Assessing the nature of lipid raft membranes

Abstract

The paradigm of biological membranes has recently gone through a major update. Instead of being fluid and homogeneous, recent studies suggest that membranes are characterized by transient domains with varying fluidity. In particular, a number of experimental studies have revealed the existence of highly ordered lateral domains rich in sphingomyelin and cholesterol (CHOL). These domains, called functional lipid rafts, have been suggested to take part in a variety of dynamic cellular processes such as membrane trafficking, signal transduction, and regulation of the activity of membrane proteins. However, despite the proposed importance of these domains, their properties, and even the precise nature of the lipid phases, have remained open issues mainly because the associated short time and length scales have posed a major challenge to experiments. In this work, we employ extensive atom-scale simulations to elucidate the properties of ternary raft mixtures with CHOL, palmitoylsphingomyelin (PSM), and palmitoyloleoylphosphatidylcholine. We simulate two bilayers of 1,024 lipids for 100 ns in the liquid-ordered phase and one system of the same size in the liquid-disordered phase. The studies provide evidence that the presence of PSM and CHOL in raft-like membranes leads to strongly packed and rigid bilayers. We also find that the simulated raft bilayers are characterized by nanoscale lateral heterogeneity, though the slow lateral diffusion renders the interpretation of the observed lateral heterogeneity more difficult. The findings reveal aspects of the role of favored (specific) lipid-lipid interactions within rafts and clarify the prominent role of CHOL in altering the properties of the membrane locally in its neighborhood. Also, we show that the presence of PSM and CHOL in rafts leads to intriguing lateral pressure profiles that are distinctly different from corresponding profiles in nonraft-like membranes. The results propose that the functioning of certain classes of membrane proteins is regulated by changes in the lateral pressure profile, which can be altered by a change in lipid content.

Figure 1. Snapshots at the End of Simulations for Systems S_A (Top), S_B (Middle), and S_C (Bottom)

POPC molecules are shown in gray, PSM in orange, CHOL in yellow, and water in cyan.

Figure 2. Snapshots Averaged over the Last 10 ns from the End of Each Simulation

The deuterium order parameters, S _CD , of selected carbons (C5–C7) of POPC and PSM chains were binned in the xy-plane (column 1, from left). The in-plane electron densities, σ, have been plotted separately for CHOL (column 2) and the selected chain carbons (column 3). The average bilayer thickness, d, was obtained from the grid of the undulation analysis (column 4). Systems S_A to S_C are represented on rows from top to bottom, respectively. Only the bottom leaflet has been used for columns 1–3, whereas both leaflets were used for column 4. The equivalent plots for the top leaflet have been presented in Figure S2.

Figure 3. 2-D Radial Distribution Functions between the Molecular Center of Mass Positions in S_A and S_B

The figures show the time evolution in the system at three different time intervals: 0–10 ns (gray, dashed), 30–40 ns (gray), and 90–100 ns (black). The error bars for the black curve indicate the average difference of the two monolayers.

Figure 4. Lateral Pressure Profiles of Systems S_A and S_B (Top) and of Previously Simulated Pure POPC/PSM Systems and a Binary DPPC–CHOL System (Bottom)

The center of the membrane is at z = 0. The graphs have been averaged to be symmetric on both sides of the center and smoothed by adaptive high-order spline fitting [90]. Error bars are statistical errors for each slab. The errors have been shown for only one of the monolayers of the DPPC–CHOL system because they are equal for both monolayers and also smaller or equal for the other systems.