Cholesterol interactions with fluid-phase phospholipids:: Effect on the lateral organization of the bilayer

The lateral organization of lipids and proteins in cell membranes is recognized as an important factor in several cellular processes. Cholesterol is thought to function as a modulator of the lateral segregation of lipids into cholesterol-poor and cholesterol-rich domains. We investigated how the affinity of cholesterol for different phospholipids, as seen in cholesterol partitioning between methyl-beta-cyclodextrin and large unilamellar vesicles, was reflected in the lateral organization of lipids in complex bilayers. We especially wanted to determine how the low-T-m lipid affected the lateral structure. Partition experiments showed that cholesterol had a higher affinity for N-oleoyl-sphingomyelin (OSM) than for palmitoyl-oleoyl- phosphatidylcholine (POPC) bilayers, but the highest preference was for N-palmitoyl-sphingomyelin (PSM)-containing bilayers. Partial phase diagrams of POPC/PSM/cholesterol and OSM/PSM/cholesterol bilayers at 23 degrees C and 37 degrees C were used to gain insight into the lateral organization of lipids in bilayers. Analysis of phase diagrams revealed that the phospholipid composition of cholesterol-poor and cholesterol-rich domains reflected the affinity that cholesterol exhibited toward bilayers composed of different lipids. Therefore, the determined affinity of cholesterol for different phospholipid bilayers was useful in predicting the cholesterol-induced lateral segregation of lipids in complex bilayers.