Structure of Cholesterol in Lipid Rafts

Rafts, or functional domains, are transient nano-or mesoscopic structures in the plasma membrane and are thought to be essential for many cellular processes such as signal transduction, adhesion, trafficking, and lipid or protein sorting. Observations of these membrane heterogeneities have proven challenging, as they are thought to be both small and short lived. With a combination of coarse-grained molecular dynamics simulations and neutron diffraction using deuterium labeled cholesterol molecules, we observe raftlike structures and determine the ordering of the cholesterol molecules in binary cholesterol-containing lipid membranes. From coarse-grained computer simulations, heterogenous membranes structures were observed and characterized as small, ordered domains. Neutron diffraction was used to study the lateral structure of the cholesterol molecules. We find pairs of strongly bound cholesterol molecules in the liquid-disordered phase, in accordance with the umbrella model. Bragg peaks corresponding to ordering of the cholesterol molecules in the raftlike structures were observed and indexed by two different structures: a monoclinic structure of ordered cholesterol pairs of alternating direction in equilibrium with cholesterol plaques, i.e., triclinic cholesterol bilayers.

Figure 1

(a) Schematic representation of DPPC and cholesterol molecules used in the simulations. (b) Snapshot of the simulation at $\mu =8.5k_{B}T$, resulting in a cholesterol concentration of $\approx 17\mathrm{mol}\%$. (c) Snapshot of the simulation at $\mu =7.8k_{B}T$ resulting in a cholesterol concentration of $\approx 60\mathrm{mol}\%$.

Figure 2

(a) Total cholesterol concentration and cholesterol concentration inside rafts for different chemical potential $\mu$. Inset shows a histogram of local cholesterol densities, taken using squares of area $25{\sigma }^2\approx 9{\mathrm{nm}}^2$. (b) Radially averaged two-dimensional lateral structure factor of cholesterol head groups for different $\mu$ as indicated. The level of molecular order increases with decreasing $\mu$, i.e., increasing cholesterol concentration.

Figure 3

(a) Schematics of DPPC and (deuterated) cholesterol molecules. (b) Sketch of the scattering geometry. $q_{\parallel }$ denotes the in-plane component of the scattering vector. (c) Diffraction measured at $\lambda =2.37\AA$ showing broad, fluidlike peaks. (d) Data measured at $\lambda =1.44$ and 1.48 Å. Several pronounced Bragg peaks are observed in addition to the broad peaks in (a). (e) Illustration of the different molecular structures: pairs of cholesterol molecules in the liquid-disordered regions of the membrane in equilibrium with highly ordered cholesterol structures such as the umbrella structure (f) and cholesterol plaques (g). An aluminum Bragg peak due to the windows of the humidity chamber and the sample holder is present at $q_{\parallel }=2.68{\AA }^{-1}$. Aluminum forms a face-centered cubic lattice with lattice parameter $a=4.04941\AA$ [45].