passages.id: split_6_test_2298_passage
passages.type: progene_text
passages.text: The following two theories for the mechanism of ABCA1 in lipid efflux to apolipoprotein acceptors have been proposed : 1 ) that ABCA1 directly binds the apolipoprotein ligand and then facilitates lipid efflux and 2 ) that ABCA1 acts as a phosphatidylserine ( PS ) translocase , increasing PS levels in the plasma membrane exofacial leaflet , and that this is sufficient to facilitate apolipoprotein binding and lipid assembly .