The curvature and cholesterol content of phospholipid bilayers alter the transbilayer distribution of specific molecular species of phosphatidylethanolamine

The curvature, cholesterol content, and transbilayer distribution of phosph olipids significantly influence the functional properties of cellular membr anes, yet little is known of how these parameters interact. In this study, the transbilayer distribution of phosphatidylethanolamine (PE) is determine d in vesicles with large (98 nm) and small (19 nm) radii of curvature and w ith different proportions of PE, phosphatidylcholine, and cholesterol. It w as found that the mean diameters of both types of vesicles were not influen ced by their lipid composition, and that the amino-reactive compound 2, 4, 6-trinitrobenzenesulphonic acid (TNBS) was unable to cross the bilayer of e ither type of vesicle. When large vesicles were treated with TNBS, similar to 40% of the total membrane PE was derivatized; in the small vesicles 55% reacted. These values are interpreted as representing the percentage of tot al membrane PE residing in the outer leaflet of the vesicle bilayer. The la rge vesicles likely contained similar to 20% of the total membrane lipid as internal membranes. Therefore, in both types of vesicles, PE as a phosphol ipid class was randomly distributed between the inner and outer leaflets of the bilayer. The proportion of total PE residing in the outer leaflet was unaffected by changes in either the cholesterol or PE content of the vesicl es. However, the transbilayer distributions of individual molecular species of PE were not random, and were significantly influenced by radius of curv ature, membrane cholesterol content, or both. For example, palmitate- and d ocosahexaenoate-containing species of PE were preferentially located in the outer leaflet of the bilayer. Membrane cholesterol content affected the tr ansbilayer distributions of stearate-, oleate-, and linoleate-containing sp ecies. The transbilayer distributions of palmitate-, docosahexaenoate-, and stearate-containing species were significantly influenced by membrane curv ature, but only in the presence of high levels of cholesterol. Thus, differ ences in membrane curvature and cholesterol content alter the array of PE m olecules present on the surfaces of phospholipid bilayers. In cells and org anelles, these differences could have profound effects on a number of criti cal membrane functions and processes.