Evidence for an ATP-independent long-chain phosphatidylcholine translocator in hepatocyte membranes

Transport of phosphatidylcholine (PC) molecules across canalicular plasma membranes of the liver is essential for their secretion into bile. To test for evidence of protein-mediated translocation of natural long-chain PCs, we investigated whether hepatocyte membrane subfractions reconstituted into proteoliposomes promoted transmembrane translocation of radiolabeled PCs. Translocation of PC molecules in proteoliposomes was measured by an assay that employed multilamellar acceptor vesicles and the specific PC transfer protein purified from liver. As inferred from the percentage of radiolabel removed from proteoliposomes, facilitated PC translocation occurred in microsomes and canalicular and basolateral plasma membranes from rat liver but not in erythrocyte ghosts, microsomes, homogenates of COS and H35 cells, or Xenopus laevis oocytes. Heat denaturation in the presence of 2-mercaptoethanol and Pronase digestion of solubilized membrane proteins inhibited translocation. In contrast to the mdr2 gene product (Mdr2), which promotes ATP-dependent, verapamil-inhibitable PC translocation, ATP did not enhance and verapamil failed to block PC translocation. These data support the possibility that an ATP-independent PC translocator, possibly distinct from Mdr2, may be present in hepatocyte canalicular plasma membranes.