Podack E R, Stoffel W, Esser A F, Müller-Eberhard H J
Proc Natl Acad Sci U S A. 1981 Jul;78(7):4544-8. doi: 10.1073/pnas.78.7.4544.
Membrane destruction by complement is effected by the membrane attack complex (MAC) which is the dimer of a fusion product of the complement proteins C5b, C6, C7, C8, and C9. Phospholipid bilayer vesicles were used as target membranes for the MAC and its intermediate complexes. The subunits of these membrane-bound complexes were explored as to their relative exposure to the hydrocarbon phase of the lipid bilayer and to water surrounding the lipid vesicles. Protein exposed to the aqueous phase was labeled with 125I; protein exposed to the hydrocarbon phase was labeled by using tritiated azido phospholipids and irradiation. Analysis of the membrane-bound MAC showed that subunits C5b, C8 beta, and C9 were exposed to the aqueous phase. The subunits C8 alpha-gamma and C9 were primarily in contact with the hydrocarbon phase. C6 and C7 were little exposed to either phase, suggesting that these proteins are inaccessible within the MAC. Analysis of the intermediate complexes showed that C5b was the subunit most exposed to water in membrane-bound C5b-7, and C5b and C8 beta were the water-exposed subunits in C5b-8. Subunit exposure to the hydrocarbon phase of the lipid bilayer changed during MAC assembly. Whereas all three subunits of C5b-7 carried the phospholipid photolabel; most of the label was bound to the C8 subunit in C5b-8 and to C9 in the MAC. It is proposed that contact with the hydrocarbon core of membranes is established by C5b-7 through each of its subunits, by C5b-8 through C8, and by the MAC through C8 and, particularly, C9.
补体介导的膜破坏是由膜攻击复合物（MAC）实现的，该复合物是补体蛋白C5b、C6、C7、C8和C9融合产物的二聚体。磷脂双层囊泡用作MAC及其中间复合物的靶膜。研究了这些膜结合复合物的亚基相对于脂质双层烃相和脂质囊泡周围水相的相对暴露情况。暴露于水相的蛋白质用125I标记；暴露于烃相的蛋白质通过使用氚化叠氮磷脂和辐射进行标记。对膜结合的MAC的分析表明，亚基C5b、C8β和C9暴露于水相。亚基C8α-γ和C9主要与烃相接触。C6和C7很少暴露于任何一相，这表明这些蛋白质在MAC中无法接近。对中间复合物的分析表明，C5b是膜结合的C5b-7中最暴露于水的亚基，C5b和C8β是C5b-8中暴露于水的亚基。在MAC组装过程中，亚基暴露于脂质双层烃相的情况发生了变化。虽然C5b-7的所有三个亚基都携带磷脂光标记；但大部分标记在C5b-8中与C8亚基结合，在MAC中与C9亚基结合。有人提出，C5b-7通过其每个亚基与膜的烃核心建立接触，C5b-8通过C8建立接触，MAC通过C8，特别是C9建立接触。
