INDUCTION OF LIPID ASYMMETRY AND EXCHANGE IN MODEL MEMBRANE SYSTEMS

Negatively-charged lipids such as fatty acids, mono- and diacyl phosphatidylglycerol, diacyl phosphatidic acid and cardiolipin redistribute across a lipid bilayer in response to an applied pH gradient to accumulate on the high pH side of the membrane thereby forming bilayers that are asymmetric with respect to lipid composition. A novel general procedure for the quantitative assessment of lipid asymmetry has been developed, based upon partition in aqueous two-phase polymer systems. Herein several examples are provided to demonstrate the utility of the partition method. The effects of temperature and lipid composition upon both the rate of formation and extent of asymmetry in model systems are described. Finally an example of one system is given in which the interaction between two membrane surfaces is modulated by the lipid asymmetry of the membranes.