Antibody precipitation of lipid vesicles containing myelin proteins: dependence on lipid composition.

Antibody binding to human CNS myelin basic protein and to rabbit sciatic nerve myelin P-2 in their lipid-bound and water-soluble conformations has been investigated. 125I-labeled basic protein or P-2 was bound to the surface of liposomes (vesicles) of different acidic lipids, phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylglycerol (PG), and cerebroside sulfate (CBS). The antibody was prepared against aqueous solutions of basic protein and P-2. Antibody binding to the proteins in liposomes was measured by precipitation of the liposomes by using a double antibody radioimmunoassay. The amount of 125I-basic protein precipitated was lest when the protein was bound to PA and increased in the order PA less than PS less than PG less than CBS less than PE approximately equal to basic protein in solution, suggesting that the antigenic determinants were lest exposed or most altered for PA and most exposed for PE. This agreed fairly well with previously published biophysical studies that suggested that hydrophobic segments of the protein penetrated into the lipid bilayer and that this penetration decreased in the order PA approximately equal to PG greater than PS greater than CBS greater than or equal to PE. The amount of 125I-P-2 precipitated was least for PA and CBS and increased in the order PA approximately equal to CBS less than PS less than PG less than PE approximately equal to P-2 in solution. However, the differences were less than for basic protein and the effect of CBS was different for the 2 proteins. Less is known about the conformation of P-2 in these lipids but it is known that lipids increase its disease-inducing activity. These results indicate that interaction with lipid may sequester or alter the conformation of antigenic determinants such that antibody binding decreases.