TERMINAL MEMBRANE C5B-9 COMPLEX OF HUMAN-COMPLEMENT - TRANSITION FROM AN AMPHIPHILIC TO A HYDROPHILIC STATE THROUGH BINDING OF THE S-PROTEIN FROM SERUM

The membrane-damaging C5b-9(m) complex of complement is a cylindrically structured, amphiphilic molecule that is generated on a target membrane [rabbit erythrocyte] during complement attack. Isolated C5b-9(m) complexes possess the capacity of binding a protein, termed S-protein, that is present in human plasma. Binding of this protein apparently shields the apolar surfaces of C5b-9(m), since the resulting SC5b-9(m) complex is hydrophilic and no longer aggregates in detergent-free solution. Dispersed SC5b-9(m) complexes exhibit an apparent sedimentation coefficient of 29S in sucrose density gradients, corresponding to a MW of .apprx. 1.4 million. SDS PAGE [sodium dodecyl sulfate-polyacrylamide gel electrophoresis] analyses indicate binding of 3-4 molecules S-protein per C5b-9(m) complex. The data are consistent with a monomer nature and MW of 1-1.1 million of the C5b-9(m) complex. Ultrastructural analysis of SC5b-9(m) shows preservation of the hollow cylindrical C5b-9(m) structure. Additional material, probably representing the S-protein itself, can be visualized attached to the originally membrane-embedded portion of the macromolecule. The topography of apolar surfaces on a molecule appears directly probed and visualized through the binding of a serum protein.