Retention of lipid asymmetry in membranes on polylysine-coated polyacrylamide beads

Phosphatidylcholine-specific exchange protein from calf liver was used to study the asymmetry and transmembrane movement of phosphatidylcholine in rat erythrocyte membranes isolated on polylysine-coated beads. While confirming previously published results for sealed ghosts, we found that for membranes attached to beads, where the cytoplasmic surface is exposed, about 36% of the total phosphatidylcholine is readily available for exchange, while the remaining 64% is exchangeable at a much slower rate. This indicates that the relative transbilayer asymmetry of phosphatidylcholine is largely maintained when red cell membranes are isolated on beads. On the other hand, transmembrane movement of phosphatidylcholine is decreased in membranes attached to cationized beads: the half-time for equilibration of phosphatidylcholine between the two monolayers of the membrane is 8 h for membranes on beads, compared to 1.5 h for sealed ghosts. Our results indicate that polylysine-derivatized beads are a useful tool for studying asymmetric properties of biological membranes.