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Ninth component of complement: self-aggregation and interaction with lipids.

J R Dankert, J W Shiver, A F Esser

    Biochemistry
    |May 21, 1985
    PubMed
    Summary
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    Human complement component C9 polymerization is triggered by trypsin, not heat. C9 interacts with lipid vesicles, forming aggregates and tubules without compromising membrane integrity.

    Area of Science:

    • Immunology
    • Biochemistry
    • Molecular Biology

    Background:

    • The ninth component (C9) of human complement is crucial for the formation of the membrane attack complex.
    • Previous studies suggested specific environmental conditions for C9 polymerization.

    Purpose of the Study:

    • To investigate environmental conditions influencing C9 polymerization.
    • To clarify the interaction of C9 with lipid membranes.

    Main Methods:

    • Incubation of C9 under various temperature conditions.
    • Proteolysis of C9 with trypsin, alpha-thrombin, and proteinase K.
    • Interaction studies of C9 with small unilamellar lipid vesicles (SUV).
    • Analysis of vesicle membrane integrity using fluorescent markers and potassium ion leakage assays.

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    Main Results:

    • C9 does not aggregate or lose hemolytic activity at 37°C, contrary to prior reports.
    • Higher temperatures induce C9 denaturation and aggregation.
    • Trypsin rapidly polymerizes C9 into tubular structures (poly-C9) at room temperature, a process inhibited by trypsin inhibitor.
    • Monomeric C9 directly interacts with SUVs, leading to vesicle agglutination and C9 aggregation into strings and tubules.
    • A portion of C9 associates with SUVs without aggregation, and membrane integrity is maintained.

    Conclusions:

    • C9 polymerization is primarily induced by specific proteolysis (trypsin), not solely by incubation temperature.
    • C9 exhibits direct interaction with lipid bilayers, leading to structural changes and aggregation.
    • The interaction of C9 with lipid vesicles does not result in membrane damage or leakage.