Spectroscopic studies of specifically deuterium labeled membrane systems. Nuclear magnetic resonance investigation of protein--lipid interactions in Escherichia coli membranes

Characterization of the In Vivo Deuteration of Native Phospholipids by Mass Spectrometry Yields Guidelines for Their Regiospecific Customization

Customization of deuterated biomolecules is vital for many advanced biological experiments including neutron scattering. However, because it is challenging to control the proportion and regiospecificity of deuterium incorporation in live systems, often only two or three synthetic lipids are mixed together to form simplistic model membranes. This limits the applicability and biological accuracy of the results generated with these synthetic membranes. Despite some limited prior examination of deuterating Escherichia coli lipids in vivo, this approach has not been widely implemented. Here, an extensive mass spectrometry-based profiling of E. coli phospholipid deuteration states with several different growth media was performed, and a computational method to describe deuterium distributions with a one-number summary is introduced. The deuteration states of 36 lipid species were quantitatively profiled in 15 different growth conditions, and tandem mass spectrometry was used to reveal deuterium localization. Regressions were employed to enable the prediction of lipid deuteration for untested conditions. Small-angle neutron scattering was performed on select deuterated lipid samples, which validated the deuteration states calculated from the mass spectral data. Based on these experiments, guidelines for the design of specifically deuterated phospholipids are described. This unlocks even greater capabilities from neutron-based techniques, enabling experiments that were formerly impossible.

Bio-membranes: Picosecond to second dynamics and plasticity as deciphered by solid state NMR

Since the first membrane models in the 1970s, the concept of biological membranes has evolved considerably. The membrane is now seen as a very complex mixture whose dynamic behavior is even more complex. Solid-state NMR is well suited for such studies as it can probe the movements of the membrane from picoseconds to seconds. Two NMR observables can be used: motionally averaged spectra and relaxation times. They bring information on order parameters, phase transitions, correlation times, activation energies and membrane elasticity. Spectra are used to determine the nature of the membrane phase. The order parameters can be measured directly from spectra that are dominated by quadrupolar, dipolar and chemical shielding magnetic interactions and allow describing the lipid membrane as being very rigid at the glycerol and chain level and very fluid at its center and surface. Correlation times and activation energies can be measured for intramolecular motions (pico to nanoseconds), molecular motions (nano to 100 ns) and collective modes of membrane deformation (microseconds). Sterols modulate membrane phases, order parameters, correlation times and membrane elasticity. In general terms, sterols tend to act to reduce the impact of environmental changes on molecular order and dynamics. They can be described as regulators of membrane dynamics by keeping them in a state of dynamics that changes very little when the temperature or other factors change. The presence of such large-scale membrane dynamics is proposed as a means of adapting to evolutionary constraints.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Biophysical analysis of the plant-specific GIPC sphingolipids reveals multiple modes of membrane regulation

The plant plasma membrane (PM) is an essential barrier between the cell and the external environment, controlling signal perception and transmission. It consists of an asymmetrical lipid bilayer made up of three different lipid classes: sphingolipids, sterols, and phospholipids. The glycosyl inositol phosphoryl ceramides (GIPCs), representing up to 40% of total sphingolipids, are assumed to be almost exclusively in the outer leaflet of the PM. However, their biological role and properties are poorly defined. In this study, we investigated the role of GIPCs in membrane organization. Because GIPCs are not commercially available, we developed a protocol to extract and isolate GIPC-enriched fractions from eudicots (cauliflower and tobacco) and monocots (leek and rice). Lipidomic analysis confirmed the presence of trihydroxylated long chain bases and 2-hydroxylated very long-chain fatty acids up to 26 carbon atoms. The glycan head groups of the GIPCs from monocots and dicots were analyzed by gas chromatograph-mass spectrometry, revealing different sugar moieties. Multiple biophysics tools, namely Langmuir monolayer, ζ-Potential, light scattering, neutron reflectivity, solid state 2H-NMR, and molecular modeling, were used to investigate the physical properties of the GIPCs, as well as their interaction with free and conjugated phytosterols. We showed that GIPCs increase the thickness and electronegativity of model membranes, interact differentially with the different phytosterols species, and regulate the gel-to-fluid phase transition during temperature variations. These results unveil the multiple roles played by GIPCs in the plant PM.

The cryoprotectant trehalose destabilises the bilayer organisation of Escherichia coli-derived membrane systems at elevated temperatures as determined by 2H and 31P-NMR

In this study, 2H and 31P-NMR techniques were used to study the effects of trehalose and glycerol on phase transitions and lipid acyl chain order of membrane systems derived from cells of E. coli unsaturated fatty acid auxotroph strain K1059, which was grown in the presence of [11,11-2H2]-oleic acid or [11,11-2H2]-elaidic acid. From an analysis of the temperature dependence of the quadrupolar splitting it could be concluded that neither 1 M trehalose or glycerol generally had any significant effect on the temperature of the lamellar gel to liquid-crystalline phase transition. In the case of the oleate-containing hydrated total lipid extract, glycerol but not trehalose caused a 5 degrees C increase of this transition temperature. In general, both cryoprotectants induced an ordering of the acyl chains in the liquid-crystalline state. Trehalose and glycerol both decrease the bilayer to non-bilayer transition temperature of the hydrated lipid extract of oleate-grown cells by about 5 degrees C, but only trehalose in addition induces an isotropic to hexagonal (HII) phase transition. In the biological membranes, trehalose and not glycerol destabilised the lipid bilayer, and in the case of the E. coli spheroplasts, part of the induced non-bilayer structures is ascribed to a hexagonal (HII) phase in analogy with the total lipids. Interestingly, 1 mM Mg2+ was a prerequisite for the destabilisation of the lipid bilayer. In the hydrated total lipid extract of E. coli grown on the more ordered elaidic acid, both transition temperatures were shifted about 20 degrees C upwards compared with the oleate-containing lipid, but the effect of trehalose on the lipid phase behaviour was similar. The bilayer destabilising ability of trehalose might have implications for the possible protection of biological systems by (cryo-)protectants during dehydration, in that protection is unlikely to be caused by preventing the occurrence of polymorphic phase transitions.

Effects of temperature variation and phenethyl alcohol addition on acyl chain order and lipid organization in Escherichia coli derived membrane systems. A 2H- and 31P-NMR study

Using 2H- and 31P-NMR techniques the effects of temperature variation and phenethyl alcohol addition were investigated on lipid acyl chain order and on the macroscopic lipid organization of membrane systems derived from cells of the Escherichia coli fatty acid auxotrophic strain K1059, which was grown in the presence of [11,11-2H2]oleic acid. Membranes of intact cells showed a gel to liquid-crystalline phase transition in the range of 4-20 degrees C, which was similar to that observed for the total lipid extract and for the dominant lipid species phosphatidylethanolamine (PE). Phosphatidylglycerol (PG) remained in a fluid bilayer throughout the whole temperature range (4-70 degrees C). At 30 degrees C acyl chain order was highest in PE, followed by the total lipid extract, PG, intact cells, and isolated inner membrane vesicles. Acyl chain order in E. coli PE and PG was much higher than in the corresponding dioleoylphospholipids. E. coli PE was found to maintain a bilayer organization up to about 60 degrees C, whereas in the total lipid extract as well as in intact E. coli cells bilayer destabilization occurred already at about 42 degrees C. It is proposed that the regulation of temperature at which the bilayer-to-non-bilayer transition occurs may be important for membrane functioning in E. coli. Addition of phenethyl alcohol did not affect the macroscopic lipid organization in E. coli cells or in the total lipid extract, but caused a large reduction in chain order of about 70% at 1 mol% of the alcohol in both membrane systems. It is concluded that while both increasing temperature and addition of phenethyl alcohol can affect membrane integrity, in the former case this is due to the induction of non-bilayer lipid structures, whereas in the latter case this is caused by an increase in membrane fluidity.

Protein heat denaturation and study of membrane lipid-protein interactions by spin label ESR

Spinach chloroplast membranes labelled with stearic acid-spin probe-bearing nitroxyl (label) moiety at 5th, 9th, 12th, 13th, 14th or 16th carbon locations with respect to the carboxylic group of stearic acid were studied (in the dark) by electron spin resonance (ESR) spectroscopy. Spectra were recorded at sample temperatures of 5, 30 and 67 degrees C. After heat denaturation of the membrane proteins for 5 min at 67 degrees C, the spectra were re-recorded at 30 and 5 degrees C for comparison. The results unequivocally show that membrane lipid fatty-acyl chains become substantially more rigid after protein heat-denaturation. The data throw light on the degree of lipid-protein interactions at various microlocations along the length of fatty-acyl chains of the membrane lipid matrix.

Deuterium nuclear magnetic resonance studies on the plasmalogens and the glycerol acetals of plasmalogens of Clostridium butyricum and Clostridium beijerinckii

Deuterium nuclear magnetic resonance was used to investigate the structure of different lipid fractions isolated from the anaerobic bacteria Clostridium butyricum and Clostridium beijerinckii. The fractions isolated from C. butyricum were (1) phosphatidylethanolamine/plasmenylethanolamine and (2) the glycerol acetal of plasmenylethanolamine, and from C. beijerinckii similar fractions containing principally (1) phosphatidyl-N-monomethylethanolamine, along with its plasmalogen, and (2) the glycerol acetal of this plasmalogen were isolated. The third fraction from both species consisted largely of the acidic lipids phosphatidylglycerol and cardiolipin along with plasmalogen forms of these lipids. Palmitic acid with deuterium labels at C-2, C-3, or C-4 or oleic acid with deuterium labels at C-2 and C-9,10 was added to the growth medium and incorporated to various extents in the lipid fractions. Biochemical analysis showed that palmitic acid and oleic acid were preferentially bound to the sn-2 and sn-1 positions, respectively, of the glycerol backbone when both fatty acids were added to the medium. From the 2H NMR spectra, the hydrocarbon chain ordering near the lipid-water interface could be determined and appeared to be similar for all three lipid fractions. The deuterium quadrupole splitting and order parameter were low at the C-2 segment and increased by almost a factor of 2 at positions C-3 and C-4 for cells fed with deuterated palmitic acid along with unlabeled oleic acid. These results agree with previous findings on pure diacyl lipids in which the sn-2 chain was found to adopt a bent conformation at the carbon segment C-2. However, two unusual quadrupole splittings could be detected for the plasmalogens.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of micellar lipids on rabbit intestinal brush-border membrane phospholipid bilayer integrity studied by 31P NMR

The effect of biliary salts and fatty acids on the bilayer structure of rabbit intestinal brush-border membranes was studied using the nonperturbing probe 31P NMR. The broad, asymmetric lineshape of the 31P NMR spectrum of isolated brush-border vesicles demonstrates that their component phospholipids are organized in extended bilayers. These membranes are not significantly perturbed by incubation with physiological concentrations of biliary salts (3, 9, 18 mM), demonstrating that the vesicles are highly stable, corresponding to their biological function. However, the emergence of a narrow peak superimposed on the broad lineshape indicates that a small proportion of the membrane phospholipids has reached isotropic motion, which may correspond to external or internal micellar structures. Incubation with mixed micelles of fatty acids and taurochlorate show that long-chain fatty acids enhance the membrane-perturbing effect of taurocholate while short-chain, water-soluble fatty acids do not, suggesting a difference in the absorption mechanisms.

Dynamic structure of membranes by deuterium NMR

Progress in our understanding of the dynamic structure of membrane lipids and proteins has recently been made possible by the advent of high-field "solid-state" nuclear magnetic resonance spectroscopic studies of specifically deuterium-labeled systems. Major features of lipid and protein dynamics have been deduced.

Surface dynamics of the integral membrane protein bacteriorhodopsin

Recently, there has been great interest in determining the three-dimensional structures of membrane proteins, particularly bacteriorhodopsin, for which a variety of possible folding arrangements have been suggested. In this paper we present nuclear magnetic resonance (NMR) spectra of deuterated bacteriorhodopsin, and use the data to help interpret the various suggested bacteriorhodopsin folding patterns. The results strongly indicate that (1) a membrane surface (+/- 1 residue) may be defined by NMR in bacteriorhodopsin; (2) all amino acids inside the surface are essentially crystalline; (3) all amino acids outside the surface (surface residues) in bacteriorhodopsin are highly mobile on the time scale of the 2H NMR experiments; (4) NMR data may be used to help evaluate the various structural models that have been proposed; (5) aggregation of purple membrane sheets may lead to an immobilization of the surface residues.

Low-frequency motion in membranes. The effect of cholesterol and proteins

Nuclear magnetic resonance (NMR) relaxation techniques have been used to study the effect of lipid-protein interactions on the dynamics of membrane lipids. Proton enhanced (PE) 13C-NMR measurements are reported for the methylene chain resonances in red blood cell membranes and their lipid extracts. For comparison similar measurements have been made of phospholipid dispersions containing cholesterol and the polypeptide gramicidin A+. It is found that the spin-lattice relaxation time in the rotating reference frame (T1 rho) is far more sensitive to protein, gramicidin A+ or cholesterol content than is the laboratory frame relaxation time (T1). Based on this data it is concluded that the addition of the second component to a lipid bilayer produces a low-frequency motion in the region of 10(5) to 10(7) Hz within the membrane lipid. The T1 rho for the superimposed resonance peaks derived from all parts of the phospholipid chain are all influenced in the same manner suggesting that the low frequency motion involves collective movements of large segments of the hydrocarbon chain. Because of the molecular co-operativity implied in this type of motion and the greater sensitivity of T1 rho to the effects of lipid-protein interactions generally, it is proposed that these low-frequency perturbations are felt at a greater distance from the protein than those at higher frequencies which dominate T1.

A 2H-NMR study of Acholeplasma laidlawii membranes highly enriched in myristic acid

Myristic acid specifically deuterated at several positions along the acyl chain was biosynthetically incorporated into the membrane lipids of Acholeplasma laidlawii B to the level of greater than or equal to 90%. 2H-NMR was used to study the molecular order and lipid phase composition of the membranes as a function of temperature. Isolated membranes and intact cells give rise to similar 2H spectra. Below 25 degrees C the spectra exhibit a broad gel phase component which at 0 degrees C reaches the rigid limit value expected for an immobilized methylene group. Spectral moments were used to determine the relative amounts of gel and liquid crystalline phase lipids throughout the gel-liquid crystal phase transition. The results indicate that at the growth temperature (37 or 30 degrees C) the A. laidlawii B membrane lipids are approximately 85-90% in the gel state, and that protein has little effect on lipid order of the liquid crystalline lipid, but leads to an increase in the linewidth by approx. 20%.

Human erythrocyte membranes are fluid down to -5 degrees C

This first observation of the deuterium nuclear magnetic resonance (2H-NMR) spectrum of phospholipid molecules incorporated into intact human erythrocyte ghosts shows that the liquid crystalline phase is stable down to a temperature of -5 degrees C. The quality of the 3H-NMR spectra indicate that it is now possible to carry out clinical studies of erythrocyte membranes using the techniques employed in this study.

31P nuclear magnetic resonance studies of the phospholipid-protein interface in cell membranes

Both native and recombined membrane systems from the human erythrocyte membrane and the rabbit sarcoplasmic reticulum have been studied with 31P Nuclear Magnetic Resonance (NMR). We compare intensities of the anisotropic 31P resonance exhibited by these membranes with the intensity expected from the known phospholipid content of the membranous sample. In a recombinant with human erythrocyte glycophorin, a component of the phospholipid is "missing" from the 31P NMR resonance, apparently due to a severe broadening of the resonance of that component. Approximately 29 phospholipid molecules were found immobilized per glycophorin molecule in the membrane, regardless of the phospholipid:protein ratio. Cholesterol may inhibit the immobilization of phospholipids by glycophorin. Recombinants with band three from the human erythrocyte membrane contain an immobilized phospholipid component, analogous to the results with glycophorin. 31P NMR data from the native sarcoplasmic reticulum membrane also revealed an immobilized phospholipid component whose magnitude is independent of temperature between 30 degrees C and 45 degrees C. Extensive papain proteolysis of the membrane completely digests the Ca++ Mg++ ATPase and removes the immobilization of phospholipids noted in the intact membrane. Limited trypsin cleavage, however, does not completely remove the immobilized component; salt reduces the immobilized component.

Raman and Fourier transform infrared spectroscopic studies of the interaction between glycophorin and dimyristoylphosphatidylcholine

Glycophorin from the human erythrocyte membrane has been isolated in pure form and reconstituted into large unilamellar vesicles with 1,2-dimyristoyl-3-sn-phosphatidylcholine at lipid/protein mole ratios ranging from 50:1 to 200:1. The effect of protein on the phospholipid phase transition has been monitored by Raman and Fourier transform infrared spectroscopy and differential scanning calorimetry. No evidence for an immobilized higher melting lipid component is observed. The gel to liquid-crystalline phas transition is significantly broadened and shifted to lower temperatures as the proportion of protein is increased, while the pretransition is abolished. At all temperatures, the mobility of the acyl chains is increased by the addition of protein while interchain lateral interactions are disrupted. However, there is no evidence for a significant change in the conformational order at low temperatures (approximately 5 degrees C) or ii the liquid-crystalline phase.

Analysis of the composition of mixed lipid phases by the moments of 2H NMR spectra

The use of (2)H NMR spectral moments to determine the composition of biphasic lipid mixtures is outlined. The analysis has been applied to phosphatidylethanolamine-cholesterol (1:1), potassium palmitate, 30% (wt/wt) water and phosphatidylcholine-cholesterol (4:1) systems, as well as to membrances of Escherichia coli during phase transitions. The advantages and disadvantages of the use of spectral moments to determine fractions of coexistent phases are discussed.

Dynamical and temperature-dependent effects of lipid-protein interactions. Application of deuterium nuclear magnetic resonance and electron paramagnetic resonance spectroscopy to the same reconstitutions of cytochrome c oxidase

2H NMR and EPR spectra have been obtained as a function of temperature and protein concentration from the same samples of beef heart cytochrome c oxidase reconstituted into 1-(16,16,16-trideuteriopalmitoyl)-2-palmitoleoyl-sn-glycero-3-phosphocholine. At all temperatures, the EPR spectra show the characteristic "bound" and "free" components, while the 2H NMR spectra show only a narrow distribution of orientational order parameters. At temperatures near the phase transition of the pure lipid, the dependence of the 2H NMR average orientational order on protein concentration fits a two-stage model in which the phospholipid molecular exchange rapidly between two states tentatively identified as sites either on or off the protein surface. From this model, the 2H NMR spectra yield a value of 0.18 mg of phospholipid per mg of protein as necessary to cover the surface of cytochrome c oxidase, which is the same value as derived from the EPR spectra at -20 degrees C. Both the 2H NMR and EPR spectra vary markedly with temperature. At temperatures well above the phase transition of the pure lipid, the average orientational parameters derived from the 2H NMR spectra are independent of protein concentration and are the same as for the lipid alone. Qualitatively, the EPR spectra show large apparent decreases in the average orientational order with increasing temperature. Analysis of 2H NMR relaxation rates indicates an additional motion in the presence of protein with a correlation time of 10(-6)-10(-7) s. If this new motion is associated with exchange between the two states, a minimum value of 10(6)-10(7) s-1 for the exchange rate is obtained, assuming that the lipids on the protein surface are much more motionally restricted than the rest of the lipid. Such an exchange rate is compatible with the observed differences in 2H NMR and EPR spectra. These results are consistent with short-lived, energetically weak interactions between cytochrome c oxidase and the phospholipids used in this study.

Structure of Escherichia coli membranes. Glycerol auxotrophs as a tool for the analysis of the phospholipid head-group region by deuterium magentic resonance

Glycerol selectively deuterated at various positions was synthesized and supplied to the growth medium of Escherichia coli strain T131 GP, which is defective in endogenous glycerol synthesis as well as glycerol degradation and lacks the ability to synthesize cardiolipin. The procedure enables the stereospecific labeling of the membrane phospholipids (approximately 80% phosphatidylethanolamine, approximately 20% phosphatdylglycerol). Deuterium magnetic resonance spectra were obtained for cell membranes and lipid dispersions either from total lipid extractions or from purified phosphitidylglycerol or -ethanolamine. When glycerol deuterated at various positions was used, all resonances of the phospholipid glycerol backbone and the terminal glycerol moiety in phosphatidylglycerol could be assigned. The results indicate that the molecular conformation of the glycerol backbone is independent of the phospholipid species investigated and is also not altered by the presence of high amounts of membrane proteins. For the quantitative interpretation of the deuterium magnetic resonance splittings, a model is proposed which assumes essentially free rotation around the glycerol C(2)-C(3) bond combined with an asymmetric and restricted jump process around the C(1)-C(2) bond. This model is compatible with known X-ray structures of phospholipids molecules. The two deuterons of both the glycerol backbone C(1) and C(3) segments were found to be magnetically inequivalent. Stereoselective monodeuteration eliminated one set of quadrupole splittings in both cases.

Orientational order of unsaturated lipids in the membranes of Acholeplasma laidlawii as observed by 2H-NMR

Oleic acid specifically deuterated at fifteen different positions along the chain, including the double bond, was biosynthetically incorporated into the membrane lipids of the microorganism Acholeplasma laidlawii B. A detailed study of the dynamic conformation of these chains was carried out using deuterium nuclear magnetic resonance. The deuterium spectra fourteen different samples were recorded as a function of temperature over the range 0-41 degrees C. Spectra were obtained down to -52 degrees C for the sample enriched with oleic acid deuterated at the C-12' position. Above 20 degrees C, where the lipids are in the liquid crystal phase, a single quadrupolar powder pattern was observed for each C2H2 segment, except for the C-2' position which gave rise to a three-component spectrum characteristic for this position in both model and biological membranes. Simulation of this spectrum indicates that there are two conformations of the lipid molecule in the region of the C-2' segment of the sn-2 chain. The orientationa fluctuations of the fatty acid chain segments in the A. laidlawii membranes are described by the deuterium order parameters, and a striking similarity is shown to exist between the oleate chain conformation of the A. laidlawii membrane and a phospholipid model membrane. Remarkable similarities are also demonstrated in the A. laidlawii membrane enriched in palmitic and oleic fatty acids when the order parameter profiles are plotted at the same reduced temperature. Below 15 degrees C a second component, due to gel phase lipid, starts to appear in the spectra. This broad gel phase component grows at the expense of the liquid crystal phase component as the temperature is reduced. The spectra indicate that the center of the phase transition is at about -12 degrees C, in good agreement with DSC studies.

Discontinuous thermotropic response of Tetrahymena membrane lipids correlated with specific lipid compositional changes

Steady-state fluorescence polarization measurements of 1,6-diphenyl-1,3,5-hexatriene in microsomal lipids from Tetrahymena pyriformis cells grown at 39 or 15 degrees C revealed discrete slope discontinuities in plots of polarization vs. temperature. Two well-defined 'break points' were present in the 0-40 degrees C temperature range examined and their precise location was dependent upon the growth temperature of the cells. By mixing phospholipids from cells grown at different temperatures, the break points at 17.5 and 32 degrees C in 39 degrees C-lipid multilayer preparations were shown to correlate with the breaks at 12 and 27 degrees C, respectively, in similar preparations from 15 degrees C-grown cells. The discrete break points were also present, but at slightly different characteristic temperatures, in a phosphatidylcholine fraction and a phosphatidylethanolamine plus 2-amino-ethylphosphonolipid fraction purified from the phospholipids and in total microsomal lipids (phospholipids plus the sterol-like triterpenoid, tetrahymanol). However, catalytic hydrogenation of the phospholipid fatty acids or mixing the non-hydrogenated phospholipids with increasing proportions or synthetic dipalmitoyl phosphatidylcholine eliminated the break points. We interpret this discontinuous thermotropic response in microsomal lipids as signalling a lipid phase separation of importance in regulating physiological events.

Lipid conformation in model membranes and biological membranes

Protein molecules in solution or in protein crystals are characterized by rather well-defined structures in which α-helical regions, β-pleated sheets, etc., are the key features. Likewise, the double helix of nucleic acids has almost become the trademark of molecular biology as such. By contrast, the structural analysis of lipids has progressed at a relatively slow pace. The early X-ray diffraction studies by V. Luzzati and others firmly established the fact that the lipids in biological membranes are predominantly organized in bilayer structures (Luzzati, 1968). V. Luzzati was also the first to emphasize the liquid-like conformation of the hydrocarbon chains, similar to that of a liquid paraffin, yet with the average orientation of the chains perpendicular to the lipid–water interface. This liquid–crystalline bilayer is generally observed in lipid–water systems at sufficiently high temperature and water content, as well as in intact biological membranes under physiological conditions (Luzzati & Husson, 1962; Luzzati, 1968; Tardieu, Luzzati & Reman, 1973; Engelman, 1971; Shipley, 1973). In combination with thermodynamic and other spectroscopic observations these investigations culminated in the formulation of the fluid mosaic model of biological membranes (cf. Singer, 1971). However, within the limits of this model the exact nature of lipid conformation and dynamics was immaterial, the lipids were simply pictured as circles with two squiggly lines representing the polar head group and the fatty acyl chains, respectively. No attempt was made to incorporate the well-established chemical structure into this picture. Similarly, membrane proteins were visualized as smooth rotational ellipsoids disregarding the possibility that protruding amino acid side-chains and irregularities of the backbone folding may create a rather rugged protein surface.

State modifications of thymocyte plasma membrane proteins and lipids by mitogenic doses of concanavalin A: a Raman study on isolated membrane vesicles

Sealed plasma membrane vesicles from rabbit thymocytes were reacted with 0.4-10 micrograms concanavalin A/ml, that is at concentrations that produce cooperative lectin-binding in vivo and in vitro and induce mitogenesis of intact cells. The effects of concanavalin A were monitored by laser Raman spectroscopy of the vesicles in the CH-stretching region. This technique revealed moderately cooperative lipid state transitions in untreated membranes centered at about -6 degrees and 25 degrees, as well as a protein state change at about 43 degrees C. Concanavalin A treatment of the membranes lowered the transition temperatures of the integral of 25 degrees an integral of 43 degrees state changes indicating a direct effect of lectin binding on membrane protein/lipid organization. It is proposed that the primary protein involved is the 55,000D transmembrane protein (Schmidt-Ullrich, R., Mikkelsen, R. B. and Wallach, D. F. H. (1978), J. Biol. Chem. 253, 6973-6978), known to be the high-affinity receptor for concanavalin A, and that the concanavalin A-sensitive integral of 25 degrees transition arises from lipids associated with this protein.