Partial purification and characterization of the human erythrocyte Mg2(+)-ATPase. A candidate aminophospholipid translocase

Transbilayer phosphatidylethanolamine movements in the yeast plasma membrane. Evidence for a protein-mediated, energy-dependent mechanism

Aminophospholipid movements in the plasma membrane of higher eukaryotic cells seem to be regulated by an ATP-dependent, protein-mediated process. To examine whether similar mechanisms exist in yeast cells, we have analysed phosphatidylethanolamine (PtdEtn) distributions in Saccharomyces cerevisiae (A184D) cells under a variety of conditions, with trinitrobenzenesulfonic acid and fluorescamine as the external membrane probes. The levels of external PtdEtn in the intact cells were reduced to about 50% by pretreatment of the cells with inhibitors of mitochondrial ATP synthesis, ATPase inhibitors or protein-sulfhydryl-group-modifying reagents, or by depletion of the cells of ATP by metabolic starvation. The levels of external PtdEtn could be restored to normal by repletion of the energy-depleted cells with ATP. Furthermore, treatment of the energy-depleted cells with sulfhydryl-modifying reagents did not cause further reduction in the external PtdEtn levels but decreased the accessibility of PtdEtn to fluorescamine after restoration of the cellular ATP levels to normal in these cells. These results demonstrate an involvement of an ATP-dependent, protein-mediated process(es) in the regulation of the PtdEtn distribution across the plasma-membrane bilayer of yeast cells. The results are discussed with regard to possible models that can generate and maintain the transbilayer phospholipid asymmetry in the yeast plasma membrane.

Adhesion of K88ab to guinea pig erythrocytes: effect on membrane enzyme activities

Nontoxigenic Escherichia coli strains bearing K88 fimbriae have been associated with diarrhea in piglets. We have used guinea pig erythrocytes as a model of the host cell to study the cellular alterations after adherence of purified K88ab fimbriae. Although Mg2+-dependent ATPase was inhibited (up to 61%), Na/K ATPase was not. Metabolic enzymes were not significantly affected.

A subfamily of P-type ATPases with aminophospholipid transporting activity

The appearance of phosphatidylserine on the surface of animal cells triggers phagocytosis and blood coagulation. Normally, phosphatidylserine is confined to the inner leaflet of the plasma membrane by an aminophospholipid translocase, which has now been cloned and sequenced. The bovine enzyme is a member of a previously unrecognized subfamily of P-type adenosine triphosphatases (ATPases) that may have diverged from the primordial enzyme before the separation of the known families of ion-translocating ATPases. Studies in Saccharomyces cerevisiae suggest that aminophospholipid translocation is a general function of members of this family.

Manipulation of the phosphatidylethanolamine pool in the human red cell membrane affects its Mg2+-ATPase activity

Decreasing the size of the outer leaflet pool of phosphatidylethanolamine (PE) in the erythrocyte membrane by treatment of intact cells with either phospholipase A2, or trinitrobenzenesulphonic acid (TNBS), causes a corresponding decrease in Mg(2+)-ATPase activity as determined in their respective ghosts. Also, incubation of ghosts with Ro09-0198, a cyclic peptide from Streptoverticillium which is known to interact specifically with PE, causes a decrease in Mg(2+)-ATPase activity which is dependent on the amount of peptide added. These three different approaches, all causing a decrease in endogenous PE, thus result in a concomitant decrease in Mg(2+)-ATPase activity which reaches a plateau level at approximately 25% residual activity. Hence, it is inferred that the complementary fraction (75%) of the total Mg(2+)-ATPase in the red cell membrane is closely related to the functioning of its aminophospholipid specific translocase as it mediates a (continuous) transport of PE molecules from outer to inner membrane leaflet. This view is supported by the observation that an increase in the total amount of PE in the membrane by decarboxylation of an appreciable fraction of its PS, results in a considerable increase in Mg(2+)-ATPase activity.

Binding and phagocytosis of apoptotic vascular smooth muscle cells is mediated in part by exposure of phosphatidylserine

Apoptosis of vascular smooth muscle cells has recently been demonstrated to occur in vitro and in vivo. Uptake of apoptotic cells into adjacent normal cells appears to be rapid and specific. We have investigated binding and phagocytosis of apoptotic vascular smooth muscle cells by normal smooth muscle cell monolayers. Vascular smooth muscle cells were infected with the proto-oncogene c-myc or the adenovirus E1A gene, induced to undergo apoptosis in low-serum conditions, and then incubated with normal smooth muscle cells. Apoptosis was accompanied by a marked increase in exposure of phosphatidylserine on the outer surface of the cell, which was recognized by binding to annexin V. Liposomes containing phosphatidylserine but not phosphatidylinositol inhibited uptake of apoptotic cells in a dose-dependent manner to a maximum of 50% inhibition; annexin V also inhibited the uptake of apoptotic cells in a dose-dependent and calcium-dependent manner. Binding of apoptotic bodies did not appear to be mediated by endogenous annexin V, as evidenced by the inability of an antibody to annexin V to inhibit uptake. Smooth muscle cells were also able to recognize exposed phosphatidylserine on other cell types, as judged by their ability to bind erythrocytes having a high degree of exposed phosphatidylserine. We conclude that smooth muscle cells express phosphatidylserine during apoptosis, and this exposure partly mediates binding and phagocytosis of dead cells. This mechanism may be important in promoting rapid cell removal in the vessel wall.

Continuous analysis of the mechanism of activated transbilayer lipid movement in platelets

Dithionite reduction of fluorescent (NBD) phospholipids was used as the basis of a continuous assay of transbilayer lipid movement to the cell surface during platelet activation. This assay reveals that virtually all previously internalized phosphatidylserine passes through the external leaflet of the membrane within 90 s after activation with Ca2+ and ionophore or with thrombin and thapsigargin. We demonstrate that this lipid scrambling is reversible, bidirectional, and insensitive to the lipid headgroup. Prolonged activation gradually results in inactivation of the scramblase. The assay also reveals that activation of the scrambling activity is sensitive to the sulfhydryl reagent pyridyldithioethylamine, suggesting the involvement of a protein in the process of activated transbilayer lipid scrambling.

Altered lipid composition and differential changes in activities of membrane-bound enzymes of erythrocytes in hepatic cirrhosis

Lipid composition, fluidity, and Na+,K(+)-adenosine triphosphatase (ATPase), Mg(2+)-ATPase, and acetylcholinesterase (AChE) activities of erythrocyte membranes were examined in comparison to plasma lipid composition and lecithin:cholesterol acyltransferase (LCAT) activities in 39 patients with hepatic cirrhosis due to viral hepatitis (Child-Pugh class A, n = 12; class B, n = 13; and class C, n = 14). Plasma LCAT activities decreased and the plasma free-cholesterol to phospholipid molar ratio (C/PL) increased with progressive severity of hepatic cirrhosis. C/PL and fluorescence polarization (inverse of fluidity) of erythrocyte membranes also increased with disease progression (C/PL: Child-Pugh A, 0.911 +/- 0.010; B, 0.941 +/- 0.011; C, 0.979 +/- 0.028; and normal, 0.798 +/- 0.010; fluorescence polarization: Child-Pugh A, 0.348 +/- 0.002; B, 0.351 +/- 0.002; C, 0.355 +/- 0.002; and normal, 0.340 +/- 0.002). There was a correlation between C/PL and fluorescence polarization of erythrocyte membranes (r = .629, P < .001). Na+,K(+)-ATPase activity of erythrocyte membranes did not differ between cirrhotic patients and normal subjects. On the other hand, Mg(2+)-ATPase activity decreased in Child-Pugh C cirrhosis. AChE activity was decreased in Child-Pugh A cirrhosis, and decreased further in Child-Pugh B and C cirrhosis. AChE and Mg(2+)-ATPase activities correlated inversely with fluorescence polarization (r = -.652, P < .001 and r = -.381, P < .01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced Mg(2+)-ATPase activity in ghosts from HS erythrocytes and in normal ghosts stripped of membrane skeletal proteins may reflect enhanced aminophospholipid translocase activity

Hereditary spherocytosis (HS) is a congenital haemolytic anaemia which is characterized by a great variety of structural defects in the red cell's membrane skeleton and/or deficiencies in particular membrane (skeletal) proteins. Enhanced (Mg2+)-dependent adenosine triphosphatase (Mg(2+)-ATPase) activities, varying from 115% to 160%, were invariably found in erythrocyte ghosts derived from 13 HS patients. Similarly, an enhancement of Mg(2+)-ATPase activity by 30% is observed in normal red cell ghosts that have been stripped of the greater part of their membrane skeletal proteins by treatment with a low ionic strength buffer. Reassociation of those stripped ghosts with spectrin reduces the enhanced Mg(2+)-ATPase activity to its original level. Since in both cases, HS ghosts and stripped normal ghosts, the stabilizing effects that the membrane skeleton exerts on the maintenance of an endofacial localization of the aminophospholipids are impaired, the enhanced Mg(2+)-ATPase activity is interpreted to reflect an increased activity of the aminophospholipid translocase. The present observations therefore support a role of the membrane skeleton in the stabilization of phospholipid asymmetry in the red cell membrane and consequently in reducing the energy consumption of the translocase.

Colocalization of Rh polypeptides and the aminophospholipid transporter in dilauroylphosphatidylcholine-induced erythrocyte vesicles

Cytoskeleton-free vesicles released from human red blood cells (RBC) transport exogenously supplied aminophospholipid analogues from the vesicle's outer to inner leaflet at rates comparable to those of normal RBC (Beleznay et al. (1993) Biochemistry 32, 3146-3152). Because polypeptides associated with the Rh blood group system have been implicated in the transbilayer movement of phosphatidylserine (PS), we investigated the relationship and co-localization of the aminophospholipid translocase and Rh in dilauroylphosphatidylcholine-induced RBC vesicles. The transbilayer movement of fluorescent (NBD-PS) and photoactivatable (125I-N3-PS) PS in RBC vesicles was ATP-and temperature-dependent. Inhibition of PS transport by sulfhydryl reagents could be accomplished by direct vesicle treatment or by treating RBC before vesiculation. In the case of diamide- and pyridyldithioethylamine-mediated inhibition, NBD-PS transport could be restored by reduction with dithiothreitol, indicating that the movement of the PS transporter into the emerging vesicle was independent of the oxidative status of membrane sulfhydryls. The presence of Rh polypeptides in the vesicles was verified by direct immunoprecipitation of isotopically-labeled Rh and semi-quantified by antibody adsorption assays. Similar to the movement of the PS transporter, localization of Rh polypeptides in the vesicle membrane was independent of the red cell's oxidative status. These results show that the PS translocase and Rh-related proteins colocalize in RBC vesicles suggesting that these proteins may be members of a multicomponent complex that plays a role in lipid movement and the generation of membrane lipid asymmetry.

Purification of deformin, an extracellular protein synthesized by Bartonella bacilliformis which causes deformation of erythrocyte membranes

A factor capable of deforming erythrocyte membranes, found in the culture supernatants of Bartonella bacilliformis, was purified 1840-fold using hydrophobic, ion exchange and gel exclusion chromatography. The final fractions contained a single detectable polypeptide species, referred to as deformin, having a molecular weight of 67000 by SDS-PAGE and a native molecular weight of 130,000 by gel exclusion chromatography or velocity sedimentation in a glycerol gradient. Erythrocytes treated with deformin acquire trenches, indentations, and invaginations which could be reversed by vanadate, dilauroylphosphatidylcholine (DLPC), or by raising the internal Ca2+ concentrations with the inophore A23187. Internal vacuoles also form. Erythrocytes treated with trypsin or neuraminidase are much more sensitive to deformin than untreated erythrocytes; erythrocytes treated with phospholipase D are less sensitive to deformin. This protein may play a role in causing the severe anemia which can result as a consequence of infection by B. bacilliformis.

A Chinese hamster ovary cell mutant defective in the non-endocytic uptake of fluorescent analogs of phosphatidylserine: isolation using a cytosol acidification protocol

Transmembrane movement of phosphatidylserine (PS) and various PS analogs at the plasma membrane is thought to occur by an ATP-dependent, protein-mediated process. To isolate mutant CHO cells defective in this activity, we first obtained conditions which inhibited the endocytic, but not the non-endocytic pathway of lipid internalization since PS may enter cells by a combination of these two pathways. We found that acidic treatment of cells, which blocks clathrin-dependent endocytosis, enhanced the energy-dependent uptake of 1-palmitoyl-2-(6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]caproyl -sn- glycero-3-phosphoserine (C6-NBD-PS) in CHO cells from donor vesicles (liposomes) by about twofold. Control experiments demonstrated that the enhanced uptake of C6-NBD-PS at acidic pH was not due to: (a) an increase in the capacity of the plasma membrane to incorporate C6-NBD-PS from the donor vesicles; (b) a decrease in the rate of loss of C6-NBD-PS from the cells; or (c) fusion or engulfment of the donor vesicles. When cytosolic acidification (to pH 6.3) was imposed without acidification of the extracellular medium, C6-NBD-PS uptake by intact cells was increased by about 50% compared to control values determined in the absence of acidification. These results suggested that a protein and energy dependent system(s) for transbilayer movement of the fluorescent PS was stimulated by cytosolic acidification. A screening method for mutant cells defective in the non-endocytic uptake of fluorescent PS analogs with replica cell colonies at acidic pH was then devised. After selection of mutagenized CHO-K1 cells by in situ screening, we obtained a mutant cell line in which uptake of fluorescent PS analogs was reduced to about 25% of the wild type level at either pH 6.0 or 7.4. Control experiments demonstrated that the reduced uptake of fluorescent PS analogs in the mutant cells was unrelated to multidrug resistance, and that endocytosis of another plasma membrane lipid marker occurred normally in the mutant cells. These results suggested that a non-endocytic pathway responsible for uptake of fluorescent PS analogs was specifically affected in the mutant cells.

Recognition of oxidatively damaged and apoptotic cells by an oxidized low density lipoprotein receptor on mouse peritoneal macrophages: role of membrane phosphatidylserine

We recently reported that oxidized low density lipoprotein (OxLDL), but not acetyl LDL (AcLDL), inhibited the binding and phagocytosis of nonopsonized, oxidatively damaged red blood cells (OxRBCs) by mouse peritoneal macrophages, implying the involvement of a "scavenger receptor" other than the AcLDL receptor. Numerous studies establish that loss of plasma membrane phospholipid asymmetry, which increases phosphatidylserine expression on the outer leaflet of the membrane, can play a key role in macrophage recognition of damaged and apoptotic cells. We report here that this recognition is in part attributable to the same mouse macrophage receptor that recognizes OxLDL. As described in an accompanying paper, this is a plasma membrane protein of 94-97 kDa. Phosphatidylserine liposomes show strong ligand binding to the same 94- to 97-kDa protein and this binding is inhibited by OxLDL but not by AcLDL. Inhibition of the RBC membrane phospholipid translocase by incubation with sodium vanadate caused a progressive increase in the appearance of phosphatidylserine on the cell surface and a parallel increase in the binding of these RBCs to macrophages, binding that was inhibited by OxLDL. Finally, OxLDL also inhibited the binding of sickled RBCs and apoptotic thymocytes to mouse macrophages. However, the latter was incomplete (approximately 50%), suggesting that other receptors are also involved. We suggest that the OxLDL receptor plays a significant role in recognition of damaged and apoptotic cells.

Reconstitution of ATP-dependent aminophospholipid translocation in proteoliposomes

In addition to ion-pumping ATPases, most plasma membranes of animal cells contain a Mg2+ ATPase activity, the function of which is unknown. This enzyme, of apparent molecular mass 110 kDa, was purified from human erythrocyte membranes by a series of column chromatographic procedures after solubilization in Triton X-100. When reincorporated into artificial bilayers formed from phosphatidylcholine, it was able to transport a spin-labeled phosphatidylserine analogue from the inner to the outer membrane leaflet provided Mg2+ ATP was present in the incubation mixture. The ATP-dependent transport of the phosphatidylethanolamine analogue required the presence of an anionic phospholipid (e.g., phosphatidylinositol) in the outer membrane leaflet. In contrast the transmembrane distribution of spin-labeled phosphatidylcholine was unaffected in the same experimental conditions. This transmembrane movement of aminophospholipid analogues was inhibited by treatment of the proteoliposomes with a sulfhydryl reagent. We conclude that the Mg2+ ATPase is sufficient for the biochemical expression of the aminophospholipid translocase activity, which is responsible for the inward transport of phosphatidylserine and phosphatidylethanolamine within the erythrocyte membrane. The presence of this transport activity in many animal cell plasma membranes provides a function for the Mg2+ ATPase borne by these membranes.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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Biochemical aspects of the blood group Rh (rhesus) antigens

Despite their importance in clinical haematology, the details of the structures and possible functions of the proteins associated with Rh antigen expression have only recently begun to emerge. The antigens are carried by a multimeric complex between a M(r) 30,000 polypeptide which is not glycosylated (the Rh30 polypeptide), and a heavily glycosylated glycoprotein (the Rh50 glycoprotein). The N-terminal amino acid sequences of the two types of proteins were determined and used to isolated cDNA clones. The Rh30 and Rh50 proteins are both very hydrophobic membrane proteins, each containing up to 12 membrane spans. The two proteins are homologous in sequence and clearly belong to the same family. They are erythroid-specific and not related to any other known family of proteins. The Rh30 polypeptides are the genetic determinants of Rh blood group antigen activity. One polypeptide (Rh30A) is probably associated with CcEe antigen activity, while another (Rh30B) is responsible for the D antigen. The proteins have structures typical of transporters but their functions are still unclear. A number of other red cell membrane proteins (LW, CD47, glycophorin B and Fy) show alterations in red cells lacking Rh antigens (Rhnull). These proteins may have a role in the biosynthesis or function of the Rh30 and Rh50 proteins.

Quantitative comparison between aminophospholipid translocase activity in human erythrocytes and in K562 cells

Spin-labeled phospholipids were used to determine the transbilayer movement of phospholipids in human erythrocytes, in K562 cells and in human neonatal red cells. The erythroleukemia cell line, K562, as well as human neonatal red cells, which are rich in reticulocytes, were considered as representative of human erythrocyte precursor cells. In the nucleated cells, the difference between outside-inside movement of aminophospholipids and that of phosphatidylcholine or sphingomyelin analogues allowed us to discriminate between lipid internalization due to aminophospholipid translocase activity and to endocytosis. From the initial rates of aminophospholipid inward movement, we inferred that the activity of the aminophospholipid translocase is higher in the precursor cells than in mature erythrocytes.

Dynamics of the membrane lipid phase

The lipid bilayer of a membrane is sometimes seen as an inert hydrophobic phase allowing the 'solubility' of transmembrane proteins and acting as a barrier between two compartments. However, the bilayer is, in fact, a highly organized system subjected to many movements leading to a dynamically equilibrated structure. A lipid within a membrane experiences intramolecular motions (movement of some segments of the molecule) and moves or diffuses in and across each monolayer. In plasma membranes, transverse diffusion is either passive (cholinecontaining phospholipids, fatty acids ...) or active via a carrier protein (amino-phospholipids). The known asymmetric transverse distribution of phospholipids between the two plasma membrane leaflets is a stationary state resulting from all these motions, especially the active transport. Nevertheless, recent studies have shown that it is also possible to obtain an uneven distribution of some lipids (e.g. fatty acid, phosphatidic acid) across a membrane via a pH gradient. Lateral diffusion within a monolayer depends on the composition of the monolayer and not on the nature of the diffusing lipid. The phospholipid asymmetry, based on the polar head groups, exists also for the corresponding fatty acids, as the nature of the acyl chains differs according to the head group. A consequence is that the cytoplasmic leaflet of plasma membranes has a different 'fluidity' from that of the outer leaflet.

The inhibition of ATP-dependent shape change of human erythrocyte ghosts correlates with an inhibition of Mg(2+)-ATPase activity by fluoride and aluminofluoride complexes

The vanadate-sensitive Mg(2+)-dependent ATPase activity of the human erythrocyte ghost is believed to be involved in the shape change events that convert echinocytic ghosts to smoothed forms (biconcave discs and stomatocytes). At physiological salt concentration, pH 7.4, 2 mM ATP, 5 mM Mg2+ and 1 mM EGTA, the Mg(2+)-ATPase activity of ghosts was inhibited strongly by millimolar concentrations of sodium fluoride: I50 = 1.31 +/- 0.23 mM (mean +/- S.D.; n = 12). The addition of aluminium chloride to 15 microM reduced the concentration of NaF required for 50% inhibition to 0.76 +/- 0.21 mM (n = 10). Aluminium alone had only a small inhibitory effect on the ATPase activity (13 +/- 9%; n = 10). Desferrioxamine, a strong chelator of tervalent aluminium ion, failed to reverse the inhibition by fluoride and reversed the inhibition in the presence of aluminium and fluoride back to those values obtained with fluoride alone. Of several metal salts tested only beryllium sulfate was able to replace aluminium as an effective inhibitor in the presence of fluoride. Inhibition of the Mg(2+)-ATPase activity by fluoride and the aluminofluoride complexes correlated with an inhibition of the rate of MgATP-dependent change in red cell ghost shape from echinocytes to smoothed forms. All gross morphological changes of the smoothing process were affected, including the production of discocytes, stomatocytes and endocyctic vesicles.

Effect of benzyl alcohol on phospholipid transverse mobility in human erythrocyte membrane

The effect of benzyl alcohol on the transverse mobility and repartition of phospholipids in the human erythrocyte membrane was investigated using electron spin resonance and morphological modification of red blood cells. Transmembrane internalization rates and equilibrium distribution in red blood cells of short-chain spin-labeled phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were strongly modified by treatment with 10-70 mM benzyl alcohol. A dual effect was observed: (a) at 4 degrees C and 37 degrees C there was an N-ethylmaleimide-sensitive, long lasting and fully reversible increase in the spin-labeled phosphatidylserine and phosphatidylethanolamine internalization rate; (b) at 37 degrees C, an enhancement of N-ethylmaleimide-insensitive fluxes of all the labeled phospholipids through the membrane occurred. Both effects were dose-dependent. Erythrocytes submitted to benzyl alcohol incubation also showed dose-dependent shape changes: an immediate one from discocytes to echinocytes, followed by a slower N-ethylmaleimide- and ATP-dependent change to stomatocytes. Moreover, benzyl alcohol treatment was shown to lead to enhanced hydrolysis of intracellular ATP. All the effects of benzyl alcohol can be described as an accumulation of labeled phosphatidylethanolamine (and labeled phosphatidylcholine at 37 degrees C) in the inner leaflet. This can be interpreted as a perturbation of the erythrocyte membrane, leading to an energy-consuming specific increase in aminophospholipid translocase activity, in addition to a slow and passive bidirectional flux of all phospholipids at 37 degrees C.

Rapid loss and restoration of lipid asymmetry by different pathways in resealed erythrocyte ghosts

The normal asymmetric distribution of phospholipids across the plasma membrane of erythrocytes can be abolished by lysing and resealing cells in the presence of Ca2+. In the present study, using flow cytometric analysis of the binding of merocyanine 540 to monitor transbilayer phospholipid distribution, Ca(2+)-induced loss of asymmetry is shown to be independent from the aminophospholipid translocase which catalyzes movement of normally internal phospholipids from the outer to the inner leaflet of the membrane. Loss of asymmetry is rapid, temperature-sensitive, and occurs in an uninterrupted, intact bilayer, rather than by diffusion of lipids through the hemolytic pore. Addition of ATP during lysis reverses loss of asymmetry, and this restoration can be blocked by inhibitors of the aminophospholipid translocase. These results suggest that the ATP-dependent translocase is essential for recovery of asymmetry, in turn suggesting that separate mechanisms mediate the loss and the recovery of lipid asymmetry in erythrocytes.

Bilayer/cytoskeleton interactions in lipid-symmetric erythrocytes assessed by a photoactivable phospholipid analogue

Two mechanisms have been proposed for maintenance of transbilayer phospholipid asymmetry in the erythrocyte plasma membrane, one involving specific interactions between the aminophospholipids of the inner leaflet of the bilayer and the cytoskeleton, particularly spectrin, and the other involving the aminophospholipid translocase. If the former mechanism is correct, then erythrocytes which have lost their asymmetric distribution of phospholipids should display altered bilayer/cytoskeleton interactions. To test this possibility, normal erythrocytes, erythrocytes from patients with chronic myelogenous leukemia or sickle disease, and lipid-symmetric and -asymmetric erythrocyte ghosts were labeled with the radioactive photoactivable analogue of phosphatidylethanolamine, 2-(2-azido-4-nitrobenzoyl)-1-acyl-sn-glycero-3-phospho[14C]ethanolamine ([14C]AzPE), previously shown to label cytoskeletal proteins from the bilayer. The labeling pattern of cytoskeletal proteins in pathologic erythrocytes and lipid-asymmetric erythrocyte ghosts was indistinguishable from normal erythrocytes, indicating that the probe detects no differences in bilayer/cytoskeleton interactions in these cells. In contrast, in lipid-symmetric erythrocyte ghosts, labeling of bands 4.1 and 4.2 and actin, and to a lesser extent ankyrin, by [14C]AzPE was considerably reduced. Significantly, however, labeling of spectrin was unaltered in the lipid-symmetric ghosts, suggesting that its relationship with the bilayer is normal in these lipid-symmetric cells. These results do not support a model in which spectrin is involved in the maintenance of an asymmetric distribution of phospholipids in erythrocytes.

Association of vanadate-sensitive Mg(2+)-ATPase and shape change in intact red blood cells

Intact human erythrocytes, initially depleted of Mg2+ by EDTA incubation in the presence of A23187, exhibit Mg(2+)-dependent phosphate production of around 1.5 mmol per liter cells.h, half-maximally activated at around 0.4 mM added free Mg2+. This appears to correspond to Mg(2+)-stimulated adenosine triphosphatase (Mg(2+)-ATPase) activity found in isolated membranes, which is known to have a similar activity and affinity for Mg2+. Vanadate (up to 100 microM) inhibited Mg(2+)-dependent phosphate production and ATP breakdown in intact cells. Over a similar concentration range vanadate (3-100 microM) transformed intact cells from normal discocytes to echinocytes within 4-8 h at 37 degrees C, and more rapidly in Mg(2+)-depleted cells. The rate of Ca(2+)-induced echinocytosis was also enhanced in Mg(2+)-depleted cells. These results support previous studies in erythrocyte ghosts suggesting that vanadate-induced shape change is associated with inhibition of Mg(2+)-ATPase activity localized in the plasma membrane of the red blood cell.

Transbilayer movement of phosphatidylserine in erythrocytes. Inhibitors of aminophospholipid transport block the association of photolabeled lipid to its transporter

The ability to cross-link [125I]iodo-azido-phosphatidylserine (125I-N3-PS) to the putative 32-kDa aminophospholipid transporter of human red blood cells (RBC) has been examined by SDS-PAGE. In the absence of transport inhibitors, 125I-N3-PS preferentially labeled the 32-kDa polypeptide, whereas treatment of the cells with pyridyldithioethylamine (PDA), a potent inhibitor of the aminophospholipid translocase, abrogated the association of the probe to this protein. ATP-depletion, low temperature, and diamide or 5,5'-dithiobis(2-nitrobenzoic acid), inhibitors that oxidize an endofacial sulfhydryl distinct from the PDA-sensitive site (Connor, J. and Schroit, A.J. (1990) Biochemistry 29, 37-43), also blocked association of the PS analogue to the protein. Once 125I-N3-PS became associated with the transporter, however, only PDA was able to partially displace it. These data suggest that sulfhydryl reactive reagents inhibit PS transport by blocking the association of PS with its transporter, a process that is also ATP- and temperature-dependent.