Reversible interactions in free solution were demonstrated to occur (a) between C5 and C8, (b) between C5, 6, 7 and C8, and (c) between C8 and C9. No interaction was observed between C8 and C6 or C7 and between C9 and C5, 6, 7. Interactions between C8 and C9 were enhanced at lowered ionic strength (0.05) and a molar excess of C8 over C9. Complex formation was independent of pH over the range of 6.5–8.5. Under optimal conditions the C8, 9 complex had a sedimentation coefficient of 10.2–10.6S, while native C8 and C9 sedimented at 8.5 and 4.8S, respectively. Specificity and reversibility of these interactions were established.

In spite of the limited number of interactions observed, all five of the native proteins of the membrane attack mechanism interacted to form an association product that sedimented at 10.8–11.2S. Demonstration of this product in free solution supports the concept that C5-9 on acquisition of cytolytic activity assemble into a stable multimolecular complex.

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