Interaction of a solubilized membrane ATPase with aqueous dispersions of bilayer lipid membranes

Abstract

1. 1. Aqueous dispersions of phosphatidylcholine have been used to study the interaction of a soluble ATPase with phospholipid bilayers. Binding of the membrane-derived ATPase to the phospholipid was demonstrated by differential centrifugation of mixtures of the soluble enzyme and multilamellar phosphatidylcholine liposomes. Mg 2+ was found to increase the ATPase binding to the model membranes. The upper limit of the molar ratio of the phosphatidylcholine to ATPase in the bilayer-ATPase complex was approximately 8000:1. 2. 2. The bilayer-ATPase interaction was also investigated using homogeneous phosphatidylcholine vesicles, 270 Å in diameter. Electron microscopy was employed to determine whether morphological changes occurred as a result of the vesicle-ATPase interaction. At short incubation times, it was found that large vesicles were generated in the mixture; after an hour, multilamellar structures appeared. The lamellar spacings of these phospholipid-ATPase structures were 54 rA compared with 44 Å in the absence of the protein. 3. 3. The addition of the ATPase was also observed to cause a line broadening and a concomitant decrease in the signal intensity in the high resolution PMR spectrum of sonicated phosphatidylcholine vesicle.