Tschopp J, Podack E R, Müller-Eberhard H J
Proc Natl Acad Sci U S A. 1982 Dec;79(23):7474-8. doi: 10.1073/pnas.79.23.7474.
The ultrastructure of the membrane attack complex (MAC) of complement had been described as representing a hollow cylinder of defined dimensions that is composed of the proteins C5b, C6, C7, C8, and C9. After the characteristic cylindrical structure was identified as polymerized C9 [poly(C9)], the question arose as to the ultrastructural identity and topology of the C9-polymerizing complex C5b-8. An electron microscopic analysis of isolated MAC revealed an asymmetry of individual complexes with respect to their length. Whereas the length of one boundary (+/- SEM) was always 16 +/- 1 nm, the length of the other varied between 16 and 32 nm. In contrast, poly(C9), formed spontaneously from isolated C9, had a uniform tubule length (+/- SEM) of 16 +/- 1 nm. On examination of MAC-phospholipid vesicle complexes, an elongated structure was detected that was closely associated with the poly(C9) tubule and that extended 16-18 nm beyond the torus of the tubule and 28-30 nm above the membrane surface. The width of this structure varied depending on its two-dimensional projection in the electron microscope. By using biotinyl C5b-6 in the formation of the MAC and avidin-coated colloidal gold particles for the ultrastructural analysis, this heretofore unrecognized subunit of the MAC could be identified as the tetramolecular C5b-8 complex. Identification also was achieved by using anti-C5 Fab-coated colloidal gold particles. A similar elongated structure of 25 nm length (above the surface of the membrane) was observed on single C5b-8-vesicle complexes. It is concluded that the C5b-8 complex, which catalyzes poly(C9) formation, constitutes a structure of discrete morphology that remains as such identifiable in the fully assembled MAC, in which it is closely associated with the poly(C9) tubule.
补体膜攻击复合物（MAC）的超微结构被描述为一个具有特定尺寸的空心圆柱体，由蛋白质C5b、C6、C7、C8和C9组成。在特征性的圆柱结构被确定为聚合的C9 [聚（C9）]之后，关于C9聚合复合物C5b - 8的超微结构特征和拓扑结构的问题就出现了。对分离出的MAC进行电子显微镜分析发现，单个复合物在长度方面存在不对称性。其中一个边界的长度（±标准误）始终为16 ± 1 nm，而另一个边界的长度则在16至32 nm之间变化。相比之下，由分离出的C9自发形成的聚（C9）具有均匀的微管长度（±标准误），为16 ± 1 nm。在检查MAC - 磷脂囊泡复合物时，检测到一种细长结构，它与聚（C9）微管紧密相关，并且在微管的环面之外延伸16 - 18 nm，在膜表面上方延伸28 - 30 nm。这种结构的宽度根据其在电子显微镜下的二维投影而有所不同。通过在MAC形成过程中使用生物素化的C5b - 6以及用于超微结构分析的抗生物素蛋白包被的胶体金颗粒，MAC中这个此前未被识别的亚基可以被鉴定为四分子C5b - 8复合物。使用抗C5 Fab包被的胶体金颗粒也实现了鉴定。在单个C5b - 8 - 囊泡复合物上观察到了类似的长度为25 nm（在膜表面上方）的细长结构。得出的结论是，催化聚（C9）形成的C5b - 8复合物构成了一种具有离散形态的结构，在完全组装的MAC中仍然可以如此识别，在其中它与聚（C9）微管紧密相关。
