Hydrophilic-amphiphilic transition of the terminal SC5b-8 complement complex through tryptic modification: biochemical and ultrastructural studies

the SC5b-8 complex of human complement is a hydrophilic molecule of mol. wt 800,000-850,000 that is seen in the electron microscope as an elongated, straight or curved structure of 50-55 nm total length and 8-9 nm width. Tryptic attack on the fluid-phase complex exposes lipid-binding surfaces on the molecule. The trypsinized complex can be incorporated into liposomal lipid bilayers, and the majority of protein is then viewed as ill-defined, larger tufts projecting exterior to the liposomal membrane. These tufts possibly represent clusters of a unit lesion, which consists of two diverging projections, each approximately 25 nm in length. The two projections are possibly joined to each other to give the membrane-bound complex a shape akin to that of an incomplete funnel. Analyses by SDS-polyacrylamide gel electrophoresis show that the polypeptide subunits C5b, C7 and C8 beta entirely resist tryptic degradation in both SC5b-8 and SC5b-9 complement complexes. Limited proteolysis of C6, C8 alpha gamma and C9, and extensive degradation of the S-protein are effected by trypsin. The results are compatible with the concept that proteolytic cleavage of the S-protein in SC5b-8 and SC5b-9 is the cause of the trypsin-dependent, hydrophilic-amphiphilic transition of the terminal, fluid-phase complement complexes.