title: 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 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.
mesh_terms: D000510:Alpha-Globulins / Q000032:analysis*; D003165:Complement System Proteins / Q000032:analysis*; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007122:Immunoelectrophoresis; D008051:Lipid Bilayers; D008081:Liposomes / Q000276:immunology; D008854:Microscopy, Electron; D010446:Peptide Fragments / Q000032:analysis; D014357:Trypsin
chemical_list: D000510:Alpha-Globulins; D008051:Lipid Bilayers; D008081:Liposomes; D010446:Peptide Fragments; C030931:SC5b-8; D003165:Complement System Proteins; D014357:Trypsin