Kim J, Kim H
Biochemistry. 1986 Dec 2;25(24):7867-74. doi: 10.1021/bi00372a012.
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 the 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.
已发现α-乳白蛋白（α-LA）、溶菌酶和核糖核酸酶在低pH值下可诱导磷脂酰丝氨酸/磷脂酰乙醇胺囊泡融合。在广泛的条件下研究了这些蛋白质之一α-LA的融合行为及其与磷脂囊泡的结合。在pH值低于6时，α-LA存在下的初始融合速率随酸度增加而增加，并且还发现α-LA与囊泡的结合程度随pH值降低而增加。一旦在酸性介质中与囊泡结合，将pH值中和至7并不能使α-LA从囊泡上解离，并且这种不可逆结合也随pH值降低而增加。当最初在低pH值下与囊泡一起孵育时，发现α-LA的一个片段对蛋白水解消化具有抗性。测定了该片段的氨基酸组成，并由此推导了似乎插入双层中的α-LA片段的序列。用丹磺酰氯进行疏水标记支持该片段确实渗透到双层的疏水内部。由于该与囊泡结合的蛋白质的N末端和C末端对于外部添加的蛋白水解酶都是可及的，因此得出结论，多肽片段的一个环进入双层。综合这些观察结果，提示α-LA片段的一个环渗透到磷脂双层中可能是融合的原因。
