EPR Studies of the Motional Characteristics of the Phospholipid in Functional Reconstituted Sarcoplasmic Reticulum Membrane Vesicles

Enhanced toxicity and cellular binding of a modified amyloid beta peptide with a methionine to valine substitution

The amyloid beta peptide (Abeta) is toxic to neuronal cells, and it is probable that this toxicity is responsible for the progressive cognitive decline associated with Alzheimer's disease. However, the nature of the toxic Abeta species and its precise mechanism of action remain to be determined. It has been reported that the methionine residue at position 35 has a pivotal role to play in the toxicity of Abeta. We examined the effect of mutating the methionine to valine in Abeta42 (AbetaM35V). The neurotoxic activity of AbetaM35V on primary mouse neuronal cortical cells was enhanced, and this diminished cell viability occurred at an accelerated rate compared with Abeta42. AbetaM35V binds Cu2+ and produces similar amounts of H2O2 as Abeta42 in vitro, and the neurotoxic activity was attenuated by the H2O2 scavenger catalase. The increased toxicity of AbetaM35V was associated with increased binding of this mutated peptide to cortical cells. The M35V mutation altered the interaction between Abeta and copper in a lipid environment as shown by EPR analysis, which indicated that the valine substitution made the peptide less rigid in the bilayer region with a resulting higher affinity for the bilayer. Circular dichroism spectroscopy showed that both Abeta42 and AbetaM35V displayed a mixture of alpha-helical and beta-sheet conformations. These findings provide further evidence that the toxicity of Abeta is regulated by binding to neuronal cells.

Metal ions, pH, and cholesterol regulate the interactions of Alzheimer's disease amyloid-beta peptide with membrane lipid

The interaction of A beta peptides with the lipid matrix of neuronal cell membranes plays an important role in the pathogenesis of Alzheimer's disease. By using EPR and CD spectroscopy, we found that in the presence of Cu(2+) or Zn(2+), pH, cholesterol, and the length of the peptide chain influenced the interaction of these peptides with lipid bilayers. In the presence of Zn(2+), A beta 40 and A beta 42 both inserted into the bilayer over the pH range 5.5-7.5, as did A beta 42 in the presence of Cu(2+). However, A beta 40 only penetrated the lipid bilayer in the presence of Cu(2+) at pH 5.5-6.5; at higher pH there was a change in the Cu(2+) coordination sphere that inhibited membrane insertion. In the absence of the metals, insertion of both peptides only occurred at pH < 5.5. Raising cholesterol to 0.2 mol fraction of the total lipid inhibited insertion of both peptides under all conditions investigated. Membrane insertion was accompanied by the formation of alpha-helical structures. The nature of these structures was the same irrespective of the conditions used, indicating a single low energy structure for A beta in membranes. Peptides that did not insert into the membrane formed beta-sheet structures on the surface of the lipid.

Alzheimer's disease amyloid-beta binds copper and zinc to generate an allosterically ordered membrane-penetrating structure containing superoxide dismutase-like subunits

Amyloid beta peptide (Abeta) is the major constituent of extracellular plaques and perivascular amyloid deposits, the pathognomonic neuropathological lesions of Alzheimer's disease. Cu(2+) and Zn(2+) bind Abeta, inducing aggregation and giving rise to reactive oxygen species. These reactions may play a deleterious role in the disease state, because high concentrations of iron, copper, and zinc have been located in amyloid in diseased brains. Here we show that coordination of metal ions to Abeta is the same in both aqueous solution and lipid environments, with His(6), His(13), and His(14) all involved. At Cu(2+)/peptide molar ratios >0.3, Abeta coordinated a second Cu(2+) atom in a highly cooperative manner. This effect was abolished if the histidine residues were methylated at N(epsilon)2, indicating the presence of bridging histidine residues, as found in the active site of superoxide dismutase. Addition of Cu(2+) or Zn(2+) to Abeta in a negatively charged lipid environment caused a conformational change from beta-sheet to alpha-helix, accompanied by peptide oligomerization and membrane penetration. These results suggest that metal binding to Abeta generated an allosterically ordered membrane-penetrating oligomer linked by superoxide dismutase-like bridging histidine residues.

Effects of melittin on lipid-protein interactions in sarcoplasmic reticulum membranes

To investigate the physical mechanism by which melittin inhibits Ca-adenosine triphosphatase (ATPase) activity in sarcoplasmic reticulum (SR) membranes, we have used electron paramagnetic resonance spectroscopy to probe the effect of melittin on lipid-protein interactions in SR. Previous studies have shown that melittin substantially restricts the rotational mobility of the Ca-ATPase but only slightly decreases the average lipid hydrocarbon chain fluidity in SR. Therefore, in the present study, we ask whether melittin has a preferential effect on Ca-ATPase boundary lipids, i.e., the annular shell of motionally restricted lipid that surrounds the protein. Paramagnetic derivatives of stearic acid and phosphatidylcholine, spin-labeled at C-14, were incorporated into SR membranes. The electronic paramagnetic resonance spectra of these probes contained two components, corresponding to motionally restricted and motionally fluid lipids, that were analyzed by spectral subtraction. The addition of increasing amounts of melittin, to the level of 10 mol melittin/mol Ca-ATPase, progressively increased the fraction of restricted lipids and increased the hyperfine splitting of both components in the composite spectra, indicating that melittin decreases the hydrocarbon chain rotational mobility for both the fluid and restricted populations of lipids. No further effects were observed above a level of 10 mol melittin/mol Ca-ATPase. In the spectra from control and melittin-containing samples, the fraction of restricted lipids decreased significantly with increasing temperature. The effect of melittin was similar to that of decreased temperature, i.e., each spectrum obtained in the presence of melittin (10:1) was nearly identical to the spectrum obtained without melittin at a temperature approximately 5 degrees C lower. The results suggest that the principal effect of melittin on SR membranes is to induce protein aggregation and this in turn, augmented by direct binding of melittin to the lipid, is responsible for the observed decreases in lipid mobility. Protein aggregation is concluded to be the main cause of inactivation of the Ca-ATPase by melittin, with possible modulation also by the decrease in mobility of the boundary layer lipids.

An electron paramagnetic resonance study of skeletal muscle membrane fluidity in malignant hyperthermia

Skeletal muscle sarcolemma (SL), transverse tubule (TT) and heavy sarcoplasmic reticulum (HSR) membranes were isolated from malignant hyperthermia susceptible (MHS) and normal pigs, and the rotational dynamics of lipid hydrocarbon chain motion was examined by electron paramagnetic resonance (EPR) spectroscopy. The stearic acid spin probe 16-SASL was incorporated into MHS and normal membranes and both the order parameter (S) and effective correlation time (tau r) of probe motion were calculated from spectra recorded over the temperature range of 2 to 40 degrees C. At any given temperature, TT membranes exhibited significantly greater values for both the S and tau r of probe motion than did SL, which exhibited significantly greater values than did HSR membranes. The order of decreasing S and tau r values for 16-SASL mobility correlated with the decreasing cholesterol content of these membranes (TT greater than SL greater than HSR), however there was no difference in the S or tau r values for a given membrane fraction isolated from both MHS and normal muscle. Arrhenius plots of 16-SASL mobility in SL, TT and HSR were linear from 2 to 40 degrees C, indicating no abrupt thermotropic change in the lipid hydrocarbon phase of any of the membrane types studied. Apparent activation energies (Ea), calculated from the Arrhenius plots, were similar for MHS and normal membranes derived from a given cellular location. However, the Ea of probe motion for TT membranes (2.3 +/- 0.1 and 2.4 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) was significantly less than for SL (3.4 +/- 0.4 and 2.9 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) which, in turn, was significantly less than the Ea for HSR (3.7 +/- 0.1 and 3.7 +/- 0.1 kcal/mol/degree for MHS and normal, respectively). Since 16-SASL motion was similar in MHS and normal membranes, we conclude that there is no evidence for a generalized membrane defect affecting lipid mobility in these MHS muscle membranes.

Fourier transform infrared spectroscopic identification of gel phase domains in reconstituted phospholipid vesicles containing Ca2+-ATPase

Ca2+-ATPase from rabbit sarcoplasmic reticulum has been isolated, purified, and reconstituted into lipid environments containing as primary components 1,2-dielaidoylphosphatidylcholine (DEPC) and acyl-chain perdeuterated 1,2-dimyristoylphosphatidylcholine (DMPC-d54). Differential scanning calorimetry (DSC) has been used to elucidate the phase behavior of this lipid pair while Fourier transform infrared spectroscopy (FT-IR) has been used to monitor the state of each lipid component in the presence of protein. The lipid mixture shows gel state miscibility over at least most of the composition range, a result in good accord with Van Dijck et al. (Biochim. Biophys. Acta 470, 58-69 (1977)), for the binary mixture with proteated DMPC. Acyl chain perdeuteration thus does not greatly alter the miscibility properties of the lipid pair. Reconstitution of Ca2+-ATPase with this lipid pair proceeds with moderate efficiency. Up to 80% of the endogenous lipid can be replaced depending on the lipid composition. Unusual composition-dependent protein-induced effects on lipid melting properties are noticed. At low levels of DMPC-d54, both the DEPC and DMPC-d54 components have their melting processes broadened and shifted to lower temperatures, compared with binary lipid mixtures of the same composition. This suggests that protein perturbs both lipids in similar fashion. At high levels of DMPC-d54, the DEPC component exhibits a highly cooperative melting process at temperatures close to that for pure DEPC. This strongly indicates that domains of DEPC are present (at least at low temperatures) in the bilayer, and that Ca2+-ATPase is excluded from these domains. The protein thus exhibits preferential interaction with the DMPC-d54 component. This work demonstrates the utility of FT-IR for identification of the molecular origin of particular domains in reasonably complex lipid mixtures. The relevance of this work to native membrane systems where lipid domains have been observed by several groups is discussed.

Role of lipids in sarcoplasmic reticulum: a higher lipid content is required to sustain phosphoenzyme decomposition than phosphoenzyme formation

Enzyme preparations with variable phospholipid contents were obtained by removing lipids from sarcoplasmic reticulum with deoxycholate. Preparations containing from 90 to 37 phospholipids per enzyme showed normal values of both Ca2+-ATPase activity and steady-state phosphoenzyme levels. Fractions containing 37 to 23 phospholipids per enzyme had a reduced ATPase activity but normal phosphoenzyme levels, showing that in this range of lipid content the ATPase reaction is inhibited in a reaction step subsequent to phosphoenzyme formation but prior to phosphoenzyme decomposition. Delipidation below 23 lipids per enzyme caused a marked reduction of the amount of phosphoenzyme formed, so that although both reactions require lipids, fewer lipids are required for phosphoenzyme formation than for decomposition. The effect of lipid removal could be completely reversed by readdition of lipids to fractions containing more than 11 lipids per enzyme. It is proposed that phosphoenzyme formation requires full occupancy of a boundary domain of 23 lipids per enzyme, and that the selective inhibition of phosphoenzyme decomposition at higher lipid contents is caused by a decrease in the rotational mobility of the enzyme.

Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Regardless of the nature of the protein constituents of membranes, the molecular arrangement of lipids interacting with them must satisfy hydrophobic, ionic, and steric requirements. Biological membranes have a great diversity of lipid constituents, and this diversity might have functional roles. It has been proposed, for example, that the hydrophobic regions of membrane proteins are stabilized in the membrane through interactions with lipids able to adopt configurations other than the bilayer structure. Progress in understanding at the molecular level how lipid-protein interactions control the properties of membrane proteins has been hindered by the lack of information concerning the structure of the hydrophobic regions of membrane proteins. Nevertheless, there are many examples in the literature describing how changes in the lipid environment affect physical and biochemical properties of membrane proteins. From these studies, discussed in this review, an overall picture of how lipids and proteins interact in membranes is beginning to emerge.

Lipid phase of transverse tubule membranes from skeletal muscle. An electron paramagnetic resonance study

The lipid phase of transverse tubule membrane was probed with a variety of fatty acid spin labels. The motion of the probe increased as the distance between the spin label and polar head group increased, in agreement with results reported in other membranes. The value of the order parameter at 37 degrees C for a fatty acid spin label containing the label attached to its fifth carbon atom was closer to values reported for bacterial membranes than to the lower values reported for other mammalian membranes. Order parameters for spin labels containing the label nearer to the center of the bilayer were closer to the values reported in other mammalian membranes than to values reported for bacterial membranes. These results indicate that the lipid segments in the vicinity of the polar head group, and less so those near the center of the bilayer, are motionally more restricted in transverse tubules than in other mammalian membranes. In particular, the lipid phase of the transverse tubule membrane is less fluid than that of the sarcoplasmic reticulum membrane. A possible role of the high cholesterol content of transverse tubules in generating the lower fluidity of its lipid phase is discussed.

Exchange rates and numbers of annular lipids for the calcium and magnesium ion dependent adenosinetriphosphatase

A spin-labeled phospholipid is used to study lipid-protein interactions in the (Ca2+,Mg2+)-ATPase of sarcoplasmic reticulum from muscle. A novel null method is used to decompose composite electron spin resonance spectra into two components, characteristic of immobilized and mobile environments. Calculations based on a random mixing model suggest that protein-protein interactions will be relatively rare in these systems and that the immobilized lipid does not represent lipid trapped between proteins but rather represents annular phospholipid at the lipid-protein interface of the adenosinetriphosphatase. The apparent decrease in the amount of immobilized lipid with increasing temperature is shown to be consistent with lipid exchange between bulk and annulus, characterized by an exchange time of 10(-7) s at 37 degrees C. A minimum number of annular phospholipid sites of 32 and 22 are calculated at 0 and 37 degrees C, respectively.

Phase behavior of membranes reconstituted from dipentadecanoylphosphatidylcholine and the Mg2+-dependent, Ca2+-stimulated adenosinetriphosphatase of sarcoplasmic reticulum: evidence for a disrupted lipid domain surrounding protein

A new method was used for reconstituting active sodium deoxycholate solubilized Ca2+-ATPase of rabbit skeletal muscle sarcoplasmic reticulum. Removal of the detergent by dialysis at the pretransition temperature of the pure lipid (22 degrees C) favored the formation of sheet-like structures with a lipid and protein content close to that of the detergent-solubilized sample. Freeze-fracture electron micrographs revealed the Ca2+-ATPase to be organized in rows corresponding to the typical banded pattern seen in low-temperature freeze-fracture micrographs of pure lipid bilayers. Incubation of the sheetlike structures at a temperature (38 degrees C) above the pure lipid main phase transition (33.5 degrees C) caused closure of the sheets into vesicles displaying homogeneous intramembranous particle distributions, at least for membranes containing less than 150 lipids per Ca2+-ATPase. However, in membranes of higher lipid content, free lipid patches were seen both above and below the lipid phase transition. By use of high-sensitivity differential scanning calorimetry, three classes of excess heat capacity peaks were observed in the vesiculated samples. A broadened "free lipid" peak occurred for samples containing between 550 and 200 lipids per protein (Tm = 33.5 degrees C, as for the order-disorder transition in pure lipid vesicles). Between 200 and 150 lipids per Ca2+-ATPase, a broad shoulder became apparent in the range of 29-32 degrees C. Below 150 lipids per Ca2+-ATPase, a peak at 26-28 degrees C became increasingly prominent with lower lipid content. At a lipid to protein ratio of about 30, no peaks in heat capacity were observed. The temperature dependence of diphenylhexatriene fluorescence anisotropy revealed a similar pattern of membrane phase behavior, except that a phase transition was detected at 33.5 degrees C in all membranes studied. On the basis of these observations, we propose that the Ca2+-ATPase is surrounded by a "lipid annulus" of motionally inhibited lipid molecules that do not contribute to a calorimetrically detectable phase transition. Beyond the annulus, "secondary domains" of disrupted lipid packing account for the peak at 26-28 degrees C and the 29-32 degrees C shoulders. At high lipid to protein ratios, the secondary domains coexist with protein-free, lipid-bilayer patches, which account for the peak at 33.5 degrees C.

Fourier transform infrared spectroscopic studies of lipid-protein interaction in native and reconstituted sarcoplasmic reticulum

Fourier transform infrared spectroscopy has been used to monitor lipid-protein interaction and protein secondary structure in native and reconstituted sarcoplasmic reticulum vesicles. Studies of the temperature dependence of the CH2 symmetric stretching frequency reveal no cooperative phase transitions in purified sarcoplasmic reticulum or in vesicles reconstituted with dioleoylphosphatidylcholine, although a continuous introduction of disorder into the lipid acyl chains is observed as the temperature is raised. In addition, temperature-dependent changes are observed in the Amide I and Amide II vibrations arising from protein peptide bonds. A comparison of lipid order in native sarcoplasmic reticulum and its lipid extract showed that the introduction of protein is accompanied by a slight increase in lipid order. Reconstitution of Ca2+-ATPase from sarcoplasmic reticulum with dipalmitoylphosphatidylcholine (lipid/protein ratio 30:1), reveals a perturbed lipid melting event broadened and reduced in midpoint temperature from multilamellar lipid vesicles. The onset of melting (27-28 degrees C) correlates well with the onset of ATPase activity and confirms a suggestion (Hesketh, T.R., Smith, G.A., Houslay, M.D., McGill, K.A., Birdsall, N.J.M., Metcalfe, J.C. and Warren, G.B. (1976) Biochemistry 15, 4145-4151) that a liquid crystalline environment is a requirement for optimal protein function. Finally, Ca2+-ATPase has been reconstituted into binary lipid mixtures of DOPC and acyl-chain perdeuterated DPPC. The effect of protein on the structure and melting behavior of each lipid component was monitored. The protein appears to preferentially interact with the DOPC component.

Competition between cholesterol and phosphatidylcholine for the hydrophobic surface of sarcoplasmic reticulum Ca2+-ATPase

A multiple equilibrium binding model is used to examine phospholipid and cholesterol binding with the transmembranous protein Ca2+-ATPase (calcium pump). The protein was reconstituted in egg phosphatidylcholine bilayers by lipid substitution of rabbit muscle sarcoplasmic reticulum. Electron spin resonance spectra of a phosphatidylcholine spin-label and a recently developed cholesterol spin-label show two major spectral contributions, a motionally restricted component consistent with interactions between the label and the protein surface and another component characteristic of motion of the label in a fluid lipid bilayer. The number of lipid binding (or contact) sites at the hydrophobic surface of the protein is calculated to be N = 22 +/- 2. Experiments with intact sarcoplasmic reticulum membranes give approximately the same value for N. The relative binding constants are Kav approximately 1 for the phosphatidylcholine label and Kav approximately 0.65 for the cholesterol spin-label. Thus, cholesterol does contact the surface of the protein, but with a somewhat lower probability than phosphatidylcholine. This is confirmed by competition experiments where unlabeled cholesterol and the phospholipid spin-label are both present in the bilayer. Evidently the flexible acyl chains of the phospholipid molecules accommodate more readily to the irregular surface of the protein than does the rigid steroid structure of cholesterol.

Dependence of cyclic-AMP induced relaxation on Ca2+ and calmodulin in skinned smooth muscle of guinea pig Taenia coli

cAMP (10(-6) - 10(-4) M) produced a dose-dependent relaxation of Ca2+-induced contraction in the guinea-pig taenia coli skinned with 1% Triton X-100. At 0.53 microM Ca2+ and 0.05 microM calmodulin (CaM), cAMP (10(-4) M) produced a maximal relaxation of 75% (pH 6.7; 25 degrees C). Increasing Ca2+ (0.8 microM) or CaM (0.37 microM) reduced cAMP-induced relaxation to 25 and 5% respectively. At high CaM (5 microM), cAMP-induced relaxation could be completely inhibited by as low as 0.25 microM Ca2+. Furthermore, small increases in Ca2+ or CaM could effectively reverse the cAMP-induced relaxation in the continuous presence of cAMP. These results demonstrate that small modulations in the Ca2+-calmodulin activity have a strong effect on the ability of cAMP to produce a direct relaxing effect on the contractile proteins in skinned fiber. It is suggested that the effects of cAMP on the cellular mechanisms that lower cytoplasmic free Ca2+ concentration may act as the important determinants of the extent of the direct inhibitory effect of cAMP on the contractile elements. These two mechanisms may act in concert in this fashion to effect cAMP-induced relaxation in smooth muscle during beta-adrenergic stimulation.