indicating a lack of high-affinity binding or direct allosteric modulation. The combined results suggest that the observed allosteric effects are largely indirect and originate from cholesterol-mediated changes in membrane properties, as shown by membrane fluidity measurements and high-pressure NMR.

High Pressure Shifts the β1-Adrenergic Receptor to the Active Conformation in the Absence of G Protein

G protein-coupled receptors (GPCRs) are versatile chemical sensors, which transmit the signal of an extracellular binding event across the plasma membrane to the intracellular side. This function is achieved via the modulation of highly dynamical equilibria of various conformational receptor states. Here we have probed the effect of pressure on the conformational equilibria of a functional thermostabilized β₁-adrenergic GPCR (β₁AR) by solution NMR. High pressure induces a large shift in the conformational equilibrium (midpoint ∼600 bar) from the preactive conformation of agonist-bound β₁AR to the fully active conformation, which under normal pressure is only populated when a G protein or a G protein-mimicking nanobody (Nb) binds to the intracellular side of the β₁AR·agonist complex. No such large effects are observed for an antagonist-bound β₁AR or the ternary β₁AR·agonist·Nb80 complex. The detected structural changes of agonist-bound β₁AR around the orthosteric ligand binding pocket indicate that the fully active receptor occupies an ∼100 ų smaller volume than that of its preactive form. Most likely, this volume reduction is caused by the compression of empty (nonhydrated) cavities in the ligand binding pocket and the center of the receptor, which increases the ligand receptor interactions and explains the ∼100-fold affinity increase of agonists in the presence of G protein. The finding that isotropic pressure induces a directed motion from the preactive to the fully active GPCR conformation provides evidence of the high mechanical robustness of this important functional switch.

The Fluidity of Phosphocholine and Maltoside Micelles and the Effect of CHAPS

The dynamics of phosphocholine and maltoside micelles, detergents frequently used for membrane protein structure determination, were investigated using electron paramagnetic resonance of spin probes doped into the micelles. Specifically, phosphocholines are frequently used detergents in NMR studies, and maltosides are frequently used in x-ray crystallography structure determination. Beyond the structural and electrostatic differences, this study aimed to determine whether there are differences in the local chain dynamics (i.e., fluidity). The nitroxide probe rotational dynamics in longer chain detergents is more restricted than in shorter chain detergents, and maltoside micelles are more restricted than phosphocholine micelles. Furthermore, the micelle microviscosity can be modulated with mixtures, as demonstrated with mixtures of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate with n-dodecylphosphocholine, n-tetradecylphosphocholine, n-decyl-β-D-maltoside, or n-dodecyl-β-D-maltoside. These results indicate that observed differences in membrane protein stability in these detergents could be due to fluidity in addition to the already determined structural differences.

Nuts and Bolts of CF3 and CH 3 NMR Toward the Understanding of Conformational Exchange of GPCRs

With the advent of efficient protein expression and functional purification protocols, it is now possible to reconstitute many G protein-coupled receptors (GPCRs) in detergent micelles at concentrations of 25 μM or more. Such concentrations are sufficient for studies of conformational states and dynamics relating to function and the mechanism of activation of GPCRs, using solution state NMR. In particular, methyl spectroscopy, in the form of one-dimensional (19)F NMR or two-dimensional ((1)H,(13)C) NMR, provides high fidelity spectra which reveal detailed features associated with conformational states and their lifetimes, as a function of ligand. While X-ray crystallography provides exquisitely detailed structures of lowest energy states associated with ligands, G proteins, and other proteins, NMR is able to validate such states, while providing insight into higher energy states that form part of the conformational landscape and are involved in activation. Through relaxation experiments spanning microseconds to seconds, lifetimes of these functional states can often be measured. By determining the effect of ligands on both equilibrium populations and rates of interconversion between states, it becomes possible to understand activation in terms of an ensemble description and in turn relate the ensemble to pharmaceutical phenomena.