title: Flipping and flopping--lipids on the move.
abstract: The rapid movement of polar lipids from one membrane leaflet to the other is facilitated by lipid flippases or translocases. Although their activity was first observed over 30 years ago, the structures, physiological roles, and molecular mechanisms of this group of proteins remain enigmatic. Lipid flippases maintain membrane lipid asymmetry, and in eukaryotes they are also intimately involved in membrane budding and vesicle trafficking. The ATP-dependent flippases are members of well-characterized protein families, whose other members transport nonlipid substrates across cell membranes. The P(4)-type ATPases carry out the inward translocation of phospholipids, and various ABC transporters are involved in outward lipid movement. The ATP-independent flippases move lipid substrates in both directions between membrane leaflets. With only a few exceptions, the molecular identity of these proteins is still unknown, despite their involvement in key biosynthetic pathways in both bacteria and eukaryotes. This review provides an overview of the different classes of flippases, and summarizes recent progress in their identification and functional characterization. The possible mechanisms of action of lipid flippases are discussed, and future directions explored.
authors.affiliation: Department of Molecular and Cellular Biology, Biophysics Interdepartmental Group, University of Guelph, Guelph, ON, Canada. fsharom@uoguelph.ca
authors.forename: Frances J
authors.initials: FJ
authors.lastname: Sharom
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001419:Bacteria / Q000201:enzymology*; D001426:Bacterial Proteins / Q000378:metabolism*; D001693:Biological Transport, Active; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D004798:Enzymes / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*; D008954:Models, Biological; D049294:Phospholipid Transfer Proteins / Q000378:metabolism* / Q000502:physiology
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D004798:Enzymes; D008563:Membrane Lipids; C079726:MsbA protein, Bacteria; D049294:Phospholipid Transfer Proteins; D000251:Adenosine Triphosphatases