Anisotropic 2H-nuclear magnetic resonance spin-lattice relaxation in cerebroside- and phospholipid-cholesterol bilayer membranes

Monitoring apoptosis in intact cells by high-resolution magic angle spinning 1 H NMR spectroscopy

Apoptosis maintains an equilibrium between cell proliferation and cell death. Many diseases, including cancer, develop because of defects in apoptosis. A known metabolic marker of apoptosis is a notable increase in ¹ H NMR-observable resonances associated with lipids stored in lipid droplets. However, standard one-dimensional NMR experiments allow the quantification of lipid concentration only, without providing information about physical characteristics such as the size of lipid droplets, viscosity of the cytosol, or cytoskeletal rigidity. This additional information can improve monitoring of apoptosis-based cancer treatments in intact cells and provide us with mechanistic insight into why these changes occur. In this paper, we use high-resolution magic angle spinning (HRMAS) ¹ H NMR spectroscopy to monitor lipid concentrations and apparent diffusion coefficients of mobile lipid in intact cells treated with the apoptotic agents cisplatin or etoposide. We also use solution-state NMR spectroscopy to study changes in lipid profiles of organic solvent cell extracts. Both NMR techniques show an increase in the concentration of lipids but the relative changes are 10 times larger by HRMAS ¹ H NMR spectroscopy. Moreover, the apparent diffusion rates of lipids in apoptotic cells measured by HRMAS ¹ H NMR spectroscopy decrease significantly as compared with control cells. Slower diffusion rates of mobile lipids in apoptotic cells correlate well with the formation of larger lipid droplets as observed by microscopy. We also compared the mean lipid droplet displacement values calculated from the two methods. Both methods showed shorter displacements of lipid droplets in apoptotic cells. Our results demonstrate that the NMR-based diffusion experiments on intact cells discriminate between control and apoptotic cells. Apparent diffusion measurements in conjunction with ¹ H NMR spectroscopy-derived lipid signals provide a novel means of following apoptosis in intact cells. This method could have potential application in enhancing drug discovery by monitoring drug treatments in vitro, particularly for agents that cause portioning of lipids such as apoptosis.

Broadband adiabatic inversion experiments for the measurement of longitudinal relaxation time constants

Accurate measurements of longitudinal relaxation time constants (T₁) in solid-state nuclear magnetic resonance (SSNMR) experiments are important for the study of molecular-level structure and dynamics. Such measurements are often made under magic-angle spinning conditions; however, there are numerous instances where they must be made on stationary samples, which often give rise to broad powder patterns arising from large anisotropic NMR interactions. In this work, we explore the use of wideband uniform-rate smooth-truncation pulses for the measurement of T₁ constants. Two experiments are introduced: (i) BRAIN-CPT1, a modification of the BRAIN-CP (BRoadband Adiabatic-INversion-Cross Polarization) sequence, for broadband CP-based T₁ measurements and (ii) WCPMG-IR, a modification of the WURST-CPMG sequence, for direct-excitation (DE) inversion-recovery experiments. A series of T₁ constants are measured for spin-1/2 and quadrupolar nuclei with broad powder patterns, such as ¹¹⁹Sn (I = 1/2), ³⁵Cl (I = 3/2), ²H (I = 1), and ¹⁹⁵Pt (I = 1/2). High signal-to-noise spectra with uniform patterns can be obtained due to signal enhancements from T₂ ^eff-weighted echo trains, and in favorable cases, BRAIN-CPT1 allows for the rapid measurement of T₁ in comparison to DE experiments. Protocols for spectral acquisition, processing, and analysis of relaxation data are discussed. In most cases, relaxation behavior can be modeled with either monoexponential or biexponential functions based upon measurements of integrated powder pattern intensity; however, it is also demonstrated that one must interpret such T₁ values with caution, as demonstrated by measurements of T₁ anisotropy in ¹¹⁹Sn, ²H, and ¹⁹⁵Pt NMR spectra.

Solid-State NMR of highly 13C-enriched cholesterol in lipid bilayers

Cholesterol (Chol) is vital for cell function as it is essential to a myriad of biochemical and biophysical processes. The atomistic details of Chol's interactions with phospholipids and proteins is therefore of fundamental interest, and NMR offers unique opportunities to interrogate these properties at high resolution. Towards this end, here we describe approaches for examining the structure and dynamics of Chol in lipid bilayers using high levels of ¹³C enrichment in combination with magic-angle spinning (MAS) methods. We quantify the incorporation levels and demonstrate high sensitivity and resolution in 2D ¹³C-¹³C and ¹H-¹³C spectra, enabling de novo assignments and site-resolved order parameter measurements obtained in a fraction of the time required for experiments with natural abundance sterols. We envision many potential future applications of these methods to study sterol interactions with drugs, lipids and proteins.

High-resolution magic angle spinning (1)H magnetic resonance spectroscopy detects choline as a biomarker in a swine obstructive chronic pancreatitis model at an early stage

Chronic pancreatitis (CP) is a progressive inflammatory and fibrotic disease of the pancreas which encompasses a variety of clinical syndromes ranging from mild to life-threatening complications. Metabolomics has increasingly been applied to identify biomarkers for disease diagnosis with particular interest in diseases at an early stage. In this study, we tested a swine obstructive CP model by subtotal ligation of the main pancreatic duct, and the metabolic profiles of the Bama miniature swine pancreas were investigated using high-resolution magic angle spinning proton magnetic resonance spectroscopy (HR MAS (1)H MRS) combined with principal components analysis (PCA). Increases in lactate and choline for mild CP and decreases in glycerophosphocholine, phosphocholine, betaine and glycine were observed from normal pancreas to mild, moderate and severe CP. PCA results showed visual separations among the groups. The increase of choline at an early stage of CP and the decrease of glycerophosphocholine, phosphocholine, betaine and glycine reveal the pathogenesis of CP at a molecular level. The MRS results presented here demonstrate the potential of metabolic profiles in discriminating a normal pancreas from different stages of CP, which may be used to achieve CP early diagnosis and timely intervention to prevent irreversible destruction of the pancreas.

Study ofTrypanosoma cruziepimastigote cell death by NMR-visible mobile lipid analysis

Cell death mechanisms inTrypanosoma cruzihave not been disclosed in detail though different conventional techniques have been used in the classification of parasite-cell death type. Nuclear magnetic resonance (NMR) has successfully been used as a tool to evaluate the onset of apoptosis in a number of higher eukaryote-cell models analysing the ratio of CH2/CH3integration from the visible mobile lipids (VML). Surprisingly, this versatile non-invasive spectroscopy technique has never been employed with this purpose inT. cruzi. In the present study it is shown that under different parasite death-conditions the ratio CH2/CH3varied drastically. Thus,T. cruziepimastigotes in apoptotic conditions increase significantly this ratio while in necrotic as well as in autophagic situations the parasites maintain the VML, CH2/CH3ratio, in normal values. Additionally, other VML markers commonly used in these studies, such as the change in the region of methyl-choline moiety, -N+(CH3)3, exhibited different particular patterns according to the type of cell death. Our results suggest that the1H NMR-VML technique is an adequate tool to discriminate differentT. cruzideath pathways.

Synthesis, purification, and characterization of single helix membrane peptides and proteins for NMR spectroscopy

Membrane proteins function as receptors, channels, transporters, and enzymes. These proteins are generally difficult to express and purify in a functional form due to the hydrophobic nature of their membrane spanning sequences. Studies on membrane proteins with a single membrane spanning helix have been particularly challenging. Single-pass membrane proteins will often form dimers or higher order oligomers in cell membranes as a result of sequence motifs that mediate specific transmembrane helix interactions. Understanding the structural basis for helix association provides insights into how these proteins function. Nevertheless, nonspecific association or aggregation of hydrophobic membrane spanning sequences can occur when isolated transmembrane domains are reconstituted into membrane bilayers or solubilized into detergent micelles for structural studies by solid-state or solution NMR spectroscopy. Here, we outline the methods used to synthesize, purify, and characterize single transmembrane segments for structural studies. Two synthetic strategies are discussed. The first strategy is to express hydrophobic peptides as protein chimera attached to the maltose binding protein. The second strategy is by direct chemical synthesis. Purification is carried out by several complementary chromatography methods. The peptides are solubilized in detergent for solution NMR studies or reconstituted into model membranes for solid-state NMR studies. We describe the methods used to characterize the reconstitution of these systems prior to NMR structural studies to establish if there is nonspecific aggregation.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Discrimination of metabolic profiles of pancreatic cancer from chronic pancreatitis by high-resolution magic angle spinning 1H nuclear magnetic resonance and principal components analysis

The metabolic profiles of Sprague-Dawley rat pancreases were investigated by high-resolution magic angle spinning proton magnetic resonance spectroscopy ((1)H NMR) combined with principal components analysis (PCA) to discriminate pancreatic cancer from chronic pancreatitis. Intact pancreatic tissue samples were obtained from Sprague-Dawley rats with histologically proven pancreatic cancer (n = 5), chronic pancreatitis (n = 5), and two matched controls (n = 5 per group). Two (1)H NMR experiments, single-pulse and Carr-Purcell-Meiboom-Gill, were carried out separately. Increases in phosphocholine and glycerophosphocholine levels and decreases in leucine, isoleucine, valine, lactate and alanine levels were observed in chronic pancreatitis, whereas the opposite trends were observed in pancreatic cancer. Increasing taurine and decreasing betaine were found both in chronic pancreatitis and in pancreatic cancer. Additionally, the lipid content in pancreatic cancer was higher than that in chronic pancreatitis. PCA was carried out for the single-pulse and Carr-Purcell-Meiboom-Gill (1)H NMR spectra, respectively, to visualize separation among the samples and to extract characteristic metabolites of pancreatic cancer and chronic pancreatitis. Decreased phosphocholine and glycerophosphocholine were suggested as unique metabolite indicators of pancreatic cancer. Furthermore, even with the disturbance of various quantities of lipid contents pancreatic cancer and chronic pancreatitis could be differentiated well by the combination of high-resolution magic angle spinning (1)H NMR and PCA. Thus this combination was demonstrated to have the potential to improve magnetic resonance spectroscopy for positive early diagnosis of pancreatic cancer in clinical settings.

Molecular dynamics of amide ions in potassium amide (KNH2) studied with orientation-dependent deuterium spin lattice relaxation

The reorientational molecular dynamics of the amide ions were investigated in three different phases of KND2 by means of 2H NMR line-shape analyses of solid-echo, T1Z as well as T1Q distorted spectra in a temperature range of 80-420 K. The correlation times of the amide dynamics cover roughly eight decades in this temperature range. Due to the nonzero asymmetry parameter (eta approximately 0.2) of the electric field gradient tensor the calculation of the orientation-dependent spectral densities Jm(theta, phi) required for the interpretation of the T1Z and T1Q distorted spectra cannot be simplified as in the case eta = 0 and a numerical approach was used for the calculation of Jm(theta, phi), which allows a maximum flexibility for simulating different models of motion. The amide ion dynamics in the low-temperature phase can be described as a superposition of a thermally activated large angle jump of the amide ions about their two-fold axes in an asymmetric four-well potential and strongly anisotropic molecular librations. The asymmetry of the potential surface of the jump process was found to be a function of temperature. Activation energy EA, attempt frequency tau0(-1) and DND bond angle epsilon were determined to 15.5(2) kJ/mol, 62(6) x 10(12) s(-1) and 104.7(3) degrees. In the middle- and high-temperature phases the amide ions perform 90 degrees jumps about the crystallographic four-fold axes. For the high-temperature modification the correlation times were observed to follow an Arrhenius law with EA = 6.3(2) kJ/mol and tau0(-1) = 32(3) x 10(12) s(-1).

Metabolite changes in BT4C rat gliomas undergoing ganciclovir-thymidine kinase gene therapy-induced programmed cell death as studied by 1H NMR spectroscopy in vivo, ex vivo, and in vitro

Programmed cell death was induced by HSV-tk gene therapy in rat BT4C glioma cells, and metabolite changes associated with cell damage were monitored in vivo by 1H NMR spectroscopy and ex vivo by high resolution magic angle spinning (HRMAS) 1H NMR, and in vitro in perchloric acid extracts of tumors. Metabolite concentrations, as quantified in vivo using water as an internal reference and in vitro in extracts, were correlated with cell density. The results showed that both in vivo and in vitro glycine and creatine concentrations followed volume-averaged cell density, whereas that of total choline-containing compounds was unaffected by a cell loss approaching 60%. Meanwhile, both saturated and unsaturated 1H NMR visible lipids increased. HRMAS 1H NMR spectroscopy of the tumor samples at 14.1 tesla demonstrated the presence of nucleotide peaks from adenosine and uridine nucleotides in glioma samples ex vivo. The assignment of a doublet at 7.95 ppm to UDP was confirmed by spiking experiments of tumor extracts in conjunction with 1H and 31P NMR spectroscopy. HRMAS also resolved the choline-containing peak at 3.2 ppm in vivo into resonances from choline (3.20 ppm), phosphocholine (3.22 ppm), glycerophosphocholine (3.24 ppm), and taurine (3.26 ppm). These resonances were uncorrelated with temporal progression through programmed cell death. Our results show that 1H NMR-detected lipids and some of the small molecular weight metabolites respond to gene therapy. However, the choline-containing compounds are unaffected by severe decline in cell density. The latter observation supports the idea that triacylglycerols, rather than membrane phospholipids, are the key components of 1H NMR visible lipids, and it also casts doubt on the validity of resonance of choline-containing compounds as a diagnostic marker of programmed cell death in vivo.

High-resolution diffusion and relaxation-edited magic angle spinning1H NMR spectroscopy of intact liver tissue

High-resolution magic angle spinning (HRMAS) (1)H NMR spectroscopy is ideal for monitoring the metabolic environment within tissues, particularly when spectra are weighted by physical properties such as T(1) and T(2) relaxation times and apparent diffusion coefficients (ADCs). In this study, spectral-editing using T(1) and T(2) relaxation times and ADCs at variable diffusion times was used in conjunction with HRMAS (1)H NMR spectroscopy at 14.1 T in liver tissue. To enhance the sensitivity of ADC measurements to low molecular weight metabolites a T(2) spin echo was included in a standard stimulated gradient spin-echo sequence. Fatty liver induced in rats by chronic orotic acid feeding was investigated using this modified sequence. An increase in the combined ADC for the co-resonant peaks glucose, betaine, and TMAO during fatty liver disease was detected (ADCs = 0.60 +/- 0.11 and 0.35 +/- 0.1 * 10(-9) m(2)s(-1) (n = 3) for rats fed with and without orotic acid), indicative of a reduction in glucose and betaine and an increase in TMAO.

Molecular modelling of membrane sterols with the use of the GROMOS 96 force field

Membrane located sterols determine the structure and function of eucariotic cell membranes. Moreover, they are targets for important antifungal antibiotic amphotericin B. Knowledge about the geometry and dynamics of sterols in the environment of lipidic membranes is necessary to understand their functions. However, due to the dynamic character of the membrane, no experimental data about sterol behaviour on the molecular level is available. Hence molecular modelling simulations could be a source of useful information. The main goal of this paper is to prove the adequacy of the GROMOS 96 force field for molecular simulations of membrane sterols. We focused our attention on the reproduction of characteristic geometrical features observed in the crystal of cholesterol hemiethanolate by molecular dynamics simulations. The results presented clearly indicate that the GROMOS 96 force field can be a useful tool to simulate the highly lipophilic systems. Moreover, interactions responsible for the stability of such systems can also be recognised.

Choline containing metabolites during cell transfection: an insight into magnetic resonance spectroscopy detectable changes

Increases in choline containing metabolites have been associated with a number of disorders, including malignant cell growth. In this study, high resolution magic angle spinning (1)H nuclear magnetic resonance spectroscopy was employed to monitor metabolite changes during cell transfection, and an increase in phosphocholine was detected. This increase appears to be correlated with cell membrane disruption associated with the insertion of plasmid DNA into cells, since the level of phosphocholine in mock transfected cells was comparable to that of control cells. These data suggest choline containing metabolite changes detected in vivo using magnetic resonance spectroscopy relate to cell membrane disruption.

The EGF receptor transmembrane domain: peptide-peptide interactions in fluid bilayer membranes

A peptide containing the transmembrane domain of the human EGF receptor was studied in fluid lipid bilayers for insight into receptor tyrosine kinase lateral associations in cell membranes. The peptide comprised the 23-amino acid hydrophobic segment thought to span the membrane (Ile(622) to Met(644) of the EGF receptor), plus the first 10 amino acids of the receptor's cytoplasmic domain (Arg(645) to Thr(654)). Probes for solid-state NMR spectroscopy were incorporated by deuteration of the methyl side chains of alanine at positions 623 and 637. (2)H-NMR spectra were recorded from 25 to 65 degrees C in membranes composed of 1-palmitoyl-2-oleoyl phosphatidylcholine, with and without 33% cholesterol, and relaxation times were measured. Peptide concentration ranged from 0. 5 to 10 mol %. The peptide behaved as predominant monomers undergoing rapid symmetric rotational diffusion; however, there was evidence of reversible side-to-side interaction among the hydrophobic transmembrane domains, particularly at physiological temperatures and in the presence of natural concentrations of cholesterol. The results of these experiments in fluid membranes are consistent with the existence of lipid-protein interactions that would predispose to receptor microdomain formation in membranes of higher animal cells.

Anisotropic motion of cholesterol in oriented DPPC bilayers studied by quasielastic neutron scattering: the liquid-ordered phase

Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.

Epidermal growth factor receptor transmembrane domain: 2H NMR implications for orientation and motion in a bilayer environment

As part of a study of receptor tyrosine kinase behavior in membranes, we have collected extensive NMR data from three well-defined probe locations within the transmembrane region of the human EGF receptor. Spectra were obtained for selectively deuterated alanine residues in a series of peptides corresponding to the putative transmembrane domain (with short extramembranous extensions). Peptides were incorporated into fluid unsonicated liposomes of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and POPC containing 33 mol % cholesterol to mimic common lipid composition of cell plasma membranes. The peptide concentration was in the range of 1-6 mol % relative to that of phospholipid. Data acquired at 35 degreesC have been analyzed quantitatively to determine their implications to receptor spatial orientation and dynamics. If it is presumed that the single transmembrane portion approximates an alpha-helix of 3.6 residues per turn, this helix was found to be tilted away from the membrane perpendicular, about which there was rapid axial diffusion. However, rotation about the peptide long axis was static on the NMR time scale of 10(-)4 s, and the peptide appeared to have a preferred direction(s) of lean. The results for this peptide, whose hydrophobic length is greater than the membrane hydrophobic thickness, were very similar between membranes of POPC and membranes of POPC containing 33 mol % cholesterol, despite considerable host matrix differences in thickness and order. Allowed values of peptide tilt occupied a narrow range: between 10 and 14 degrees in POPC and between 10 and 12 degrees in POPC/cholesterol. Although the existence of some preferred direction(s) of lean was demanded by the results, the direction of lean was not uniquely determined. We have interpreted these results, which were essentially unchanged at 65 degreesC, as reflecting the behavior of peptide monomers undergoing rapidly reversible peptide-peptide interactions. For transmembrane monomers, interference with rotation about the peptide long axis might be understood to arise from an energy benefit (in a tilted peptide) to prevention of particular amino acid side chains near the membrane surfaces from moving in and out of hydrophobic or hydrophilic environments. It will be desirable to test the conclusion of preferential lean of a monomeric receptor since such behavior could provide a mechanism for modulating monomer association with other species (i.e., signal transduction).

Sequence-related behaviour of transmembrane domains from class I receptor tyrosine kinases

2H NMR spectroscopy and freeze-fracture electron microscopy were used to compare the transmembrane domains of two Class I protein receptor tyrosine kinases (the EGF receptor and Neu/erbB-2) regarding overall behaviour in fluid lipid bilayer membranes. The 34-residue peptide, EGFRtm, was synthesised to contain the 23 amino acid hydrophobic stretch (Ile622 to Met644) thought to span the membrane of the human EGF receptor, plus the first 10 amino acids (Arg645 to Thr654) of the cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at sites corresponding to Ala623, Met644, and Val650. The 38-residue peptide, Neutm, was synthesised having the 21 residue hydrophobic stretch (Ile660 to Ile680) calculated to span the membrane in rat Neu/erbB-2, plus residues Lys681 to Thr691 of the contiguous cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at Ala661, Leu667, and Val676. A third peptide, Neutm*, was also prepared, corresponding to the transmembrane domain of a constitutively-activating Neu/erbB-2 transformant in which Val664 is replaced by Glu: it was deuterated in a manner identical to Neutm. Peptides were studied by 2H NMR spectroscopy at 1 mol% and 6 mol% in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and in POPC containing 33 mol% cholesterol, over the range 12 degrees to 65 degreesC. Overall motion was found to be different for each of the three peptides under a given set of conditions. EGFRtm spectra were characteristic of axially symmetric motion in membranes of POPC alone, and in POPC/cholesterol at 35 degreesC and above. In contrast, spectra of the transmembrane peptides, Neutm and Neutm*, were characteristic of significantly axially asymmetric motion under all conditions studied (and regardless of sample preparation method). Addition of 33% cholesterol to membranes was accompanied by spectral changes consistent with increased formation of peptide dimers/oligomers in all cases. The transformant peptide, Neutm*, showed greater spectral evidence of immobilisation than did the wild type - probably reflecting a greater tendency to form large oligomers. Sequence-related details within the transmembrane domains of Class I receptor tyrosine kinases appear to exert important control over their associations within membranes. Freeze-fracture electron microscopy of the NMR samples demonstrated their liposomal nature. Peptide-related intramembranous particles (IMPs) were present which likely represent oligomers of the transmembrane peptide. IMP size and distribution were similar under a given set of conditions for all three peptides, suggesting that the differences seen by NMR spectroscopy reflect structures smaller than the 2 nm resolution limit of freeze-fracture EM and peptide relationships within its 20 nm accuracy of identifying lateral position.

The EGF receptor transmembrane domain: 2H NMR study of peptide phosphorylation effects in a bilayer environment

Phosphorylation events are considered to be key control points in receptor tyrosine kinase function. We have used wide-line 2H NMR spectroscopy to look for physical effects of phosphorylating a threonine residue within the cytoplasmic domain of the human EGF receptor, as sensed at a distant site in the transmembrane portion. Modifications were made to Thr654 (a cytoplasmic residue suggested to be involved in regulation of EGF binding and of cytoplasmic domain function), and effects were sought at Ala623 (near the extracellular membrane surface but within the membrane-spanning region). The study was carried out on synthetic peptides corresponding to the EGF receptor transmembrane domain plus 10 or 11 residues of the cytoplasmic domain, assembled into lipid bilayer membranes. Three peptides were compared that differed only at Thr654. This residue was alternately: nonphosphorylated but left as a (-)-charged C-terminus (-Thr654COO-), nonphosphorylated and with a neighboring amidated glycine residue as the C-terminus (-Thr654GlyCONH2), or phosphorylated and with a neighboring amidated glycine residue as the C-terminus (-Thr654PO4-GlyCONH2). Bilayer membranes were composed of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or 2:1 POPC/cholesterol, containing 6 mol % peptide relative to phospholipid. The deuterated site, Ala623, was intrinsically conformationally sensitive; yet spatial orientation and motional order of the probe location were found not to be obviously influenced by phosphorylation.

Pilin C-terminal peptide binds asialo-GM1 in liposomes: a 2H-NMR study

Wideline 2H-NMR observations are described demonstrating the interaction of a synthetic peptide (PAK), representing residues 128-144 of the binding domain of pilin surface protein from Pseudomonas aeruginosa, with a complex glycosphingolipid thought to be its natural receptor. The receptor glycolipid (asialo-GM1) carried 2H probe nuclei on the terminal and next-to-terminal carbohydrate residues and was present as a minor component in fluid phosphatidylcholine liposomes. The peptide induced spectral changes that could be understood as arising from receptor motional changes, without receptor immobilization on the NMR time scale of 10(4) s-1. Spectral effects were reversed by reduction of the single peptide disulfide bond--a structural feature previously shown to be a determinant of PAK conformation (Campbell AP, McInnes C, Hodges RS, Sykes BD. 1995. Biochemistry 34:16255-16268). This is the first demonstration of PAK interaction with its epithelial cell receptor in liposomes.

Oligomerization of the EGF receptor transmembrane domain: a 2H NMR study in lipid bilayers

During the course of a previous study by wideline 2H NMR, we noted spectral features suggesting the possibility of monitoring homodimer/oligomer interactions between transmembrane domains of the EGF receptor in lipid bilayers [Rigby, A. R., Shaw, G. S., Barber, K. R., & Grant, C. W. M. (1996) Biochemistry 35, 12591-12601]. In the present work this possibility was explored using the 34-residue peptide EGFRtm. The peptide sequence included the 23 amino acid hydrophobic stretch thought to span the membrane (Ile622-Met644 of the EGF receptor), plus the first 10 amino acids of the receptor's cytoplasmic domain (Arg645-Thr654). Selective deuteration was carried out at sites corresponding to Ala623, Met644, and Val650. Samples were studied from 12 to 65 degrees C by 2H NMR in fluid membranes having low peptide concentration (1 mol %) or high peptide concentration (6 mol %). Methyl groups proved to be technically particularly attractive probe locations. Reversible homodimer/oligomer interactions were detected in membranes of the common natural phospholipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), without cholesterol. Effects on the EGF receptor transmembrane domain included alterations in peptide backbone motional order and/or conformation at the site of Ala623 within the membrane, and alterations in motional properties of the Val650 side chain in the cytoplasmic domain. There was little spectral evidence of stable oligomer formation except at the lowest temperature studied. Addition of 33% cholesterol to these membranes was accompanied by spectral changes consistent with the formation of more stable peptide oligomers, and by evidence that peptide-peptide interactions were sensed at all three probe locations. Peptide-peptide interactions remained easily reversible, particularly at higher temperatures. Freeze-fracture electron microscopy of the NMR samples demonstrated peptide-related intramembranous particles traversing the membranes. To our knowledge, this is the first electron microscopy description of receptor tyrosine kinases or their fragments in model membranes. In the presence of cholesterol, the peptide-related particles were generally larger, more sharply demarcated, and showed a tendency to cluster. These observations relate to models of receptor lateral association as an aspect of signal transduction, and to forces that may determine protein sorting and organization in cell membranes. We suggest that the cholesterol effects reflect a general phenomenon rather than one specific to the EGF receptor.

Globoside as a membrane receptor: a consideration of oligosaccharide communication with the hydrophobic domain

Recognition of macromolecules by glycosphingolipids is closely correlated with the nature of the glycolipid carbohydrate; however, it is also thought to be secondarily modulated by the structure of the single fatty acid. In the present work, we sought insight into what physical effect a change in this fatty acid has on the extramembranous portion of globosides at liposomal surfaces mimicking systems for which modulated receptor function has been recorded in the past. Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of globotriaosylceramide (Gb3) were replaced with deuterium. In this location, the nonperturbing probe nuclei sampled cumulative conformational and orientational characteristics of the oligosaccharide chain at a sugar residue that is critical in specific binding of verotoxins. Deuterated Gb3 having 18:1 fatty acid was compared to the same species having 22:1 fatty acid, at 6.3 mol % in unsonicated bilayers of dipalmitoylphosphatidylcholine/cholesterol. Both produced narrow, apparently axially asymmetric 2H NMR spectra over a wide temperature range. Motional properties of the terminal sugar were measurably influenced by the fluidity of the host matrix; however, evidence was not found for conformational or orientational variation in this sugar brought about by the fatty acid alteration. In related experiments, acetate protons on the terminal N-acetyl galactosamine (GalNAc) residue of globotetraosylceramide (Gb4) were substituted with deuterium, and the natural fatty acid was replaced with 18:0 or 24:0 species deuterated at C2. Once again, species with short vs long fatty acid were examined for evidence of headgroup differences. Spectra of Gb4 were compared at 10 mol % in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine, and at 5 mol % in membranes containing 33 mol% cholesterol. Spectral splittings reflecting cumulative effects on conformation and order at the terminal deuterated sugar remained unchanged between species having 18:0 vs 24:0 fatty acid in POPC/cholesterol. In a pure POPC host matrix, there was clear evidence of a motional difference between the two--the longer chain Gb4 demonstrating spectral asymmetry--but the spectral width was unchanged. Transverse relaxation times, T2, were measured. Our findings appear to help correlate the conclusions of a number of workers dealing with the molecular basis of crypticity. We suggest that changes in glycolipid receptor function based on ceramide fatty acid variation have a major origin in the fatty acid's ability to determine the thermodynamics of interaction with the host matrix, as reflected in such parameters as glycolipid motional properties, local membrane curvature, and likely glycolipid time-dependent lateral associations. The result at low concentrations of glycolipid may often be only a subtly altered collective surface epitope, best detected by a specific recognition event.

Transmembrane region of the epidermal growth factor receptor: behavior and interactions via 2H NMR

The first wide-line 2H NMR investigation of a receptor tyrosine kinase is reported. Selectively deuterated peptides from the membrane-associated portion of the human epidermal growth factor (EGF) receptor were synthesized for examination in lipid bilayers mimicking certain natural membrane features. The peptide sequence included the 23-amino acid hydrophobic stretch thought to span the membrane (Ile622-Met644 of the EGF receptor), plus the first 10 amino acids of the receptor's cytoplasmic domain (Arg645-Thr654). Dispersion of the peptide with lipid in the lipomimetic solvent, trifluoroethanol (TFE), was found to be a very useful initial step for sample preparation. TFE readily dissolved all components and was then easily removed in vacuo to yield thin films which could be subsequently hydrated to produce bilayers incorporating homogeneously dispersed peptide. Samples extensively studied consisted of 6 mol % peptide in multilamellar liposomes of 1-palmitoyl-2-oleoylphosphatidylcholine and similar liposomes containing cholesterol. 2H NMR spectra of the resulting unsonicated model membranes indicated the existence of peptide monomers undergoing rapid axially symmetric diffusion. It was possible to examine structural and behavioral effects of events often suggested as pivotal in signaling mechanisms and to consider by wide-line NMR for the first time the effect of cholesterol on hydrophobic peptides. When it was incorporated into bilayers by an alternative method involving dialysis of aqueous solutions prepared using a cationic detergent, spectra suggested that the peptide existed primarily as irreversibly aggregated oligomers which were relatively immobile on a time scale of 10(-3)-10(-4) s. For liposomes prepared by hydration of thin films, deuterated methyl groups on the peptide at locations corresponding to Ala623, Met644, and Val650 of the human EGF receptor were individually distinguishable. In highly fluid matrices, spectra suggested the presence of peptide monomers, diffusing symmetrically about axes perpendicular to the membrane. Studied as a function of temperature, 2H NMR spectra of such samples permitted independent consideration of membrane/peptide relationships at separate locations in the receptor tyrosine kinase. None of the locations probed demonstrated significant conformational sensitivity to temperature over a wide range. Effects seen at Ala623 and Met644, at opposite ends of the putative membrane-spanning domain, suggested slight increases in motional order with decreasing temperature. Addition of 33% cholesterol to the membrane caused little apparent conformational change at Val650 or Met644. However, in the presence of the sterol, Met644 and Ala623 exhibited nonaxially symmetric motion at low temperatures, perhaps as a result of peptide oligomerization. Moreover, the presence of cholesterol led to considerable change in spatial arrangement or order at Ala623. There was little evidence to support transmission of conformational changes along the peptide segment probed.

Minor influence of sialic acid on conformation of a membrane-bound oligosaccharide recognition site

Wideline 2H NMR spectroscopy was used to assess the conformational and orientational effects of N-acetylneuraminic acid (NeuAc) (sialic acid) as a component of a particular oligosaccharide chain at a bilayer membrane surface. For this purpose, three glycosphingolipids, sharing a neutral core tetrasaccharide and differing only in the number of sialic acid residues, were compared. The starting compound was GD1A, which has terminal sialic acid attached to the second and fourth sugars of its neutral tetrasaccharide core. GD1A was probe-labeled in a non-perturbing fashion on both of these sialic acid residues and on its single GalNAc residue by replacement of -COCH3 with -COCD3 giving [(d3NeuAc)2,d3-GalNAc]GA1a. This represents the most complex glycolipid to have been studied by 2H NMR spectroscopy at a bilayer membrane surface. The sialic acid residue on the fourth sugar from the membrane was subsequently removed to produce the glycolipid [d3NeuAc,d3GalNAc]GM1, deuterated at the two remaining amino sugars. The neutral glycolipid [d3GalNAc]asialo-GM1 was then generated by removal of the second sialic acid residue, leaving an uncharged species deuterated at one (internal) oligosaccharide chain site (GalNAc). The effect of sialic acid was futher examined by selective deuteration of GM1 and asialo-GM1 at C6 of the terminal Gal residue, giving [d2Gal]GM1 and [d2Gal]asialo-GM1. Spectra of the three glycosphingolipids were compared at 7.7 mol % in unsoncicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine containing 23 mol % cholesterol. For liposomes suspend in buffered salt solutions with 2 mM Ca2+, 2H NMR spectra demonstrated the presence of well defined average conformation for each oligosaccharide chain. This preferred average conformation persisted over a wide temperature range, consistent with there being a single major oligosaccharide conformer in each case. Spectral features arising from both deuterated amino sugar (GalNAc) of asialo-GM1 could be identified, little changed, in spectra of GM1 and GD1A. Similarly, deuterons in the terminal Gal residue of asialo-Gm1 produced the same spectrum seen for this residue in GM1. Our findings indicate that certain major conformational and orientational features of this complex oligosaccharide recognition site are preserved, within maximum angular deviation + or -5 degrees or less upon addition or removal of a sialic acid residue.

New approach to study fast and slow motions in lipid bilayers: application to dimyristoylphosphatidylcholine-cholesterol interactions

Natural abundance 13C solid-state nuclear magnetic resonance spectroscopy was used to investigate the effect of the incorporation of cholesterol on the dynamics of dimyristoylphosphatidylcholine (DMPC) bilayers in the liquid-crystalline phase. In particular, the use of a combination of the cross-polarization and magic angle spinning techniques allows one to obtain very high resolution spectra from which can be distinguished several resonances attributed to the polar head group, the glycerol backbone, and the acyl chains of the lipid molecule. To examine both the fast and slow motions of the lipid bilayers, 1H spin-lattice relaxation times as well as proton and carbon spin-lattice relaxation times in the rotating frame were measured for each resolved resonance of DMPC. The use of the newly developed ramped-amplitude cross-polarization technique results in a significant increase in the stability of the cross-polarization conditions, especially for molecular groups undergoing rapid motions. The combination of T1 and T1 rho measurements indicates that the presence of cholesterol significantly decreases the rate and/or amplitude of both the high and low frequency motions in the DMPC bilayers. This effect is particularly important for the lipid acyl chains and the glycerol backbone region.

Deuteron nuclear magnetic resonance study of the dynamic organization of phospholipid/cholesterol bilayer membranes: molecular properties and viscoelastic behavior

The influence of cholesterol on the dynamic organization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers was studied by deuteron nuclear magnetic resonance (2H NMR) using unoriented and macroscopically aligned samples. Analysis of the various temperature- and orientation-dependent experiments were performed using a comprehensive NMR model based on the stochastic Liouville equation. Computer simulations of the relaxation data obtained from phospholipids deuterated at the 6-, 13- and 14-position of the sn-2 chain and cholesterol labeled at the 3 alpha-position of the rigid steroid ring system allowed the unambiguous assignment of the various motional modes and types of molecular order present in the system. Above the phospholipid gel-to-liquid-crystalline phase transition, TM, 40 mol % cholesterol was found to significantly increase the orientational and conformational order of the phospholipid with substantially increased trans populations even at the terminal sn-2 acyl chain segments. Lowering the temperature continuously increases both inter- and intramolecular ordering, yet indicates less ordered chains than found for the pure phospholipid in its paracrystalline gel phase. Trans-gauche isomerization rates on all phospholipid alkyl chain segments are slowed down by incorporated cholesterol to values characteristic of gel-state lipid. However, intermolecular dynamics remain fast on the NMR time scale up to 30 K below TM, with rotational correlation times tau R parallel for DMPC ranging from 10 to 100 ns and an activation energy of ER = 35 kJ/mol. Below 273 K a continuous noncooperative condensation of both phospholipid and cholesterol is observed in the mixed membranes, and at about 253 K only a motionally restricted component is left, exhibiting slow fluctuations with correlation times of tau R perpendicular greater than 1 microsecond. In the high-temperature region (T greater than TM), order director fluctuations are found to constitute the dominant transverse relaxation process. Analysis of these collective lipid motions provides the viscoelastic parameters of the membranes. The results (T = 318 K) show that cholesterol significantly reduces the density of the cooperative motions by increasing the average elastic constant of the membrane from K = 1 x 10(-11) N for the pure phospholipid bilayers to K = 3.5 x 10(-11) N for the mixed system.

Physical properties of the fluid lipid-bilayer component of cell membranes: a perspective

The motivation for this review arises from the conviction that, as a result of the mass of experimental data and observations collected in recent years, the study of the physical properties of membranes is now entering a new stage of development. More and more, experiments are being designed to answer specific, detailed questions about membranes which will lead to a quantitative understanding of the way in which the physical properties of membranes are related to and influence their biological function.

Slow motions in lipid bilayers. Direct detection by two-dimensional solid-state deuterium nuclear magnetic resonance

Two-dimensional solid-state 2H NMR spectroscopy of specifically deuteriated lipids is used to detect and to characterize the rate and mode of slow motions in two lipid bilayer systems. Lateral diffusion of lipid molecules over the curved surface of dipalmitoylphosphatidylcholine liposomes can be detected by two-dimensional exchange 2H NMR and it is shown that molecular orientational exchange is complete on the timescale of 100 ms. In contrast, it is shown that for the glycolipid 1,2-di-O-tetradecyl-3-O-Beta-D-glucopyranosyl)-sn-glycerol (beta-DTGL), there is no evidence of a corresponding orientational exchange in the liquid-crystalline phase suggesting that this lipid forms relatively flat bilayers. In the gel phase of hydrated multibilayers of beta-DTGL, a slow (10(3) s(-1)) whole molecule axial motion is demonstrated at 40 degrees C. Comparison of the experimental and simulated 2D-NMR ridge patterns suggests that large angle jumps about the long molecular axis, rather than small step Brownian diffusion, can best account for the 2D-exchange spectra of beta-DTGL in the gel phase. The significance of this technique for the study of dynamics in other biological systems is discussed.

Glycerolipids: common features of molecular motion in bilayers

In the present study, analysis of 2H NMR line-shape and spin-lattice relaxation behavior has been used to investigate the dynamics of several glycolipid and phospholipid bilayers. The gel-phase spectra of these lipids labeled at the C3 position of the glycerol backbone are broad (approximately 90 kHz) and characteristic of fast-limit axially asymmetric motion. Moreover, anisotropic spin-lattice relaxation is observed in all of these systems. The line-shape and relaxation features of the lipids in the gel phase were best simulated by using a fast-limit three-site jump model, with relative site populations of 0.46, 0.34, and 0.20. This motion is associated with an internal jump about the C2-C3 bond of the glycerol backbone. A second motion, rotation about the long axis of the molecule, is needed to account for the observed temperature dependence of the quadrupolar echo amplitude and the spectral line shape above and below the gel to liquid-crystalline phase transition temperature. On the other hand, the gel-phase spectra of phospholipids labeled at the C2 position of the glycerol backbone are also characterized by a fast internal motion, which is simulated by a two-site librational jump. The results indicate that the glycerol backbone dynamics of the glycolipid and phospholipid systems investigated in this study can be described in terms of common fast internal motions and a slower whole molecule axial motion. These results are compared with previous dynamic studies of similar systems.

Cholesterol dynamics in membranes

Time-resolved fluorescence anisotropy of the sterol analogue, cholestatrienol, and 13C nuclear magnetic resonance (NMR) spin lattice relaxation time (T1c) measurements of [13C4] labeled cholesterol were exploited to determine the correlation times characterizing the major modes of motion of cholesterol in unsonicated phospholipid multilamellar liposomes. Two modes of motion were found to be important: (a) rotational diffusion and (b) time dependence of the orientation of the director for axial diffusion, or "wobble." From the time-resolved fluorescence anisotropy decays of cholestatrienol in egg phosphatidylcholine (PC) bilayers, a value for tau perpendicular, the correlation time for wobble, of 0.9 x 10(-9) s and a value for S perpendicular, the order parameter characterizing the same motion, of 0.45 s were calculated. Both tau perpendicular and S perpendicular were relatively insensitive to temperature and cholesterol content of the membranes. The T1c measurements of [13C4] labeled cholesterol did not provide a quantitative determination of tau parallel, the correlation time for axial diffusion. T1c from the lipid hydrocarbon chains suggested a value for tau perpendicular similar to that for cholesterol. Steady-state anisotropy measurements and time-resolved anisotropy measurements of cholestatrienol were used to probe sterol behavior in a variety of pure and mixed lipid multilamellar liposomes. Both the lipid headgroups and the lipid hydrocarbons chains contributed to the determination of the sterol environment in the membrane, as revealed by these fluorescence measurements. In particular, effects of the phosphatidylethanolamine (PE) headgroup and of multiple unsaturation in the lipid hydrocarbon chains were observed. However, while the steady-state anisotropy was sensitive to these factors, the time-resolved fluorescence analysis indicated that tau perpendicular was not strongly affected by the lipid composition of the membrane. S perpendicular may be increased by the presence of PE. Both steady-state anisotropy measurements and time-resolved anisotropy measurements of cholestatrienol were used to probe sterol behavior in three biological membranes: bovine rod outer segment (ROS) disk membranes, human erythrocyte plasma membranes, and light rabbit muscle sarcoplasmic reticulum membranes. In the ROS disk membranes the value for S perpendicular was marginally higher than in the PC membranes, perhaps reflecting the influence of PE. The dramatic difference noted was in the value for tau perpendicular. In both the ROS disk membranes and the erythrocyte membranes, tau perpendicular was one-third to one-fifth of tau perpendicular in the phospholipid bilayers. This result may reveal an influence of membrane proteins on sterol behavior.

Dynamics of the phosphate group in phospholipid bilayers. A 31P angular dependent nuclear spin relaxation time study

To understand 31P relaxation processes and hence molecular dynamics in the phospholipid multilayer it is important to measure the dependence of the 31P spin-lattice relaxation time on as many variables as the physical system allows. Such measurements of the 31P spin-lattice relaxation rate have been reported both as a function of Larmor frequency and temperature for egg phosphatidylcholine liposomes (Milburn, M.P., and K.R. Jeffrey. 1987. Biophys. J. 52:791-799). In principle, the spin-lattice relaxation rate in an anisotropic environment such as a bilayer will be a function of the angle between the bilayer normal and the magnetic field. However, the measurement of this angular dependence has not been possible because the rapid (on the time-scale of the spin-lattice relaxation rate) diffusion of the lipid molecules over the curved surface of the liposome average this dependence (Milburn, M.P., and K.R. Jeffrey. 1987. Biophys. J. 52:791-799; Brown, M.F., and J.H. Davis. 1981. Chem. Phys. Lett. 79:431-435). This paper reports the results of the measurement of the 31P spin-lattice relaxation rate as a function of this angle, beta', (the angle between the bilayer normal and the external magnetic field) using samples oriented between glass plates. These measurements were made at high field (145.7 MHz) where the spin-lattice relaxation processes are dominated by the chemical shielding interaction (Milburn, M.P., and K.R. Jeffrey. 1987. Biophys. J. 52:791-799). A model of molecular motion that includes a fast axially symmetric rotation of the phosphate group (tau i approximately 10(-9) s) and a wobble of the head group tilt with respect to this rotation axis has been used to describe both the angular dependence of the spin-lattice relaxation and the spectral anisotropy. Cholesterol is seen to have a negligible effect on the motional properties of the phospholipid phosphate segment as measured by the orientation dependence of the spin-lattice relaxation.