Re-incorporation of the terminal C5b-9 complement complex into lipid bilayers: formation and stability of reconstituted liposomes.

The formation and stability of lecithin liposomes carrying re-incorporated C5b-9(m) complexes prepared through a detergent-dialysis procedure was studied. Confluent aggregates of phospholipid and protein formed at low initial lipid-protein ratios (1:1, w/w), higher lipid-protein ratios (e.g. 5:1, w/w) were required for formation of lipid vesicles with recognizable bilayer structure. The unfractionated preparations comprised a heterogeneous population of vesicles that sedimented according to their lipid-protein content to varying positions upon centrifugation in CsCl density gradients. The vesicles were stable and C5b-9(m) complexes did not detach from the membranes during centrifugation through CsCl. They also resisted elution from the bilayer by treatment with salt solutions of low or high ionic strength, or of high pH. The results indicate anchorage of C5b-9(m) in the membrane through apolar interactions, akin to that of an integral membrane protein, and are compatible with the channel concept of complement lysis.