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 during complement attack. Isolated C5b-9(m) complexes are shown here to 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 detergentfree solution. Dispersed SC5b-9(m) complexes exhibit an apparent sedimentation coefficient of 29S in sucrose density gradients, corresponding to a molecular weight of approximately 1.4 million. SDS PAGE analyses indicate binding of 3-4 molecules of S-protein per C5b-9(m) complex. These data are consistent with a monomer nature and molecular weight 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 thus appears directly probed and visualized through the binding of a serum protein.
D003165:Complement System Proteins / Q000502:physiology*; D004910:Erythrocyte Membrane / Q000276:immunology; D006023:Glycoproteins / Q000502:physiology*; D006801:Humans; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000502:physiology; D012995:Solubility; D019096:Vitronectin
D003165:Complement System Proteins; D006023:Glycoproteins; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D019096:Vitronectin