title: Interaction of complement proteins C5b-6 and C5b-7 with phospholipid vesicles: effects of phospholipid structural features.
abstract: Complement components 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.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism; D002151:Calorimetry; D003182:Complement C5; D003165:Complement System Proteins / Q000378:metabolism*; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008081:Liposomes; D010743:Phospholipids / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship
chemical_list: D003182:Complement C5; D008081:Liposomes; D010743:Phospholipids; D015060:1,2-Dipalmitoylphosphatidylcholine; C050974:complement C5b-6 complex; C037453:complement C5b-7 complex; D003165:Complement System Proteins; D004134:Dimyristoylphosphatidylcholine