Protein-catalyzed phospholipid exchange between gel and liquid-crystalline phospholipid vesicles

Bovine liver phospholipid exchange protein catalyzes the transfer of phosphatidylcholine between two populations of single bilayer phospholipid vesicles. Donor vesicles are prepared from egg phosphatidylcholine--phosphatidic acid--lactosylceramide (90:2:8) mol %); acceptor vesicles are prepared from phosphatidylcholine--phosphatidic acid (98:2 mol %). Activity is determined from the rate of transfer of 3H-labeled egg phosphatidylcholine from donor to acceptor vesicles in the presence of phospholipid exchange protein. Donor vesicles are quantitatively precipitated by Ricinus communis agglutinin, while acceptor vesicles remain in the supernate. When egg phosphatidylcholine acceptor vesicles over the temperature range 11--45 degrees C are used, a linear Arrhenius plot is obtained, in keeping with the observation that these membranes exist only in the liquid-crystalline state. When dimyristoylphosphatidylcholine acceptor vesicles under the same conditions are used, however, a biphasic plot is seen with decreasing transfer activity at lower temperatures. The discontinuity occurs at 31 degrees C and corresponds with the onset of the liquid-crystalline to gel phase transition. The incorporation of cholesterol into dimyristoylphosphatidylcholine vesicles at a concentration sufficient to abolish the thermotropic phase transition yields a monophasic Arrhenius plot of transfer activity. The results indicate that bovine liver phospholipid exchange protein interacts catalytically with phospholipid bilayer vesicles composed of saturated or unsaturated phosphatidylcholines but preferentially with liquid-crystalline membranes.