Ultrastructure of the membrane attack complex of complement: detection of the tetramolecular C9-polymerizing complex C5b-8.

Abstract

The ultrastructural identity and topology of the C9[complement component 9]-polymerizing complex C5b-8 of the rabbit membrane attack complex [MAC] of complement was examined. EM analysis of isolated MAC revealed an asymmetry of individual complexes with respect to their length. While the length of 1 boundary (.+-. SEM [standard error of the mean]) was always 16 .+-. 1 nm, the length of the other varied between 16 and 32 nm. Poly(C9) [polymerized 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 2-dimensional projection by EM. By using biotinyl C5b-6 in the formation of the MAC and avidin-coated colloidal, Au particles for the ultrastructural analysis, this heretofore unrecognized subunit of the MAC, was identified as the tetramolecular C5b-8 complex. Identification also was achieved by using anti-C5 Fab-coated colloidal Au particles. A similar elongated structure of 25 nm length (above the surface of the membrane) was observed on single C5b-8-vesicle complexes. The C5b-8 complex, which catalyzes poly(C9) formation, apparently constitutes a structure of discrete morphology; it remains as such identifiable in the fully assembled MAC in which it is closely associated with the poly(C9) tubule.