Transbilayer movement of a fluorescent phosphatidylethanolamine analogue across the plasma membranes of cultured mammalian cells.

The internalization of a fluorescent analogue of phosphatidylethanolamine following its insertion into the plasma membrane of cultured Chinese hamster fibroblasts was examined. When liposomes composed of 50 mol % 1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)-aminocaproyl phosphatidylethanolamine (C6-NBD-PE) and dioleoylphosphatidylcholine were incubated with monolayer cell cultures at 2 degrees C, a spontaneous transfer of the fluorescent lipid from liposomes to cells occurred. As long as the cells were kept at 2 degrees C, the fluorescent lipid remained at the plasma membrane. However, if, after removing the fluorescent liposomes, the cultures were warmed to 37 degrees C, the C6-NBD-PE was internalized and resided in the nuclear envelope, mitochondria, and Golgi apparatus in addition to the plasma membrane. Delivery of the fluorescent lipid to the Golgi apparatus could be blocked by the addition of 2-deoxyglucose plus sodium azide to the incubation medium. Evidence is presented suggesting that while delivery of the fluorescent lipid to the Golgi apparatus was mainly dependent on endocytosis, delivery to the nuclear envelope and mitochondria occurred by rapid transbilayer movement of the lipid across the plasma membrane followed by translocation of lipid monomers. Rapid transbilayer movement of C6-NBD-PE across the plasma membrane was found to be a temperature-dependent process that was blocked below 7 degrees C.