Protein-mediated lipid transfer between monolayers and bilayers

Abstract

An exchange of phosphatidylcholine (PC) is observed between two separate monolayers and between monolayer and vesicles in the presence of phosphatidylcholine transfer protein from beef liver (PC-TP). A coupled transfer of phosphatidylinositol (PI) from monolayer to phosphatidylcholine vesicles and a phosphatidylcholine transfer in the reverse direction is observed in the presence of bovine brain transfer protein (PI-TP). This protein shows an affinity for phosphatidylinositol which is more than 8 times higher than for phosphatidylcholine. PI-TP shows a lower specificity for phosphatidylcholine than PC-TP. However, sphingomyelin, which has a polar head group very similar to phosphatidylcholine, inhibits the lipid transfer as catalyzed by PI-TP. A transfer of cholesterol from monolayer to lipids, particularly those which form nonbilayer structures, is catalyzed by the nonspecific phospholipid transfer protein (ns-PL-TP). Monolayer studies indicate that the ns-PL-TP causes adsorption or fusion of vesicles with the lipid monolayer. A transfer of cholesterol from monolayer to vesicles was also catalyzed by the serum apoprotein apo E.