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

Abstract

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 C8β entirely resist tryptic degradation in both SC5b-8 and SC5b-9 complement complexes. Limited proteolysis of C6, C8αγ 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.