The Effect of Cholesterol on the Phase Structure of Glucolipids from Acholeplasma laidlawii Membranes

Abstract

The packing properties in lipid mixtures containing cholesterol and membrane glucolipids from A. laidlawii are modified by varying the amounts of cholesterol, diacylmonoglucosylglycerol and diacyldiglucosylglycerol in the mixtures as well as the temperature and the degree of acyl chain desaturation. These changes affect both the proportions of different lipids having dissimilar molecular geometries and the geometry of the lipid molecules themselves. All mixtures containing glucolipids with equal amounts of palmitoyl and oleoyl chains formed a lamellar liquid-crystalline phase in the growth temperature range of A. laidlawii, while a reversed cubic liquid-crystalline phase dominated in mixtures containing dioleoyl glucolipids. These lipids formed a reversed hexagonal phase together with 27% cholesterol. Mixtures with lipid compositions occurring in the membranes of living Acholeplasma cells formed a lamellar liquid-crystalline phase. Large amounts of cholesterol and diacylmonoglucosylglycerol, high temperatures and a high degree of cis desaturation favored the formation of cubic or hexagonal liquid-crystalline phase structures of the investigated lipid mixtures. Diacylmonoglucosylglycerol and cholesterol are both wedge-shaped. Temperature and cis desaturation accentuate the wedge-shape properties of the glucolipid molecules. The changes in the lipid composition of A. laidlawaii membranes as a response to cholesterol incorporation can be explained by the geometry and packing characteristic of the sterol molecule; the concept of fluidity does not need to be involved.