Regulation of transbilayer plasma membrane phospholipid asymmetry
2003; Elsevier BV; Volume: 44; Issue: 2 Linguagem: Inglês
10.1194/jlr.r200019-jlr200
ISSN1539-7262
Autores Tópico(s)Cellular transport and secretion
ResumoLipids 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 membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells.It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry. 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 membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry. The transbilayer distribution of lipids across biological membranes is asymmetric (1Bretscher M.S. Asymmetric lipid bilayer structure for biological membranes.Nature (New Biol.). 1972; 236: 11-12Google Scholar). The choline-containing lipids, phosphatidylcholine (PC) and sphingomyelin (SM), are enriched primarily on the external leaflet of the plasma membrane or the topologically equivalent lumenal leaflet of internal organelles. In contrast, the amine-containing glycerophospholipids, phosphatidylethanolamine (PE) and phosphatidylserine (PS), are located preferentially on the cytoplasmic leaflet. Other minor phospholipids, such as phosphatidic acid (PA), phosphatidylinositol (PI), phosphatidylinositol-4-monophosphate (PIP), and phosphatidylinositol-4,5-bisphosphate (PIP2), are also enriched on the cytofacial side of the membrane. This lipid asymmetry has been most well-characterized in the erythrocyte membrane, the outer monolayer of which contains 75–80% of the PC and SM, 20% of the PE, PA, PI, and PIP2, and no detectable PS or PIP (2Op den Kamp J.A.F. Lipid asymmetry in membranes.Annu. Rev. Biochem. 1979; 48: 47-71Google Scholar, 3Rothman J.E. Lenard J. Membrane asymmetry.Science. 1977; 195: 743-753Google Scholar, 4Bretscher M.S. Phosphatidyl-ethanolamine: Differential labelling in intact cells and cell ghosts of human erythrocytes by a membrane-impermeable reagent.J. Mol. Biol. 1972; 71: 523-528Google Scholar, 5Gascard P. Tran D. Sauvage M. Sulpice J-C. Fukami K. Takenawa T. Claret M. Giraud F. Asymmetric distribution of phosphoinositides and phosphatidic acid in the human erythrocyte membrane.Biochim. Biophys. Acta. 1991; 1069: 27-36Google Scholar, 6Bütikofer P. Lin Z.W. Chiu D.T-Y. Kuypers F.A. Transbilayer distribution and mobility of phosphatidylinositol in human red blood cells.J. Biol. Chem. 1990; 265: 16035-16038Google Scholar) [methods for measuring transbilayer lipid asymmetry have been reviewed recently (7Boon J.M. Smith B.D. Chemical control of phospholipid distribution across bilayer membranes.Med. Res. Rev. 2002; 22: 251-281Google Scholar)]. The distribution of glycosylsphingolipids, another significant membrane component, favors the external leaflet of the plasma membrane (8Kolter T. Proia R.L. Sandhoff K. Combinatorial ganglioside biosynthesis.J. Biol. Chem. 2002; 277: 25859-25862Google Scholar). Loss of transmembrane phospholipid asymmetry, with consequent exposure of PS in the external monolayer, occurs in both normal and pathologic conditions. PS externalization is induced early in the process of apoptosis (9Fadok V.A. Voelker D.R. Campbell P.A. Cohen J.J. Bratton D.L. Henson P.M. Exposure of phosphatidylserine on the surface of apoptotic lymphocytes triggers specific recognition and removal by macrophages.J. Immunol. 1992; 148: 2207-2216Google Scholar) and during platelet activation (10Bevers E.M. Comfurius P. van Rijn J.L. Hemker H.C. Zwaal R.F. Generation of prothrombin-converting activity and the exposure of phosphatidylserine at the outer surface of platelets.Eur. J. Biochem. 1982; 122: 429-436Google Scholar). This perturbation results in a change in cell surface properties, including conversion to a procoagulant state (11Lubin B. Chiu D. Bastacky J. Roelofsen B. Van Deenen L.L. Abnormalities in membrane phospholipid organization in sickled erythrocytes.J. Clin. Invest. 1981; 67: 1643-1649Google Scholar), increased adhesion (12Schlegel R.A. McEvoy L. Williamson P. Membrane phospholipid asymmetry and the adherence of loaded red blood cells.Bibl. Haematol. 1985; 51: 150-156Google Scholar), increased aggregation (13Wali R.K. Jaffe S. Kumar D. Sorgente N. Kalra V.K. Increased adherence of oxidant treated human and bovine erythrocytes to cultured endothelial cells.J. Cell. Physiol. 1987; 133: 25-36Google Scholar), and recognition by phagocytic cells (14Fadok V.A. Bratton D.L. Henson P.M. Phagocyte receptors for apoptotic cells: recognition, uptake, and consequences.J. Clin. Invest. 2001; 108: 957-962Google Scholar, 15Fadok V.A. Bratton D.L. Frasch S.C. Warner M.L. Henson P.M. The role of phosphatidylserine in recognition of apoptotic cells by phagocytes.Cell Death Differ. 1998; 5: 551-562Google Scholar). While these processes are essential for normal cell development and hemostasis, unregulated loss of PS asymmetry may contribute significantly to heart disease and stroke and has been associated with diseases that have high cardiovascular risk, such as diabetes (16Wali R.K. Jaffe S. Kumar D. Kalra V.K. Alterations in organization of phospholipids in erythrocytes as a factor in adherence to endothelial cells in diabetes mellitus.Diabetes. 1988; 37: 104-111Google Scholar, 17Wilson M.J. Richter-Lowney K. Daleke D.L. Hyperglycemia induces a loss of phospholipid asymmetry in human erythrocytes.Biochemistry. 1993; 32: 11302-11310Google Scholar). A number of recent reviews contain excellent discussions of lipid asymmetry (18Pomorski T. Hrafnsdottir S. Devaux P.F. van Meer G. Lipid distribution and transport across cellular membranes.Semin. Cell Dev. Biol. 2001; 12: 139-148Google Scholar), lipid transporters (19Borst P. Zelcer N. van Helvoort A. ABC transporters in lipid transport.Biochim. Biophys. Acta. 2000; 1486: 128-144Google Scholar, 20Borst P. Elferink R.O. Mammalian abc transporters in health and disease.Annu. Rev. Biochem. 2002; 71: 537-592Google Scholar, 21Bevers E.M. Comfurius P. Dekkers D.W. Zwaal R.F. Lipid translocation across the plasma membrane of mammalian cells.Biochim. Biophys. Acta. 1999; 1439: 317-330Google Scholar), and the consequences of a loss of asymmetry (22Zwaal R.F. Schroit A.J. Pathophysiologic implications of membrane phospholipid asymmetry in blood cells.Blood. 1997; 89: 1121-1132Google Scholar, 23Schlegel R.A. Williamson P. Phosphatidylserine, a death knell.Cell Death Differ. 2001; 8: 551-563Google Scholar). This review will describe the role of transbilayer lipid transporters, with emphasis on their substrate specificity in the maintenance of lipid asymmetry across the bilayer of the plasma membrane. Lipid biosynthesis is inherently asymmetric. The enzymes responsible for lipid synthesis are localized typically only on the one side of the membrane in which biosynthesis occurs. For the major glycerophospholipids (PS, PE, PC, and PI), de novo synthesis occurs on the cytosolic side of the endoplasmic reticulum (ER)(24Bell R.M. Ballas L.M. Coleman R.A. Lipid topogenesis.J. Lipid Res. 1981; 22: 391-403Google Scholar). With the exception of PC, this places the newly synthesized lipids on the side of the membrane in which they are ultimately enriched in the plasma membrane. Because of the thermodynamic barrier to spontaneous transbilayer movements, these lipids should remain enriched on the cytoplasmic side of the membrane, provided that there is no perturbation to the membrane. However, the asymmetric addition of newly synthesized phospholipids to one leaflet of the bilayer generates an unstable membrane. Accumulation of lipid on one side of the membrane can induce membrane bending and consequent membrane shape changes (25Daleke D.L. Huestis W.H. Incorporation and translocation of aminophospholipids in human erythrocytes.Biochemistry. 1985; 24: 5406-5416Google Scholar, 26Ferrell Jr., J.E. Lee K.J. Huestis W.H. Membrane bilayer balance and erythrocyte shape: a quantitative assessment.Biochemistry. 1985; 24: 2849-2857Google Scholar, 27Farge E. Devaux P.F. Shape changes of giant liposomes induced by an asymmetric transmembrane distribution of phospholipids.Biophys. J. 1992; 61: 347-357Google Scholar). In addition, evidence indicates that ER and Golgi membranes may be less asymmetric than the plasma membrane (28Zachowski A. Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement.Biochem. J. 1993; 294: 1-14Google Scholar). These problems are rectified by the presence of a lipid transporter that redistributes ER phospholipids across the membrane (29Bishop W.R. Bell R.M. Assembly of the endoplasmic reticulum phospholipid bilayer: The phosphatidylcholine transporter.Cell. 1985; 42: 51-60Google Scholar, 30Buton X. Morrot G. Fellmann P. Seigneuret M. Ultrafast glycerophospholipid-selective transbilayer motion mediated by a protein in the endoplasmic reticulum membrane.J. Biol. Chem. 1996; 271: 6651-6657Google Scholar, 31Hrafnsdottir S. Menon A.K. Reconstitution and partial characterization of phospholipid flippase activity from detergent extracts of the Bacillus subtilis cell membrane.J. Bacteriol. 2000; 182: 4198-4206Google Scholar, 32Gummadi S.N. Menon A.K. Transbilayer movement of dipalmitoylphosphatidylcholine in proteoliposomes reconstituted from detergent extracts of endoplasmic reticulum. Kinetics of transbilayer transport mediated by a single flippase and identification of protein fractions enriched in flippase activity.J. Biol. Chem. 2002; 277: 25337-25343Google Scholar). Although de novo glycerophospholipid synthesis is asymmetric, the action of this transporter defeats vectoral biosynthesis and results in a more random distribution of lipids across the bilayer. Sphingolipids are localized predominately on the external leaflet of the plasma membrane. Unlike PC synthesis, sphingolipid synthesis occurs predominantly on the side of the membrane in which these lipids ultimately reside. With the exception of glucosylceramide (Glc-Cer), which is synthesized on the cytofacial side of the Golgi, all of the sphingolipids are synthesized on the lumenal surface of the ER or Golgi, including SM, galactosylceramide, and complex sugar-linked sphingolipids (8Kolter T. Proia R.L. Sandhoff K. Combinatorial ganglioside biosynthesis.J. Biol. Chem. 2002; 277: 25859-25862Google Scholar, 33Holthuis J.C. Pomorski T. Raggers R.J. Sprong H. Van Meer G. The organizing potential of sphingolipids in intracellular membrane transport.Physiol. Rev. 2001; 81: 1689-1723Google Scholar, 34Lannert H. Gorgas K. Meissner I. Wieland F.T. Jeckel D. Functional organization of the Golgi apparatus in glycosphingolipid biosynthesis. Lactosylceramide and subsequent glycosphingolipids are formed in the lumen of the late Golgi.J. Biol. Chem. 1998; 273: 2939-2946Google Scholar). Because Glc-Cer is a precursor of many glycosylsphingolipids, a mechanism must exist to transport this lipid to the lumenal surface of the ER or Golgi. A transporter that catalyzes the transbilayer movement of short-chain analogs of Glc-Cer has been discovered (35Burger K.N. van der Bijl P. van Meer G. Topology of sphingolipid galactosyltransferases in ER and Golgi: transbilayer movement of monohexosyl sphingolipids is required for higher glycosphingolipid biosynthesis.J. Cell Biol. 1996; 133: 15-28Google Scholar, 36Buton X. Herve P. Kubelt J. Tannert A. Burger K.N. Fellmann P. Muller P. Herrmann A. Seigneuret M. Devaux P.F. Transbilayer movement of monohexosylsphingolipids in endoplasmic reticulum and Golgi membranes.Biochemistry. 2002; 41: 13106-13115Google Scholar) that may serve this function. The selective accumulation of glycerophospholipids on one side of the plasma membrane requires that during, or as a result of, membrane trafficking from the ER to the plasma membrane that the transbilayer randomizing process be inhibited or that an asymmetry-generating process be activated. Thermodynamic considerations require an input of energy to generate, or to maintain, a transbilayer lipid gradient. Both inward and outward ATP-dependent lipid transport activities have been discovered that selectively move lipids across the plasma membrane. The asymmetric distribution of phospholipids in the plasma membrane may be the result of the selective trafficking or regulation of lipid transporting proteins. The retention of ATP-independent nonselective lipid transporters in the ER, combined with the trafficking of substrate-specific ATP-dependent transporters to the plasma membrane may account for the creation of a highly asymmetric plasma membrane from the more symmetric ER and Golgi membranes. Alternatively, lipid randomizing and asymmetry generating lipid transporters may coexist in multiple membranes, but be differentially regulated. Discrimination between these models awaits the positive identification, verification of intracellular location, and characterization of the biochemical regulation of these transporters. Once lipid asymmetry has been established, it is maintained by a combination of slow transbilayer diffusion, protein-lipid interactions, and protein-mediated transport. The presence of binding sites for acidic lipids, including PS, on the cytoskeletal proteins spectrin and band 4.1 (37Mombers C. Verkleij A.J. de Gier J. van Deenen L.L.M. The interaction of spectrin-actin and synthetic phospholipids. II. The interaction with phosphatidylserine.Biochim. Biophys. Acta. 1979; 551: 271-281Google Scholar, 38Cohen A.M. Liu S.C. Lawler J. Derick L. Palek J. Identification of the protein 4.1 binding site to phosphatidylserine vesicles.Biochemistry. 1988; 27: 614-619Google Scholar, 39Sato S.B. Ohnishi S.I. Interaction of a peripheral protein of the erythrocyte membrane, band 4.1, with phosphatidylserine-containing liposomes and erythrocyte inside-out vesicles.Eur. J. Biochem. 1983; 130: 19-25Google Scholar) and soluble membrane-binding proteins such as annexins (40Meers P. Mealy T. Relationship between annexin V tryptophan exposure, calcium, and phospholipid binding.Biochemistry. 1993; 32: 5411-5418Google Scholar) suggest that cytofacial protein-membrane interactions may play a role in sequestering PS in the cytofacial monolayer. Indeed, lipid-symmetric membranes bind cytoskeletal proteins more poorly than lipid-asymmetric membranes at low ionic strength and have lower mechanical stability (41Manno S. Takakuwa Y. Mohandas N. Identification of a functional role for lipid asymmetry in biological membranes: Phosphatidylserine-skeletal protein interactions modulate membrane stability.Proc. Natl. Acad. Sci. USA. 2002; 99: 1943-1948Google Scholar). However, the number and magnitude of the available binding sites is not sufficient to trap PS (42Maksymiw R. Sui S.F. Gaub H. Sackmann E. Electrostatic coupling of spectrin dimers to phosphatidylserine containing lipid lamellae.Biochemistry. 1987; 26: 2983-2990Google Scholar, 43Gudi S.R. Kumar A. Bhakuni V. Gokhale S.M. Gupta C.M. Membrane skeleton-bilayer interaction is not the major determinant of membrane phospholipid asymmetry in human erythrocytes.Biochim. Biophys. Acta. 1990; 1023: 63-72Google Scholar, 44O'Toole P.J. Wolfe C. Ladha S. Cherry R.J. Rapid diffusion of spectrin bound to a lipid surface.Biochim. Biophys. Acta. 1999; 1419: 64-70Google Scholar, 45O'Toole P.J. Morrison I.E. Cherry R.J. Investigations of spectrin-lipid interactions using fluoresceinphosphatidylethanolamine as a membrane probe.Biochim. Biophys. Acta. 2000; 1466: 39-46Google Scholar). In addition, spectrin-depleted membranes (46Calvez J.Y. Zachowski A. Herrmann A. Morrot G. Devaux P.F. Asymmetric distribution of phospholipids in spectrin-poor erythrocyte vesicles.Biochemistry. 1988; 27: 5666-5670Google Scholar) and pathologic cells with defective or deficient cytoskeletal proteins (47Kuypers F.A. Lubin B.H. Yee M. Agre P. Devaux P.F. Geldwerth D. The distribution of erythrocyte phospholipids in hereditary spherocytosis demonstrates a minimal role for erythrocyte spectrin on phospholipid diffusion and asymmetry.Blood. 1993; 81: 1051-1057Google Scholar, 48de Jong K. Larkin S.K. Eber S. Franck P.F. Roelofsen B. Kuypers F.A. Hereditary spherocytosis and elliptocytosis erythrocytes show a normal transbilayer phospholipid distribution.Blood. 1999; 94: 319-325Google Scholar) are capable of generating and maintaining a PS gradient. These data indicate that, although the plastic properties of the erythrocyte membrane require close association with cytofacial lipids, this interaction does not play a major role in the maintenance of lipid asymmetry. The thermodynamic barrier to passive lipid flip-flop prevents rapid spontaneous transbilayer diffusion of phospholipids. The half time for phospholipid flip-flop is approximately several hours to days (49Kornberg R.D. McConnell H.M. Inside-outside transitions of phospholipids in vesicle membranes.Biochemistry. 1971; 10: 1111-1120Google Scholar) and depends on the nature of the lipid and the membrane. In the human erythrocyte, flip-flop rates are dependent on phospholipid acyl chain length and degree of unsaturation (50Fuji T. Tamura A. Dynamic behaviour of amphiphilic lipids to penetrate into membrane of intact erythocytes and to induce change in the cell shape.Biomed. Biochim. Acta. 1983; 42: S81-S85Google Scholar, 51Middelkoop E. Lubin B.H. Op den Kamp J.A.F. Roelofsen B. Flip-flop rates of individual molecular species of phosphatidylcholine in the human red cell membrane.Biochim. Biophys. Acta. 1986; 855: 421-424Google Scholar, 52Van Meer G. Op den Kamp J.A.F. Transbilayer movement of various phosphatidylcholine species in intact human erythrocytes.J. Cell. Biochem. 1982; 19: 193-204Google Scholar). Considering that the half time of flip is much shorter that the average lifespan of most cell types, it is unlikely that this phenomenon could account for the maintenance of phospholipid asymmetry. Other perturbations to membrane structure may induce a rapid reorientation of lipids. For example, chronic in vitro hyperglycemia (17Wilson M.J. Richter-Lowney K. Daleke D.L. Hyperglycemia induces a loss of phospholipid asymmetry in human erythrocytes.Biochemistry. 1993; 32: 11302-11310Google Scholar) or diabetes (53Wali R.K. Jaffe S. Kumar D. Kalra V.K. Alterations in organization of phospholipids in erythrocytes as factor in adherence to endothelial cells in diabetes mellitus.Diabetes. 1988; 37: 104-111Google Scholar) induces the exposure of inner monolayer lipids on the surface of the erythrocyte plasma membrane and may contribute to the vascular damage associated with this disease (54Manodori A.B. Kuypers F.A. Altered red cell turnover in diabetic mice.J. Lab. Clin. Med. 2002; 140: 161-165Google Scholar). Although the barrier to rapid spontaneous flip-flop contributes to the maintenance of lipid asymmetry, other mechanisms must be responsible for the regeneration of lipid asymmetry or the activation-induced rapid loss of asymmetry. Perhaps the most significant contributors to the maintenance and dissipation of transbilayer lipid asymmetry are proteins that catalyze the movement of lipids across the membrane (Table 1). These activities are classified according to substrate specificity, requirements for energy, and direction of transport (Fig. 1). Two classes of transport activities have been described that are responsible for the ATP-dependent transport of lipids. The most well-characterized activity is the aminophospholipid translocase or "flippase," which transports PS from the outer monolayer to the cytoplasmic surface of the plasma membrane. A second ATP-dependent activity, catalyzed by "floppases," transport lipids in the opposite direction. The most well-characterized floppase activities have been shown to catalyze the inner-to-outer monolayer transport of short-chain fluorescent lipids and the selective efflux of PC or cholesterol. Three ATP-independent and relatively nonspecific scramblase activities have been reported; a plasma membrane Ca2+-activated transporter, an ER glycerophospholipid-specific transporter, and an ER monohexosyl-lipid transporter.TABLE 1Lipid specificity of transbilayer lipid transportersClassProteinSpecificityReferenceFlippasesP4-ATPasesamphipaths(102Halleck M.S. Pradhan D. Blackman C. Berkes C. Williamson P. Schlegel R.A. Multiple members of a third subfamily of P-type ATPases identified by genomic sequences and ESTs.Genome Res. 1998; 8: 354-361Google Scholar, 103Axelsen K.B. Palmgren M.G. Evolution of substrate specificities in the P-type ATPase superfamily.J. Mol. Evol. 1998; 46: 84-101Google Scholar)erythrocyte Mg2+-ATPasePS(94Daleke D.L. Lyles J.V. Identification and purification of aminophospholipid flippases.Biochim. Biophys. Acta. 2000; 1486: 108-127Google Scholar)ABCRN-retinylidene-PE(137Weng J. Mata N.L. Azarian S.M. Tzekov R.T. Birch D.G. Travis G.H. Insights into the function of Rim protein in photoreceptors and etiology of Stargardt's disease from the phenotype in abcr knockout mice.Cell. 1999; 98: 13-23Google Scholar)FloppasesABCA1cholesterol(117–119)ABCB1none(12Schlegel R.A. McEvoy L. Williamson P. Membrane phospholipid asymmetry and the adherence of loaded red blood cells.Bibl. Haematol. 1985; 51: 150-156Google Scholar3)ABCB4PC(123van Helvoort A. Smith A.J. Sprong H. Fritzsche I. Schinkel A.H. Borst P. van Meer G. MDR1 P-glycoprotein is a lipid translocase of broad specificity, while MDR3 P-glycoprotein specifically translocates phosphatidylcholine.Cell. 1996; 87: 507-517Google Scholar)ABCC1short-chain phospholipids(133Kamp D. Haest C.W. Evidence for a role of the multidrug resistance protein (MRP) in the outward translocation of NBD-phospholipids in the erythrocyte membrane.Biochim. Biophys. Acta. 1998; 1372: 91-101Google Scholar)ScramblasesPLSCR1none(149Sims P.J. Wiedmer T. Unraveling the mysteries of phospholipid scrambling.Thromb. Haemost. 2001; 86: 266-275Google Scholar)ER flippasenone(29Bishop W.R. Bell R.M. Assembly of the endoplasmic reticulum phospholipid bilayer: The phosphatidylcholine transporter.Cell. 1985; 42: 51-60Google Scholar, 30Buton X. Morrot G. Fellmann P. Seigneuret M. Ultrafast glycerophospholipid-selective transbilayer motion mediated by a protein in the endoplasmic reticulum membrane.J. Biol. Chem. 1996; 271: 6651-6657Google Scholar, 31Hrafnsdottir S. Menon A.K. Reconstitution and partial characterization of phospholipid flippase activity from detergent extracts of the Bacillus subtilis cell membrane.J. Bacteriol. 2000; 182: 4198-4206Google Scholar, 32Gummadi S.N. Menon A.K. Transbilayer movement of dipalmitoylphosphatidylcholine in proteoliposomes reconstituted from detergent extracts of endoplasmic reticulum. Kinetics of transbilayer transport mediated by a single flippase and identification of protein fractions enriched in flippase activity.J. Biol. Chem. 2002; 277: 25337-25343Google Scholar) Open table in a new tab The ultimate transbilayer distribution of lipids is determined by the specificity of the lipid transporters located in the membrane. Each of the transport activities described above displays a unique specificity or nonspecificity that defines its function in the determination of lipid organization. A number of excellent reviews have surveyed the subject of lipid transporters recently (7Boon J.M. Smith B.D. Chemical control of phospholipid distribution across bilayer membranes.Med. Res. Rev. 2002; 22: 251-281Google Scholar, 18Pomorski T. Hrafnsdottir S. Devaux P.F. van Meer G. Lipid distribution and transport across cellular membranes.Semin. Cell Dev. Biol. 2001; 12: 139-148Google Scholar, 19Borst P. Zelcer N. van Helvoort A. ABC transporters in lipid transport.Biochim. Biophys. Acta. 2000; 1486: 128-144Google Scholar, 21Bevers E.M. Comfurius P. Dekkers D.W. Zwaal R.F. Lipid translocation across the plasma membrane of mammalian cells.Biochim. Biophys. Acta. 1999; 1439: 317-330Google Scholar). The following summarizes the current state of knowledge regarding the specificity of these transport activities and, where evidence is available, the protein(s) involved. Aminophospholipid flippase activity was first reported by Devaux and coworkers who measured the ATP-dependent uptake of spin-labeled lipid analogs in human erythrocytes (55Seigneuret M. Devaux P.F. ATP-dependent asymmetric distribution of spin-labeled phospholipids in the erythrocyte membrane: relation to shape changes.Proc. Natl. Acad. Sci. USA. 1984; 81: 3751-3755Google Scholar). Phospholipids labeled with fluorescent fatty acids, particularly 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives, have also been used extensively to study this transporter (56Connor J. Schroit A.J. Determination of lipid asymmetry in human red cells by resonance energy transfer.Biochemistry. 1987; 26: 5099-5105Google Scholar, 57Sleight R.G. Pagano R.E. Transbilayer movement of a fluorescent phosphatidylethanolamine analogue across the plasma membranes of cultured mammalian cells.J. Biol. Chem. 1985; 260: 1146-1154Google Scholar, 58Colleau M. Herve P. Fellmann P. Devaux P.F. Transmembrane diffusion of fluorescent phospholipids in human erythrocytes.Chem. Phys. Lipids. 1991; 57: 29-37Google Scholar). The addition of these polar, bulky substituents to the fatty acid component of lipids may potentially alter transporter-lipid interactions, thus questioning whether movements measured with these lipids reflect the behavior of endogenous lipids. These spin and fluorescent probes are powerful tools, but their use requires careful interpretation (59Devaux P.F. Fellmann P. Herve P. Investigation on lipid asymmetry using lipid probes. Comparison between spin-labeled lipids and fluorescent lipids.Chem. Phys. Lipids. 2002; 116: 115-134Google Scholar, 60Maier O. Oberle V. Hoekstra D. Fluorescent lipid probes: some properties and applications (a review).Chem. Phys. Lipids. 2002; 116: 3-18Google Scholar) and independent verification that their movements reflect those of endogenous lipids. In addition to spin-labeled and fluorescent lipids, native and radiolabeled short (25Daleke D.L. Huestis W.H. Incorporation and translocation of aminophospholipids in human erythrocytes.Biochemistry. 1985; 24: 5406-5416Google Scholar, 61Daleke D.L. Huestis W.H. Erythrocyte morphology reflects the transbilayer distribution of incorporated phospholipids.J. Cell Biol. 1989; 108: 1375-1385Google Scholar, 62Anzai K. Yoshioka Y. Kirino Y. Novel radioactive phospholipid probes as a tool for measurement of phospholipid translocation across biomembranes.Biochim. Biophys. Acta. 1993; 1151: 69-75Google Scholar) and long (63Tilley L. Cribier S. Roelofsen B. Op den Kamp J.A.F. van Deenen L.L.M. ATP-dependent translocation of aminophospholipids across the human erythrocyte membrane.FEBS Lett. 1986; 194: 21-27Google Scholar) chain fatty acid-containing species have been used to measure flippase activity. The use of these lipids is more difficult and restricted, but their behavior may reflect more accurately the behavior of endogenous lipids. Flippase-catalyzed transport is linked to an ATPase; transport activity requires ATP (25Daleke D.L. Huestis W.H. Incorporation and translocation of aminophospholipids in human erythrocytes.Biochemistry. 1985; 24: 5406-5416Google Scholar, 55Seigneuret M. Devaux P.F. ATP-dependent asymmetric distribution of spin-labeled phospholipids in the erythrocyte membrane: relation to sh
Referência(s)