Membrane lipids can modulate guanylate cyclase activity of rat intestinal microvillus membranes

Studies of the temperature dependence (10-40 degrees C) of guanylate cyclase in rat intestinal microbillus membranes reveal a change in energy of activation (slope of the Arrhenius plot) at 30 +/- 1 degree C. The break point temperature corresponds to the lipid thermotropic transition in these membranes previously characterized by differential scanning calorimetry (range: 23-39 degrees C; peak temperature, 31 degrees C). The break point temperature for guanylate cyclase also corresponds to that of a number of other microbillus membrane enzymes and of D-glucose transport. These activities are defined as "intrinsic" membrane activities by this operational criterion. Treatment with the nonionic detergent Lubrol WX increased the guanylate cyclase activity 4- to 8-fold and removed the discontinuity in the Arrhenius plot.     

Thermally induced changes in reconstituted and membranous cytochrome c oxidase.

The Arrhenius plots of electron transport activity in cytochrome c oxidase reconstituted with well-defined phospholipids have been shown to display a change in slope at 20--25 degrees C regardless of the chemical nature of the incorporated lipid. In native membranous cytochrome c oxidase, the discontinuity in Arrhenius activity plot occurred at 16--18 degrees C. These temperature breaks were found to correlate with changes in spin-label mobilities but not with the bulk lipid transition observed by differential scanning calorimetry. Temperature-dependent reciprocal equilibrium between the immobilized and fluid pools is demonstrated. It is suggested that the changes in kinetic and spin-label spectral characteristics in cytochrome c oxidase membranes are related very likely to a lipid-protein interaction prompted by a thermally induced change in the physical state of the lipids that does not involve a gel to liquid crystalline transition.     

Temperature dependence of D-glucose transport in reconstituted liposomes

Sodium-dependent D-glucose uptake into proteoliposomes reconstituted from dimyristoylphosphatidylcholine (DMPC) and hog kidney brush border membrane extract is strongly affected by temperature and the physical state of the membranes. This dependence is defined by a nonlinear Arrhenius plot with a break point at 23 degrees C, a temperature not significantly different from the phase transition temperature of the pure lipid (24 degrees C). The transport process is characterized by different activation energies: 35.1 kcal/mol below and 5.5 kcal/mol above the transition temperature. The shift in the break point for the D-glucose transport activity from 15 degrees C, in the brush border membranes, to 23 degrees C in the reconstituted system leads us to conclude that the lipids surrounding the sodium/D-glucose cotransport system can exchange readily with the bulk lipid used for reconstitution. The results thus provide no evidence for the presence of an annulus of specific lipids surrounding the transport system.     

Calorimetric and spectroscopic studies of lipid thermotropic phase behavior in liver inner mitochondrial membranes from a mammalian hibernator

Arrhenius plots of various enzyme and transport systems associated with the liver mitochondrial inner membranes of ground squirrels exhibit changes in slope at temperatures of 20-25 degrees C in nonhibernating but not in hibernating animals. It has been proposed that the Arrhenius breaks observed in nonhibernating animals are the result of a gel to liquid-crystalline phase transition of the mitochondrial membrane lipids, which also occurs at 20-25 degrees C, and that the absence of such breaks in hibernating animals is due to a major depression of this lipid phase transition to temperatures below 4 degrees C. In order to test this hypothesis, we have examined the thermotropic phase behavior of liver inner mitochondrial membranes from hibernating and nonhibernating Richardson's ground squirrels, Spermophilus richardsonii, by differential scanning calorimetry and by 19F nuclear magnetic resonance and fluorescence polarization spectroscopy. Each of these techniques indicates that no lipid phase transition occurs in the membranes of either hibernating or nonhibernating ground squirrels within the physiological temperature range of this animal (4-37 degrees C). Moreover, differential scanning calorimetric measurements indicate that only a small depression of the lipid gel to liquid-crystalline phase transition, which is centered at about -5 degrees C in nonhibernating animals and at about -9 degrees C in hibernators, occurs. We thus conclude that the Arrhenius plot breaks observed in some membrane-associated enzymatic and transport activities of nonhibernating animals are not the result of a lipid phase transition and that a major shift in the gel to liquid-crystalline lipid phase transition temperature is not responsible for seasonal changes in the thermal behavior of these inner mitochondrial membrane proteins.     

Physico-chemical mechanisms of organic acid transport in the apical membrane cells of the proximal kidney tubules in rats. I. Kinetic transport parameters and effect of the lipid phasic state on transport

The kinetics of the transport of 3H-para-aminohippuric acid (PAH) and the influence of the temperature on the initial rate of transport were studied on the vesicles of a purified fraction of the apical membrane isolated from cells of kidney proximal tubules. The PAH transport is accomplished owing to the facilitate diffusion mechanism. The apparent Michaelis constant at 36 degrees C was equal to 7.0 + 1.0 mM, the maximum rate was 15 nmol/min on 1 mg of protein, the inhibition constant for the PAH transport by probenecid being 0.5 mM. At 22 degrees C the apparent Michaelis constant was drastically increased. When the temperature dependence of the initial rate of PAH transport into vesicles was replotted in the form of the Arrhenius plot, there was a turning-point of the line at 28-30 degrees C. The same turning-point is shown on the Arrhenius plot for temperature dependence of alkaline phosphatase activity (a marker enzyme for the apical membrane). The electron paramagnetic resonance spectra analysis of 5-doxylstearate-labeled apical membrane preparation reveals a thermotropic transition near 21-29 degrees C. It is concluded that the function of the carrier and the activity of alkaline phosphatase depend on the phasic state of membrane lipids; the normal function of membrane proteins is possible under the liquid-crystalline state of the lipid bilayer.     

Temperature dependence of the apparent affinity and the maximum velocity of the membrane-bound monosaccharide transport system in the yeast Rhodotorula gracilis

Analysis of the temperature dependence of the monosaccharide transport system in the yeast Rhodotorula gracilis (ATCC 26194, CBS 6681), as tested with D-xylose, revealed that the apparent affinity of the transport system, measured as the reciprocal of the half-saturation constant KT, increased when transport velocity was stimulated by temperature (15--30 degrees C) and decreased when the rate of uptake was reduced at temperatures aboce 30 degrees C. Breaks in Arrhenius plots were accompanied by corresponding breaks in van't Hoff plots. Whereas untreated cells exhibited in the van't Hoff plot a discontinuity at 28--30 degrees C this was not observed in heat-treated cells (at either 37 or 45 degrees C). In heat-treated cells the maximum transport velocity was always lower and the apparent affinity higher than in untreated cells at the same temperature; the optimum temperature for both transport velocity and apparent affinity was shifted to higher values. The data are interpreted in terms of a reversible phase transition of membrane lipids effecting an irreversible alteration of membrane structure. The temperature-induced reversible alkalinization of unbuffered yeast suspensions supports this interpretation.     

Temperature dependence of rotational dynamics of protein and lipid in sarcoplasmic reticulum membranes

We have investigated the relationship between function and molecular dynamics of both the lipid and the Ca-ATPase protein in sarcoplasmic reticulum (SR), using temperature as a means of altering both activity and rotational dynamics. Conventional and saturation-transfer electron paramagnetic resonance (EPR) was used to probe rotational motions of spin-labels attached either to fatty acid hydrocarbon chains or to the Ca-ATPase sulfhydryl groups in SR. EPR studies were also performed on aqueous dispersions of extracted SR lipids, in order to study intrinsic lipid properties independent of the protein. While an Arrhenius plot of the Ca-ATPase activity exhibits a clear change in slope at 20 degrees C, Arrhenius plots of lipid hydrocarbon chain mobility are linear, indicating that an abrupt thermotropic change in the lipid hydrocarbon phase is not responsible for the Arrhenius break in enzymatic activity. The presence of protein was found to decrease the average hydrocarbon chain mobility, but linear Arrhenius plots were observed both in the intact SR and in extracted lipids. Lipid EPR spectra were analyzed by procedures that prevent the production of artifactual breaks in the Arrhenius plots. Similarly, using sample preparations and spectral analysis methods that minimize the temperature-dependent contribution of local probe mobility to the spectra of spin-labeled Ca-ATPase, we find that Arrhenius plots of overall protein rotational mobility also exhibit no change in slope. The activation energy for protein mobility is the same as that of ATPase activity above 20 degrees C; we discuss the possibility that overall protein mobility may be essential to the rate-limiting step above 20 degrees C.     

Effect of temperature on nuclear membranes and nucleo-cytoplasmic RNA-transport in Tetrahymena grown at different temperatures.

The effect of temperature on the nuclear envelope structure and the transport of total RNA and ribosomal subunits from nucleus to cytoplasm was examined in Tetrahymena cells propagated at two different temperatures. Freeze-etch electron microscopy of cells grown at 23 and 18 degrees C detects the emergence of smooth areas on the fracture faces of the nuclear membranes upon lowering the temperature below approximately 15 and approximately 12 degrees C, respectively. Coincident with these freeze-etch changes, a discontinuous decrease is observed in the nucleocytoplasmic RNA-transport; this is probably not due to a cease in RNA-synthesis. Below the thermotropic discontinuity observed in the transport of total RNA in 18 degrees-cells the nucleocytoplasmic transport of the small and large ribosomal subunits is equally retarded. Recent temperature studies on the endoplasmic reticulum membranes of Tetrahymena suggest that the freeze-etch changes in the nuclear membranes are induced by a thermotropic clustering of the membrane lipids. We conclude that this lipid clustering induces the permanent protein constituents in the nuclear envelope pore complexes to change from a relatively "open" into a relatively "closed" state thus causing the observed decrease in RNA-transport.     

Lipid peroxidation as a major factor in the modification of the catalytic function of Ca-ATPase of the sarcoplasmic reticulum in hypercholesterolemia

A comparative study of the effect of an experimental hypercholesterolemia and in vitro induced lipid peroxidation (LPO) on the temperature dependence of the activity of sarcoplasmic reticular Ca-ATPase from rabbit skeletal muscle (SR) has been performed. A control Arrhenius plot of ATPase activity determined in the presence of alamethicin was characterized by discontinuity in the 20 degrees C area. Both in vitro induced LPO and hypercholesterolemia resulted in a shift of discontinuity to 30 degrees C area. The replacement of lipid Ca-ATPase membrane environment by egg yolk lecithin did not affect the temperature dependence of the activity in control SR and failed to restore the original nature of the Arrhenius plot for Ca-ATPase modified by hypercholesterolemia or the in vitro induced LPO.     

Effects of temperature on cytochrome oxidase activity in solubilized form and in lipid vesicle systems.

Isolated mammalian cytochrome oxidase gave an Arrhenius plot with a break (Tb) at about 20 degrees C when assayed in a medium containing Emasol. The activation energies above and below 20 degrees C were 9.3 (EH) and 18.9 kcal/mol (EL), respectively. Isolated cytochrome oxidase was also incorporated into vesicles of dipalmitoyl phosphatidylcholine (DPPC, phase transition temperature Tt = 40 degrees C), dimyristoyl phosphatidylcholine (DMPC, Tt = 23 degrees C) and dioleoyl phosphatidylcholine (DOPC, Tt = -22 degrees C). The DPPC system showed a nearly linear Arrhenius plot between 9 and 36 degrees C with E = 22.8 kcal/mol. When cytochrome oxidase was resolubilized from the DPPC vesicles and assayed in solution a biphasic plot was obtained again. Cytochrome oxidase-DOPC was more active than the solubilized enzyme and exhibited a biphasic Arrhenius plot with Tb = 23 degrees C. EH and EL were 6.6 and 15.8 kcal/mol, respectively. The plot for the oxidase-DMPC also showed a break (Tb = 26 degrees C) with EH = 6.6 and EL = 26.6 kcal/mol. These results indicate that the break in the Arrhenius plot reflects primarily a structural transition in the cytochrome oxidase molecule between the "hot" and "cold" conformations, as proposed previously. This transition, as well as the molecular state of cytochrome oxidase, is affected by the physical state of the membrane lipids as reflected by changes in the kinetic properties.     

Alanine transport by Chinese hamster ovary cells with altered phospholipid acyl chain composition

The Na+-dependent transport of alanine has been examined in Chinese hamster ovary (CHO) cells as a function of the fatty acid composition of their membrane lipids. Significant changes in the fatty acid composition of the CHO cell phospholipids were achieved by supplementation of the growth medium with specific saturated (palmitate) or monoenoic (oleate) free fatty acids. Arrhenius plots of the temperature-dependent uptake of alanine were constructed for cells of altered fatty acid composition. Alanine uptake was characterized by a single discontinuity in the Arrhenius plot. The temperature of this break was observed to be dependent upon the fatty acid composition of the cell phospholipids, ranging from 16 degrees C for cells enriched with oleate to 32 degrees C for cells enriched in palmitate. Calculation of the Km value for the uptake process showed no significant change with temperature or fatty acid supplementation. Correlations are made between the physical state of the membrane lipids and the temperature-dependence for alanine transport. The results are discussed in terms of membrane fatty acid composition, ordered in equilibrium fluid phase transitions and amino acid transport.     

Annular lipids determine the ATPase activity of a calcium transport protein complexed with dipalmitoyllecithin.

Pure complexes of dipalmitoyllecithin (DPL, 16:0) which Ca2+, Mg2+ dependent ATPase from sarcoplasmic reticulum are unusual in retaining significant ATPase activity down to about 30 degrees C, well below the transition temperature of the pure lipid at 41 degrees C. A minimum of about 35 lipid molecules per ATPase is required to maintain maximal ATPase activity, but the complexes are progressively and irreversibly inactivated at lower lipid to protein ratios. Complexes containing more than the minimum lipid requirement show very similar temperature profiles of activity about 30 degrees C over a wide range of lipid to protein ratios, up to 1500:1. Spin-label studies indicate that, at lipid to protein ratios of less than about 30 lipids per ATPase, no DPL phase transition can be detected, but at all higher ratios, a phase transition occurs at about 41 degrees C. In all of these complexes there are breaks in the Arrhenius plots of ATPase activity at 27--32 degrees C and at 37.5--38.5 degrees C. Experiments with perturbing agents, such as cholesterol and benzyl alcohol which have well-defined effects on the DPL phase transition, indicate that these breaks in the Arrhenius plots of ATPase activity cannot be attributed to a depressed and broadened phase transition in the lipids near the protein molecules. These results are interpreted as evidence for a phospholipid annulus of at least 30 lipid molecules with interact directly with the ATPase and cannot undergo a phase transition at 41 degrees C. This structural interaction of the ATPase with the annular DPL molecules has a predominant effect in determining the form of the temperature-activity profiles. However, the perturbation of the DPL phase transition does not extend significantly beyond the annulus since a phase transition which starts at 41 degrees C can be detected as soon as extraannular lipid is present in the complexes. We suggest that it may be a general feature of membrane structure that penetrant membrane proteins interact with their immediate lipid environment so as to cause only a minimal perturbation of the lipid bilayer.     

Regulation of UDP-glucuronosyltransferase by lipid-protein interactions. Comparison of the thermotropic properties of pure reconstituted enzyme with microsomal enzyme

The temperature dependence of two kinetic properties of the GT2P isoform of microsomal UDP-glucuronosyltransferase was studied for enzyme in intact microsomes and for pure enzyme reconstituted into different types of lipid bilayers. The properties studied were the non-Michaelis-Menten kinetics of the enzyme and activity at Vmax(app). For enzyme in intact microsomes, the pattern of non-Michaelis-Menten kinetics was seen at all temperatures in the range tested (23 to 48 degrees C), and the slopes of the Hill plots of the data were constant across this range of temperatures. Although non-Michaelis-Menten kinetics were present for pure enzyme in bilayers of 1,2-dimyristoylphosphatidylcholine or 1,2-dipalmitoylphosphatidylcholine only in the gel phase (Hockman, Y., Kelley, M., and Zakim, D. (1983) J. Biol. Chem. 258, 6509-6519), it was not possible to reconstitute this pattern of kinetics for enzyme at T greater than 40 degrees C. For example, GT2P displayed Michaelis-Menten kinetics in bilayers of 1,2-distearoylphosphatidylcholine at 44 degrees C. For enzyme in microsomes, activities at Vmax(app) increased with increasing temperature in the range 23 to 48 degrees C, with a discontinuity in the slope of the Arrhenius plot at 34 degrees C. This thermotropic property also could not be reconstituted with pure GT2P. Instead, activities at Vmax(app) for GT2P reconstituted in 1,2-dioleoylphosphatidylcholine, 1,2-distearoylphosphatidylcholine, or 1,2-stearoyl oleoylphosphatidylcholine increased in the range 23 to 37 degrees C, but then decreased at T greater than 37 degrees C. The fall in activity at T greater than 37 degrees C was reversible, indicating that GT2P undergoes a reversible change at 37 degrees C to a less active form of the enzyme. The differences between pure, reconstituted GT2P and microsomal GT2P indicate that the thermotropic properties of the microsomal enzyme are not properties of the enzyme per se but depend on interactions between it and other components in the microsome. The data suggest, therefore, that the properties of GT2P in microsomes results in part from an organization of components in the microsomal membrane.     

Temperature dependence of d-glucose transport in reconstituted liposomes

Sodium-dependent d-glucose uptake into proteoliposomes reconstituted from dimyristoylphosphatidylcholine (DMPC) and hog kidney brush border membrane extract is strongly affected by temperature and the physical state of the membranes. This dependence is defined by a nonlinear Arrhenius plot with a break point at 23°C, a temperature not significantly different from the phase transition temperature of the pure lipid (24°C). The transport process is characterized by different activation energies: 35.1 kcal/mol below and 5.5 kcal/mol above the transition temperature. The shift in the break point for the d-glucose transport activity from 15°C, in the brush border membranes, to 23°C in the reconstituted system leads us to conclude that the lipids surrounding the sodium/d-glucose cotransport system can exchange readily with the bulk lipid used for reconstitution. The results thus provide no evidence for the presence of an annulus of specific lipids surrounding the transport system.     

A saturation transfer electron spin resonance study on the break in the Arrhenius plot for the rotational motion of Ca2+-dependent adenosine triphosphatase molecules in purified and lipid-replaced preparations of rabbit skeletal muscle sarcoplasmic reticulum

By means of saturation transfer electron spin resonance spectroscopy the rotational motion of spin-labeled Ca2+-dependent ATPase molecules has been investigated for three kinds of preparations of rabbit skeletal muscle sarcoplasmic reticulum: MacLennan's enzyme (purified ATPase preparation), DOPC- and egg PC-ATPase (purified ATPase preparations in which endogenous lipids are replaced with dioleoyl and egg yolk phosphatidylcholine, respectively). The rotational mobility of the enzyme in these preparations is somewhat lower than that in the intact membrane, probably due to the reduced amount of lipids. For all the preparations, however, the Arrhenius plot for rotational mobility showed a break at about 18 degrees C, the same temperature at which a break in the Arrhenius plot for Ca2+-ATPase activity occurs. This result provides further evidence that the break in the Arrhenius plot is not related to a lipid phase transition but to a change in the physical state of the Ca2+-ATPase molecule existing in fluid lipids.     

Temperature dependence of the energy-linked monosaccharide transport across the cell membrane of Rhodotorula gracilis.

The temperature dependence of the active monosaccharide transport across the cell membrane of the yeast Rhodotorula gracilis has been studied between 0 and 55 degrees C with D-xylose as the transported substrate: (i) Between 0 and 10 degrees C there is virtually no transport. (ii) The initial velocity of transport increases exponentially from 15 to 30 degrees C (deltaE equal to 32 plus or minus 2 kcal/mol). (iii) At 30 degrees C a sharp "break" occurs in the Arrhenius plot and with increasing temperature the transport becomes inactivated, with a positive slope of the corresponding straight line ("deltaE equal to minus 15 kcal/mol"). (iv) In the temperature range of 50-55 degrees C, both the transport and the metabolic activity cease. In order to account for the abrupt changes of the membrane permeability, we attempted to ascribe them to phase transitions in the membrane structure: the first one, between 10 and 15 degrees C, to the crystalline: liquid-crystalline phase change; the second one, around 30 degrees C, to a change from highly ordered (low entropy) to less ordered (high entropy) membrane structure. Whereas the former phase transition is reversible, the latter appears to be irreversible. Arrhenius plots of the cell respiration exhibit a "break" at 30 degrees C, as well. However, at higher temperatures there is no thermal inactivation of the respiratory activity. The importance of a proper organization of the cell membrane constituents for the efficient transport function is discussed.     

Effect of cholesterol and temperature perturbations on membrane hydrolases and transport of calcium and glucose in guinea pig brush border membrane vesicles

The function of membrane cholesterol (chol) in the regulation of membrane-bound hydrolases and transport proteins has been investigated in chol-enriched membranes of guinea pig intestinal brush borders. Chol-enrichment is accomplished by non-invasive means i.e., dietary manipulation by high-chol diet feeding. Activities of sucrase, lactase and maltase enzyme systems, Na+-dependent and -independent glucose transport and calcium uptake are found to be greatly inhibited by chol both at 22 degrees C and 37 degrees C. Glucose and calcium uptake in native membranes are found to be temperature sensitive processes and produce nonlinear Arrhenius plots with a transition temperature around 22 degrees C. The discontinuity in the Arrhenius expression is lost in chol enriched membranes which is interpreted as the increase in microviscosity imparted by chol in the bulk lipid phase environment where these proteins operate.     

Physicochemical mechanisms of organic acid transport in the apical membrane vesicles of the cells of proximal kidney tubules in rats. II. Kinetic parameters of transport and phase state of the lipid bilayer in mutant Campbell strain rats

The kinetics of 14C-para-aminohippuric acid (PAH) transport in the vesicles and the influence of the temperature on the initial rate of this transport were studied using a purified fraction of the apical membrane isolated from the kidney cortex of the Campbell strain rats with an autosomic recessive gene. The transport was brought about owing to the facilitate diffusion mechanism. At 36 degrees C the apparent Michaelis constant was equal to 29 mM, the maximum rate--62 nmol/min on 1 mg of protein, the inhibition constant for the PAH-transport by probenecid--1.5 mM. The temperature dependence of the initial rate of PAH-transport in vesicles and that of the rate of substrate splitting by alkaline phosphatase show the break point on the Arrhenius plot at 36 degrees C-38 degrees C. The analysis of electron magnetic resonance reveals the thermotropic transition at temperatures near 30 degrees-35 degrees C. Therefore the affinity of the carrier to its substrates in vesicles of the Campbell strain rats is strongly reduced and the lipid layer is more viscous than in the normal rats. We decide therefore that the mutation taking place in the Campbell strain leads to pleotropic membrane reconstructions in different organs (eye, kidney). The discovery of such a mutation is of considerable biological interest and promotes bases for development of the membrane biochemical genetics.     

Ascorbic acid-Fe2+ treatment mimics effect of vitamin E deficiency on sarcoplasmic Ca-ATPase of rabbit muscle

After 90 min treatment with ascorbic acid and FeSO4 at 4 degrees C, the activity of rabbit sarcoplasmic reticulum Ca-ATPase was reduced to 22% and the Arrhenius plot of enzyme activity showed an absence of a discontinuity. The presence of vitamin E restored enzyme activity (60%) and the discontinuity in the Arrhenius plot. Ca-ATPase reconstituted with delipidated protein from ascorbic acid-Fe-treated preparation and normal lipid exhibited properties similar to the intact treated enzyme, whereas that reconstituted with delipidated normal protein and lipid from treated preparation exhibited reduced activity but retained the Arrhenius discontinuity. These properties are similar to those observed for sarcoplasmic reticulum Ca-ATPase from the vitamin E-deficient muscular dystrophic rabbit.     

Order-disorder phase transition and lipid dynamics in rabbit small intestinal brush border membranes. Effect of proteins

The total lipids extracted from brush border membranes form smectic lamellar phases when dispersed in water. 31P broad-band nuclear magnetic resonance (NMR) shows that between body temperature (37 degrees C) and freezing of the solvent, the extracted lipids form bilayers with the lipid molecules undergoing fast anisotropic motion. This is also true for the lipids present in the brush border membrane. The electron spin resonance (ESR) results obtained with various hydrophobic spin probes incorporated in either brush border vesicle membranes or their extracted lipids are consistent with this interpretation. By use of a variety of chemically different spin-labels, the temperature dependence of brush border membranes and their extracted lipids was probed. The temperature dependence of various ESR spectral parameters shows discontinuities that, by comparison with differential scanning calorimetry, are assigned to a lipid thermotropic phase transition. Differential scanning calorimetry shows that the lipid in brush border membranes undergoes a broad, reversible phase transition of low enthalpy between 10 and 30 degrees C, with a peak temperature of about 25 degrees C. Hence, the brush border membrane of rabbit small intestine functions in the liquid-crystalline state, well above the peak temperature and also above the upper limit of the lipid phase transition. Therefore, in itself, the thermotropic lipid phase transition is unlikely to play a physiological role. The low enthalpy of the lipid phase transition, indicative of a lack of cooperativity, is primarily attributed to the relatively high cholesterol content and to heterogeneity in the lipid composition of this membrane [Hauser, H., Howell, K., Dawson, R. M. C., & Bowyer, D. E. (1980) Biochim. Biophys. Acta 602, 567-577].(ABSTRACT TRUNCATED AT 250 WORDS)