The Journal of Immunology, Vol 139, Issue 6 1960-1964, Copyright © 1987 by American Association of Immunologists

ARTICLES

Evidence that C5b recognizes and mediates C8 incorporation into the cytolytic complex of complement

JL Stewart, WP Kolb and JM Sodetz

The aim of this study was to identify constituents of the intermediate C5b-7 complex of human complement that mediate binding of C8 and formation of C5b-8. Analysis of interactions between purified C8 and C5, C6, or C7 indicate that C5 and C8 associate to form a dimer in solution. This interaction is specific and involves a single C5 binding site located on the beta-subunit of C8. Simultaneous interaction of C8 with C5 and C9 in solution suggests that during assembly of the cytolytic C5b-9 complex on membranes, C8 binds to C5b-7 through association of beta with C5b, after which C9 associates through interaction with the previously identified C9-specific site on the alpha-subunit. Other evidence of interaction with C5b was provided by the fact that C8 can bind purified C5b6. Also, in situ cross-linking experiments showed that within C5b-8, the beta-subunit is in close proximity to C5b. These results indicate that C8 binding to C5b-7 is mediated by a specific C5b recognition site on beta, thus explaining the requirement for this subunit in C5b-8 formation. They also reveal that C5b contains a specific site for interaction with beta.

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