Cholesterol affects spectrin-phospholipid interactions in a manner different from changes resulting from alterations in membrane fluidity due to fatty acyl chain composition

We previously showed that erythrocyte and brain spectrins bind phospholipid vesicles and monolayers prepared from phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (Review: A.F. Sikorski, B. Hanus-Lorenz, A. Jezierski, A. R. Dluzewski, Interaction of membrane skeletal proteins with membrane lipid domain, Acta Biochim. Polon. 47 (2000) 565). Here, we show how changes in the fluidity of the phospholipid monolayer affect spectrin-phospholipid interaction. The presence of up to 10%-20% cholesterol in the PE/PC monolayer facilitates the penetration of the monolayer by both types of spectrin. For monolayers constructed from mixtures of PI/PC and cholesterol, the effect of spectrins was characterised by the presence of two maxima (at 5 and 30% cholesterol) of surface pressure for erythroid spectrin, and a single maximum (at 20% cholesterol) for brain spectrin. The binding assay results indicated a small but easily detectable decrease in the affinity of erythrocyte spectrin for FAT-liposomes prepared from a PE/PC mixture containing cholesterol, and a 2- to 5-fold increase in maximal binding capacity (B_max) depending on the cholesterol content. On the other hand, the results from experiments with a monolayer constructed from homogenous synthetic phospholipids indicated an increase in Δπ change with the increase in the fatty acyl chain length of the phospholipids used to prepare the monolayer. This was confirmed by the results of a pelleting experiment. Adding spectrins into the subphase of raft-like monolayers constructed from DOPC, SM and cholesterol (1/1/1) induced an increase in surface pressure. The Δπ change values were, however, much smaller than those observed in the case of a natural PE/PC (6/4) monolayer. An increased binding capacity for spectrins of liposomes prepared from a “raft-like” mixture of lipids could also be concluded from the pelleting assay. In conclusion, we suggest that the effect of membrane lipid fluidity on spectrin-phospholipid interactions is not simple but depends on how it is regulated, i.e., by cholesterol content or by the chemical structure of the membrane lipids.     

Mitoxantrone changes spectrin-aminophospholipid interactions

Understanding drug-membrane and drug-membrane protein interactions would be a crucial step towards understanding the action and biological properties of anthracyclines, as the cell membrane with its integral and peripheral proteins is the first barrier encountered by these drugs. In this paper, we briefly describe mitoxantrone-monolayer and mitoxantrone-bilayer interactions, focusing on the effect of mitoxantrone on the interactions between erythroid or nonerythroid spectrin with phosphatidylethanolamine-enriched mono- and bilayers. We found that mitoxantrone markedly modifies the interaction of erythroid and nonerythroid spectrins with phosphatidylethanolamine/phosphatidylcholine (PE/PC) monolayers. The change in delta pi induced by spectrins is several-fold larger in the presence of 72 nM mitoxantrone than in its absence: spectrin/mitoxantrone complexes induced a strong compression of the monolayer. Spin-labelling experiments showed that spectrin/mitoxantrone complexes caused significant changes in the order parameter measured using a 5'-doxyl stearate probe in the bilayer, but they practically did not affect the mobility of 16'-doxyl stearate. These results indicate close-to-surface interactions/penetrations without significant effect on the mid-region of the hydrophobic core of the bilayer. The obtained apparent equilibrium dissociation constants indicated relatively similar mitoxantrone-phospholipid and mitoxantrone-spectrin (erythroid and nonerythroid) binding affinities. These results might in part, explain the effect of mitoxantrone on spectrin distribution in the living cells.     

Phosphatidylethanolamine Is a Key Regulator of Membrane Fluidity in Eukaryotic Cells

Adequate membrane fluidity is required for a variety of key cellular processes and in particular for proper function of membrane proteins. In most eukaryotic cells, membrane fluidity is known to be regulated by fatty acid desaturation and cholesterol, although some cells, such as insect cells, are almost devoid of sterol synthesis. We show here that insect and mammalian cells present similar microviscosity at their respective physiological temperature. To investigate how both sterols and phospholipids control fluidity homeostasis, we quantified the lipidic composition of insect SF9 and mammalian HEK 293T cells under normal or sterol-modified condition. As expected, insect cells show minimal sterols compared with mammalian cells. A major difference is also observed in phospholipid content as the ratio of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) is inverted (4 times higher in SF9 cells). In vitro studies in liposomes confirm that both cholesterol and PE can increase rigidity of the bilayer, suggesting that both can be used by cells to maintain membrane fluidity. We then show that exogenously increasing the cholesterol amount in SF9 membranes leads to a significant decrease in PE:PC ratio whereas decreasing cholesterol in HEK 293T cells using statin treatment leads to an increase in the PE:PC ratio. In all cases, the membrane fluidity is maintained, indicating that both cell types combine regulation by sterols and phospholipids to control proper membrane fluidity.     

Mitoxantrone changes spectrin-aminophospholipid interactions

Understanding drug-membrane and drug-membrane protein interactions would be a crucial step towards understanding the action and biological properties of anthracyclines, as the cell membrane with its integral and peripheral proteins is the first barrier encountered by these drugs. In this paper, we briefly describe mitoxantrone-monolayer and mitoxantrone-bilayer interactions, focusing on the effect of mitoxantrone on the interactions between erythroid or nonerythroid spectrin with phosphatidylethanolamine-enriched mono- and bilayers. We found that mitoxantrone markedly modifies the interaction of erythroid and nonerythroid spectrins with phosphatidylethanolamine/phosphatitydcholine (PE/PC) monolayers. The change in Δπ induced by spectrins is several-fold larger in the presence of 72?nM mitoxantrone than in its absence: spectrin/mitoxantrone complexes induced a strong compression of the monolayer. Spin-labelling experiments showed that spectrin/mitoxantrone complexes caused significant changes in the order parameter measured using a 5′-doxyl stearate probe in the bilayer, but they practically did not affect the mobility of 16′-doxyl stearate. These results indicate close-to-surface interactions/penetrations without significant effect on the mid-region of the hydrophobic core of the bilayer. The obtained apparent equilibrium dissociation constants indicated relatively similar mitoxantrone-phospholipid and mitoxantrone-spectrin (erythroid and nonerythroid) binding affinities. These results might in part, explain the effect of mitoxantrone on spectrin distribution in the living cells.     

Brain spectrin exerts much stronger effect on anionic phospholipid monolayers than erythroid spectrin

Red blood cell spectrin and its nonerythroid analogues are linked to integral proteins of the membrane by several skeletal protein receptors, such as ankyrin and protein 4.1 together with p55. However, there are also many reasons for believing that they are insufficient to engender all the properties that characterise the native membrane. Therefore, we are concerned with the mechanism by which brain spectrin interacts with phospholipids of the membrane bilayer. Brain and erythrocyte spectrin were shown previously to bind phospholipid vesicles as well as monolayers prepared from aminophospholipids: phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (PC).In the present study, it is shown that brain spectrin binds to monolayers prepared from anionic phospholipids, such as phosphatidylinositol (PI), phosphatidic acid (PA), phosphatidyl glycerol, diphosphatidylglycerol, and their mixtures with PC. Brain spectrin injected into the subphase to reach nanomolar concentration induced a substantial increase in the surface pressure of monolayers prepared from the phospholipids and their mixtures mentioned above, possibly by penetrating them. This effect is stronger in the case of monolayers prepared from anionic phospholipids alone and weaker when monolayers were prepared from mixtures with PC. The weakest effect was observed in the case of phosphatidylinositol-4,5-bisphosphate monolayers. An interaction of brain spectrin with monolayers prepared from anionic phospholipids (PI/PC 7:3 and PA/PC 7:3) was inhibited (PI/PC much stronger than PA/PC) by purified erythrocyte ankyrin, which indicates that the binding site for those lipids is located in the β-subunit, possibly in, or in close proximity of, the ankyrin-binding site.In contrast, erythrocyte spectrin injected into the subphase induced a change in the surface pressure of monolayers prepared from anionic phospholipids, which was equal or smaller than the value of surface pressure change induced by protein without a monolayer. This effect was different from what had been observed previously for monolayers prepared from aminophospholipids and their mixtures with PC, and from the data for nonerythroid spectrin presented here.     

Brain spectrin exerts much stronger effect on anionic phospholipid monolayers than erythroid spectrin

Red blood cell spectrin and its nonerythroid analogues are linked to integral proteins of the membrane by several skeletal protein receptors, such as ankyrin and protein 4.1 together with p55. However, there are also many reasons for believing that they are insufficient to engender all the properties that characterise the native membrane. Therefore, we are concerned with the mechanism by which brain spectrin interacts with phospholipids of the membrane bilayer. Brain and erythrocyte spectrin were shown previously to bind phospholipid vesicles as well as monolayers prepared from aminophospholipids: phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (PC).In the present study, it is shown that brain spectrin binds to monolayers prepared from anionic phospholipids, such as phosphatidylinositol (PI), phosphatidic acid (PA), phosphatidyl glycerol, diphosphatidylglycerol, and their mixtures with PC. Brain spectrin injected into the subphase to reach nanomolar concentration induced a substantial increase in the surface pressure of monolayers prepared from the phospholipids and their mixtures mentioned above, possibly by penetrating them. This effect is stronger in the case of monolayers prepared from anionic phospholipids alone and weaker when monolayers were prepared from mixtures with PC. The weakest effect was observed in the case of phosphatidylinositol-4,5-bisphosphate monolayers. An interaction of brain spectrin with monolayers prepared from anionic phospholipids (PI/PC 7:3 and PA/PC 7:3) was inhibited (PI/PC much stronger than PA/PC) by purified erythrocyte ankyrin, which indicates that the binding site for those lipids is located in the beta-subunit, possibly in, or in close proximity of, the ankyrin-binding site.In contrast, erythrocyte spectrin injected into the subphase induced a change in the surface pressure of monolayers prepared from anionic phospholipids, which was equal or smaller than the value of surface pressure change induced by protein without a monolayer. This effect was different from what had been observed previously for monolayers prepared from aminophospholipids and their mixtures with PC, and from the data for nonerythroid spectrin presented here.     

Lipid-binding role of betaII-spectrin ankyrin-binding domain

It is known that erythroid and non-erythroid spectrins binding of vesicles and monolayers containing PE proved sensitive to inhibition by red blood cell ankyrin. We now show that the bacterially-expressed recombinant peptides representing betaII(brain)-spectrin's ankyrin-binding domain and its truncated mutants showed lipid-binding activity, although only those containing a full-length amino terminal fragment showed high to moderate affinity towards phospholipid mono- and bilayers and a substantial sensitivity of this binding to inhibition by ankyrin. These results are in accordance with our published data on betaI-spectrin's ankyrin-binding domain [Hryniewicz-Jankowska A, et al. Mapping of ankyrin-sensitive, PE/PC mono- and bilayer binding site in erythroid beta-spectrin. Biochem J 2004;382:677-85]. Moreover, we tested also the effect of transient transfection of living cells of several cell-lines with vectors coding for GFP-conjugates including betaII and also betaI full-length ankyrin-binding domain and their truncated fragments on the membrane skeleton organization. The transfection with constructs encoding full-length ankyrin-binding domain of betaII and betaI spectrin resulted in increased aggregation of membrane skeleton and its punctate appearance in contrast to near normal appearance of membrane skeleton of cells transiently transfected with GFP control or construct encoding ankyrin-binding domain truncated at their N-terminal region. Our results therefore indicate the importance of N-terminal region for lipid-binding activity of the beta-spectrin ankyrin-binding domain and its substantial role in maintaining the spectrin-based skeleton distribution.     

Effects of physical states of phospholipids on the incorporation and cytochalasin B binding activity of human erythrocyte membrane proteins in reconstituted vesicles

Proteoliposomes were reconstituted from a Triton extract of human erythrocyte membrane proteins and a mixture of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of varying ratios. With mixtures of egg PC and soybean PE, the protein/lipid ratio of the reconstituted vesicles was maximal at 25% PC and 75% PE, the composition which is known to have a maximum bilayer disruption (highest occurrence of lipidic particles seen by freeze-fracture electron microscopy). With mixtures of 1-palmitoyl-2-oleoyl-PC and dilinoleoyl-PE, which gave vesicles with few isolated lipidic particles at room temperature, the effect was less pronounced. The specific activity of the cytochalasin B (CB) binding protein in the reconstituted vesicles, on the other hand, was increased monotonically up to severalfold as the PC content was increased in the egg PC/soybean PE mixture. A similar increase was observed when soybean PE was partially substituted by dimyristoyl-PC, cholesterol, or transphosphatidylated PE from egg PC. These findings indicate that preexisting defects in the lipid bilayer promote protein incorporation into the bilayer during reconstitution whereas reduction of the bilayer fluidity facilitates the CB binding activity in the reconstituted vesicles.     

A special lipid mixture for membrane fluidization

The potency for membrane fluidization of mixtures containing neutral lipids (NL), phosphatidylcholine (PC) and phosphatidylethanolamine (PE) from hen egg yolk was tested on human erythrocytes and lymphocytes. A specific mixture consisting of 70% NL, 20% PC and 10% PE was found to be a potent membrane fluidizer operating almost exclusively by extracting membrane cholesterol. Spectral results and electron micrographs indicate that aqueous dispersion of this mixture consists of chylomicron-like assemblies where the neutral lipids provide the hydrophobic core on the surface of which phospholipids are spread as a monolayer.     

Self-adaptive modification of red-cell membrane lipids in lecithin: Cholesterol acyltransferase deficiency. Lipid analysis and spin labeling

In a patient with lecithin: cholesterol acyltransferase deficiency, free cholesterol was markedly increased, and esterified cholesterol was diminished. In the patient's plasma, an increase in phosphatidylcholine (PC) and a decrease in sphingomyelin were observed. Concomitantly, an increase in a shorter acyl chain 16:0 was noted in PC, sphingomyelin and phosphatidylethanolamine (PE). In contrast to these results, longer chains such as 22:0 and 24:0 were decreased, especially in sphingomyelin. Unsaturated double bonds such as 18:1 was also increased in PC and PE. In the red-cell membrane lipids, the increase in free cholesterol was counteracted by an increase in PC and by a decrease in sphingomyelin and PE, reflecting changes in the patient's plasma lipids. Increased 16:0 (in PC) and decreased 18:0 and 24:0 were observed. The increased plasma free cholesterol due to metabolic defect (lecithin:cholesterol acyltransferase deficiency) led to decreased red-cell membrane fluidity. This effect appeared to be counteracted by changing phospholipid composition (increased PC and decreased sphingomyelin and PE), by increasing shorter chains (16:0), by decreasing longer chains (18:0 and 24:0) and by increasing unsaturated double bonds (18:2). These results can be interpreted as a self-adaptive modification of lecithin:cholesterol acyltransferase deficiency-induced red-cell membrane abnormalities, to maintain normal membrane fluidity. This speculation was supported by the ESR spin-label studies on the patient's membrane lipids. The normal order parameters in intact red cells and in total lipid liposomes were decreased if cholesterol-depleted membrane liposomes were prepared. Thus, the hardening effect of cholesterol appeared to be counteracted by the softening effects described above. Overall membrane fluidity in intact red cells of the lecithin:cholesterol acyltransferase-deficient patient was maintained normally, judged by order parameters in ESR spin-label studies.     

Phospholipid asymmetry in cardiac sarcolemma. Analysis of intact cells and 'gas-dissected' membranes

The investigation focuses on the phospholipid composition of the sarcolemma of cultured neonatal rat heart cells and on the distribution of the phospholipid classes between the two monolayers of the sarcolemma. The plasma membranes are isolated by 'gas-dissection' technique and 38% of total cellular phospholipid is present in the sarcolemma with the composition: phosphatidylethanolamine (PE) 24.9%, phosphatidylcholine (PC) 52.0%, phosphatidylserine/phosphatidylinositol (PS/PI) 7.2%, sphingomyelin 13.5%. The cholesterol/phospholipid ratio of the sarcolemma is 0.5. The distribution of the phospholipids between inner and outer monolayer is defined with the use of two phospholipases A2, sphingomyelinase C or trinitrobenzene sulfonic acid as lipid membrane probes in whole cells. The probes have access to the entire sarcolemmal surface and do not produce detectable cell lysis. The phospholipid classes are asymmetrically distributed: (1) the negatively charged phospholipids, PS/PI are located exclusively in the inner or cytoplasmic leaflet; (2) 75% of PE is in the inner leaflet; (3) 93% of sphingomyelin is in the outer leaflet; (4) 43% of PC is in the outer leaflet. The predominance of PS/PI and PE at the cytoplasmic sarcolemmal surface is discussed with respect to phospholipid-ionic binding relations between phospholipids and exchange and transport of ions, and the response of the cardiac cell on ischemia-reperfusion.     

Effect of hormones on phospholipid metabolism in human cultured fibroblasts

The effect of hormones on phospholipid metabolism, pool size, 32P labeling and changes in fatty acid of human adult fibroblasts was determined. Simultaneously the change in membrane fluidity of single cells was recorded via fluorescence recovery after photobleaching under the influence of hormones. From all substances tested (isoproterenol, phenylephrine, adrenalin, histamine, angiotensin II, dansylcadaverine, propranolol) only isoproterenol and adrenalin slightly decreased total amount of phosphatidylcholine (PC). The amount of the other phospholipids analyzed remained unchanged. The 32P incorporation rate into phospholipids (PC, phosphatidylinositol (PI), phosphatidylethanolamine (PE)) was affected basicly different analyzing either PC, PI or PE. Histamine and propranolol provoked the highest incorporation of 32P (240% increase in PI labeling). Isoproterenol and adrenalin decreased PC labeling (45% and 18%) whereas isoproterenol decreased 32P incorporation into PI (18%), and adrenalin led to an increase (37%). PE labeling showed no or a slight increase in 32P incorporation applying the other agonists or antagonists. The fatty acid pattern of the respective phospholipids changed only to a minor extend. A decrease in hexadecanoic acid content of PI was found after administration of either isoproterenol, adrenalin or histamine. Parallel determination of membrane fluidity of single cells by fluorescence recovery after photobleaching showed an increase in the diffusion coefficient of a fluorescent lipid probe sticking in the membrane, following administration of isoproterenol and adrenalin, other substances tested exerted no effect. A relationship to changes in phospholipid metabolism became obvious. These results are discussed considering known mechanisms of receptor coupling and change in phospholipid metabolism and fluidity.     

Fluidity and lipid composition of oat and rye shoot plasma membrane: effect of sterol perturbation by xenobiotics

Oat and rye plants were treated with either tetcyclacis (an experimental plant growth regulator), nuarimol (a fungicide) or gamma-ketotriazole (an experimental herbicide). These treatments reduced shoot growth and changed the lipid composition of the shoot plasma membranes. In oat, both tetcyclacis and nuarimol treatments increased plasma membrane cholesterol and increased the phosphatidylethanolamine/phosphatidylcholine (PE/PC) ratio, whereas gamma-ketotriazole treatment reduced cholesterol and the PE/PC ratio. In rye, all treatments reduced the PE/PC ratio. Generally, the sterol/phospholipid ratio was less in oat than in rye but the cholesterol/phospholipid ratio was greater. With all treatments in oat and rye, increases were observed in unsaturation of the phospholipid acyl chains. The fluidity of membranes was measured by steady-state fluorescence polarisation of the probe diphenylhexatriene; oat membranes were more fluid than rye. Membrane fluidity was greater in plasma membranes from plants treated with the xenobiotics than the controls. The results are discussed in the context of the effect of plasma membrane lipid composition on membrane fluidity, and it is concluded that there appears to be no overall simple relationship between membrane lipid composition and fluidity that holds for all treatments in both species.     

Differential temperature acclimatization responses in the membrane phospholipids of Posthodiplostomum minimum and its second intermediate host, Lepomis macrochirus

The effects of temperature change on phospholipid content in metacercariae of Posthodiplostomum minimum and their second intermediate hosts, Lepomis macrochirus, were examined to gauge similarities in the homeoviscous adaptation of host and parasite membranes to environmental thermal change. Heart, liver, and muscle tissues from individual L. macrochirus responded to environmental temperature declines with a decrease in the ratio of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Increases in membrane PE concentration increase membrane fluidity, maintaining fish membrane function as environmental temperature declines. However, the metacercariae of P. minimum exhibit changes in cholesterol levels, total lipid levels, and lipid composition (PE/PC) that contrast the normal changes for homeoviscous membrane adaptation exhibited by their fish intermediate hosts. The parasites seem to rely on their hosts for homeoviscous adaptation within normal developmental temperature ranges, pooling both cholesterol and PE as energetic stores for development and ontological transitions signaled by elevated temperatures.     

Influence of Phospholipid Species on Membrane Fluidity: A Meta-analysis for a Novel Phospholipid Fluidity Index

Generalized membrane lipid composition determinants of fluidity have been widely investigated, including phospholipid/cholesterol ratio and unsaturation index. Individual phospholipids differ in their physical characteristics, including their interaction with cholesterol and level of unsaturation, emphasizing the importance of examining their individual influence on membrane fluidity. Thus, the purpose of this study was to examine the dominant phospholipids of biological membranes (phosphatidylcholine, PC; phosphatidylethanolamine, PE; sphingomyelin, SM) through a meta-analysis to assess the validity of an inclusive phospholipid fluidity index (PFI = PC/(PE + SM)) as a determinant for membrane fluidity (expressed as polarization of fluorescent probe 1,6 diphenyl-1,3,5-hexatriene) in comparison to previous phospholipid ratios (PC/PE and PC/SM). The results demonstrate that all indices significantly predicted membrane fluidity at 25°C (based on 10–13 data points). In contrast, only PFI approached significance when predicting membrane fluidity at 37°C (P = 0.10 based on five points). As a result, PFI appears to be the only phospholipid index close to significantly predicting membrane fluidity at mammalian physiological temperature. Because this meta-analysis only assessed studies using mammalian membranes, future work should experimentally assess the validity of the PFI utilizing membranes from mammals and a variety of other species and tissues at their respective physiological temperatures.     

Interactions of Bax and tBid with Lipid Monolayers

The release of cytochrome c from mitochondria to the cytosol is a crucial step of apoptosis that involves interactions of Bax and tBid proteins with the mitochondrial membrane. We investigated Bax and tBid interactions with (i) phosphatidylcholine (PC) monolayer as the main component of the outer leaflet of the outer membrane, (ii) with phosphatidylethanolamine (PE) and phosphatidylserine (PS) that are present in the inner leaflet and (iii) with a mixed PC/PE/Cardiolipin (CL) monolayer of the contact sites between the outer and inner membranes. These interactions were studied by measuring the increase of the lipidic monolayer surface pressure induced by the proteins. Our measurements suggest that tBid interacts strongly with the POPC/DOPE/CL, whereas Bax interaction with this monolayer is about 12 times weaker. Both tBid and Bax interact moderately half as strongly with negatively charged DOPS and non-lamellar DOPE monolayers. TBid also slightly interacts with DOPC. Our results suggest that tBid but not Bax interacts with the PC-containing outer membrane. Subsequent insertion of these proteins may occur at the PC/PE/CL sites of contact between the outer and inner membranes. It was also shown that Bax and tBid being mixed in solution inhibit their insertion into POPC/DOPE/CL monolayer. The known 3-D structures of Bax and Bid allowed us to propose a structural interpretation of these experimental results.     

The effects of protein hydrophobic exposure on the slope of the line in the deltapi vs pi(i) plot

The deltapi vs pi(i) curve from monolayer surface pressure detection is a powerful method to characterize the membrane insertion. The deltapi vs pi(i) curve is fixed by two parameters, the pi(c) and the slope. The intersected point (pi(c)) of deltapi vs pi(i) curve with abscissa is generally used as a quantitative measure of the membrane insertion ability of a protein. In the current work we demonstrate a correlation between the variation in the slope of the deltapi vs pi(i) curve with protein hydrophobic exposure by performing monolayer experiments on three different proteins, human apolipoprotein H, trichosanthin, and mutant trichosanthin. The value of /slope/ increases gradually following the degree of hydrophobic exposure. These findings suggest that the two parameters, the pi(c) and the slope, will complement each other to interpret the lipid/protein interaction involved in membrane insertion.     

Hepatic plasma membrane fluidity and dietary proteins.

Effect of protein deficient diet on hepatic plasma membrane fluidity has been studied in rats using (i) steady state fluorescence polarization and anisotropy, (ii) phospholipid and cholesterol contents, (iii) phospholipid fatty acid composition, (iv) turnover of phosphatidyl choline (PC), and (v) activities of membrane-bound enzymes as parameters and rats fed casein (20%) diet as standard group. A significant increase in steady state fluorescence and anisotropy values was registered in the deficient group, indicating increased resistance and hence decrease in fluidity of the plasma membrane. Supplementation of the diet with lysine and threonine improved these values, thereby suggesting the significance of diet for membrane fluidity. Simultaneous significant alterations in other parameters, viz. (i) decrease in PC, PE and free cholesterol and increase in esterified cholesterol contents, (ii) decrease in unsaturation of fatty acids of PC, (iii) decrease in incorporation of NaH2 32PO4, [CH3-14C]choline and [CH3-14C]methionine into plasma membrane PC, and (iv) decrease in activities of plasma membrane 5'-nucleotidase and phosphodiesterase along with increase of (Na(+)-K+)ATPase and adenyl cyclase, were observed in the deficient group which on supplementation with lysine and threonine showed improvement over alterations.     

Characteristics of spectrin-induced leakage of extruded, phosphatidylserine vesicles

At neutral pH spectrin induces modest leakage of trapped calcein from reverse-phase or extruded, but not sonicated, vesicles composed of phosphatidylserine, but not phosphatidylcholine. The extent of leakage from extruded vesicles is not or is only slightly affected by magnesium ions at a physiological concentration or calcium ions at a greater than physiological concentration, respectively. In addition to accounting for several previously discrepant observations on the lytic effects of spectrin, these findings indicate that some proteins like spectrin may destabilize vesicles with low curvature more readily than vesicles of high curvature, in contrast to certain amphiphilic peptides. 60% less leakage is induced from phosphatidylserine vesicles by heat-denatured than by native spectrin. In contrast, both trypsin- and subtilisin-treated spectrins, if sufficiently digested, induce several-fold more leakage than undigested spectrin. Since spectrin prepared either by 1 M Tris dissociation of Triton-extracted cytoskeletons or by low ionic strength extraction of ghosts released the same amounts of calcein from vesicles of various compositions, these effects are unlikely to reflect artifacts of spectrin preparation. Furthermore, spectrin is unlikely to promote leakage in vivo, since vesicles composed of phosphatidylserine, cholesterol and/or phosphatidylethanolamine, which constitute the lipid composition of the inner monolayer of the red cell membrane, did not leak on addition of spectrin, whereas vesicles composed of phosphatidylserine and phosphatidylcholine, did leak in the presence of spectrin.     

Interaction of vesicular stomatitis virus with lipid vesicles: depletion of cholesterol and effect on virion membrane fluidity and infectivity.

Interaction with excess unilamellar phosphatidylcholine (PC) vesicles resulted in depletion of as much as 90% of the cholesterol from the membrane of intact vesicular stomatitis (VS) virus. The cholesterol depletion was not significantly influenced by the proteolytic removal of virion glycoprotein spikes, but it was temperature dependent. Cholesterol depletion caused substantial reduction in anisotropy of the VS virion membrane as measured by fluorescence depolarization of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene; residual adsorbed vesicles represent a significant factor in this apparent increase in virion membrane fluidity. Interaction with PC vesicles resulted in a substantial loss of VS viral infectivity as measured by plating efficiency on L-cell monolayers. Reduction in infectivity appeared to be related to temperature-dependent depletion of virion cholesterol by PC vesicles. Interaction of VS virions with cholesterol-containing PC vesicles resulted in significantly less decline in infectivity, but attempts to restore cholesterol and infectivity to depleted VS virions were unsuccessful. Depletion of virion cholesterol apparently results through collision with PC vesicles rather than movement of cholesterol monomers or micelles through the aqueous phase, because PC vesicle-virion interaction in the presence of cholesterol oxidase did not result in substantial oxidation of translocated cholesterol.