Membrane proteins in human erythrocytes during cell fusion induced by oleoylglycerol

Association of Physical Activity With Bioactive Lipids and Cardiovascular Events

BACKGROUND

To clarify the mechanisms underlying physical activity (PA)-related cardioprotection, we examined the association of PA with plasma bioactive lipids (BALs) and cardiovascular disease (CVD) events. We additionally performed genome-wide associations.

METHODS

PA-bioactive lipid associations were examined in VITAL (VITamin D and OmegA-3 TriaL)-clinical translational science center (REGISTRATION: URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT01169259; N=1032) and validated in JUPITER (Justification for the Use of statins in Prevention: an Intervention Trial Evaluating Rosuvastatin)-NC (NCT00239681; N=589), using linear models adjusted for age, sex, race, low-density lipoprotein-cholesterol, total-C, and smoking. Significant BALs were carried over to examine associations with incident CVD in 2 nested CVD case-control studies: VITAL-CVD (741 case-control pairs) and JUPITER-CVD (415 case-control pairs; validation).

RESULTS

We detected 145 PA-bioactive lipid validated associations (false discovery rate <0.1). Annotations were found for 6 of these BALs: 12,13-diHOME, 9,10-diHOME, lysoPC(15:0), oxymorphone-3b-D-glucuronide, cortisone, and oleoyl-glycerol. Genetic analysis within JUPITER-NC showed associations of 32 PA-related BALs with 22 single-nucleotide polymorphisms. From PA-related BALs, 12 are associated with CVD.

CONCLUSIONS

We identified a PA-related bioactive lipidome profile out of which 12 BALs also had opposite associations with incident CVD events.

Fusogenic mechanisms

Recent work is described on the fusion of human erythrocytes induced by chlorpromazine, which is accompanied by the endogenous proteolysis of integral and skeletal membrane proteins. This fusion reaction is of interest because it occurs at a high incidence in the presence of EGTA. It thus contrasts with the emphasis that has been placed on the role of Ca2+ in the fusion of lipid bilayers as a model for its behaviour in biomembrane fusion reactions. A survey of fusion in cells induced by chemicals and by Sendai virus, and of fusion in liposomes, in fact reveals numerous reported instances of fusion occurring in the absence of Ca2+. The finding that endogenous proteolysis of erythrocyte membrane proteins accompanies cell fusion induced by chlorpromazine leads to the suggestion that the products of proteolysis may in some cases be fusogenic, and that their formation may provide a general mechanism for the fusion of lipid bilayers in biomembrane fusion reactions, aspects of which are imitated by the fusogenic viruses.

Interactions between metal ions and poly(ethylene glycol) in the fusion of human erythrocytes

Diffusion of the fluorescent membrane probe, Dil-C16 (3), from labelled to unlabelled human erythrocytes has been employed to monitor hemi-fusion (membrane fusion) in monolayers of cells exposed to poly(ethylene glycol) (PEG). Diffusion of the cytoplasmic probe, 6-carboxyfluorescein, was used similarly to monitor cell fusion (cytoplasmic mixing). Hemi-fusion, which is normally seen when erythrocytes are exposed to dehydrating concentrations of commercial PEG 6000, did not occur when the PEG was pretreated with Chelex 100 resin to remove metal ions. Cytoplasmic mixing, which is normally observed when the dehydrated erythrocytes are substantially rehydrated, also failed to occur when both PEG 6000 and the rehydrating buffer had been treated with Chelex 100. The re-addition to Chelex-treated PEG of components removed by the resin, and the addition of 10 mu mM concentrations of La3+ or Al3+, restored its ability to induce hemi-fusion and cell fusion. Higher concentrations of several other metals, including Ca2+, were also effective. These observations show that metal ions are required for hemi-fusion with erythrocytes in the presence of PEG, and that dehydration alone is insufficient to induce hemi-fusion. Phosphatidylserine was apparently not accessible in erythrocytes treated with PEG 6000 until the cells were rehydrated. This indicates that metal ions do not assist the hemi-fusion of erythrocytes by forming trans complexes with surface phosphatidylserine when the cells are dehydrated by PEG.(ABSTRACT TRUNCATED AT 250 WORDS)

Divalent cations, phospholipid asymmetry and osmotic swelling in electrically-induced lysis, cell fusion and giant cell formation with human erythrocytes

We have previously reported that acidic phospholipids are exposed at the surface of human erythrocytes when the cells are subjected to electrical breakdown. It has now been shown that the prothrombinase assay, which was used previously for the determination of acidic phospholipids, is specific for phosphatidylserine under the conditions of our experiments. In the light of this finding, we have investigated and characterised factors that govern cell lysis, cell fusion, and the formation of giant cells induced by electrical breakdown with human erythrocytes in media of low ionic strength. Divalent cations (1.1 mM) protected the cells against haemolysis, in the order Mn2+ > Ca2+ > Ba2+ > Mg2+ >> Zn2+, whereas about 99% of the cells lysed immediately on breakdown in the presence of Na+ or K+ (2.1 mM), or Al3+ (0.95 mM). The lengths of pearl chains of fused erythrocytes formed was similarly greatest with Mn2+ and decreased progressively with Ba2+, Zn2+, Ca2+ and Mg2+. No cell fusion occurred with Na+, K+, or Al3+. It is suggested that interactions with phosphatidylserine, which is exposed at the cell surface by electrical breakdown, may enable Mn2+, Ba2+ and Ca2+ ions to inhibit cell lysis (via membrane resealing) and facilitate cell fusion. Following electrically-induced cell fusion, erythrocytes round-up into giant cells. It has previously been proposed that Ca2+ ions accelerate the rounding-up process. However, data are presented which show that, as with erythrocytes treated with Sendai virus, the formation of rounded, giant cells following cell fusion depends on the osmotic swelling properties of permeabilised erythrocytes. Osmotic swelling may also have induced any hemi-fused cells present to fuse completely. Zn2+ ions anomalously enabled erythrocytes to round-up very rapidly into giant cells following electrical breakdown. This phenomenon may result from an interaction of Zn2+ ions with cysteine groups in membrane proteins, which decreases the immediate loss of ions that occurs when erythrocytes are subjected to electrical breakdown in low-ionic-strength media.

Relationships between the surface exposure of acidic phospholipids and cell fusion in erythrocytes subjected to electrical breakdown

The procoagulant activity of human erythrocytes, which provides a measure of the translocation of acidic phospholipids from the inner to the outer monolayer of the plasma membrane, has been compared with the percentage cell fusion in experiments on the effects of electrical breakdown pulses under differing experimental conditions. After treatment with breakdown pulses of 20 microseconds or longer (5 kV cm-1), the plasma membranes of erythrocytes in 250 mM sucrose exhibited an almost complete loss of asymmetry with respect to acidic phospholipids. As the breakdown voltage was increased from 2 to 5 kV cm-1 (with breakdown pulses of 99 microseconds), the surface exposure of acidic phospholipids and cell fusion increased approximately in parallel. Furthermore, with 99 microseconds pulses and a voltage of 3 kV cm-1, a decrease in the osmolarity from 250 to 150 mM of the sucrose medium was accompanied by an increase in both the surface exposure of acidic phospholipids and the extent of cell fusion. Breakdown pulses of 2-5 microseconds were sufficient to cause a marked loss of asymmetry, but no cell fusion was observed unless the pulse length was at least 20 microseconds. Kinetic experiments indicated that exposure of the acidic phospholipids at the cell surface was more likely to be due to a direct effect of the electric field pulses on plasma membrane structure than to secondary effects, such as the action of endogenous proteinases on the membrane skeleton. It seems possible that a localised, surface exposure of acidic phospholipids may contribute to the 'long-lived fusogenic state' (Sowers, A.E. (1986) J. Cell Biol. 102, 1358-1362) and the 'transient permeant structures' (Teissié, J. and Rols, M.P. (1986) Biochem. Biophys. Res. Commun. 140, 258-266) that enable cell fusion to occur when contact between cells is established after they have been subjected to field pulses. Our observations also provide circumstantial support for the concept that changes in the phospholipid asymmetry of membranes may be important in physiologically-occurring instances of biomembrane fusion.

Membrane fusion without cytoplasmic fusion (hemi-fusion) in erythrocytes that are subjected to electrical breakdown

There are many reports of hemi-fusion in phospholipid vesicles but few published studies on hemi-fusion in cells. We report evidence from both fluorescence microscopy and freeze-fracture electron microscopy for hemi-fusion in the electrofusion of human erythrocytes. We have also characterised the conditions that favour hemi-fusion as opposed to complete fusion, and discuss the possibility that hemi-fusion might precede complete electrically-induced cell fusion. A membrane probe (DiIC16) and a cytoplasmic probe (6-carboxyfluorescein) were used to investigate the behaviour of doubly-labelled human erythrocytes which were aligned in chains by dielectrophoresis and then exposed to high voltage breakdown pulses. Some of the cells were fused by the pulses, as shown by diffusion of both membrane and cytoplasmic probes from labelled to unlabelled cells. With other cells, the membrane probe diffused into unlabelled cells after the breakdown pulses, without the cytoplasmic probe diffusing into unlabelled cells or leaking into the medium. Membrane fusion (hemi-fusion) thus occurred without cytoplasmic fusion in these erythrocytes. Such cells were irreversibly, but fragilely, attached to their neighbours by the breakdown pulses. There was an inverse relationship between conditions that permit complete fusion and those that favour hemi-fusion, with respect to breakdown pulse length, breakdown voltage and, in particular, osmolarity and temperature. The incidence of hemi-fusion in 250 mM erythritol was twice that in 150 mM erythritol, and hemi-fusion was 5-fold greater at 25 degrees C than at 20 degrees C. Hemi-fused erythrocytes occasionally fused completely on heating to 50 degrees C, demonstrating that hemi-fusion can proceed to complete cell fusion. Freeze fracture electron micrographs of preparations of hemi-fused cells revealed long-lived, complementary depressions and protrusions on the E- and P-fracture faces, respectively, of tightly apposed cells that may mediate hemi-fusion. The possibility that the fusion of closely adjacent human erythrocytes by electrical breakdown pulses may involve an intermediate, shared bilayer structure, which is stable in certain conditions but which can be ruptured by osmotic swelling of the permeabilised cells, is discussed.

Surface exposure of phosphatidylserine is associated with the swelling and osmotically-induced fusion of human erythrocytes in the presence of Ca2+

An assay for procoagulant activity has been used to investigate the Ca2(+)-dependent exposure of phosphatidylserine at the surface of human erythrocytes that were induced to swell and to fuse osmotically. Since the phosphatidylserine of human erythrocytes is located in the inner leaflet of the plasma membrane, it is inaccessible in intact cells which therefore had no procoagulant activity in an isotonic solution of sucrose. The procoagulant activity of erythrocytes incubated in increasingly hypotonic, sucrose solutions containing Mg2+ paralleled the percentage haemolysis, reflecting the accessibility of phosphatidylserine in an increasing number of lysed cells. However, cells in mildly hypotonic sucrose solutions containing Ca2+ had an abnormally high procoagulant activity indicating that phosphatidylserine was exposed in intact cells under these conditions. Erythrocytes that were subjected to continuous swelling at 37 degrees C, which was induced by entry of the permeant molecule poly(ethylene glycol) 400 (PEG 400) developed procoagulant activity in the presence of Ca2+ prior to extensive lysis. Cells treated in this way also fused. With Mg2+, PEG 400-treated erythrocytes lysed without fusing, and the development of procoagulant activity paralleled the rate of lysis. Erythrocytes incubated with ionophore A23187, subtilisin, and Ca2+ developed procoagulant activity (with less than 20% lysis), and they fused on subsequent exposure to a hypotonic medium. The procoagulant activity reached its maximum before fusion could be induced in the hypotonic medium. It is concluded that the entry of Ca2+ facilitates a translocation of phosphatidylserine to the outer leaflet of the erythrocyte plasma membrane that plays an important role in fusion protocols that involve cell swelling. It is also suggested that transbilayer movements of phosphatidylserine could be an important control factor in the cell biology of membrane fusion phenomena.

Trypanosoma cruzi: involvement of proteolytic activity during cell fusion induced by epimastigote form

Human erythrocytes were fused by Trypanosoma cruzi from 7 and 14 day old culture (stationary and declination phases, respectively) while only lysis was induced by 4 day old culture parasite (exponential phase). Lysis and erythrocyte fusion were studied by phase contrast microscopy, measuring of hemolysis and gel electrophoresis. The fusogenicity is Ca2+-dependent while lysis is delayed in the absence of exogenous Ca2+. The proteolysis of erythrocyte protein bands 1, 2, 2.1, 2.3 and 3 are common features of both fusion and lysis processes. Nevertheless the breakdown rate of ankyrin (band 2.1) and band 3 are different in fused or in lysed cells. The lysis process is associated with a faster degradation of band 2.1 and increase of band 2.3 than in the case of the fusion process. By contrast, degradation of band 3 occurs faster in the fusion than in the lytic event. Treatment of fusogenic parasites but not erythrocytes with TPCK, soybean trypsin inhibitor or FCS inhibited to some extent the fusion process and the decrease of bands 1, 2, 2.1, 2.3 and 3. The results suggest that proteases from fusogenic parasites may be directly or indirectly involved in the proteolysis of band 2.1 in a way related to induction of fusion.

Osmotic forces in artificially induced cell fusion

The importance of cell swelling in the fusion of erythrocytes by three different chemical treatments has been investigated with cells that were cytoplasmically labelled with 6-carboxyfluorescein. Hen erythrocytes, which had been pre-incubated with ionophore A23187 and 5 mM Ca2+ to cause a proteolytic breakdown of the membrane skeleton, were induced to fuse by applying an osmotic shock. Human erythrocytes that had been incubated in an isotonic salt/buffer solution, which was progressively diluted and which contained 0.5 mM La3+ to minimise cell lysis, were also fused. In addition, the fusion of human erythrocytes by 40% poly(ethylene glycol) began only when the poly(ethylene glycol) was diluted, and it mostly occurred when the diluted polymer solution was subsequently replaced by isotonic buffer. In related experiments, the effect of an osmotic gradient on electrically induced cell fusion has been studied. Human erythrocytes in 150 mM erythritol fused more readily than less swollen cells in 200-400 mM erythritol when subjected to a 20 microseconds pulse of 3.5 kV X cm-1, indicating that the extent of cell fusion induced by the breakdown pulse is governed by the combined electrical-compressive and osmotic forces. Since osmotic phenomena are already known to be important in exocytosis, we suggest that these observations on cell fusion indicate that osmotic forces may provide the driving force for many membrane fusion reactions in biological systems.

An osmotic model for the fusion of biological membranes

A molecular model for fusion-fission reactions in membranes is proposed that is based on data from studies on artificially induced cell fusion and on the behaviour of phospholipid bilayers: it is put forward as a framework for further investigations into this fundamental property of biological systems.

Roles of lipids and proteins in the Ca2+-PO4-induced aggregation of cytoskeleton-free erythrocyte vesicle membranes

The roles of lipids and proteins in Ca2+-PO4-induced membrane aggregation were investigated. Cytoskeleton-free vesicles derived from intact human and rabbit erythrocytes (HEves and REves, respectively) were employed as a model system. The HEves and REves have a simplified membrane protein composition [band 3 proteins and glycoproteins PAS-1, -2, and -3 (HEves)] and normal lipid composition. Optimal experimental conditions for pH, [PO4], and [CaCl2] were determined for quantitatively examining the dynamics and extent of HEves and REves aggregation, measured turbidimetrically. The aggregation process was found to be quite sensitive to small changes in pH and [PO4] and much less sensitive to [CaCl2]. The roles of membrane proteins in vesicle aggregation were examined by selectively modifying the proteins enzymatically. The roles of lipids were studied by using sonicated lipid vesicles [small unilamellar vesicles (SUVs)] made from Dodge ghost lipid extracts. Enzymatic treatment with trypsin, chymotrypsin, or Pronase had no effect on either the rates or the extent of vesicle aggregation (2-min incubation period). Neuraminidase treatment reduced both factors by approximately 20%. SUVs aggregated with Ca2+-PO4 in a way which depended on the PO4/lipid ratio. Together the results suggest the following: (1) PO4 is associated with the vesicle surface, involving the membrane lipids; (2) the vesicle + PO4 incubation time component of the PO4 effect is eliminated by enzymatically modifying the vesicle membrane proteins; (3) qualitative, rather than quantitative, properties of sialic acid containing molecules affect vesicle aggregation; and (4) with the exception of the incubation time effect, membrane proteins seem neither to promote nor to inhibit Ca2+-PO4-induced HEves or REves aggregation.

Proteolysis of ankyrin and of band 3 protein in chemically induced cell fusion. Ca2+ is not mandatory for fusion

Human erythrocytes were fused by incubation with 0.5-2 mM-chlorpromazine hydrochloride at pH 6.8-7.6. Fusogenic preparations of chlorpromazine were cloudy suspensions of microdroplets, and below pH 6.8 chlorpromazine gave clear solutions that were inactive. Unlike control cells, the lateral mobility of the intramembranous particles of the PF-fracture face of chlorpromazine-treated cells was relatively unrestricted, since the particles were partly clustered at 37 degrees C and they exhibited extensive cold-induced clustering. Ca2+ stimulated fusion, but fusion was only very weakly inhibited by EGTA (10 mM) and by N-ethylmaleimide (50 mM); pretreatment of the cells with Tos-Lys-CH2Cl (7-amino-1-chloro-3-L-tosylamidoheptan-2-one) (7.5 mM) markedly inhibited fusion. Changes in the membrane proteins of erythrocytes fused by chlorpromazine, before and after treatment with chymotrypsin to remove band 3 protein, were investigated. The several observations made indicate that the Ca2+-insensitive component of fusion is associated with degradation of ankyrin (band 2.1 protein) to band 2.3-2.6 proteins and to smaller polypeptides by a serine proteinase that is inhibited by Tos-Lys-CH2Cl, and that the component of fusion inhibited by EGTA and N-ethylmaleimide is associated with degradation of band 3 protein to band 4.5 protein by a Ca2+-activated cysteine proteinase. Proteolysis of ankyrin appeared to be sufficient to permit the chlorpromazine-induced fusion of human erythrocytes, but fusion occurred more rapidly when band 3 protein was also degraded in the presence of Ca2+. Since other cells have structures comparable with the spectrin-actin skeleton of the erythrocyte membrane, the observations reported may be relevant to the initiation of naturally occurring fusion reactions in biomembranes. It is also suggested that, should polypeptides with fusogenic properties be produced from integral and skeletal membrane proteins by endogenous proteolysis, their formation would provide a general mechanism for the fusion of lipid bilayers in biomembrane fusion reactions.

Do hydrophobic sequences cleaved from cellular polypeptides induce membrane fusion reactions in vivo?

The concept that a direct interaction between Ca2+ and phospholipids is a major factor in membrane fusion reactions is questioned. Attention is drawn to a number of findings on associations between fusion and the proteolysis of membrane proteins. It is proposed that hydrophobic polypeptides, which are functionally comparable to the fusogenic proteins of certain viruses but which are produced in cells by the endogenous proteolysis of membrane and cellular proteins, may induce membrane fusion reactions in vivo.

Trout red blood cells treated with proteases fuse when placed on glass slides

Treatment of trout red blood cells (RBC) with proteases and polyethylene glycol (PEG) either successively or concurrently caused cell fusion. Neither PEG nor protease treatment alone brought about the fusion of cells in suspension. However, incubation of RBC on glass slides with proteases caused extensive fusion.

Purification of Plasmodium lophurae cathepsin D and its effects on erythrocyte membrane proteins

The cathepsin D of Plasmodium lophurae was purified using a combination of CM-Sephadex, pepstatin-agarose and Sephadex G-100 chromatography. The plasmodial enzyme was distinct from that of the host red cell and bovine spleen in its low isoelectric point (pI 4.3). The cathepsin D of P. lophurae, as well as plasmodial extracts demonstrating such proteinase activity, were able to digest the membrane proteins of duckling and human red cells at pH 7.4; proteolysis was not inhibited in phosphate-buffered saline by 100 microM pepstatin. Membrane proteins most susceptible to proteolysis were those of the cytoskeleton, notably bands 1 and 2 (spectrin), bands 2.1-2.6 (spectrin-binding proteins) and band 3. Membrane protein degradation by crude plasmodial extracts was partially inhibited by a combination of 10 mM FeCl3, and 10 mM phenylmethylsulfonyl fluoride in phosphate-buffered saline. The changes induced in erythrocyte membrane proteins by exposure to plasmodial cathepsin D parallel the alterations observed in red cell membranes obtained from malaria infected cells. Since the action of the plasmodial protease was confined to the inner surface of the red cell membrane, it is possible that protease-induced modifications in the red cell cytoskeleton could lead to merozoite release.

Optimized procedures for investigating changes in human erythrocyte membrane proteins using treatment with calcium and various proteases

In order to establish reproducible sensitive techniques for investigating diseases in which membrane defects have been implicated, human red cell membrane proteins were studied by SDS electrophoresis in ghosts prepared from the blood of 35 separate adult donors using treatment with 1 mmol/l calcium and proteolytic dissection under standardized conditions. The effects of calcium were consistent with increased binding of cytosolic proteins in membranes from fresh blood and endogenous proteolysis in membranes from aged blood. Exogenous proteolysis was most effective with trypsin and was facilitated both by pre-binding this protease to the membrane and by inclusion of 1 mmol/l Ca2+. The nature and extent of these effects was highly dependent on the storage time of the blood. Preliminary indications of cleavage fragment length polymorphism was obtained after treatment with the S. aureus V8 protease but not after trypsin or alpha chymotrypsin treatment.

Limited breakdown of cytoskeletal proteins by an endogenous protease controls Ca2+-induced membrane fusion events in chicken erythrocytes

The profound morphological changes which follow the treatment of chicken erythrocytes with the ionophore A23187 and Ca2+ are associated with a concomitant breakdown of certain membrane-associated proteins including alpha-spectrin, goblin and microtubule-associated proteins (MAPS) which undergo a limited proteolysis to give large, well-defined fragments. The Ca2+-sensitive protease responsible for these changes appears to be present in the soluble fraction of the cells. Treatment with TLCK or iodoacetamide inhibits both the major morphological changes and the proteolytic events but these agents do not prevent the dissociation of microtubules or the activation of endogenous sphingomyelinase which occur in cells with raised levels of intracellular Ca2+. It is suggested that the sphingomyelinase is activated as a consequence of a Ca2+-induced loss of phospholipid asymmetry in the plasma membrane.

Isolation and characterization of large (0.5 - 1.0 micron) cytoskeleton-free vesicles from human and rabbit erythrocytes

Large (0.5 - 1.0 micron) cytoskeleton-free vesicles were obtained, by 'budding', from fresh human and rabbit erythrocytes incubated at 45 degrees C and titrated with EDTA and CaCl2. This process occurs without hemolysis. The isolated vesicles maintain their cytoplasmic integrity and normal membrane orientation, and are resistant to hemolysis over the pH range 5.0 - 11.0 and temperature range 4-50 degrees C. The only membrane proteins detected in vesicles from human erythrocytes were band 3 region polypeptides and bands PAS-1, PAS-2 and PAS-3. Vesicles obtained from rabbit erythrocytes were similarly simple. Because of their size and stability these vesicles are amenable to both kinetic and quantitative analysis using the same experimental techniques employed in studies of synthetic lipid membranes. The results obtained in this study indicate that these vesicles are essentially markedly simplified biological cells, and thus may be useful as a biologically relevant model membrane system for examining the molecular interactions which occur within, across and between cell membranes.

Degradation of band-3 glycoprotein in vitro by a protease isolated from human erythrocyte membrane

The use of soybean-trypsin-inhibitor-Sepharose-4B to purify a protease present in human erythrocyte membranes is described. The fraction bound in the presence of calcium to the affinity absorbent is active on band-3 glycoprotein in a non-ionic detergent solution at neutral pH. Band-3 glycoprotein is degraded into components having the mobilities of the proteins of bands 4.5, 7 and of lower molecular weights. When calcium is omitted from the membrane extract, an inactive form of this enzyme can be purified. By DEAE-cellulose chromatography this inactive form can be converted into the active form, presumably by dissociation of an enzyme-inhibitor complex.

Calcium-activated thiol-proteinase activity in the fusion of rat erythrocytes induced by benzyl alcohol

1. Rat erythrocytes were fused by incubation with benzyl alcohol and Ca2+. 2. Cell fusion was inhibited by EGTA, N-ethylmaleimide, tetrathionate, iodoacetamide, cystamine, Tos-Lys-CH2Cl, and to a lesser extent by Tos-Phe-CH2Cl. Phenylmethanesulphonyl fluoride, Tos-Arg-OMe and histamine did not inhibit cell fusion. 3. Gel electrophoresis of membrane proteins from "ghosts" of the erythrocytes treated with benzyl alcohol showed that a high-molecular-weight polymer was present: this was consistent with the entry into the cells of Ca2+ and the activation of a transglutaminase enzyme. 4. In the treated cells the proteins corresponding to bands 2 and 3 in human erythrocytes were decreased, and a polypeptide with a slightly greater mobility than band 3 was produced. 5. These changes were inhibited by EGTA, N-ethylmaleimide, tetrathionate, iodoacetamide, cystamine, and Tos-Lys-CH2Cl, but not by phenylmethanesulphonyl fluoride, Tos-Arg-OMe, or histamine. 6. The intramembraneous particles of the P-fracture face of cells treated with benzyl alcohol to induce fusion were decreased in number and were susceptible to cold-induced aggregation; both of these phenomena were markedly inhibited to EGTA, and partially inhibited by Tos-Lys-CH2Cl and N-ethylmaleimide. 7. These several observations indicate that a Ca2+-activated thiol-proteinase, which acts to degrade membrane proteins and to give freedom of lateral movement to intramembranous particles, may be essential feature of membrane fusion in this system. 8. It is suggested that this proteinase may act to degrade spectrin-binding proteins that attach band-3 protein to the erythrocyte cytoskeleton.

Components involved in virally mediated membrane fusion and permeability changes

1. Intact F glycoprotein is required to induce permeability changes in Lettrée cells or in erythrocytes. Some HN glycoproteins may also be required. Permeability changes thus offer a simple, accurate and rapid means of assaying the integrity of F glycoprotein in certain viral preparations. 2. The '1-day' virus (which contains intact F glycoprotein but which differs morphologically from '3 day' virus) does not cause permeability changes; it can be rendered active by various physical treatments. It is concluded that the environment in which F glycoprotein is embedded is a determining factor for permeability changes. 3. The entry of fluorescently labelled peptides into cells made permeable by virus has been measured. Peptides having a molecular weight in excess of 1000 enter poorly, suggesting a 'pore' size of approx. 1 nm in diameter. 4. Two novel assay methods concerned with virus--cell fusion are described. The first measures the fluorescence enhancement that occurs when anthroylstearate is transferred from anthroylstearate-labelled virus to cells. The second measures the giant-cell formation that occurs when partially fused erythrocytes are exposed to hypo-osmotic treatment. The '1-day' virus is active in these assays. In contrast with permeability changes, virus--cell fusion is insensitive to changes in external Ca2+-concentration. 5. The results are compatible with a model [Knutton & Pasternak (1979) Trends Biochem. Sci. 4, 220--223; Impraim, Foster, Micklem & Pasternak (1980) Biochem. J. 186, 847--860] in which virus--cell fusion is a prerequisite for permeability changes, and in which permeability changes are the cause of haemolysis and giant-cell (polykaryon) formation.

Identification of proteolytic activities in the cytosolic compartment of mature human erythrocytes

Individual lysates from human erythrocyte suspensions, completely deprived of leucocytes and were assayed for a number of proteolytic activities using both naturally occurring and synthetic substrates. Removal of hemoglobin by batchwise DEAE-cellulose chromatography did not modify the complement of the various proteolytic activities which were then fractionated by means of chromatography on a column of DEAE-cellulose, followed by conventional techniques such as gel chromatography and preparative electrophoresis. This procedure allowed a number of proteinases to be identified in the erythrocyte cytosol while providing a tool for their selective though partial separation. The following peptidases were found to be present in the soluble fraction of mature human erythrocytes: (a) a neutral endopeptidase having an approximate molecular weight of 110 000; (b) three acidic endopeptidases, with pH optima between 2.5 and 3.5, showing molecular and functional properties almost identical with those of the three proteinases previously purified from solubilized erythrocyte membranes [Pontremoli et al. (1979) Biochem. J. 181, 559--568]; (c) two dipeptidylaminopeptidases whose molecular weights are around 80 000 and tentatively identified as dipeptidyl aminopeptidases II and III, respectively, on the basis of their substrate specificities and pH optima; (d) presumably two aminopeptidases, having an approximate molecular weight of 80 000 and classified as an aminopeptidase with broad substrate specificity and an aminopeptidase B, respectively. No evidence for any carboxypeptidase activity was found in the cytosolic compartment of mature human erythrocytes.

The isolation and characterization of 60 nm vesicles ('nanovesicles') produced during ionophore A23187-induced budding of human erythrocytes

In addition to the microvesicles released during the treatment of human erythrocytes with Ca2+ and ionophore A23187, a new subpopulation of still smaller dense vesicles ('nanovesicles') has now been identified. Nanovesicles are about 60 nm in diameter, have an acetylcholinesterase activity higher than that of microvesicles and appear to be relatively enriched in sphingomyelin and correspondingly depleted of phosphatidylethanolamine. They have a polypeptide composition quite different from those of erythrocyte membranes or microvesicles, consisting largely of components of mol.wts. 60 000 and 26 000 in addition to haemoglobin. These two major polypeptides do not appear to represent contaminating cytoplasmic proteins or proteolytic subfragments of a larger protein.

The effect of endogenous proteases on the spectrin binding proteins of human erythrocytes

We have demonstrated that in human erythrocyte ghosts endogenous proteolytic activity is responsible for the digestion of the spectrin binding proteins (bands 2.1 to 2.6). The pH optimum, cofactor requirements and inhibitor sensitivity have been established. Our results indicate that proteolysis of bands 2.1 to 2.6 and the formation of 3', a fragment containing an active spectrin binding site, can occur through two enzymatic pathways: a cascade of consecutive proteolytic cleavages of the spectrin binding proteins inhibited by phenylmethylsulfonyl fluoride or a Ca2+-stimulated, phenylmethylsulfonyl fluoride-insensitive, EDTA-inhibited cleavage of band 2.1 to band 2.3, followed by digestion to band 3' by phenylmethylsulfonyl fluoride-inhibitable enzymes. These findings may provide the techniques necessary to prevent proteolysis of the spectrin binding proteins during purification and reconstitution experiments and provide insight into how they are formed in vivo.

Isolation and partial characterization of three acidic proteinases in erythrocyte membranes

1. The distribution of proteolytic activity in membranes from human erythrocytes and from rabbit reticulocytes and erythrocytes was investigated, after removal of leucocytes and platelets from the cell suspensions. 2. All membrane preparations displayed proteolytic activity in the acidic pH region only. Membranes from human and rabbit mature erythrocytes showed latent activity, which could be increased when extracted with a number of detergents. 3. Three active fractions were resolved either by gel chromatography of solubilized membrane extracts or by standard polyacrylamide-gel electrophoresis. The three proteinase activities (designated proteinases I, II and III) were purified from solubilized extracts of human erythrocyte membranes. 4. The relevant mol.wts. were around 80000, 40000 and 30000, respectively, and each of the three proteinases appeared to be composed of a single polypeptide chain. 5. Distinctive pH optima (in the range pH2.8-3.9) and different saturation profiles with globin as substrate were observed for proteinases I, II and III. 6. Dithioerythritol, Hg(2+) and Cu(2+) inhibited each of the three human enzymes, but more selective inhibitory effects were exerted by other modifiers of proteolytic enzymes and by haemin. Similar effects were observed with the three proteinases from rabbit cells. 7. The activity of the three human proteinases seems to be restricted to naturally occurring protein substrates, although with poor specificity, and none of them was active on synthetic substrates. 8. Digestion of globin by each of the three enzymes yielded similar polypeptide fragments in all cases, this indicating an endopeptidase type of activity.

Calcium ions and cell fusion. Effects of chemical fusogens on the permeability of erythrocytes to calcium and other ions

1. Fusogenic and non-fusogenic chemicals were tesetd for their ability to allow 45Ca2+ and 3H2O to enter hen and human erythrocytes. 2. The ratio of 45Ca2+/3H2O in treated cells to that in untreated cells is referred to as the entry ratio. 3. Within 1 min at 37 degrees C both water-soluble and lipid-soluble fusogens increased the value of the entry ratio, which reached maximum values in 5--10 min. 4. Values of the entry ratio in the range of 4--12 were found under conditions that led to cell fusion. 5. Closely related but non-fusogenic chemicals did not significantly alter the entry ratio. 6. The entry ratios for 86Rb+, 22Na+ and 35SO42- were also significantly increased by both lipid-soluble and water-soluble fusogens, though the increases were not as large as those for 45Ca2+. 7. It is suggested that fusogenic compounds increase the permeability of biological membranes to ions, and that an increase in the concentration of intracellular Ca2+ initiates or facilitates events that lead to the chemically induced fusion of erythrocytes.

Effects of pH, Ca2+, temperature, and protease pretreatment on interkingdom fusion

The incubation of carrot protoplasts and cultured Xenopus cells in a protease solution has been shown to enhance their subsequent interkingdom fusion by a high pH/high Ca2+ method. The effects of Ca2+ concentration, pH, and temperature on the frequency of heterokaryon formation have also been studied. Potentially viable heterokaryons have been repeatedly produced at high frequencies (consistently greater than 10%), far exceeding those so far achieved in PEG-mediated fusion. Cell aggregates are readily dispersed after this method of fusion, permitting the accurate estimation of fusion frequencies.