The terminal, membrane-derived C5b-9 complex of human complement (C) is an apparently hollow, cylindrical macromolecule vertically oriented on the target membrane. In the present study, an antiserum to the complex has been used to probe its immunobiochemical properties. “Neoantigenic” determinants characteristic of the complex have been detected, which are absent on native C5-C9 molecules. Evidence that the C5b-9 complex is an amphiphilic molecule that possesses apolar, detergent-binding surfaces has been obtained by using charge-shift crossed immunoelectrophoresis, and by direct demonstration of Triton X-100 binding to the complex in quantitative immunoelectrophoresis. By the same criteria, serum C5, C6, and C9 are hydrophilic molecules. The results indicate that assembly of C5-C9 into the terminal membrane C5b-9 complex is accompanied by conformational changes in the individual C components that lead to the exposure of apolar molecular regions in the complex. It is proposed that this constitutes the basis for the lipid-binding properties of the macromolecule, which enable it to become inserted into biologic and artificial lipid membranes with apparent generation of a transmembrane pore.

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