A hypothetical transporter model for transmembrane “flipping” of cholesterol molecules by the human ABCG5/ABCG8 heterodimer. The figure is based on the x-ray crystallogrophic structure of MsbA, the lipid flippase from Escherichia coli, which is also composed of 2 half-transporters, but forms a homodimer (19). The organization of the ABCG5/ABCG8 heterodimer is such that the two subunits, each containing 6 transmembrane α-helices, physically interact to form a truncated cone within the membrane lipid bilayer. For thermodynamic reasons, membrane lipid is likely present within as well as outside this structure. It is believed that transport is initiated (a) upon binding ATP at a nucleotide-binding domain located on the cytoplasmic face of the half-transporters (19). Through side entrances of the chamber, the transporter captures a cholesterol molecule from the inner leaflet of the canalicular membrane (a). The transport dynamics then require that ATP-hydrolysis induce a conformational change forcing the two half-transporters to close partially at the cytoplasmic side (b), rendering the cholesterol molecule thermodynamically uncomfortable on the inner leaflet. This induces “flip-flop” of cholesterol across the membrane lipid within the transporter, bringing the sterol and its hydroxyl function to lie in a correct orientation at the lipid-water interface of the outer membrane leaflet (19). With continued ATP hydrolysis (c), the cholesterol molecules are expelled into nascent bile from the exoplasmic face of the transporter as monomers (solubility 10^–8 M), to diffuse to solubilization sites within bile salt micelles and phospholipid vesicles in the canalicular space (10, 21). This transporter model is based on x-ray crystallographic work of Chang and Roth (19) and incorporates theoretical suggestions of Higgins and Linton (25). Unilamellar vesicle and mixed micelle models, with close-packing of the lipids, are based on cryophotoelectromicrographs of Crawford et al. (21) and molecular dynamics simulations of Marrink and Mark (22), respectively. The molecular models are drawn to conform to space-filling dimensions for cholesterol (CH), phospholipid (PL), and bile salt (BS). Filled circles, hydroxyl functions; filled ovals, glycerol; wavy lines, acyl chains; inscribed positive signs, cationic groups; inscribed negative signs, anionic groups; large scalloped cross-hatched ovals, saturated (BS) or unsaturated (CH) steroid nucleus.