Phospholipid Flippases (2006)

Lipids in biological membranes are organized non-randomly across the bilayer. In eukaryotic cells, the cytoplasmic surface is enriched in anionic and primary amine-containing phospholipids (phosphoinositides (PIPn), 2The abbreviations used are: PIPn, phosphatidylinositol-n-phosphate, where n = 0–3; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine; PI, phosphatidylinositol; PG, phosphatidylglycerol; CL, cardiolipin; ER, endoplasmic reticulum; NBD, 7-nitrobenz-2-oxa-1,3-diazole-4-yl; GlcN-PI, glucosamine phosphatidylinositol; MDR, multidrug resistance; MRP, multidrug resistance related protein. phosphatidic acid, phosphatidylethanolamine (PE), phosphatidylserine (PS)), whereas the extracytoplasmic leaflet and the topologically equivalent lumenal surface of internal organelles are enriched in choline-containing phospholipids (phosphatidylcholine (PC) and sphingomyelin) and glycosphingolipids. In prokaryotic cells, PE is enriched in the cytoplasmic leaflet of the plasma membrane, phosphatidylglycerol (PG) is localized predominantly to the outer leaflet, and cardiolipin (CL) has a symmetric transbilayer distribution (1Op den Kamp J.A.F. Annu. Rev. Biochem. 1979; 48: 47-71Crossref PubMed Scopus (978) Google Scholar). The origin of lipid asymmetry stems primarily from vectoral biosynthesis. Most eukaryotic glycerophospholipids are synthesized on the cytoplasmic face of the ER, whereas sphingolipids are either synthesized or modified on the lumenal surface of the ER and Golgi. To maintain bilayer balance, some newly synthesized lipids must traverse to the side of the membrane opposite that of the site of synthesis. Once synthesized, lipids must also move to other membranes within the cell (2Voelker D.R. Biochem. Cell Biol. 2004; 82: 156-169Crossref PubMed Scopus (19) Google Scholar), which occurs primarily by a vesicular process. Lipids form the basic structure of the outer coat of Gram-negative bacteria and serve as carriers of oligosaccharide residues destined for attachment to lipids and proteins. The assembly of these units requires the collection of material from both sides of the membrane and the delivery of the final product to the correct location, processes that require the transbilayer movement of lipids with bulky or hydrophilic headgroups. Once generated, transverse phospholipid asymmetry is preserved by the combination of slow transbilayer movement of lipids (3Kornberg R.D. McConnell H.M. Biochemistry. 1971; 10: 1111-1120Crossref PubMed Scopus (780) Google Scholar) and the presence of selective lipid transporters. The thermodynamic barrier for transbilayer movement must be overcome for the regulated and rapid exchange of complex lipids across the membrane bilayer. The task of moving a large polar group, attached to a hydrophobic tail, across the membrane requires transporters with the capability of interacting with amphipathic compounds. These transporters must provide a sizeable hydrophilic pathway for the lipid headgroup to pass through the membrane yet must also be able to accommodate the hydrophobic character of the lipid. Some of the proteins that have been assigned lipid transport activity are relatively nonspecific. Others exhibit a high degree of specificity. Interestingly, the generation and the dissipation of the transbilayer lipid gradient rely on the activity of a relatively small number of lipid transporter families that move lipids and their precursors from one side of the membrane to the other. Mark Bretscher first coined the term "flippase" to refer to lipid transporters that serve to equilibrate newly synthesized lipid across biogenic membranes such as the ER (4Bretscher M.S. Moscona A.A. The Cell Surface in Development. John Wiley & Sons, Inc., New York1974: 17-24Google Scholar). Although the definition of flippases has come to include all types of lipid transporters, transbilayer lipid transporters can be classified based on their substrate specificity, direction of transport, and requirement for ATP. Transporters that move lipids to the cytoplasmic face of the membrane are commonly called "flippases," whereas those that transport lipids from the cytofacial surface to the opposite side of the membrane are called "floppases" (Fig. 1). The degree of substrate selectivity for lipids varies, but both classes of these transporters typically require an input of energy (usually in the form of ATP), allowing them to pump lipids against a concentration gradient. In contrast, "scramblases" transport lipids in both directions, are driven by a pre-existing transbilayer lipid gradient, and in the plasma membrane are Ca2+-activated. Scramblases play an important role in the controlled randomization of lipid distribution that occurs during apoptosis or during cell activation, and by externalizing PS, scramblases signal the engulfment of dead or dying cells by phagocytes (5Balasubramanian K. Schroit A.J. Annu. Rev. Physiol. 2003; 65: 701-734Crossref PubMed Scopus (325) Google Scholar, 6Williamson P. Schlegel R.A. Biochim. Biophys. Acta. 2002; 1585: 53-63Crossref PubMed Scopus (179) Google Scholar, 7Zwaal R.F. Comfurius P. Bevers E.M. Cell. Mol. Life Sci. 2005; 62: 971-988Crossref PubMed Scopus (644) Google Scholar). Biogenic membrane flippases are energy-independent and belong to the class of transporters first envisioned by Bretscher. These are distinct from the ATP-dependent flippases, although the term "flippase" is commonly used to refer to both classes of transporters, and are associated with lipid biosynthetic processes. These include eukaryotic transporters in the ER and Golgi (glycolipids), as well as prokaryotic transporters. A number of excellent recent reviews have summarized in depth the role of ABC transporters (8Borst P. Elferink R.O. Annu. Rev. Biochem. 2002; 71: 537-592Crossref PubMed Scopus (1355) Google Scholar, 9Pohl A. Devaux P.F. Herrmann A. Biochim. Biophys. Acta. 2005; 1733: 29-52Crossref PubMed Scopus (124) Google Scholar), P4-ATPases (10Graham T.R. Trends Cell Biol. 2004; 14: 670-677Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 11Paulusma C.C. Oude Elferink R.P.J. Biochim. Biophys. Acta. 2005; 1741: 11-24Crossref PubMed Scopus (89) Google Scholar), and biogenic flippases (12Kol M.A. de Kroon A.I. Killian J.A. de Kruijff B. Biochemistry. 2004; 43: 2673-2681Crossref PubMed Scopus (95) Google Scholar) in lipid-mediated transport (see also Refs. 13Pomorski T. Holthuis J.C. Herrmann A. van Meer G. J. Cell Sci. 2004; 117: 805-813Crossref PubMed Scopus (153) Google Scholar and 14Devaux P.F. Lopez-Montero I. Bryde S. Chem. Phys. Lipids. 2006; 141: 119-132Crossref PubMed Scopus (61) Google Scholar). The goal of the present review is to summarize the current state of knowledge of each of these families of putative lipid transporters, with emphasis on the biogenic and the unidirectional ATP-dependent transporters. Lipid biosynthetic pathways rely on the communication of substrates and products between the leaflets of biogenic membranes, such as the ER, Golgi, and prokaryotic plasma membrane. The substrates are diverse and include glycerophospholipids and their precursors, glycolipids, and dolichol-linked sugars. Eukaryotic Glycerophospholipid Transporters—One of the first flippase activities to be identified was the ER flippase. Using a novel reconstitution assay based on the density shift of flippase-containing liposomes due to transport of brominated lipids, Backer and Dawidowicz reported the activity of a rat liver microsomal PC flippase (15Backer J.M. Dawidowicz E.A. Nature. 1987; 327: 341-343Crossref PubMed Scopus (75) Google Scholar). Bell and co-workers demonstrated the existence of a protease and N-ethylmaleimide-sensitive ER flippase activity in liver microsomes that was specific for short chain PC (dibutyryl PC) and its metabolites (16Bishop W.R. Bell R.M. Cell. 1985; 42: 51-60Abstract Full Text PDF PubMed Scopus (186) Google Scholar, 17Kawashima Y. Bell R.M. J. Biol. Chem. 1987; 262: 16495-16502Abstract Full Text PDF PubMed Google Scholar). Using spin-labeled and fluorescent lipids, ER transport was shown to be fast (t½ ≤ 1 min) and non-selective for glycerophospholipids (18Buton X. Morrot G. Fellmann P. Seigneuret M. J. Biol. Chem. 1996; 271: 6651-6657Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar). Transport activity has been reconstituted from detergent extracts of ER microsomes, and recent evidence indicates that the transport of fluorescent analogs of PI in the ER is independent of inositol and glycerol stereochemistry (19Vishwakarma R.A. Vehring S. Mehta A. Sinha A. Pomorski T. Herrmann A. Menon A.K. Org. Biomol. Chem. 2005; 3: 1275-1283Crossref PubMed Scopus (58) Google Scholar). An implication of these data is that transport of PC and PI in biogenic membranes may occur through the same nonspecific transporter. Prokaryotic Transporters—The synthesis of phospholipids in prokaryotic membranes occurs on the cytoplasmic face of the plasma membrane, generating the same issues of bilayer balance encountered by phospholipid synthesis in the ER. In 1977, Rothman and Kennedy demonstrated that newly synthesized PE in Bacillus megaterium equilibrated with the outer monolayer pool of this lipid much faster than spontaneous diffusion, implying that transbilayer PE movement was facilitated by membrane proteins (20Rothman J.E. Kennedy E.P. Proc. Natl. Acad. Sci. U. S. A. 1977; 74: 1821-1825Crossref PubMed Scopus (132) Google Scholar). Subsequent studies have shown that fluorescent analogs of PE, PC, PG, and CL and newly synthesized PE, PG, and CL are rapidly transported across the cytoplasmic membrane of B. megaterium and Escherichia coli, indicating that this transport mechanism is nonselective (for a recent review see Kol et al. (12Kol M.A. de Kroon A.I. Killian J.A. de Kruijff B. Biochemistry. 2004; 43: 2673-2681Crossref PubMed Scopus (95) Google Scholar)). The identity of this transporter remains unknown, although flippase activity has been reconstituted from extracts of B. subtilis (21Hrafnsdóttir S. Menon A.K. J. Bacteriol. 2000; 182: 4198-4206Crossref PubMed Scopus (44) Google Scholar) and from E. coli (22Watkins W.E. Menon A.K. Biol. Chem. 2002; 383: 1435-1440Crossref PubMed Scopus (11) Google Scholar) inner membranes. Interestingly, MsbA, an ABC transporter responsible for the export of lipid A in E. coli, also transports PE, indicating that MsbA may be a non-selective lipid transporter (23Doerrler W.T. Gibbons H.S. Raetz C.R. J. Biol. Chem. 2004; 279: 45102-45109Abstract Full Text Full Text PDF PubMed Scopus (198) Google Scholar). Glycolipid Transporters—The movement of sugar-linked lipids across the membrane is essential for protein glycosylation and the synthesis of cell surface protein membrane anchors. Phosphatidylinositol forms the root of the PI-glycans, which tether some cell surface proteins to the membrane. Using a reconstitution method similar to the one described for the ER flippase, the transport of fluorescent (7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD)) analogs of PI-glycan precursors (NBD-GlcN-PI) have been demonstrated in proteoliposomes containing an extract of ER proteins (24Vishwakarma R.A. Menon A.K. Chem. Commun. 2005; : 453-455Crossref PubMed Scopus (51) Google Scholar). The synthesis of glycosphingolipids involves the assembly of the monohexosylsphingolipids GalCer and GlcCer on the cytoplasmic face of the Golgi. These lipids are transported to the lumenal surface where additional monosaccharides are added to create more elaborate glycolipids, which then become trapped in the Golgi lumen. The transport of monohexosylsphingolipids (GlcCer and GalCer) across ER and cis-Golgi membranes has been shown to be fast, ATP-independent, bidirectional, suppressible by 4,4′-diisothiocyanatostilbene-2,2′-disulfonate and distinct from PC transport (25Buton X. Herve P. Kubelt J. Tannert A. Burger K.N. Fellmann P. Muller P. Herrmann A. Seigneuret M. Devaux P.F. Biochemistry. 2002; 41: 13106-13115Crossref PubMed Scopus (62) Google Scholar). Although these transporters have not been positively identified, some ABC transporters have been shown to transport GlcCer across the Golgi and plasma membranes (see below). N-Linked protein oligosaccharides are synthesized by the formation of a mannose-rich tetradecasaccharide linked to dolichol pyrophosphate in the lumen of the ER, which is transferred to asparagine residues on nascent proteins. The synthesis of this core structure requires transporters to move the precursor Man5GlcNAc2-P-P-dolichol and the sugar donors Man-P-dolichol and Glc-P-dolichol from the cytoplasmic to the lumenal surface of the ER. A yeast protein, Rft1, has been proposed to be the Man5GlcNAc2-P-P-dolichol transporter (26Helenius J. Ng D.T. Marolda C.L. Walter P. Valvano M.A. Aebi M. Nature. 2002; 415: 447-450Crossref PubMed Scopus (218) Google Scholar), whereas a rat liver Man-P-dolichol transporter has been partially purified and reconstituted using a short chain lipid analog (Man-P-Dol10) to measure transport activity (27Rush J.S. Waechter C.J. Biochemistry. 2004; 43: 7643-7652Crossref PubMed Scopus (17) Google Scholar). Homologs of Rft1 have been found in the genomes of other eukaryotes, including humans, indicating that this is likely to be a ubiquitous family of proteins in organisms that are capable of N-glycosylation (28Helenius J. Aebi M. Semin. Cell Dev. Biol. 2002; 13: 171-178Crossref PubMed Scopus (65) Google Scholar). Sequence analysis of Rft1 does not reveal an ATP binding site, consistent with the ATP-independent transport of Man5GlcNAc2-P-P-dolichol. The ABC class of transporters catalyze the ATP-dependent transport of a variety of substrates, including amphipathic compounds, xenobiotics, ions, and peptides (for recent reviews see Refs. 8Borst P. Elferink R.O. Annu. Rev. Biochem. 2002; 71: 537-592Crossref PubMed Scopus (1355) Google Scholar and 9Pohl A. Devaux P.F. Herrmann A. Biochim. Biophys. Acta. 2005; 1733: 29-52Crossref PubMed Scopus (124) Google Scholar). These proteins are widely distributed in prokaryotes and eukaryotes, and defects in human isoforms have been associated with a number of diseases. They have a common functional structure consisting of two hexahelical transmembrane domains and two nucleotide binding domains. This review will address those ABC transporters for which endogenous lipid transport activity has been measured or suspected (see supplemental Table 1). The ABCA family has been implicated in lipid and sterol transport. ABCA1 is a ubiquitous plasma membrane protein that transports cholesterol to the extracellular surface of cells in extrahepatic tissues, where it is harvested by high density lipoprotein for circulation to the liver. ABCA1 may also catalyze PS efflux (29Hamon Y. Broccardo C. Chambenoit O. Luciani M.F. Toti F. Chaslin S. Freyssinet J.M. Devaux P.F. McNeish J. Marguet D. Chimini G. Nat. Cell Biol. 2000; 2: 399-406Crossref PubMed Scopus (467) Google Scholar, 30Smith J.D. Waelde C. Horwitz A. Zheng P. J. Biol. Chem. 2002; 277: 17797-17803Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar) and assist in the binding of apoA-1 to the surface of the cell, although ABCA1-mediated efflux of NBD-PE and NBD-PS does not require an apoA-1 (31Alder-Baerens N. Muller P. Pohl A. Korte T. Hamon Y. Chimini G. Pomorski T. Herrmann A. J. Biol. Chem. 2005; 280: 26321-26329Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). A highly homologous family member, ABCA7, has a more limited tissue distribution, but it may also play a role in cholesterol and PS externalization (32Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar, 33Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). Defects in ABCA3 have been shown to produce severe surfactant deficiency, implying that this protein is involved in lipid export into the lung alveolar space (34Shulenin S. Nogee L.M. Annilo T. Wert S.E. Whitsett J.A. Dean M. N. Engl. J. Med. 2004; 350: 1296-1303Crossref PubMed Scopus (558) Google Scholar). ABCA4 is localized to retinal rod outer segment disk membranes and transports all-trans-N-retinylidene PE to the cytosolic side of the disk for recycling of the all-trans-retinal to the cis form (35Weng J. Mata N.L. Azarian S.M. Tzekov R.T. Birch D.G. Travis G.H. Cell. 1999; 98: 13-23Abstract Full Text Full Text PDF PubMed Scopus (729) Google Scholar). The ABCB family includes the multidrug resistance (MDR) or P-glycoprotein transporters. First discovered as xenobiotic transporters, proteins in this family have also been found to transport lipids. ABCB1 is localized to the apical surface of polarized cells and is capable of transporting a wide variety of lipids, including fluorescent (NBD) or short chain glycerophospholipids, sphingolipids, and platelet-activating factor (9Pohl A. Devaux P.F. Herrmann A. Biochim. Biophys. Acta. 2005; 1733: 29-52Crossref PubMed Scopus (124) Google Scholar, 36van Helvoort A. Smith A.J. Sprong H. Fritzsche I. Schinkel A.H. Borst P. van Meer G. Cell. 1996; 87: 507-517Abstract Full Text Full Text PDF PubMed Scopus (791) Google Scholar). In contrast, ABCB4 is a selective PC transporter (36van Helvoort A. Smith A.J. Sprong H. Fritzsche I. Schinkel A.H. Borst P. van Meer G. Cell. 1996; 87: 507-517Abstract Full Text Full Text PDF PubMed Scopus (791) Google Scholar) and may play a key role in the export of this lipid into the bile (37Carlton V.E. Pawlikowska L. Bull L.N. Ann. Med. 2004; 36: 606-617Crossref PubMed Scopus (49) Google Scholar, 38Smit J.J. Schinkel A.H. Oude Elferink R.P. Groen A.K. Wagenaar E. van Deemter L. van van M.A. Cell. Full Text PDF PubMed Scopus Google Scholar). The family proteins that have a structure and than ABC transporters. This family transmembrane and as well as transporters of and lipids (8Borst P. Elferink R.O. Annu. Rev. Biochem. 2002; 71: 537-592Crossref PubMed Scopus (1355) Google Scholar). is localized to the surface of polarized cells, and it has been shown to transport Comfurius P. van Bevers E.M. R.F. Biochem. J. 2000; 350: PubMed Scopus Google Scholar, D. Biochim. Biophys. Acta. PubMed Scopus Google Scholar), sphingolipids van Helvoort A. van Meer G. J. Cell Sci. 1999; PubMed Google Scholar), and I. G. U. D. J. 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Biol. 2003; Google Scholar) and other ATP-dependent The of PS flippase activity in to the and of the X. M.S. Schlegel R.A. P. 1996; PubMed Scopus Google Scholar). This was found to be homologous to proteins in the and homologous to a yeast that been associated with A of data that these of a novel class of that may be involved in transport. of the have been identified, all of which are found in eukaryotic including a human isoforms C.C. Oude Elferink R.P.J. Biochim. Biophys. Acta. 2005; 1741: 11-24Crossref PubMed Scopus (89) Google Scholar, M.S. J.J. S. L. P. C. S. Y. H. D. P. Schlegel R.A. Physiol. 1999; PubMed Scopus Google Scholar) Table the family into M.S. Schlegel R.A. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). The of has linked of these to the of membrane and transport. two have been associated with human 11Paulusma C.C. Oude Elferink R.P.J. Biochim. Biophys. Acta. 2005; 1741: 11-24Crossref PubMed Scopus (89) Google Scholar and The and P4-ATPases include the the yeast proteins and and human and studies have shown that is enriched in the but is present in limited in the plasma membrane T. H. Devaux P.F. van Meer G. Holthuis J.C. Mol. Biol. Cell. 2003; 14: PubMed Scopus Google Scholar). studies have shown that a related NBD-PS and NBD-PE transport in P. J. T.R. Proc. Natl. Acad. Sci. U. S. A. 2004; PubMed Scopus (162) Google Scholar, N. L. Pomorski T. Holthuis J.C. Mol. Biol. Cell. 2006; PubMed Scopus (124) Google Scholar) as well as the of these proteins in the formation of W.E. M.F. K. Chen T.R. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). and may play a role in membrane the the plasma membrane, and in to their role in transmembrane phospholipid and have been shown to the of fluorescent analogs of PC, PE, and PS in the plasma membrane T. H. Devaux P.F. van Meer G. Holthuis J.C. Mol. Biol. Cell. 2003; 14: PubMed Scopus Google Scholar). of one of these is not indicating that have the is not (10Graham T.R. Trends Cell Biol. 2004; 14: 670-677Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, P. T.R. Mol. Biol. Cell. 2002; 13: PubMed Scopus Google Scholar). activity of the is by PS Y. N. M. M. Biochem. Biophys. PubMed Scopus Google Scholar), and this may be by key in isoforms of the presence of a in but not activity J. Y. X. C. L. J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). In the of the is by PS K. L. M.S. Schlegel R.A. P. Biochemistry. 2006; PubMed Scopus (61) Google Scholar), but not by PE or PC, and the same for the of PS that the human flippase and J. Lipid 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). P4-ATPases include the yeast protein and the human Although is involved in of the pathway and play a role in plasma membrane transport and is involved in formation and from the Golgi to the ER and is essential for P. T.R. Mol. Biol. Cell. 2002; 13: PubMed Scopus Google Scholar, S. H. B. Mol. Cell. Biol. 2004; PubMed Scopus (97) Google Scholar). P4-ATPases have also been shown to with an essential class of membrane proteins that two transmembrane and an (10Graham T.R. Trends Cell Biol. 2004; 14: 670-677Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 11Paulusma C.C. Oude Elferink R.P.J. Biochim. Biophys. Acta. 2005; 1741: 11-24Crossref PubMed Scopus (89) Google Scholar). and with whereas as with K. K. N. U. M. K. Mol. Biol. Cell. 2004; PubMed Scopus Google Scholar). of the of whereas to K. K. N. U. M. K. Mol. Biol. Cell. 2004; PubMed Scopus Google Scholar), including the of the of and the plasma membrane L. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar, U. K. C. H. A. T. K. T. M. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). These may the in of the of the with an protein for or of activity (see supplemental Table and human have been discovered Y. M. 2004; Google Scholar). the family is for activity or of the P4-ATPases is an important of current The putative of P4-ATPases in membrane and in lipid transport may be PS transport across the human membrane a bilayer that in membrane and M. Devaux P.F. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, J. Cell Biol. PubMed Scopus Google Scholar). may occur in more complex cells and or The of yeast P4-ATPases with nucleotide exchange that small involved in binding coat proteins to the membrane may also be important (10Graham T.R. Trends Cell Biol. 2004; 14: 670-677Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar). This is likely not of direction an of formation W.E. M.F. K. Chen T.R. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). have been associated with a number of human and which is by including and M. A. K. M. I. S. M. Nat. PubMed Google Scholar), and such as T. L.M. S.E. J.S. Mol. 2003; PubMed Scopus Google Scholar), have been linked to A of has been associated with and may be a for M.S. T. S. J. 2004; PubMed Scopus (62) Google Scholar). the putative role of the P4-ATPases in membrane structure and these remains to be The generation and of the transbilayer distribution of lipids require the of a number of specific and nonspecific transbilayer lipid transporters. Although these transporters are to as "flippases," distinct of transporter In to a phospholipid flippases may also either by membrane or by an for the binding of coat proteins. The selective or transport of each of these diverse families of lipid transporters the membrane pathway may be responsible for the generation of lipid asymmetry as the membranes from the ER to the plasma membrane. Although a of evidence has implicated a number of proteins as "flippases," their and of transport