Transmembrane orientation of lipophilin in phosphatidylcholine vesicles

Lipophilin, a hydrophobic myelin protein, was incorporated into phosphatidylcholine vesicles by dialysis from 2-chloroethanol which has been shown to produce single-layered lipid-protein vesicles. These vesicles were labeled with a nonpenetrating surface-labeling reagent, 4,4-diisothiocyano-2,2-ditritiostilbene disulfonic acid, ([³H]DIDS), in order to determine if the protein completely spans the bilayer. After labeling the vesicles, lipophilin was isolated. At least 88% of the protein was labeled with [³H]DIDS. Dextran (mol wt 250,000–275,000) was converted to the dialdehyde form and reacted with lipophilin-PC vesicles. In this case greater than 90% of the protein was complexed to the dextran. The high degree of labeling obtained with both compounds was consistent with a model in which lipophilin was considered to span the bilayer completely.