Solid-state NMR approaches for studying the interaction of peptides and proteins with membranes

Lipid chain-driven interaction of a lipidated Src-family kinase Lyn with the bilayer membrane

N-Myristoylation is a process of ubiquitous protein modification, which promotes the interaction of lipidated proteins on cell surfaces, in conjunction with reversible S-palmitoylation. We report the cooperative lipid-lipid interaction of two acyl chains of proteins, which increases the protein-membrane interaction and facilitates selective targeting of membranes containing anionic lipids. Lyn is a member of the Src family kinases distributed on the membrane surface by N-myristoyl and neighbouring S-palmitoyl chain anchors at the unique N-terminus domain. We prepared N-terminal short segments of lipidated Lyn to investigate the behaviour of each acyl chain in the lipid composition-dependent membrane interaction by solid-state nuclear magnetic resonance (NMR) analysis. Solid-state ³¹P-NMR studies revealed that S-palmitoylation of N-myristoylated Lyn peptides increased the interaction between peptides and phospholipid head groups, particularly with the anionic phosphatidylserine-containing bilayers. The solid-state ²H-NMR of Lyn peptides with a perdeutero N-myristoyl chain indicated an increase (0.6-0.8 Å) in the extent of the N-myristoyl chain in the presence of nearby S-palmitoyl chains, probably through the interaction via the acyl chains. The cooperative hydrocarbon chain interaction of the two acyl chains of Lyn increased membrane binding by extending the hydrocarbon chains deeper into the membrane interior, thereby promoting the peptide-membrane surface interaction between the cationic peptide side chains and the anionic lipid head groups. This lipid-driven mechanism by S-palmitoylation promotes the partition of the lipidated proteins to the cytoplasmic surface of the cell membranes and may be involved in recruiting Lyn at the signalling domains rich in anionic lipids.

Membrane Active Peptides and Their Biophysical Characterization

In the last 20 years, an increasing number of studies have been reported on membrane active peptides. These peptides exert their biological activity by interacting with the cell membrane, either to disrupt it and lead to cell lysis or to translocate through it to deliver cargos into the cell and reach their target. Membrane active peptides are attractive alternatives to currently used pharmaceuticals and the number of antimicrobial peptides (AMPs) and peptides designed for drug and gene delivery in the drug pipeline is increasing. Here, we focus on two most prominent classes of membrane active peptides; AMPs and cell-penetrating peptides (CPPs). Antimicrobial peptides are a group of membrane active peptides that disrupt the membrane integrity or inhibit the cellular functions of bacteria, virus, and fungi. Cell penetrating peptides are another group of membrane active peptides that mainly function as cargo-carriers even though they may also show antimicrobial activity. Biophysical techniques shed light on peptide–membrane interactions at higher resolution due to the advances in optics, image processing, and computational resources. Structural investigation of membrane active peptides in the presence of the membrane provides important clues on the effect of the membrane environment on peptide conformations. Live imaging techniques allow examination of peptide action at a single cell or single molecule level. In addition to these experimental biophysical techniques, molecular dynamics simulations provide clues on the peptide–lipid interactions and dynamics of the cell entry process at atomic detail. In this review, we summarize the recent advances in experimental and computational investigation of membrane active peptides with particular emphasis on two amphipathic membrane active peptides, the AMP melittin and the CPP pVEC.

Probing the interaction of the potassium channel modulating KCNE1 in lipid bilayers via solid-state NMR spectroscopy

KCNE1 is known to modulate the voltage-gated potassium channel α subunit KCNQ1 to generate slowly activating potassium currents. This potassium channel is essential for the cardiac action potential that mediates a heartbeat as well as the potassium ion homeostasis in the inner ear. Therefore, it is important to know the structure and dynamics of KCNE1 to better understand its modulatory role. Previously, the Sanders group solved the three-dimensional structure of KCNE1 in LMPG micelles, which yielded a better understanding of this KCNQ1/KCNE1 channel activity. However, research in the Lorigan group showed different structural properties of KCNE1 when incorporated into POPC/POPG lipid bilayers as opposed to LMPG micelles. It is hence necessary to study the structure of KCNE1 in a more native-like environment such as multi-lamellar vesicles. In this study, the dynamics of lipid bilayers upon incorporation of the membrane protein KCNE1 were investigated using ³¹ P solid-state nuclear magnetic resonance (NMR) spectroscopy. Specifically, the protein/lipid interaction was studied at varying molar ratios of protein to lipid content. The static ³¹ P NMR and T₁ relaxation time were investigated. The ³¹ P NMR powder spectra indicated significant perturbations of KCNE1 on the phospholipid headgroups of multi-lamellar vesicles as shown from the changes in the ³¹ P spectral line shape and the chemical shift anisotropy line width. ³¹ P T₁ relaxation times were shown to be reversely proportional to the molar ratios of KCNE1 incorporated. The ³¹ P NMR data clearly indicate that KCNE1 interacts with the membrane. Copyright © 2017 John Wiley & Sons, Ltd.

Probing Hydronium Ion Histidine NH Exchange Rate Constants in the M2 Channel via Indirect Observation of Dipolar-Dephased 15N Signals in Magic-Angle-Spinning NMR

Water-protein chemical exchange in membrane-bound proteins is an important parameter for understanding how proteins interact with their aqueous environment, but has been difficult to observe in membrane-bound biological systems. Here, we demonstrate the feasibility of probing specific water-protein chemical exchange in membrane-bound proteins in solid-state MAS NMR. By spin-locking the 1H magnetization along the magic angle, the 1H spin diffusion is suppressed such that a water-protein chemical exchange process can be monitored indirectly by dipolar-dephased 15N signals through polarization transfer from 1H. In the example of the Influenza A full length M2 protein, the buildup of dipolar-dephased 15N signals from the tetrad of His37 side chains have been observed as a function of spin-lock time. This confirms that hydronium ions are in exchange with protons in the His37 NH bonds at the heart of the M2 proton conduction mechanism, with an exchange rate constant of ∼1750 s-1 for pH 6.2 at -10 °C.

Structural Changes and Proapoptotic Peroxidase Activity of Cardiolipin-Bound Mitochondrial Cytochrome c

The cellular process of intrinsic apoptosis relies on the peroxidation of mitochondrial lipids as a critical molecular signal. Lipid peroxidation is connected to increases in mitochondrial reactive oxygen species, but there is also a required role for mitochondrial cytochrome c (cyt-c). In apoptotic mitochondria, cyt-c gains a new function as a lipid peroxidase that catalyzes the reactive oxygen species-mediated chemical modification of the mitochondrial lipid cardiolipin (CL). This peroxidase activity is caused by a conformational change in the protein, resulting from interactions between cyt-c and CL. The nature of the conformational change and how it causes this gain-of-function remain uncertain. Via a combination of functional, structural, and biophysical experiments we investigate the structure and peroxidase activity of cyt-c in its membrane-bound state. We reconstituted cyt-c with CL-containing lipid vesicles, and determined the increase in peroxidase activity resulting from membrane binding. We combined these assays of CL-induced proapoptotic activity with structural and dynamic studies of the membrane-bound protein via solid-state NMR and optical spectroscopy. Multidimensional magic angle spinning (MAS) solid-state NMR of uniformly (13)C,(15)N-labeled protein was used to detect site-specific conformational changes in oxidized and reduced horse heart cyt-c bound to CL-containing lipid bilayers. MAS NMR and Fourier transform infrared measurements show that the peripherally membrane-bound cyt-c experiences significant dynamics, but also retains most or all of its secondary structure. Moreover, in two-dimensional and three-dimensional MAS NMR spectra the CL-bound cyt-c displays a spectral resolution, and thus structural homogeneity, that is inconsistent with extensive membrane-induced unfolding. Cyt-c is found to interact primarily with the membrane interface, without significantly disrupting the lipid bilayer. Thus, membrane binding results in cyt-c gaining the increased peroxidase activity that represents its pivotal proapoptotic function, but we do not observe evidence for large-scale unfolding or penetration into the membrane core.

(13) C-TmDOTA as versatile thermometer compound for solid-state NMR of hydrated lipid bilayer membranes

Recent advances in solid-state nuclear magnetic resonance (NMR) techniques, such as magic angle spinning and high-power decoupling, have dramatically increased the sensitivity and resolution of NMR. However, these NMR techniques generate extra heat, causing a temperature difference between the sample in the rotor and the variable temperature gas. This extra heating is a particularly crucial problem for hydrated lipid membrane samples. Thus, to develop an NMR thermometer that is suitable for hydrated lipid samples, thulium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (TmDOTA) was synthesized and labeled with (13) C (i.e., (13) C-TmDOTA) to increase the NMR sensitivity. The complex was mixed with a hydrated lipid membrane, and the system was subjected to solid-state NMR and differential scanning calorimetric analyses. The physical properties of the lipid bilayer and the quality of the NMR spectra of the membrane were negligibly affected by the presence of (13) C-TmDOTA, and the (13) C chemical shift of the complex exhibited a large-temperature dependence. The results demonstrated that (13) C-TmDOTA could be successfully used as a thermometer to accurately monitor temperature changes induced by (1) H decoupling pulses and/or by magic angle spinning and the temperature distribution of the sample inside the rotor. Thus, (13) C-TmDOTA was shown to be a versatile thermometer for hydrated lipid assemblies. Copyright © 2015 John Wiley & Sons, Ltd.

Solid-state NMR spectroscopy to study protein-lipid interactions

The appropriate lipid environment is crucial for the proper function of membrane proteins. There is a tremendous variety of lipid molecules in the membrane and so far it is often unclear which component of the lipid matrix is essential for the function of a respective protein. Lipid molecules and proteins mutually influence each other; parameters such as acyl chain order, membrane thickness, membrane elasticity, permeability, lipid-domain and annulus formation are strongly modulated by proteins. More recent data also indicates that the influence of proteins goes beyond a single annulus of next-neighbor boundary lipids. Therefore, a mesoscopic approach to membrane lipid-protein interactions in terms of elastic membrane deformations has been developed. Solid-state NMR has greatly contributed to the understanding of lipid-protein interactions and the modern view of biological membranes. Methods that detect the influence of proteins on the membrane as well as direct lipid-protein interactions have been developed and are reviewed here. Examples for solid-state NMR studies on the interaction of Ras proteins, the antimicrobial peptide protegrin-1, the G protein-coupled receptor rhodopsin, and the K(+) channel KcsA are discussed. This article is part of a Special Issue entitled Tools to study lipid functions.

Investigating the interaction between peptides of the amphipathic helix of Hcf106 and the phospholipid bilayer by solid-state NMR spectroscopy

The chloroplast twin arginine translocation (cpTat) system transports highly folded precursor proteins into the thylakoid lumen using the protonmotive force as its only energy source. Hcf106, as one of the core components of the cpTat system, is part of the precursor receptor complex and functions in the initial precursor-binding step. Hcf106 is predicted to contain a single amino terminal transmembrane domain followed by a Pro-Gly hinge, a predicted amphipathic α-helix (APH), and a loosely structured carboxy terminus. Hcf106 has been shown biochemically to insert spontaneously into thylakoid membranes. To better understand the membrane active capabilities of Hcf106, we used solid-state NMR spectroscopy to investigate those properties of the APH. In this study, synthesized peptides of the predicted Hcf106 APH (amino acids 28-65) were incorporated at increasing mol.% into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) and POPC/MGDG (monogalactosyldiacylglycerol; mole ratio 85:15) multilamellar vesicles (MLVs) to probe the peptide-lipid interaction. Solid-state (31)P NMR and (2)H NMR spectroscopic experiments revealed that the peptide perturbs the headgroup and the acyl chain regions of phospholipids as indicated by changes in spectral lineshape, chemical shift anisotropy (CSA) line width, and (2)H order SCD parameters. In addition, the comparison between POPC MLVs and POPC/MGDG MLVs indicated that the lipid bilayer composition affected peptide perturbation of the lipids, and such perturbation appeared to be more intense in a system more closely mimicking a thylakoid membrane.

Helical membrane protein conformations and their environment

Evidence that membrane proteins respond conformationally and functionally to their environment is growing. Structural models, by necessity, have been characterized in preparations where the protein has been removed from its native environment. Different structural methods have used various membrane mimetics that have recently included lipid bilayers as a more native-like environment. Structural tools applied to lipid bilayer-embedded integral proteins are informing us about important generic characteristics of how membrane proteins respond to the lipid environment as compared with their response to other nonlipid environments. Here, we review the current status of the field, with specific reference to observations of some well-studied α-helical membrane proteins, as a starting point to aid the development of possible generic principles for model refinement.

Ionization behavior of polyphosphoinositides determined via the preparation of pH titration curves using solid-state 31P NMR

Detailed knowledge of the degree of ionization of lipid titratable groups is important for the evaluation of protein-lipid and lipid-lipid interactions. The degree of ionization is commonly evaluated by acid-base titration, but for lipids localized in a multicomponent membrane interface this is not a suitable technique. For phosphomonoester-containing lipids such as the polyphosphoinositides, phosphatidic acid, and ceramide-1-phosphate, this is more conveniently accomplished by (31)P NMR. Here, we describe a solid-state (31)P NMR procedure to construct pH titration curves to determine the degree of ionization of phosphomonoester groups in polyphosphoinositides. This procedure can also be used, with suitable sample preparation conditions, for other important signaling lipids. Access to a solid-state, i.e., magic angle spinning, capable NMR spectrometer is assumed. The procedures described here are valid for a Bruker instrument, but can be adapted for other spectrometers as needed.

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.

Specific membrane binding of neurotoxin II can facilitate its delivery to acetylcholine receptor

The action of three-finger snake alpha-neurotoxins at their targets, nicotinic acetylcholine receptors (nAChR), is widely studied because of its biological and pharmacological relevance. Most such studies deal only with ligands and receptor models; however, for many ligand/receptor systems the membrane environment may affect ligand binding. In this work we focused on binding of short-chain alpha-neurotoxin II (NTII) from Naja oxiana to the native-like lipid bilayer, and the possible role played by the membrane in delivering the toxin to nAChR. Experimental (NMR and mutagenesis) and molecular modeling (molecular-dynamics simulation) studies revealed a specific interaction of the toxin molecule with the phosphatidylserine headgroup of lipids, resulting in the proper topology of NTII on lipid bilayers favoring the attack of nAChR. Analysis of short-chain alpha-neurotoxins showed that most of them possess a high positive charge and sequence homology in the lipid-binding motif of NTII, implying that interaction with the membrane surrounding nAChR may be common for the toxin family.

Biophysics and function of phosphatidic acid: a molecular perspective

Phosphatidic acid is the simplest (diacyl)glycerophospholipid present in cells and is now a well established second messenger with direct biological functions. It is specifically recognized by diverse proteins and plays an important role in cellular signaling and membrane dynamics in all eukaryotes. An important determinant of the biological functions of phosphatidic acid is its anionic headgroup. In this review we will focus on the peculiar ionization properties of phosphatidic acid and their crucial role in lipid-protein interactions. We will take a molecular approach focusing entirely on the physical chemistry of the lipid and develop a model explaining the ionization properties of phosphatidic acid, termed the electrostatic-hydrogen bond switch model. Diverse examples from recent literature in support of this model will be presented and the broader implications of our findings will be discussed.

Participation of the surface structure of Pharaonis phoborhodopsin, ppR and its A149S and A149V mutants, consisting of the C-terminal alpha-helix and E-F loop, in the complex-formation with the cognate transducer pHtrII, as revealed by site-directed 13C solid-state NMR

We have recorded 13C solid state NMR spectra of [3-13C]Ala-labeled pharaonis phoborhodopsin (ppR) and its mutants, A149S and A149V, complexed with the cognate transducer pharaonis halobacterial transducer II protein (pHtrII) (1-159), to gain insight into a possible role of their cytoplasmic surface structure including the C-terminal alpha-helix and E-F loop for stabilization of the 2:2 complex, by both cross-polarization magic angle spinning (CP-MAS) and dipolar decoupled (DD)-MAS NMR techniques. We found that 13C CP-MAS NMR spectra of [3-13C]Ala-ppR, A149S and A149V complexed with the transducer pHtrII are very similar, reflecting their conformation and dynamics changes caused by mutual interactions through the transmembrane alpha-helical surfaces. In contrast, their DD-MAS NMR spectral features are quite different between [3-13C]Ala-A149S and A149V in the complexes with pHtrII: 13C DD-MAS NMR spectrum of [3-13C]Ala-A149S complex is rather similar to that of the uncomplexed form, while the corresponding spectral feature of A149V complex is similar to that of ppR complex in the C-terminal tip region. This is because more flexible surface structure detected by the DD-MAS NMR spectra are more directly influenced by the dynamics changes than the CP-MAS NMR. It turned out, therefore, that an altered surface structure of A149S resulted in destabilized complex as viewed from the 13C NMR spectrum of the surface areas, probably because of modified conformation at the corner of the helix E in addition to the change of hydropathy. It is, therefore, concluded that the surface structure of ppR including the C-terminal alpha-helix and the E-F loops is directly involved in the stabilization of the complex through conformational stability of the helix E.

The lipidated membrane anchor of full length N-Ras protein shows an extensive dynamics as revealed by solid-state NMR spectroscopy

Many proteins involved in signal transduction are equipped with covalently attached lipid chains providing a hydrophobic anchor targeting these molecules to membranes. Despite the considerable biological significance of this membrane binding mechanism for 5-10% of all cellular proteins, to date very little is known about structural and dynamical features of lipidated membrane binding domains. Here we report the first comprehensive study of the molecular dynamics of the C-terminus of membrane-associated full-length lipidated Ras protein determined by solid-state NMR. Fully functional lipid-modified N-Ras protein was obtained by chemical-biological synthesis ligating the expressed water soluble N-terminus with a chemically synthesized (2)H or (13)C labeled lipidated heptapeptide. Dynamical parameters for the lipid chain modification at Cys 181 were determined from static (2)H NMR order parameter and relaxation measurements. Order parameters describing the amplitude of motion in the protein backbone and the side chain were determined from site-specific measurements of (1)H-(13)C dipolar couplings for all seven amino acids in the membrane anchor of Ras. Finally, the correlation times of motion were determined from temperature dependent relaxation time measurements and analyzed using a modified Lipari Szabo approach. Overall, the C-terminus of Ras shows a versatile dynamics with segmental fluctuations and axially symmetric overall motions on the membrane surface. In particular, the lipid chain modifications are highly flexible in the membrane.

Investigating the interaction of saposin C with POPS and POPC phospholipids: a solid-state NMR spectroscopic study

The interaction of Saposin C (Sap C) with negatively charged phospholipids such as phosphatidylserine (PS) is essential for its biological function. In this study, Sap C (initially protonated in a weak acid) was inserted into multilamellar vesicles (MLVs) consisting of either 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-L-serine] (negatively charged, POPS) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (neutrally charged, POPC). The MLVs were then investigated using solid-state NMR spectroscopy under neutral pH (7.0) conditions. The (2)H and (31)P solid-state NMR spectroscopic data of Sap C-POPS and Sap C-POPC MLVs (prepared under the same conditions) were compared using the (2)H order parameter profiles of the POPC-d(31) or POPS-d(31) acyl chains as well as the (31)P chemical shift anisotropy width and (31)P T(1) relaxation times of the phospholipids headgroups. All those solid-state NMR spectroscopic approaches indicate that protonated Sap C disturbs the POPS bilayers and not the POPC lipid bilayers. These observations suggest for the first time that protonated Sap C inserts into PS bilayers and forms a stable complex with the lipids even after resuspension under neutral buffer conditions. Additionally, (31)P solid-state NMR spectroscopic studies of mechanically oriented phospholipids on glass plates were conducted and perturbation effect of Sap C on both POPS and POPC bilayers was compared. Unlike POPC bilayers, the data indicates that protonated Sap C (initially protonated in a weak acid) was unable to produce well-oriented POPS bilayers on glass plates at neutral pH. Conversely, unprotonated Sap C (initially dissolved in a neutral buffer) did not interact significantly with POPS phospholipids allowing them to produce well-oriented bilayers at neutral pH.

Study of the interaction of an α-helical transmembrane peptide with phosphatidylcholine bilayer membranes by means of densimetry and ultrasound velocimetry

We applied precise densimetry and ultrasound velocimetry methods to study the interaction of a synthetic alpha-helical transmembrane peptide, acetyl-K(2)-L(24)-K(2)-amide (L(24)), with model bilayer lipid membranes. The large unilamellar vesicles (LUVs) utilized were composed of a homologous series of n-saturated diacylphosphatidylcholines (PCs). PCs whose hydrocarbon chains contained from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us to analyze how the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer influences the thermodynamical and mechanical properties of the membranes. We showed that the incorporation of L(24) decreases the temperature and cooperativity of the main phase transition of all LUVs studied. The presence of L(24) in the bilayer also caused an increase of the specific volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide concentration (mole ratio L(24):PC=1:50). The overall volume compressibility of the peptide-containing lipid bilayers in the liquid-crystalline state was in general higher in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs and large multilamellar vesicles (MLVs) with and without L(24). As expected, a higher cooperativity of the changes of the thermodynamical and mechanical parameters took place for MLVs in comparison with LUVs. These results are in agreement with previously reported DSC and (2)H NMR spectroscopy study of the interaction of the L(24) and structurally related peptides with phosphatidylcholine bilayers. An apparent discrepancy between (2)H NMR spectroscopy and compressibility data in the liquid crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L(24) suggest that around each peptide a distorted region exists that involves at least 2 layers of lipid molecules.

The infectivity of transmissible spongiform encephalopathy agent at low doses: the importance of phospholipid

The issue of whether the mechanism of infection is independent or co-operative for low doses of transmissible spongiform encephalopathy (TSE) agent is critical for risk assessment. The susceptibility (and hence ID(50)) of individuals with the same prion protein (PrP) genotype may vary considerably with a small proportion being very susceptible. Assuming independent action, the incubation period (IP) would continue to increase until the dose is below the ID(50) of the most susceptible individuals in the experiment, at which point it would become constant. This may explain the observed increase in IP with decreasing dose below the apparent ID(50) in experiments with untreated TSE agent. In contrast, IPs for autoclaved or NaOH-treated TSE agent increase greatly at doses <ID(50) suggesting strong co-operative action, or even a threshold. It is proposed here that the unit of infectivity for prion disease is a nucleation seed comprised of PrP and host phospholipid (PL). PL would play a structural role through mediating protein/lipid interactions with PrP. Heating or alkali treatment destroys the PL breaking up the nucleation seeds, which require long IPs to reform at low doses. Replenishing those inactivated PLs with host PL would explain how the phenotypic effect of long IP at low dose is lost on subpassage. It is proposed here that strain thermostability is controlled by the nature and strength of the PrP/PL interactions, which are independent of the host PrP genotype. Although repeated oral exposure to low doses of scrapie is less harmful than a single large exposure, this effect may reflect interference by competition rather than diminished risks due to a co-operative effect, and is of little importance for 'one-off' low-dose environmental exposures.

Solution Structure and Interaction of Cupiennin 1a, a Spider Venom Peptide, with Phospholipid Bilayers†

The solution structure of cupiennin 1a, a 35 residue, basic antibacterial peptide isolated from the venom of the spider Cupiennius salei, has been determined by nuclear magnetic resonance (NMR) spectroscopy. The peptide was found to adopt a helix-hinge-helix structure in a membrane mimicking solvent. The hinge may play a role in allowing the amphipathic N-terminal helix and polar C-terminal helix to orient independently upon membrane binding, in order to achieve maximal antibacterial efficacy. Solid-state 31P and 2H NMR was used to further study the effects of cupiennin 1a on the dynamic properties of lipid membranes, using zwitterionic chain deuterated dimyristoylphosphatidylcholine (d54-DMPC) and anionic dimyristoylphosphatidylglycerol (DMPG) multilamellar vesicles. In d54-DMPC alone, cupiennin 1a caused a decrease in the 31P chemical shift anisotropy, indicating some interaction with the lipid head groups, and a decrease in order over the entire acyl chain. In contrast, for the mixed (d54-DMPC/DMPG) lipid system cupiennin 1a appeared to induce lateral separation of the two lipids as evidenced by the 31P spectra, in which the peptide preferentially interacted with DMPG. Little effect was observed on the deuterated acyl chain order parameters in the d54-DMPC/DMPG model membranes. Furthermore, 31P NMR relaxation measurements confirmed a differential effect on the lipid motions depending upon the membrane composition. Therefore, subtle differences are likely in the mechanism by which cupiennin 1a causes membrane lysis in either prokaryotic or eukaryotic cells, and may explain the specific spectrum of activity.

Phospholamban and its phosphorylated form interact differently with lipid bilayers: a 31P, 2H, and 13C solid-state NMR spectroscopic study

Phospholamban (PLB) is a 52-amino acid integral membrane protein that helps to regulate the flow of Ca(2+) ions in cardiac muscle cells. Recent structural studies on the PLB pentamer and the functionally active monomer (AFA-PLB) debate whether its cytoplasmic domain, in either the phosphorylated or dephosphorylated states, is alpha-helical in structure as well as whether it associates with the lipid head groups (Oxenoid, K. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10870-10875; Karim, C. B. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 14437-14442; Andronesi, C.A. (2005) J. Am. Chem. Soc. 127, 12965-12974; Li, J. (2003) Biochemistry 42, 10674-10682; Metcalfe, E. E. (2005) Biochemistry 44, 4386-4396: Clayton, J. C. (2005) Biochemistry 44, 17016-17026). Comparing the secondary structure of the PLB pentamer and its phosphorylated form (P-PLB) as well as their interaction with the lipid bilayer is crucial in order to understand its regulatory function. Therefore, in this study, the full-length wild-type (WT) PLB and P-PLB were incorporated into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) phospholipid bilayers and studied utilizing solid-state NMR spectroscopy. The analysis of the (2)H and (31)P solid-state NMR data of PLB and P-PLB in POPC multilamellar vesicles (MLVs) indicates that a direct interaction takes place between both proteins and the phospholipid head groups. However, the interaction of P-PLB with POPC bilayers was less significant compared that with PLB. Moreover, the secondary structure using (13)C=O site-specific isotopically labeled Ala15-PLB and Ala15-P-PLB in POPC bilayers suggests that this residue, located in the cytoplasmic domain, is a part of an alpha-helical structure for both PLB and P-PLB.

Spatial Structure and Activity Mechanism of a Novel Spider Antimicrobial Peptide,

Latarcins (Ltc), linear peptides (ca. 25 amino acid long) isolated from the venom of the Lachesana tarabaevi spider, exhibit a broad-spectrum antibacterial activity, most likely acting on the bacterial plasmatic membrane. We study the structure-activity relationships in the series of these compounds. At the first stage, we investigated the spatial structure of one of the peptides, Ltc2a, and its mode of membrane perturbation. This was done by a combination of experimental and theoretical methods. The approach includes (i) structural study of the peptide by CD spectroscopy in phospholipid liposomes and by (1)H NMR in detergent micelles, (ii) determination of the effect on the liposomes by a dye leakage fluorescent assay and (31)P NMR spectroscopy, (iii) refinement of the NMR-derived spatial structure via Monte Carlo simulations in an implicit water-octanol slab, and (iv) calculation of the molecular hydrophobicity potential. The molecule of Ltc2a was found to consist of two helical regions (residues 3-9 and 13-21) connected via a poorly ordered fragment. The effect of the peptide on the liposomes suggests the carpet mechanism of the membrane deterioration. This is also supported by the analysis of hydrophobic/hydrophilic characteristics of Ltc2a and homologous antimicrobial peptides. These peptides exhibiting a helix-hinge-helix structural motif are characterized by a distinct and feebly marked amphiphilicity of their N- and C-terminal helices, respectively, and by a hydrophobicity gradient along the peptide chain. The approach we suggested may be useful in studying not only other latarcins but also a wider class of membrane-active peptides.

Modeling of 31P-NMR spectra of magnetically oriented phospholipid liposomes: A new analytical solution

31P-NMR spectroscopy is widely used for studies of phospholipid liposomes, a commonly used model of a biological membrane. For the correct analysis of 31P-NMR spectra of the liposomes it is necessary to take into account that they are deformed by the magnetic field of the spectrometer. The liposomes become ellipsoidal and this affects the lineshape of the spectrum. In the present communication we suggest a new analytical formula for modeling of 31P-NMR spectra of the prolate phospholipid liposomes. The formula assumes a Lorentzian broadening function and exactly ellipsoidal shape of the liposomes. Based on the formula a program called P-FIT is designed for the practical analysis of the experimental multicomponent spectra of the prolate liposomes. The versatility of the program developed in a Mathematica environment is demonstrated by simulations of a number of 31P-NMR spectra with different complexity.

Functionally Relevant Coupled Dynamic Profile of Bacteriorhodopsin and Lipids in Purple Membranes

The dynamics of bacteriorhodopsin (bR) and the lipid headgroups in oriented purple membranes (PMs) was determined at various temperatures and relative humidity (rh) using solid-state NMR spectroscopy. The 31P NMR spectra of the alpha- and gamma-phosphate groups in methyl phosphatidylglycerophosphate (PGP-Me), which is the major phospholipid in the PM, changed sensitively with hydration levels. Between 253 and 233 K, the signals from a fully hydrated sample became broadened similarly to those of a dry sample at 293 K. The 15N cross polarization (CP) NMR spectral intensities from [15N]Gly bR incorporated into fully hydrated PMs were suppressed in 15N CP NMR spectra at 293 K compared with those of dry membranes but gradually recovered at low temperatures or at lower hydration (75%) levels. The suppression of the NMR signals, which is due to interference with proton decoupling frequency (approximately 45 kHz), coupled with short spin-spin relaxation times (T2) indicates that the loops of bR, in particular, have motional components around this frequency. The motion of the transmembrane alpha-helices in bR was largely affected by the freezing of excess water at low temperatures. While between 253 and 233 K, where a dynamic phase transition-like change was observed in the 31P NMR spectra for the phosphate lipid headgroups, the molecular motion of the loops and the C- and N-termini slowed, suggesting lipid-loop interactions, although protein-protein interactions between stacks cannot be excluded. The results of T2 measurements of dry samples, which do not have proton pumping activity, were similar to those for fully hydrated samples below 213 K where the M-intermediates can be trapped. These results suggest that motions in the 10s micros correlation regime may be functionally important for the photocycle of bR, and protein-lipid interactions are motionally coupled in this dynamic regime.

Exploring membrane selectivity of the antimicrobial peptide KIGAKI using solid-state NMR spectroscopy

The designed antimicrobial peptide KIGAKIKIGAKIKIGAKI possesses enhanced membrane selectivity for bacterial lipids, such as phosphatidylethanolamine and phosphatidylglycerol. The perturbation of the bilayer by the peptide was first monitored using oriented bilayer samples on glass plates. The alignment of POPE/POPG model membranes with respect to the bilayer normal was severely altered at 4 mol% KIGAKI while the alignment of POPC bilayers was retained. The interaction mechanism between the peptide and POPE/POPG bilayers was investigated by carefully comparing three bilayer MLV samples (POPE bilayers, POPG bilayers, and POPE/POPG 4/1 bilayers). KIGAKI induces the formation of an isotropic phase for POPE/POPG bilayers, but only a slight change in the (31)P NMR CSA line shape for both POPE and POPG bilayers, indicating the synergistic roles of POPE and POPG lipids in the disruption of the membrane structure by KIGAKI. (2)H NMR powder spectra show no reduction of the lipid chain order for both POPG and POPE/POPG bilayers upon peptide incorporation, supporting the evidence that the peptide acts as a surface peptide. (31)P longitudinal relaxation studies confirmed that different dynamic changes occurred upon interaction of the peptide with the three different lipid bilayers, indicating that the strong electrostatic interaction between the cationic peptide KIGAKI and anionic POPG lipids is not the only factor in determining the antimicrobial activity. Furthermore, (31)P and (2)H NMR powder spectra demonstrated a change in membrane characteristics upon mixing of POPE and POPG lipids. The interaction between different lipids, such as POPE and POPG, in the mixed bilayers may provide the molecular basis for the KIGAKI carpet mechanism in the permeation of the membrane.

Impact on lipid membrane organization by free branched-chain fatty acids

Here, we exploit the non-invasive techniques of solid-state NMR (nuclear magnetic resonance) and differential scanning calorimetry (DSC) to study the effect of free iso and ante-iso branched chain fatty acids (BCFAs) on the physicochemical properties of lipid membranes. Free fatty acids are present in biological membranes at low abundance, but can influence the cellular function by modulating the membrane organization. Solid state NMR spectra of dimyristoylphosphatidylcholine (DMPC) lipid membranes containing either free 12-methyltetradecanoic acid (a15:0) or free 13-methyltetradecanoic acid (i15:0), show significant differences in their impact on the lipid bilayer. Chain order profiles obtained by deuterium NMR on fully deuterated DMPC-d(67) bilayers revealed an ordering effect induced by both fatty acids on the hydrophobic membrane core. This behavior was also visible in the corresponding DSC thermograms where the main phase transition of DMPC bilayers-indicative of the hydrophobic membrane region-was shifted to higher temperatures, with the iso isomer triggering more pronounced changes as compared to the ante-iso isomer. This is probably due to a higher packing density in the core of the lipid bilayer, which causes reduced diffusion across membranes. By utilizing the naturally occurring spin reporters nitrogen-14 and phosphorus-31 present in the hydrophilic DMPC headgroup region, even fatty acid induced changes at the membrane interface could be detected, an observation reflecting changes in the lipid headgroup dynamics.

Differential stiffness and lipid mobility in the leaflets of purple membranes

Purple membranes (PM) are two-dimensional crystals formed by bacteriorhodopsin and a variety of lipids. The lipid composition and density in the cytoplasmic (CP) leaflet differ from those of the extracellular (EC) leaflet. A new way of differentiating the two sides of such asymmetric membranes using the phase signal in alternate contact atomic force microscopy is presented. This method does not require molecular resolution and is applied to study the stiffness and intertrimer lipid mobility in both leaflets of the PM independently over a broad range of pH and salt concentrations. PM stiffens with increasing salt concentration according to two different regimes. At low salt concentration, the membrane Young's normal modulus grows quickly but differentially for the EC and CP leaflets. At higher salt concentration, both leaflets behave similarly and their stiffness converges toward the native environment value. Changes in pH do not affect PM stiffness; however, the crystal assembly is less pronounced at pH > or = 10. Lipid mobility is high in the CP leaflet, especially at low salt concentration, but negligible in the EC leaflet regardless of pH or salt concentration. An independent lipid mobility study by solid-state NMR confirms and quantifies the atomic force microscopy qualitative observations.

Solid-state nuclear magnetic resonance relaxation studies of the interaction mechanism of antimicrobial peptides with phospholipid bilayer membranes

An 18-residue peptide, KWGAKIKIGAKIKIGAKI-NH(2) was designed to form amphiphilic beta-sheet structures when bound to lipid bilayers. The peptide possesses high antimicrobial activity when compared to naturally occurring linear antimicrobial peptides, most of which adopt an amphipathic alpha-helical conformation upon binding to the lipids. The perturbation of the bilayer by the peptide was studied by static (31)P and (2)H solid-state NMR spectroscopy using POPC and POPG/POPC (3/1) bilayer membranes with sn-1 chain perdeuterated POPC and POPG as the isotopic labels. (31)P NMR powder spectra exhibited two components for POPG/POPC bilayers upon addition of the peptide but only a slight change in the line shape for POPC bilayers, indicating that the peptide selectively disrupted the membrane structure consisting of POPG lipids. (2)H NMR powder spectra indicated a reduction in the lipid chain order for POPC bilayers and no significant change in the ordering for POPG/POPC bilayers upon association of the peptide with the bilayers, suggesting that the peptide acts as a surface peptide in POPG/POPC bilayers. Relaxation rates are more sensitive to the motions of the membranes over a large range of time scales. Longer (31)P longitudinal relaxation times for both POPG and POPC in the presence of the peptide indicated a direct interaction between the peptide and the POPG/POPC bilayer membranes. (31)P longitudinal relaxation studies also suggested that the peptide prefers to interact with the POPG phospholipids. However, inversion-recovery (2)H NMR spectroscopic experiments demonstrated a change in the relaxation rate of the lipid acyl chains for both the POPC membranes and the POPG/POPC membranes upon interaction with the peptide. Transverse relaxation studies indicated an increase in the spectral density of the collective membrane motion caused by the interaction between the peptide and the POPG/POPC membrane. The experimental results demonstrate significant dynamic changes in the membrane in the presence of the antimicrobial peptide and support a carpet mechanism for the disruption of the membranes by the antimicrobial peptide.

Diffusion measurements of water, ubiquinone and lipid bilayer inside a cylindrical nanoporous support: A stimulated echo pulsed-field gradient MAS-NMR investigation

Stimulated echo pulsed-field gradient 1H magic angle spinning NMR has been used to investigate the mobility of water, ubiquinone and tethered phospholipids, components of a biomimetic model membrane. The diffusion constant of water corresponds to an isotropic motion in a cylinder. When the lipid bilayer is obtained after the fusion of small unilamellar vesicles, the extracted value of lipid diffusion indicates unrestricted motion. The cylindrical arrangement of the lipids permits a simplification of data analysis since the normal bilayer is perpendicular to the gradient axis. This feature leads to a linear relation between the logarithm of the attenuation of the signal intensity and a factor depending on the gradient strength, for lipids covering the inner wall of aluminium oxide nanopores as well as for lipids adsorbed on a polymer sheet rolled into a cylinder. The effect of the bilayer formation on water diffusion has also been observed. The lateral diffusion coefficient of ubiquinone is in the same order of magnitude as the lipid lateral diffusion coefficient, in agreement with its localization within the bilayer.

Changes of enzyme activity in lipid signaling pathways related to substrate reordering

The static fluid mosaic model of biological membranes has been progressively complemented by a dynamic membrane model that includes phospholipid reordering in domains that are proposed to extend from nanometers to microns. Kinetic models for lipolytic enzymes have only been developed for homogeneous lipid phases. In this work, we develop a generalization of the well-known surface dilution kinetic theory to cases where, in a same lipid phase, both domain and nondomain phases coexist. Our model also allows understanding the changes in enzymatic activity due to a decrease of free substrate concentration when domains are induced by peptides. This lipid reordering and domain dynamics can affect the activity of lipolytic enzymes, and can provide a simple explanation for how basic peptides, with a strong direct interaction with acidic phospholipids (such as beta-amyloid peptide), may cause a complex modulation of the activities of many important enzymes in lipid signaling pathways.

Interaction of three-finger toxins with phospholipid membranes: comparison of S- and P-type cytotoxins

The CTs (cytotoxins) I and II are positively charged three-finger folded proteins from venom of Naja oxiana (the Central Asian cobra). They belong to S- and P-type respectively based on Ser-28 and Pro-30 residues within a putative phospholipid bilayer binding site. Previously, we investigated the interaction of CTII with multilamellar liposomes of dipalmitoylphosphatidylglycerol by wide-line (31)P-NMR spectroscopy. To compare interactions of these proteins with phospholipids, we investigated the interaction of CTI with the multilamellar liposomes of dipalmitoylphosphatidylglycerol analogously. The effect of CTI on the chemical shielding anisotropy and deformation of the liposomes in the magnetic field was determined at different temperatures and lipid/protein ratios. It was found that both the proteins do not affect lipid organization in the gel state. In the liquid crystalline state of the bilayer they disturb lipid packing. To get insight into the interactions of the toxins with membranes, Monte Carlo simulations of CTI and CTII in the presence of the bilayer membrane were performed. It was found that both the toxins penetrate into the bilayer with the tips of all the three loops. However, the free-energy gain on membrane insertion of CTI is smaller (by approximately 7 kcal/mol; 1 kcal identical with 4.184 kJ) when compared with CTII, because of the lower hydrophobicity of the membrane-binding site of CTI. These results clearly demonstrate that the P-type cytotoxins interact with membranes stronger than those of the S-type, although the mode of the membrane insertion is similar for both the types.

Dominant formation of a single-length channel by amphotericin B in dimyristoylphosphatidylcholine membrane evidenced by 13C-31P rotational echo double resonance

(13)C-Labeled amphotericin B (AmB) was prepared by feeding the producing organism Streptomyces nodosus with [3-(13)C]propionate. The REDOR experiments for dimyristoylphosphatidylcholine (DMPC) membrane using the (13)C-labeled AmB showed the prominent dephasing effects between the phosphate group in PC and C41 carboxyl carbon in the polar head. In addition, C39/C40 methyl carbons also gave rise to the significant reduction of their (13)C NMR signals, implying that both terminal parts of AmB reside close to the surface of the DMPC membrane. Conversely, the same REDOR experiments with use of distearoylphosphatidylcholine (DSPC) showed no dephasing for the C39/C40 methyl signals while a marked reduction of the C41 carbonyl signal was again observed. These findings should be most reasonably accounted for by the notion that AmB can span across the DMPC membrane with a single-length interaction but cannot span the DSPC membrane due to its greater thickness. To our knowledge, the results provide the first direct spectroscopic evidence for the formation of a single-length channel across a biomembrane, which was previously suggested by channel current recording experiments.

The alignment, structure and dynamics of membrane-associated polypeptides by solid-state NMR spectroscopy

Solid-state NMR spectroscopy is being developed at a fast pace for the structural investigation of immobilized and non-crystalline biomolecules. These include proteins and peptides associated with phospholipid bilayers. In contrast to solution NMR spectroscopy, where complete or almost complete averaging leads to isotropic values, the anisotropic character of nuclear interactions is apparent in solid-state NMR spectra. In static samples the orientation dependence of chemical shift, dipolar or quadrupolar interactions, therefore, provides angular constraints when the polypeptides have been reconstituted into oriented membranes. Furthermore, solid-state NMR spectroscopy of aligned samples offers distinct advantages in allowing access to dynamic processes such as topological equilibria or rotational diffusion in membrane environments. Alternatively, magic angle sample spinning (MAS) results in highly resolved NMR spectra, provided that the sample is sufficiently homogenous. MAS spinning solid-state NMR spectra allow to measure distances and dihedral angles with high accuracy. The technique has recently been developed to selectively establish through-space and through-bond correlations between nuclei, similar to the approaches well-established in solution-NMR spectroscopy.

Investigating structural changes in the lipid bilayer upon insertion of the transmembrane domain of the membrane-bound protein phospholamban utilizing 31P and 2H solid-state NMR spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca(2+) ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state (31)P and (2)H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30 degrees C to 60 degrees C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state (31)P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. (31)P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the (31)P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S(CD)) were calculated by performing (2)H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.

The state of lipid rafts: from model membranes to cells

Lipid raft microdomains were conceived as part of a mechanism for the intracellular trafficking of lipids and lipid-anchored proteins. The raft hypothesis is based on the behavior of defined lipid mixtures in liposomes and other model membranes. Experiments in these well-characterized systems led to operational definitions for lipid rafts in cell membranes. These definitions, detergent solubility to define components of rafts, and sensitivity to cholesterol deprivation to define raft functions implicated sphingolipid- and cholesterol-rich lipid rafts in many cell functions. Despite extensive work, the basis for raft formation in cell membranes and the size of rafts and their stability are all uncertain. Recent work converges on very small rafts <10 nm in diameter that may enlarge and stabilize when their constituents are cross-linked.

Interaction of the P-type cardiotoxin with phospholipid membranes

The cardiotoxin (cytotoxin II, or CTII) isolated from cobra snake (Naja oxiana) venom is a 60-residue basic membrane-active protein featuring three-finger beta sheet fold. To assess possible modes of CTII/membrane interaction 31P- and 1H-NMR spectroscopy was used to study binding of the toxin and its effect onto multilamellar vesicles (MLV) composed of either zwitterionic or anionic phospholipid, dipalmitoylglycerophosphocholine (Pam2Gro-PCho) or dipalmitoylglycerophosphoglycerol (Pam2Gro-PGro), respectively. The analysis of 1H-NMR linewidths of the toxin and 31P-NMR spectral lineshapes of the phospholipid as a function of temperature, lipid-to-protein ratios, and pH values showed that at least three distinct modes of CTII interaction with membranes exist: (a) nonpenetrating mode; in the gel state of the negatively charged MLV the toxin is bound to the surface electrostatically; the binding to Pam2Gro-PCho membranes was not observed; (b) penetrating mode; hydrophobic interactions develop due to penetration of the toxin into Pam2Gro-PGro membranes in the liquid-crystalline state; it is presumed that in this mode CTII is located at the membrane/water interface deepening the side-chains of hydrophobic residues at the tips of the loops 1-3 down to the boundary between the glycerol and acyl regions of the bilayer; (c) the penetrating mode gives way to isotropic phase, stoichiometrically well-defined CTII/phospholipid complexes at CTII/lipid ratio exceeding a threshold value which was found to depend at physiological pH values upon ionization of the imidazole ring of His31. Biological implications of the observed modes of the toxin-membrane interactions are discussed.

Deciphering the role of individual acyl chains in the interaction network between phosphatidylserines and a single-spanning membrane protein

PMP1 is a small single-spanning membrane protein functioning as a regulatory subunit of the yeast plasma membrane H(+)-ATPase. This protein forms a unique helix and exhibits a positively charged cytoplasmic domain that is able to specifically segregate phosphatidylserines (PSs). A marked groove formed at the helix surface is thought to play a major role in the related lipid-protein interaction network. Mutational analysis and (1)H NMR experiments were therefore performed on a synthetic PMP1 fragment using DPC-d(38) micelles as a membrane-like environment, in the presence of small amounts of POPS. A mutation designed for altering the helix groove was shown to disfavor the POPS binding specificity as much as that affecting the electrostatic interaction network. From POPS titration experiments monitored by a full set of one- and two-dimensional NOESY spectra, the association between the phospholipids and the PMP1 peptide has been followed. Our data reveal that the clustering of POPS molecules is promoted from a stabilized framework obtained by coupling the PMP1 helix groove to a POPS sn-2 chain. To our knowledge, the NOE-based titration plots displayed in this report constitute the first NMR data that directly distinguish the role of the sn-1 and sn-2 acyl chains in a lipid-protein interaction. The results are discussed while taking into account our accurate knowledge of the yeast plasma membrane composition and its ability to form functional lipid rafts.

Solid-state NMR spectroscopic methods in chemistry

Over the last decades, NMR spectroscopy has grown into an indispensable tool for chemical analysis, structure determination, and the study of dynamics in organic, inorganic, and biological systems. It is commonly used for a wide range of applications from the characterization of synthetic products to the study of molecular structures of systems such as catalysts, polymers, and proteins. Although most NMR experiments are performed on liquid-state samples, solid-state NMR is rapidly emerging as a powerful method for the study of solid samples and materials. This Review outlines some of the developments of solid-state NMR spectroscopy, including techniques such as cross-polarization, magic-angle spinning, multiple-pulse sequences, homo- and heteronuclear decoupling and recoupling techniques, multiple-quantum spectroscopy, and dynamic angle spinning, as well as their applications to structure determination. Modern solid-state NMR spectroscopic techniques not only produce spectra with a resolution close to that of liquid-state spectra, but also capitalize on anisotropic interactions, which are often unavailable for liquid samples. With this background, the future of solid-state NMR spectroscopy in chemistry appears to be promising, indeed.

Effect of variations in the structure of a polyleucine-based alpha-helical transmembrane peptide on its interaction with phosphatidylcholine bilayers

High-sensitivity differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the interaction of an alpha-helical transmembrane peptide, acetyl-Lys2-Leu24-Lys2-amide (L24), and odd-chain members of the homologous series of n-saturated diacylphosphatidylcholines. An analogue of L24, in which the lysine residues were all replaced by 2,3-diaminopropionic acid, and another, in which a leucine residue at each end of the polyLeu sequence was replaced by a tryptophan, were also studied. At low peptide concentrations, the DSC thermograms exhibited by these lipid/peptide mixtures are resolvable into two components. One of these components is fairly narrow, highly cooperative, and exhibits properties which are similar to but not identical with those of the pure lipid. In addition, the transition temperature and cooperativity of this component, and its fractional contribution to the total enthalpy change, decrease with an increase in peptide concentration, more or less independently of phospholipid acyl chain length. The other component is very broad and predominates at high peptide concentrations. These two components have been assigned to the chain-melting phase transitions of populations of peptide-poor and peptide-enriched lipid domains, respectively. Moreover, when the mean hydrophobic thickness of the PC bilayer is less than the peptide hydrophobic length, the peptide-associated lipid melts at higher temperatures than does the bulk lipid and vice versa. In addition, the chain-melting enthalpy of the broad endotherm does not decrease to zero even at high peptide concentrations, suggesting that these peptides reduce somewhat but do not abolish the cooperative gel/liquid-crystalline phase transition of the lipids with which it is in contact. Our DSC results indicate that the width of the broad phase transition observed at high peptide concentration is inversely but discontinuously related to hydrocarbon chain length. Our FTIR spectroscopic data indicate that these peptides form a very stable alpha-helix under all of our experimental conditions but that small distortions of their alpha-helical conformation are induced in response to mismatch between peptide hydrophobic length and gel-state bilayer hydrophobic thickness. We also present evidence that these distortions are localized to the N- and C-terminal regions of these peptides. Interestingly, replacing the terminal Lys residues of L24 by 2,3-diaminopropionic acid residues actually attenuates the hydrophobic mismatch effects of the peptide on the thermotropic phase behavior of the host PC bilayer, in contrast to the predictions of the snorkel hypothesis. We rationalize this attenuated hydrophobic mismatch effect by postulating that the 2,3-diaminopropionic acid residues are too short to engage in significant electrostatic and hydrogen-bonding interactions with the polar headgroups of the host phospholipid bilayer, even in the absence of any hydrophobic mismatch between incorporated peptide and the bilayer. Similarly, the reduced hydrophobic mismatch effect also observed when the two terminal Leu residues of L24 are replaced by Trp residues is rationalized by considering the lower energetic cost of exposing the Trp as opposed to the Leu residues to the aqueous phase in thin PC bilayers and the higher cost of inserting the Trp as opposed to the Leu residues into the hydrophobic cores of thick PC bilayers.

Mechanisms of the interaction of alpha-helical transmembrane peptides with phospholipid bilayers

The synthetic peptide acetyl-K(2)-G-L(24)-K(2)-A-amide (P(24)) and its analogs have been successfully utilized as models of the hydrophobic transmembrane alpha-helical segments of integral membrane proteins. The central polyleucine region of these peptides was designed to form a maximally stable, very hydrophobic alpha-helix which will partition strongly into the hydrophobic environment of the lipid bilayer core, while the dilysine caps were designed to anchor the ends of these peptides to the polar surface of the lipid bilayer and to inhibit the lateral aggregation of these peptides. Moreover, the normally positively charged N-terminus and the negatively charged C-terminus have both been blocked in order to provide a symmetrical tetracationic peptide, which will more faithfully mimic the transbilayer region of natural membrane proteins and preclude favorable electrostatic interactions. In fact, P(24) adopts a very stable alpha-helical conformation and transbilayer orientation in lipid model membranes. The results of our recent studies of the interaction of this family of alpha-helical transmembrane peptides with phospholipid bilayers are summarized here.

Bilayer sample for fast or slow magic angle oriented sample spinning solid-state NMR spectroscopy

An alternative setup for Magic Angle Oriented Spinning Spectroscopy is proposed. Samples were prepared by orienting lipid bilayers onto polymer films, which were wrapped into a spiral so as to fit into 4 or 7 mm MAS rotors. This geometry resulted in narrow line widths and a higher upper spinning limit when compared to the conventional MAOSS setup with stacked glass plates. Whereas orientational information was extracted from low spinning spectra, fast spinning will be applicable to high-resolution multidimensional NMR pulse sequences.

Polyelectrolyte molecular weight and electrostatically-induced domains in lipid bilayer membranes

Polyelectrolyte-induced domain formation in charged lipid bilayer membranes was investigated as a function of polyelectrolyte molecular weight using 2H nuclear magnetic resonance (NMR) spectroscopy. Lipid bilayers consisting of mixtures of alpha- or beta-choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-alpha-d2 or POPC-beta-d2) plus the cationic amphiphile 1,2-dioleoyl-3-(dimethylamino)propane (DODAP) were exposed to the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS) of various molecular weights. Regardless of molecular weight, PSSS produced dual component 2H NMR spectra, indicating two distinct POPC populations, corresponding to PSSS-bound and PSSS-free lipid, in slow exchange with one another. Analysis of the 2H NMR subspectra quadrupolar splittings and intensities showed the PSSS-bound domain to be enriched in DODAP, with the PSSS-free domain correspondingly depleted. At polyelectrolyte loadings below global charge equivalence, PSSS bound DODAP stoichiometrically for all PSSS molecular weights, indicating that the polyelectrolyte chain lies flat upon the membrane surface. At higher PSSS loadings the domains dissipated, leading to single component 2H NMR spectra. At high NaCl concentrations PSSS dissociated from the bilayer surface. Domain size on a per PSSS chain basis increased while the degree of enrichment with DODAP decreased progressively as the PSSS chain length decreased. Such molecular weight-dependent domain characteristics have not been predicted theoretically and need to be taken into account in future refinements of domain models.