Solid-State NMR of highly 13C-enriched cholesterol in lipid bilayers

Segmental Dynamics of Membranous Cholesterol are Coupled

Cholesterol promotes the structural integrity of the fluid cell membrane and interacts dynamically with many membrane proteins to regulate function. Understanding site-resolved cholesterol structural dynamics is thus important. This long-standing challenge has thus far been addressed, in part, by selective isotopic labeling approaches. Here we present a new 3D solid-state NMR (SSNMR) experiment utilizing scalar 13C-13C polarization transfer and recoupling of the 1H-13C interactions in order to determine average dipolar couplings for all 1H-13C vectors in uniformly 13C-enriched cholesterol. The experimentally determined order parameters (OP) agree exceptionally well with molecular dynamics (MD) trajectories and reveal coupling among several conformational degrees of freedom in cholesterol molecules. Quantum chemistry shielding calculations further support this conclusion and specifically demonstrate that ring tilt and rotation are coupled to changes in tail conformation and that these coupled segmental dynamics dictate the orientation of cholesterol. These findings advance our understanding of physiologically relevant dynamics of cholesterol, and the methods that revealed them have broader potential to characterize how structural dynamics of other small molecules impact their biological functions.

Analysis of the orientation of cholesterol in high-density lipoprotein nanodiscs using solid-state NMR

Cholesterol is an essential component of eukaryotic cellular membranes that regulates the order and phase behaviour of dynamic lipid bilayers. Although cholesterol performs many vital physiological roles, hypercholesterolaemia and the accumulation of cholesterol in atherosclerotic plaques can increase the risk of coronary heart disease morbidity. The risk is mitigated by the transportation of cholesterol from peripheral tissue to the liver by high-density lipoprotein (HDL), 6-20 nm-diameter particles of lipid bilayers constrained by an annular belt of the protein apolipoprotein A-I (apoA-I). Information on the dynamics and orientation of cholesterol in HDL is pertinent to the essential role of HDL in cholesterol cycling. This work investigates whether the molecular orientation of cholesterol in HDL differs from that in the unconstrained lipid bilayers of multilamellar vesicles (MLVs). Solid-state NMR (ssNMR) measurements of dynamically-averaged ¹³C-¹³C and ¹³C-¹H dipolar couplings were used to determine the average orientation of triple ¹³C-labelled cholesterol in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers in reconstituted HDL (rHDL) nanodiscs and in MLVs. Individual ¹³C-¹³C dipolar couplings were measured from [2,3,4-¹³C₃]cholesterol in a one-dimensional NMR experiment, by using a novel application of a method to excite double quantum coherence at rotational resonance. The measured dipolar couplings were compared with average values calculated from orientational distributions of cholesterol generated using a Gaussian probability density function. The data were consistent with small differences in the average orientation of cholesterol in rHDL and MLVs, which may reflect the effects of the constrained and unconstrained lipid bilayers in the two environments. The calculated distributions of cholesterol in rHDL and MLVs that were consistent with the NMR data also agreed well with orientational distributions extracted from previous molecular dynamics simulations of HDL nanodiscs and planar POPC bilayers.

2H,13C-Cholesterol for Dynamics and Structural Studies of Biological Membranes

We present a cost-effective means of ²H and ¹³C enrichment of cholesterol. This method exploits the metabolism of ²H,¹³C-acetate into acetyl-CoA, the first substrate in the mevalonate pathway. We show that growing the cholesterol producing strain RH6827 of Saccharomyces cerevisiae in ²H,¹³C-acetate-enriched minimal media produces a skip-labeled pattern of deuteration. We characterize this cholesterol labeling pattern by mass spectrometry and solid-state nuclear magnetic resonance spectroscopy. It is confirmed that most ²H nuclei retain their original ²H-¹³C bonds from acetate throughout the biosynthetic pathway. We then quantify the changes in ¹³C chemical shifts brought by deuteration and the impact upon ¹³C-¹³C spin diffusion. Finally, using adiabatic rotor echo short pulse irradiation cross-polarization (^RESPIRATIONCP), we acquire the ²H-¹³C correlation spectra to site specifically quantify cholesterol dynamics in two model membranes as a function of temperature. These measurements show that cholesterol acyl chains at physiological temperatures in mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), sphingomyelin, and cholesterol are more dynamic than cholesterol in POPC. However, this overall change in motion is not uniform across the cholesterol molecule. This result establishes that this cholesterol labeling pattern will have great utility in reporting on cholesterol dynamics and orientation in a variety of environments and with different membrane bilayer components, as well as monitoring the mevalonate pathway product interactions within the bilayer. Finally, the flexibility and universality of acetate labeling will allow this technique to be widely applied to a large range of lipids and other natural products.

A Cholesterol Dimer Stabilizes the Inactivated State of an Inward-Rectifier Potassium Channel

Cholesterol oligomers reside in multiple membrane protein X-ray crystal structures. Yet, there is no direct link between these oligomers and a biological function. Here we present the structural and functional details of a cholesterol dimer that stabilizes the inactivated state of an inward-rectifier potassium channel KirBac1.1. K⁺ efflux assays confirm that high cholesterol concentration reduces K⁺ conductance. We then determine the structure of the cholesterol-KirBac1.1 complex using Xplor-NIH simulated annealing calculations driven by solid-state NMR distance measurements. These calculations identified an α-α cholesterol dimer docked to a cleft formed by adjacent subunits of the homotetrameric protein. We compare these results to coarse grain molecular dynamics simulations. This is one of the first examples of a cholesterol oligomer performing a distinct biological function and structural characterization of a conserved promiscuous lipid binding region.

An Inward-Rectifier Potassium Channel Coordinates the Properties of Biologically Derived Membranes

KirBac1.1 is a prokaryotic inward-rectifier K⁺ channel from Burkholderia pseudomallei. It shares the common inward-rectifier K⁺ channel fold with eukaryotic channels, including conserved lipid-binding pockets. Here, we show that KirBac1.1 changes the phase properties and dynamics of the surrounding bilayer. KirBac1.1 was reconstituted into vesicles composed of ¹³C-enriched biological lipids. Two-dimensional liquid-state and solid-state NMR experiments were used to assign lipid ¹H and ¹³C chemical shifts as a function of lipid identity and conformational degrees of freedom. A solid-state NMR temperature series reveals that KirBac1.1 lowers the primary thermotropic phase transition of Escherichia coli lipid membranes while introducing both fluidity and internal lipid order into the fluid phases. In B. thailandensis liposomes, the bacteriohopanetetrol hopanoid, and potentially ornithine lipids, introduce a similar primary lipid-phase transition and liquid-ordered properties. Adding KirBac1.1 to B. thailandensis lipids increases B. thailandensis lipid fluidity while preserving internal lipid order. This synergistic effect of KirBac1.1 in bacteriohopanetetrol-rich membranes has implications for bilayer dynamic structure. If membrane proteins can anneal lipid translational degrees of freedom while preserving internal order, it could offer an explanation to the nature of liquid-ordered protein-lipid organization in vivo.

Determining Cholesterol Binding to Membrane Proteins by Cholesterol 13C Labeling in Yeast and Dynamic Nuclear Polarization NMR

We present a general strategy for determining the cholesterol-binding site of eukaryotic membrane proteins in native-like lipid membranes by NMR spectroscopy. The strategy combines yeast biosynthetic 13C enrichment of cholesterol with detection of protein-cholesterol 13C-13C cross peaks in 2D correlation NMR spectra under the dynamic nuclear polarization (DNP) condition. Low-temperature DNP not only allows high-sensitivity detection of weak protein-cholesterol cross peaks in 2D spectra but also immobilizes cholesterol and protein to enable intermolecular distance measurements. We demonstrate this approach on the influenza M2 protein, which utilizes cholesterol to conduct membrane scission in the last step of virus budding and release from the host cell plasma membrane. A 13C-13C double-quantum filter was employed to significantly simplify the 2D 13C-13C correlation spectra and facilitate the identification of protein-cholesterol cross peaks. A number of cross peaks between the M2 transmembrane residues' side chains and the cholesterol sterol group were detected, which complement recently measured protein contacts to the isooctyl tail of cholesterol to define an extended binding interface. These results provide atomic-level evidence of M2-cholesterol interaction to cause membrane curvature and scission, and the approach is generally applicable to other eukaryotic membrane proteins for understanding the influence of cholesterol on membrane protein function.