Membrane attack complex of complement: generation of high-affinity phospholipid binding sites by fusion of five hydrophilic plasma proteins.

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Abstract
The molecular basis of the membranolytic activity of the membrane attack complex (MAC) of [human] complement [C] was investigated. By using density gradient equilibrium ultracentrifugation, the binding of egg yolk lecithin to the isolated MAC and to its intermediate complexes and precursor proteins was measured. No stable phospholipid-protein complexes were formed with the MAC precursor components C5b-6, C7, C8 and C9. Stable complexes of phospholipid and protein were formed by C5b-7, C5b-8, C5b-9 and the MAC (C5b-9 dimer) and they exhibited densities of 1.2164, 1.184, 1.2055 and 1.2275 g/ml, respectively. The molar phospholipid/protein ratios for the 4 complexes were C5b-7,399:1; C5b-8,841:1; C5b-9, 918:1; and C5b-9 dimer, 1460:1. EM of the isolated phospholipid-protein complexes revealed no lipid bilayer structures. Considering the magnitude of the phospholipid binding capacity of the MAC, it probably forms phospholipid-protein mixed in micelles in lipid bilayers and biological membranes and causes formation of hydrophilic lipid channels.