M tivities seem to work in concert to maintain a nonrandom ANY STUDIES HAVE led to the concept that membrane phospholipid asymmetry is ubiquitous. In gen- transbilayer phospholipid orientation. Inhibition of these aceral terms, the outer leaflet of eukaryotic plasma membranes tivities stops lipid movement, but it does not result in loss is formed predominantly with the cholinephospholipids ofasymmetryforatleastseveraldaysinvitro. InfluxofCa2/(sphingomyelin and phosphatidylcholine [PC]), whereas the into the cytoplasm, on the other hand, activates a scramblase majority of the aminophospholipids (phosphatidylserine [PS] activity that results in rapid transbilayer phospholipid mixing and phosphatidylethanolamine [PE]) are confined to the thatleadstoanearlysymmetricdistributionofphospholipids membrane’s inner leaflet. This selective localization dictates across the membrane bilayer (Fig 1). that asymmetric biomembranes are assembled and main- Aminophospholipid translocase. The discovery of an tained by specific mechanisms that control transbilayer lipid ATP-dependent aminophospholipid translocase in red blood sidedness. In 1984, it became clear that asymmetry was gen- cells has provided direct evidence for the existence of mecherated by the activity of an adenosine triphosphate (ATP)- anisms that generate and maintain membrane asymmetry dependent aminophospholipid translocase that specifically through the transport of specific lipids across the cell’s memtransports PS and PE between bilayer leaflets. 1 This discov- brane. 1, 22, 23 This activity is distinguished by its ability to ery underscored the prevailing concept that membrane lipid transport PS and PE from the outer to inner leaflet of plasma asymmetry was of major physiologic importance, because it membranes against the concentration gradient. Cholinephosshowed that cells invest energy to catalyze lipid movement pholipids are not moved. Competition experiments have in order to maintain a specific transmembrane phospholipid shown that the same protein transports both PS and PE, distribution. although PS is transported much faster, with half-times of 5 Although asymmetry is the rule for normal cells, loss of to 10 minutes. 24, 25 This process consumes one molecule of asymmetry, especially the appearance of PS at the cell sur- ATP per molecule of lipid transported. 26 Transport is stereoface, is associated with many physiologic and pathologic specific for naturally occurring L-isomers of the glycerophenomena. Bevers et al2, 3 were the first to report that the backbone27 and is inhibited by vanadate, 1 sulfhydryl-reactive asymmetric orientation of phospholipids in blood platelets reagents, 28, 29 and the histidine-reactive reagent bromophenwas rapidly lost upon influx of calcium during their activa- acylbromide. 25 In addition, activity is abrogated when cytotion, a finding that suggested a critical role for PS in throm- plasmic Ca2/levels reach micromolar concentrations. 30, 31 bosis. 4, 5 Apoptotic6-9 and tumorigenic cells10, 11 also express Although these observations clearly indicate that lipid transrelatively large amounts of outer-leaflet PS that may serve port is catalyzed by one or more membrane proteins, its identity as a trigger for macrophage recognition and promote the is still uncertain. A 110-kD Mg2/-ATPase has been partially cells’ phagocytosis. 12-15 purified32-34 and reconstituted into artificial lipid vesicles with In this essay, we summarize recent observations on the at least a fraction of its active center at the outer face. 35 These perturbation of membrane phospholipid asymmetry and vesicles transported a spin-labeled PS analog from the inner to present a …