Interaction of complement proteins C5b-6 and C5b-7 with phospholipid vesicles: effects of phospholipid structural features

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Abstract
Complement components of C5b-6 and C7 assemble to form C5b-7, which then interacts with membranes and commits the membrane attack complex to a target site. This protein-membrane association event was investigated to determine possible structural features that could contribute to a selective membrane attack. This system may also suggest general properties of protein-membrane insertion events. Initial binding of C5b-6 to membranes could potentially determine the site of assembly. However, binding of C5b-6 to membranes required phosphatidylglycerol or phosphatidic acid produced from egg phosphatidylcholine while binding of C5b-6 to phosphatidylcholine, phosphatidylserine, or phosphatidylinositol was undetectable. Binding to phosphatidic acid was irreversible, and the bound C5b-6 could no longer interact with C7. In contrast C5b-7 interacted with all phospholipids tested. The rate-limiting process was the interaction of C5b-6 and C7, which displayed bimolecular properties and an activation energy of 37 kcal/mol. The C5b-7 complex showed 20-fold selectivity for small unilamellar phospholipid vesicles over large unilamellar vesicles. Vesicles carrying high negative charge densities were selected over neutral vesicles by a factor of about 5. Vesicles formed from phospholipids with short, saturated hydrocarbon side chains (dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine) were about 5-fold less effective than those formed from phospholipids with natural fatty acid distributions. The gel vs. fluid state had little influence on C5b-7 insertion. While a number of explanations for these selectivities were considered, the selectivity of C5b-7-membrane insertion, in every case, could potentially arise from the degree of exposure of the hydrocarbon portion of the bilayer membrane which interacted rapidly with an exposed hydrophobic protein segment on the C5b-7 complex. These properties may influence the specificity of attack on biological membranes. In vitro studies on assembly will be strongly influenced by the selection of membrane components.