Asymmetric Structural Features in Single Supported Lipid Bilayers Containing Cholesterol and G M1 Resolved with Synchrotron X-Ray Reflectivity

Abstract

The cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor G M1 affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl- sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10 Å thicker and have a higher acyl-chain electron density (〈 ρ chain〉 = 0.33 e −/Å 3) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (〈 ρ chain〉 = 0.28 e −/Å 3). Incorporating G M1 into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4–9 Å. The structural effect of G M1 is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl- sn-glycero-3-phosphocholine, where the distal chains show a high electron density (〈 ρ chain〉 = 0.33 e −/Å 3) and the lipid diffusion constant is reduced by ∼50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer.