Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-
containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane,
while the choline-containing and sphingolipids are enriched on the outer surface. The
maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and
disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid
asymmetry is generated primarily by selective synthesis of lipids on one side of the …