Dielectric breakdown of the erythrocyte membrane enhances transbilayer mobility of phospholipids

Dielectric breakdown of erythrocytes is shown to result in a loss of asymmetry of phosphatidylethanolamine and in a markedly enhanced transbilayer mobility of exogenous lysophosphatidylcholine. The effect is much more pronounced in non-resealed cells than in cells resealed after the breakdown. A casual relationship between the structural defects in the lipid phase, indicated by these results, and fusion by dielectric breakdown is discussed.     

Cross-linking of phospholipids to proteins in the erythrocyte membrane

Erythrocytes treated with the cross-linking agents difluorodinitrobenzene and suberimidate are rendered refractory to lysis. When ghosts are treated with these reagents 8.4% and 2.3% of the total lipid phosphate is cross-linked to protein by difluorodinitrobenzene and suberimidate respectively. This represents 20 and 5.8% of the amino-phospholipids. The lipids extracted from treated ghosts do not react with ninhydrin as do lipids extracted from control ghosts. Thus essentially all the amino-phospholipids of the ghosts react with these cross-linking agents and up to 20% becomes cross-linked to proteins.     

Gramicidin-induced enhancement of transbilayer reorientation of lipids in the erythrocyte membrane

Incorporation of the channel-forming antibiotic gramicidin into the membrane of human erythrocytes highly (up to 30-fold) enhances rates of reorientation (flip) of lysophosphatidylcholine and palmitoylcarnitine to the inner membrane layer after their primary incorporation into the outer layer. Despite the high increase of flip rates by gramicidin, the asymmetric orientation of the inner membrane layer phospholipids phosphatidylethanolamine and phosphatidylserine is stable as demonstrated by the lack of accessibility of these lipids toward cleavage by exogenous phospholipase A2. On the other hand, gramicidin enhances the rate of cleavage of outer membrane layer phosphatidylcholine by phospholipase A2, which indicates changes in the packing of phosphatidylcholine following gramicidin binding. The increase of flip becomes detectable when about 10(5) copies of gramicidin per cell have been bound (gramicidin to membrane phospholipid ratio of 1:2000). This is a 1000-fold higher concentration than that required for an increase of K+ permeability mediated by the gramicidin channel. Acceleration of flip is thus not simply correlated with channel formation. The enhancement of flip is markedly dependent on structural details of gramicidin. Formylation of its four tryptophan residues abolishes the effect. Even at high concentrations of formylated gramicidin at which the extents of binding of native and of formylated gramicidin to the membrane are comparable, no flip acceleration is produced. Enhancement of flip by gramicidin occurs after a temperature-dependent lag phase. At 37 degrees C, flip rates begin to increase within a few minutes and at 25 degrees C, only after 3 h. This lag phase is most likely not due to limitations by the rate of binding of gramicidin to the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)     

Binding sites for calcium-activated neutral protease on erythrocyte membranes are not membrane phospholipids

In order to explore the binding sites for calcium-activated neutral protease (CANP) with high calcium sensitivity (muCANP) on the inner surface of human erythrocyte membranes, we analyzed the binding of muCANP to two kinds of membranes modified by treatment with phospholipase C or Triton X-100. Binding analyses were performed using an immunoblot technique. The amount of muCANP bound to phospholipase C-treated inside-out vesicles was essentially the same as that bound to untreated inside-out vesicles. It was also observed that muCANP binds to Triton X-100-treated membranes, in which most of the integral proteins and glycerophospholipids are removed while the lining proteins remain intact. In both types of modified membrane, the bound muCANP was rapdily converted to an active form by autolysis at physiological free Ca2+ concentrations. These results indicate that the binding sites for muCANP on the inner surface of erythrocyte membranes consist of components other than membrane phospholipids. In addition, it is suggested that one of the binding sites for muCANP is some lining protein.     

Nitrobenzylthionionosine binding sites in the erythrocyte membrane.

Nitrobenzylthioinosine binds tightly, but reversibly, to sites in the human erythrocyte membrane; occupancy of these sites blocks the transport of uridine and of other nucleosides. This report described the inhibition of nitrobenzylthioinosine binding at these sites by substrates of the uridine transport mechanism and by compounds related to nitrobenzylthioinosine. For some of these compounds dissociation constants for binding at the nitrobenzylthioinosine sites were determined, assumming competition with nitrobenzylthioinosine. Deoxycytidine, a substrate for the uridine transport mechanism, did not inhibit binding of nitrobenzylthioinosine, suggesting that binding sites for the latter are distinct from nucleoside sites directly involved in transport.     

Mathematical modelling of lipid transbilayer movement in the human erythrocyte plasma membrane

A model is presented to simulate transverse lipid movement in the human erythrocyte membrane. The model is based on a system of differential equations describing the time-dependence of phospholipid redistribution and the steady state distribution between the inner and outer membrane monolayer. It takes into account several mechanisms of translocation: (i) ATP-dependent transport via the aminophospholipid translocase; (ii) protein-mediated facilitated and (iii) carrier independent transbilayer diffusion. A reasonable modelling of the known lipid asymmetry could only be achieved by introducing mechanism (iii). We have called this pathway the compensatory flux, which is proportional to the gradient of phospholipids between both membrane leaflets. Using realistic model parameters, the model allows the calculation of the transbilayer motion and distribution of endogenous phospholipids of the human erythrocyte membrane for several biologically relevant conditions. Moreover, the model can also be applied to experiments usually performed to assess phospholipid redistribution in biological membranes. Thus, it is possible to simulate transbilayer motion of exogenously added phospholipid analogues in erythrocyte membranes. Those experiments have been carried out here in parallel using spin labeled lipid analogues. The general application of this model to other membrane systems is outlined.Abbreviations PBS phosphate buffered saline - DFP diisopropyl fluorophosphate - ESR electron spin resonance - RBC red blood cells - PC phosphatidylcholine - PE phosphatidylethanolamine - PS phosphatidylserine - SM sphingomyelin - (0,2)PC 1-palmitoyl-2(4doxylpentanoyl)-PC - (0,2)PE 1-palmitoyl-2(4-doxylpentanoyl)-PE - (0,2) PS 1-palmitoyl-2(4-doxylpentanoyl)-PS     

Complement proteins C5b-9 induce transbilayer migration of membrane phospholipids.

Transbilayer migration of membrane phospholipid arising from membrane insertion of the terminal human complement proteins has been investigated. Asymmetric vesicles containing pyrene-labeled phosphatidylcholine (pyrenePC) concentrated in the inner monolayer were prepared by outer monolayer exchange between pyrenePC-containing large unilamellar vesicles and excess (unlabeled) small unilamellar vesicles, using bovine liver phosphatidylcholine-specific exchange protein. After depletion of pyrenePC from the outer monolayer, the asymmetric large unilamellar vesicles were isolated by gel filtration and exposed to the purified C5b-9 proteins at 37 degrees C. Transbilayer exchange of phospholipid between inner and outer monolayers during C5b-9 assembly was monitored by changes in pyrene excimer and monomer fluorescence. Membrane deposition of the C5b67 complex (by incubation with C5b6 + C7) caused no change in pyrenePC fluorescence. Addition of C8 to the C5b67 vesicles resulted in a dose-dependent decrease in the excimer/monomer ratio. This change was observed both in the presence and absence of complement C9. No change in fluorescence was observed for control vesicles exposed to C8 (in the absence of membrane C5b67), or upon C5b-9 addition to vesicles containing pyrenePC symmetrically distributed between inner and outer monolayers. These data suggest that a transbilayer exchange of phospholipid between inner and outer monolayers is initiated upon C8 binding to C5b67. The fluorescence data were analyzed according to a "random walk" model for excimer formation developed for the case where pyrenePC is asymmetrically distributed between lipid bilayers. Based on this analysis, we estimate that a net transbilayer migration of approximately 1% of total membrane phospholipid is initiated upon C8 binding to C5b67. The potential significance of this transbilayer exchange of membrane phospholipid to the biological activity of the terminal complement proteins is considered.     

The major human erythrocyte membrane protein. Evidence for an S-shaped structure which traverses the membrane twice and contains a duplicated set of sites.

The structure of the major human erythrocyte membrane protein (protein E) was investigated by studying the products of proteolysis of the native protein in the membrane. The distribution and location of the tyrosine residues labelled by radioiodination by lactoperoxidase was determined. Proteolysis of the extracellular region of the protein by thermolysin released four tyrosine-containing peptides, all of which were also found to remain in the major fragment that is retained in the membrane. The presence of these duplicated sites in the extracellular region of the protein was confirmed by limited trypsin digestion of the intracellular region of the protein. Two groups of fragments were obtained. Both groups contained a set of the extracellular labelled sites, but they differed in containing distinct groups of intracellular sites, showing that the two sets of extracellular sites are linked by an intracellular region of the protein. The polypeptide chain thus traverses the membrane twice. An S-shaped model which is consistent with these data is proposed.     

Nitrobenzylthioinosine binding sites in the erythrocyte membrane

Nitrobenzylthioinosine binds tightly, but reversibly, to sites in the human erythrocyte membrane; occupancy of these sites blocks the transport of uridine and of other nucleosides. This report describes the inhibition of nitrobenzylthioinosine binding at these sites by substrates of the uridine transport mechanism and by compounds related to nitrobenzylthioinosine. For some of these compounds dissociation constant for binding at the nitrobenzylthioinosine sites were determined, assuming competition with nitrobenzylthioinosine.Deoxycytidine, a substrate for the uridine transport mechanism, did not inhibit binding of nitrobenzylthioinosine, suggesting that binding sites for the latter are distinct from nucleoside sites directly involved in transport.     

Cross-linking of erythrocyte membrane proteins by periodate and intramembrane particle distribution.

Treatment of isolated human erythrocyte membranes at pH 7.4 with 0.1-0.5 mM-sodium periodate specifically cross-linked some of the spectrin polypeptides. Treatment with 2 mM-periodate resulted in complete cross-linking of spectrin and partial cross-linking of other polypeptides. The latter treatment also caused aggregation of the intramembrane particles made visible by freeze-fracturing. When membranes that had been treated with 2 mM-periodate were depleted of spectrin by treatment with 0.1 mM-EDTA, extensive aggregation of the intramembrane particles occurred.     

Bacterial cytotoxins, amphotericin B and local anesthetics enhance transbilayer mobility of phospholipids in erythrocyte membranes. Consequences for phospholipid asymmetry

Incorporation of the channel-forming polyene antibiotic amphotericin B and of cytotoxins from Staphylococcus aureus (alpha-toxin) or Pseudomonas aeruginosa into erythrocyte membranes results in a concentration-dependent enhancement of the flip rates of exogenous lysophosphatidylcholine. The flip rate is also enhanced by incorporation of tetracaine and dibucaine. Removal of tetracaine and amphotericin B from the cells normalizes the flip rates. In parallel to the enhancement of flip rates, alpha-toxin produces a loss of transmembrane asymmetry of both phosphatidylethanolamine and phosphatidylserine. Pretreatment of cells with amphotericin or high concentrations (over 2.5 mmol . l-1) of tetracaine, followed by removal of the perturbing agent by washing, produces a selective loss of the asymmetric orientation of phosphatidylethanolamine to the inner membrane layer, as evaluated by the accessibility of the lipid towards cleavage by phospholipase A2. The extent to which asymmetry is lost depends on the time of pretreatment with amphotericin or tetracaine, indicating a limitation by the rate of reorientation of phosphatidylethanolamine to the outer membrane surface. Evaluation of the accessibility of phosphatidylethanolamine towards cleavage by phospholipase A2 in the presence of local anesthetics indicates accessible fractions much higher than those obtained after removal of the perturbant. In the presence of tetracaine, endofacial phosphatidylethanolamine seems somehow to become accessible to phospholipase A2. Phosphatidylserine does not exhibit this peculiarity. The results indicate that various types of perturbation of the lipid domain of the erythrocyte membrane may enhance the transbilayer mobility of phospholipids as well as destabilize the asymmetric distribution of aminophospholipids. However, as in other instances reported previously (Haest, C.W.M., Erusalimsky, J., Dressler, V., Kunze, I. and Deuticke B. (1983) Biomed. Biochim. Acta 42, 17-21), there is no tight coupling between transbilayer mobility and destabilization of asymmetry of the transbilayer distribution of phospholipids.     

The asymetric arrangement of phospholipids in the human erythrocyte membrane

In erythrocytes treated with 2,4,6-trinitrobenzenesulfonate (a non-penetrating probe) for 24 hours, a maximum of 33% of the phosphatidylethanolamine and none of the phosphatidylserine reacts with this reagent. In erythrocyte ghosts, however, 95% of the phosphatidylethanolamine and over 50% of the phosphatidylserine reacts in 90 minutes under the same conditions. When extracted erythrocyte lipids are treated with 2,4,6-trinitrobenzenesulfonate in either a chloroform-methanolbicarbonate or a sonicated aqueous bicarbonate system, both phosphatidylethanolamine and phosphatidylserine react essentially to completion within minutes. We interpret these results to indicate the localization of nearly all of the phosphatidylserine on the interior surface of the membrane thus demonstrating an asymmetric distribution of phospholipids in the erythrocyte membrane.     

Translocation of oleic acid across the erythrocyte membrane. Evidence for a fast process

To clarify divergent views concerning the mechanism of fatty acid translocation across biomembranes this issue was now investigated in human erythrocytes. Translocation rates of exogenously inserted radioactive oleic acid across the membrane of native cells were derived from the time-dependent increase of the fraction of radioactivity becoming non-extractable by albumin. No accumulation of non-extractable unesterified oleic acid occurred. The rate of transfer was markedly suppressed by SH-reagents and by ATP-depletion. The suppression, however, resulted from a mere decrease of incorporation of oleic acid into phospholipids and was not accompanied by an increase of non-extractable unesterified oleic acid. These findings were reconcilable with the concept of a slow, possibly carrier-mediated fatty acid transfer as well as a very fast presumably, diffusional process not resolvable by the albumin extraction procedure. This ambiguity was resolved by using resealed ghosts, which are unable to incorporate oleic acid into phospholipids. In such ghosts all of the oleic acid inserted into the membrane remains extractable by albumin even after prolonged incubation. On the other hand, ghosts containing albumin accumulated non-extractable oleic acid. The rate of accumulation was beyond the time resolution of the albumin extraction procedure at 4 degrees C. Oleic acid uptake into albumin-containing ghosts became kinetically resolvable when the fatty acid was added as a complex with albumin. Correspondingly, time-resolvable release of oleic acid, originally complexed to internal albumin, into an albumin-containing medium was demonstrated at 4 degrees C. Rate and extent of these redistributions of oleic acid were dependent on the concentrations of internal and external albumin. This indicates limitation by the dissociation of oleic acid from albumin and not its translocation across the membrane. Translocation of oleic acid, which is probably a simple diffusive flip-flop process, must therefore occur with a half-time of less than 15 s. These findings raise doubts on the physiological role of presently discussed concepts of a carrier-mediated translocation of fatty acids across plasma membranes.     

The transbilayer movement of phosphatidylcholine in vesicles reconstituted with intrinsic proteins from the human erythrocyte membrane

Vesicles have been prepared from 18 : 1_c/18 : 1_c-phosphatidylcholine with or without purified glycophorin or partially purified band 3 (obtained by organomercurial gel chromatography). The vesicles have been characterized by freeze-fracture electron microscopy, binding studies to DEAE-cellulose, ³¹P-NMR and K⁺ trap measurements. Pools of phosphatidylcholine available for exchange have been investigated using phosphatidylcholine exchange protein from bovine liver. The protein-containing vesicles both exhibit exchangeable pools larger than the fraction of phosphatidylcholine in the outer monolayer, whereas in the protein-free vesicles the exchangeable pool is consistent with the outer monolayer. The results indicate that both glycophorin and the partially purified band 3 preparation enhance the transbilayer movement of phosphatidylcholine.     

ATP-dependent translocation of amino phospholipids across the human erythrocyte membrane

Trace amounts of radiolabeled phospholipids were inserted into the outer membrane leaflet of intact human erythrocytes, using a non-specific lipid transfer protein. Phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine were transferred from the donor lipid vesicles to the membrane of the intact red cell with equal ease, whilst sphingomyelin was transferred 6-times less efficiently. The transbilayer mobility and equilibrium distribution of the labeled phospholipids were assessed by treatment of the intact cells with phospholipases. In fresh erythrocytes, the labeled amino phospholipids appeared to move rapidly towards the inner leaflet. The choline phospholipids, on the other hand, approached an equilibrium distribution which strongly favoured the outer leaflet. In ATP-depleted erythrocytes, the relocation of the amino phospholipids was markedly retarded.