Fusion of phospholipid vesicles induced by .alpha.-lactalbumin at acidic pH

Abstract

.alpha.-Lactalbumin (.alpha.-LA), lysozyme, and ribonuclease are found to induce fusion of phosphatidylserine/phosphatidylethanolamine vesicles at low pH. The fusogenic behavior and the binding to phospholipid vesicles of one of these proteins, .alpha.-LA, are studied at a wide range of conditions. The initial rate of fusion in the presence of .alpha.-LA increases with increasing acidity below pH 6, and the extent of .alpha.-LA binding to the vesicles is also found to increase with decreasing pH. Once bound to the vesicles in acidic media, the neutralization to pH 7 fails to dislodge the .alpha.-LA from the vesicles, and this irreversible binding also increases with decreasing pH. A segment of .alpha.-LA is found to be resistant to the proteolytic digestion when initially incubated with the vesicles at low pH. The amino acid composition of this fragment was determined, and from this the sequence of .alpha.-LA fragment, which appears to be inserted into the bilayer, is deduced. Hydrophobic labeling with dansyl chloride renders support that this segment indeed penetrates into the hydrophobic interior of bilayer. Since both the N-terminal and the C-terminal of this vesicle-bound protein are accessible to the externally added proteolytic enzymes, it is concluded that a loop of the polypeptide segment goes into bilayer. These observations, taken together, suggest a possibility that the penetration by a loop of .alpha.-LA segment into the phospholipid bilayer is responsible for the fusion.