Lipid transport by mammalian ABC proteins

ABC (ATP-binding cassette) proteins actively transport a wide variety of substrates, including peptides, amino acids, sugars, metals, drugs, vitamins and lipids, across extracellular and intracellular membranes. Of the 49 hum an ABC proteins, a significant number are known to mediate the extrusion of lipids from membranes or the flipping of membrane lipids across the bilayer to generate and maintain membrane lipid asymmetry. Typical lipid substrates include phospholipids, sterols, sphingolipids, bile acids and related lipid conjugates. Members of the ABCA subfamily of ABC transporters and other ABC proteins such as ABCB4, ABCG1 and ABCG5/8 implicated in lipid transport play important roles in diverse biological processes such as cell signalling, membrane lipid asymmetry, removal of potentially toxic compounds and metabolites, and apoptosis. The importance of these ABC lipid transporters in cell physiology is evident from the finding that mutations in the genes encoding many of these proteins are responsible for severe inherited diseases. For example, mutations in ABCA1 cause Tangier disease associated with defective efflux of cholesterol and phosphatidylcholine from the plasma membrane to the lipid acceptor protein apoA1 (apolipoprotein AI), mutations in ABCA3 cause neonatal surfactant deficiency associated with a loss in secretion of the lipid pulmonary surfactants from lungs of newborns, mutations in ABCA4 cause Stargardt macular degeneration, a retinal degenerative disease linked to the reduced clearance of retinoid compounds from photoreceptor cells, mutations in ABCA12 cause harlequin and lamellar ichthyosis, skin diseases associated with defective lipid trafficking in keratinocytes, and mutations in ABCB4 and ABCG5/ABCG8 are responsible for progressive intrafamilial hepatic disease and sitosterolaemia associated with defective phospholipid and sterol transport respectively. This chapter highlights the involvement of various mammalian ABC transporters in lipid transport in the context of their role in cell signalling, cellular homoeostasis, apoptosis and inherited disorders.