Incorporation of exogenous phosphatidylcholine in the plasma membrane of MDCK cells by a specific transfer protein.

The possibility to introduce exogenous phosphatidylcholine (PC) in the plasma membrane of Madin-Darby canine kidney (MDCK) cells other than by fusion of liposomes with virus-infected cells (Van Meer, G. and Simons, K. (1983) J. Cell Biol. 97, 1365-1374) was studied. Monolayers of confluent MDCK cells grown on a permeable support were exposed to unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC), a phospholipid that does not exchange spontaneously, and were incubated with or without the PC-specific transfer protein (PC-TP), at 4 and 37 degrees C. Added either on the apical or basolateral side of monolayers grown in the presence of [14C]choline, PC-TP stimulated the transfer of 14C-labeled PC from the cell membrane to the liposomes, even at 4 degrees C. Conversely, PC-TP promoted the transfer, by a temperature-dependent process, of [3H]DPPC from liposomes to the cell plasma membrane. The amount of DPPC imported at 37 degrees C was higher than 100 pmol/well for apical incubations. The data demonstrate that, in MDCK cells: (a) PC-TP can modify the PC species present in the plasma membrane; (b) PC accounts for a significant amount of the polar lipids present in the external leaflet of the apical membrane domain.