Inhibition of C9 polymerization within the SC5b-9 complex of complement by S-protein.
The effect of S-protein on the polymerization of C9 during assembly of the C5b-9 complex was examined. Utilizing SDS polyacrylamide gradient slab gel electrophoresis, tubular poly C9 was quantitated as SDS resistant protein of 1.1 to 1.3 X 10(6) molecular weight. Poly C9 formation occurred upon incubation of purified C5b-6, C7, C8 and C9 at molar ratios 1:1:1:12. Addition of purified S-protein to the protein mixture or to preassembled C5b-7 or C5b-8 blocked formation of poly C9 in a dose dependent fashion and gave rise to SC5b-9. SC5b-9 assembled from purified proteins or in zymosan-activated serum was visualized in the electron microscope as a wedge-shaped structure of 350 to 400 A length and 30 to 250 A width which lacked tubular poly C9 seen in images of the membrane attack complex (MAC). Using biotinyl-S-protein and colloidal gold particles coated with avidin, S-protein was located at the wide end of the wedge-like SC5b-9 complex. It is concluded that S-protein has a dual function in SC5b-9 assembly. It blocks the membrane site of C5b-7 and it inhibits C9 polymerization by SC5b-8. Accordingly, the main structural difference between SC5b-9 and the MAC is the lack of tubular poly C9.
D001665:Binding Sites; D003167:Complement Activation; D003186:Complement C9 / Q000378:metabolism*; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000378:metabolism*; D006023:Glycoproteins / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008562:Membrane Glycoproteins; D008970:Molecular Weight; D011108:Polymers / Q000378:metabolism; D011487:Protein Conformation; D019096:Vitronectin
D003186:Complement C9; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D011108:Polymers; D019096:Vitronectin