Morphology and dynamics of domains in ergosterol or cholesterol containing membranes

The effect of cholesterol and ergosterol on supported lipid bilayers composed of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and egg sphingomyelin (eSM) in a 1/1 M ratio was studied using atomic force microscopy. The addition of ergosterol or cholesterol to these membranes considerably modifies both the structure and the dynamics of the domains present in them. The height of the eSM enriched domains increases with concentration of both sterols, but more markedly with ergosterol. The height of the POPC enriched domains increases with concentration in a similar manner for both sterols. This effect is larger for eSM than for POPC when ergosterol, not cholesterol, is present. Domain coverage increases with both sterols at 5 mol% but decreases at 20 mol% and almost disappears at 40 mol%. The size of the eSM enriched domains decreases with sterol concentration, more markedly with cholesterol. Bilayer rupture forces show that overall stiffness increases with the addition of 5 mol% cholesterol, but only for the eSM enriched domains with ergosterol at the same concentration. At larger sterol concentrations the stiffness of both regions becomes reduced. At 40 mol% sterol concentration, both membranes present the same rupture force value. To gain mechanistic insight into these observations we performed Quantum Mechanical calculations and Molecular Dynamics simulations of the sterol molecules. We found that conformational freedom for the sterol molecules is quite different. This difference might be behind the observed phenomena. Finally, the different action of sterols on membrane properties is related to the sterol-dependent ionophoretic activity of polyene antibiotics.