The membrane attack complex of complement: C5b-8 complex as accelerator of C9 polymerization.
Polymerization of C9 occurs spontaneously or can be induced by the tetramolecular complex C5b-8. Spontaneous C9 (0.15 mg/ml) polymerization required more than 3 days at 37 degrees C. In the presence of C5b-8, C9 polymerization was complete within 10 min. The molar C9:C5b-8 ratio determined the extent of tubular poly C9 formation by C5b-8-bearing phospholipid vesicles. When this ratio was 9:1 or 12:1, 72% of complex-bound C9 was present as SDS resistant tubular poly C9 (Mr = 1.1 X 10(6]. At lower C9:C5b-8 ratios, poly C9 was bound primarily in nontubular form. Tubular poly C9, as part of C5b-9, could also be generated on rabbit erythrocytes by using whole human serum as a complement source. At limiting serum concentration (molar C9 to C8 ratio approximately 2), no SDS-resistant tubular poly C9 was detected. At high serum concentration or when using serum that was supplemented with C9, up to 40% of the C9 was SDS-resistant tubular poly C9, and the rest was poly C9, which was incompletely polymerized. It is suggested that the C5b-8 complex acts as an accelerator of C9 polymerization, and that its relative concentration to C9 determines the ultrastructure of the C5b-9 complex.
D000818:Animals; D003186:Complement C9 / Q000502:physiology*; D003165:Complement System Proteins / Q000302:isolation & purification / Q000502:physiology*; D004591:Electrophoresis, Polyacrylamide Gel; D004910:Erythrocyte Membrane / Q000276:immunology*; D006461:Hemolysis; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight; D011817:Rabbits
D003186:Complement C9; D046911:Macromolecular Substances; C042295:complement C5b-8 complex; D003165:Complement System Proteins