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 106 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 Å length and 30 to 250 Å 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.
|Original language||English (US)|
|Number of pages||8|
|Journal||Acta Pathologica Microbiologica et Immunologica Scandinavica - Supplementum|
|State||Published - 1984|
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