The distribution of lipid attached spin probes in bilayers: application to membrane protein topology

Heterogeneous micellar solubilization within lyotropic liquid crystals interfaces

The solubilization of sodium diclofenac (Na-DFC) in a glycerol monooleate-based emulsion triggers series of structural changes. Incorporation of Na-DFC, leads to formation of a reverse hexagonal mesophase between 2 and 5 wt% Na-DFC. Between 6 and 9 wt% Na-DFC, the hexagonal symmetry gradually transitions to a disordered lamellar mesophase. These structural shifts impact the system's storage modulus, structuring enthalpy, and structural diffusivity. Despite these transitions, the driving force for Na-DFC release remains consistent, leading to hypothesize that the interfacial structure remains unchanged during Na-DFC release. The nano-structural modifications imposed by the Na-DFC load and release were assessed by small-angle X-ray diffraction (SAXD), spin-probe electron paramagnetic resonance (EPR), and nuclear quadrupole resonance (NQR). The selective solubilization of Na-DFC was demonstrated by SAXD peak fittings, revealing an increase of hexagonally oriented rods at the expense of non-oriented micelles, rather than gradual micellar elongation. Computation of the EPR spectra also showcased the selective solubilization of Na-DFC at an enhanced free energy interface (γ), evidenced by step-wise variations in polarity, microviscosity, and order parameters. Additionally, NQR analysis highlighted a higher anisotropy for sodium compared to deuterium, linking the selective solubilization of Na-DFC to heterogeneous structural transformations. These findings underscore the heterogeneous nature of solubilization-release processes, driven by locally increased micellar free energy. Consequently, the loaded Na-DFC interfaces maintain a constant γ, ensuring a consistent release driving force despite the structural transitions affecting the matrix. The ability to selectively solubilize guest molecules may herald a new era in the utilization of selective molecular interfacial loading.

Localization of the ibuprofen molecule in model lipid membranes revealed by spin-label-enhanced NMR relaxation

Non-steroidal anti-inflammatory drugs (NSAIDs) have antipyretic, anti-inflammatory and analgesic effects, and can be used in the treatment of various diseases. These drugs have also a number of side effects, which may be related to their interaction with lipid membranes. In this study, we use the spin-labeled NSAID ibuprofen (ibuprofen-SL) as a relaxation enhancer to study its interaction with model lipid membranes employing liquid-state 1H NMR at 500 MHz. The high magnetic moment of unpaired electron in the spin label made it possible to reduce the concentration of the studied drug in the membrane to tenths of a mole percent. As model membranes, unilamellar POPC liposomes and bicelles consisting of a 2:1 mixture of DHPC:DMPC or DHPC:POPC lipids were used. An increase in the rate of proton spin-lattice relaxation, T1-1, selectively detected for protons at different positions in the lipid molecule, showed that ibuprofen-SL is localized in the hydrophobic part of the lipid bilayer. As the concentration of ibuprofen-SL increases to 0.5 mol%, the distribution of positions of ibuprofen-SL across the bilayer becomes wider. In the presence of 20 mol% of cholesterol, ibuprofen-SL is displaced from the core of the membrane to a region closer to the head group of the bilayer. This displacement was also confirmed by the NMR NOESY experiment conducted with unlabeled ibuprofen. For bilayers containing unsaturated POPC lipids, the distribution of ibuprofen positions across the bilayer becomes narrower compared to the presence of saturated DMPC lipids.

Myr-Arf1 conformational flexibility at the membrane surface sheds light on the interactions with ArfGAP ASAP1

ADP-ribosylation factor 1 (Arf1) interacts with multiple cellular partners and membranes to regulate intracellular traffic, organelle structure and actin dynamics. Defining the dynamic conformational landscape of Arf1 in its active form, when bound to the membrane, is of high functional relevance and key to understanding how Arf1 can alter diverse cellular processes. Through concerted application of nuclear magnetic resonance (NMR), neutron reflectometry (NR) and molecular dynamics (MD) simulations, we show that, while Arf1 is anchored to the membrane through its N-terminal myristoylated amphipathic helix, the G domain explores a large conformational space, existing in a dynamic equilibrium between membrane-associated and membrane-distal conformations. These configurational dynamics expose different interfaces for interaction with effectors. Interaction with the Pleckstrin homology domain of ASAP1, an Arf-GTPase activating protein (ArfGAP), restricts motions of the G domain to lock it in what seems to be a conformation exposing functionally relevant regions.

Natamycin sequesters ergosterol and interferes with substrate transport by the lysine transporter Lyp1 from yeast

Natamycin is a polyene macrolide, widely employed to treat fungal keratitis and other yeast infections as well as to protect food products against fungal molds. In contrast to other polyene macrolides, such as nystatin or amphotericin B, natamycin does not form pores in yeast membranes, and its mode of action is not well understood. Here, we have employed a variety of spectroscopic methods, computational modeling, and membrane reconstitution to study the molecular interactions of natamycin underlying its antifungal activity. We find that natamycin forms aggregates in an aqueous solution with strongly altered optical properties compared to monomeric natamycin. Interaction of natamycin with model membranes results in a concentration-dependent fluorescence increase which is more pronounced for ergosterol- compared to cholesterol-containing membranes up to 20 mol% sterol. Evidence for formation of specific ergosterol-natamycin complexes in the bilayer is provided. Using nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, we find that natamycin sequesters sterols, thereby interfering with their well-known ability to order acyl chains in lipid bilayers. This effect is more pronounced for membranes containing the sterol of fungi, ergosterol, compared to those containing mammalian cholesterol. Natamycin interferes with ergosterol-dependent transport of lysine by the yeast transporter Lyp1, which we propose to be due to the sequestering of ergosterol, a mechanism that also affects other plasma membrane proteins. Our results provide a mechanistic explanation for the selective antifungal activity of natamycin, which can set the stage for rational design of novel polyenes in the future.

Lipids-Fluorophores Interactions Probed by Combined Nonlinear Polarized Microscopy

Studying the structural dynamics of lipid membranes requires methods that can address both microscopic and macroscopic characteristics. Fluorescence imaging is part of the most used techniques to study membrane properties in various systems from artificial membranes to cells: It benefits from a high sensitivity to local properties such as polarity and molecular orientational order, with a high spatial resolution down to the single-molecule level. The influence of embedded fluorescent lipid probes on the lipid membrane molecules is however poorly known and relies most often on molecular dynamics simulations, due to the challenges faced by experimental approaches to address the molecular-scale dimension of this question. In this work we develop an optical microscopy imaging method to probe the effect of fluorophores embedded in the membrane as lipid probes, on their lipid environment, with a lateral resolution of a few hundreds of nanometers. We combine polarized-nonlinear microscopy contrasts that can independently address the lipid probe, by polarized two-photon fluorescence, and the membrane lipids, by polarized coherent Raman scattering. Using trimethylamino derivative 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS) as model probes, we show that both probes tend to induce an orientational disorder of their surrounding lipid CH-bonds in 1,2-dipalmitoylphosphatidylcholine (DPPC) lipids environments, while there is no noticeable effect in more disordered 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid membranes.

Interaction of the pitavastatin with model membranes

Pitavastatin is a statin drug that, by competitively inhibiting 3-hydroxy-3-methylglutaryl-coenzyme A reductase, can lower serum cholesterol levels of low-density lipoprotein (LDL) accompanied by side effects due to pleiotropic effects leading to statin intolerance. These effects can be explained by the lipophilicity of statins, which creates membrane affinity and causes statin localization in cellular membranes. In the current report, the interaction of pitavastatin with POPC model membranes and its influence on the membrane structure were investigated using H, H and P solid-state NMR spectroscopy. Our experiments show the average localization of pitavastatin at the lipid/water interface of the membrane, which is biased towards the hydrocarbon core in comparison to other statin molecules. The membrane binding of pitavastatin also introduced an isotropic component into the ³¹P NMR powder spectra, suggesting that some of the lamellar POPC molecules are converted into highly curved structures.

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Solid-state NMR spectroscopy shows pitavastatin effect on the bilayer

•Pitavastatin lowers the POPC order parameters

•Pitavastatin localize in the upper chain of the POPC bilayer

•Isotropic membrane phases are observed in the presence of pitavastatin

The pro-apoptotic domain of BIM protein forms toxic amyloid fibrils

BIM is a key apoptotic protein, participating in diverse cellular processes. Interestingly, recent studies have hypothesized that BIM is associated with the extensive neuronal cell death encountered in protein misfolding diseases, such as Alzheimer's disease. Here, we report that the core pro-apoptotic domain of BIM, the BIM-BH3 motif, forms ubiquitous amyloid fibrils. The BIM-BH3 fibrils exhibit cytotoxicity, disrupt mitochondrial functions, and modulate the structures and dynamics of mitochondrial membrane mimics. Interestingly, a slightly longer peptide in which BIM-BH3 was flanked by four additional residues, widely employed as a model of the pro-apoptotic core domain of BIM, did not form fibrils, nor exhibited cell disruptive properties. The experimental data suggest a new mechanistic role for the BIM-BH3 domain, and demonstrate, for the first time, that an apoptotic peptide forms toxic amyloid fibrils.

Doxyl Nitroxide Spin Probes Can Modify Toxicity of Doxorubicin towards Fibroblast Cells

The biological properties of doxyl stearate nitroxides (DSs): 5-DS, Met-12-DS, and 16-DS, commonly used as spin probes, have not been explored in much detail so far. Furthermore, the influence of DSs on the cellular changes induced by the anticancer drug doxorubicin (DOX) has not yet been investigated. Therefore, we examined the cytotoxicity of DSs and their ability to induce cell death and to influence on fluidity and lipid peroxidation (LPO) in the plasma membrane of immortalised B14 fibroblasts, used as a model neoplastic cells, susceptible to DOX-induced changes. The influence of DSs on DOX toxicity was also investigated and compared with that of a natural reference antioxidant α-Tocopherol. By employing the trypan blue exclusion test and double fluorescent staining, we found a significant level of cytotoxicity for DSs and showed that their ability to induce apoptosis and modify plasma membrane fluidity (measured fluorimetrically) is more potent than for α-Tocopherol. The most cytotoxic nitroxide was 5-DS. The electron paramagnetic resonance (EPR) measurements revealed that 5-DS was reduced in B14 cells at the fastest and Met-12-DS at the slowest rate. In the presence of DOX, DSs were reduced slower than alone. The investigated compounds, administered with DOX, enhanced DOX-induced cell death and demonstrated concentration-dependent biphasic influence on membrane fluidity. A-Tocopherol showed weaker effects than DSs, regardless the mode of its application-alone or with DOX. High concentrations of α-Tocopherol and DSs decreased DOX-induced LPO. Substantial cytotoxicity of the DSs suggests that they should be used more carefully in the investigations performed on sensitive cells. Enhancement of DOX toxicity by DSs showed their potential to act as chemosensitizers of cancer cells to anthracycline chemotherapy.

Mitochondria membrane transformations in colon and prostate cancer and their biological implications

Mitochondria have emerged as important determinants in cancer progression and malignancy. However, the role of mitochondrial membranes in cancer onset and progression has not been thoroughly investigated. This study compares the structural and functional properties of mitochondrial membranes in prostate and colon cancer cells in comparison to normal mitochondria, and possible therapeutic implications of these membrane changes. Specifically, isolation of cell mitochondria and preparation of inverted sub-mitochondrial particles (SMPs) illuminated significant cancer-induced modulations of membrane lipid compositions, fluidity, and activity of cytochrome c oxidase, one of the key mitochondrial enzymes. The experimental data further show that cancer-associated membrane transformations may account for mitochondria targeting by betulinic acid and resveratrol, known anti-cancer molecules. Overall, this study probes the relationship between cancer and mitochondrial membrane transformations, underlying a potential therapeutic significance for mitochondrial membrane targeting in cancer.

Aβ42 Double Mutant Inhibits Aβ42-Induced Plasma and Mitochondrial Membrane Disruption in Artificial Membranes, Isolated Organs, and Intact Cells

Destabilization of plasma and inner mitochondrial membranes by extra- and intracellular amyloid β peptide (Aβ42) aggregates may lead to dysregulated calcium flux through the plasma membrane, mitochondrial-mediated apoptosis, and neuronal cell death in patients with Alzheimer's disease. In the current study, experiments performed with artificial membranes, isolated mitochondria, and neuronal cells allowed us to understand the mechanism by which a nonaggregating Aβ42 double mutant (designated Aβ42_DM) exerts its neuroprotective effects. Specifically, we showed that Aβ42_DM protected neuronal cells from Aβ42-induced accumulation of toxic intracellular levels of calcium and from apoptosis. Aβ42_DM also inhibited Aβ42-induced mitochondrial membrane potential depolarization in the cells and abolished the Aβ42-mediated decrease in cytochrome c oxidase activity in purified mitochondrial particles. These results can be explained in terms of the amelioration by Aβ42_DM of Aβ42-mediated changes in membrane fluidity in DOPC and cardiolipin/DOPC phospholipid vesicles, mimicking plasma and mitochondrial membranes, respectively. These observations are also in agreement with the inhibition by Aβ42_DM of phospholipid-induced conformational changes in Aβ42 and with the fact that, unlike Aβ42, the Aβ42-Aβ42_DM complex could not permeate into cells but instead remained attached to the cell membrane. Although most of the Aβ42_DM molecules were localized on the cell membrane, some penetrated into the cytosol in an Aβ42-independent process, and, unlike Aβ42, did not form intracellular inclusion bodies. Overall, we provide a mechanistic explanation for the inhibitory activity of Aβ42_DM against Aβ42-induced membrane permeability and cell toxicity and provide confirmatory evidence for its protective function in neuronal cells.

Membrane Localization and Lipid Interactions of Common Lipid-Conjugated Fluorescence Probes

Lateral segregation of lipids in model and biological membranes has been studied intensively in the last decades using a comprehensive set of experimental techniques. Most methods require a probe to report on the biophysical properties of a specific molecule in the lipid bilayer. Because such probes can adversely affect the results of the measurement and perturb the local membrane structure and dynamics, a detailed understanding of probe behavior and its influence on the properties of its direct environment is important. Membrane phase-selective and lipid-mimicking molecules represent common types of probes. Here, we have studied how the fluorescent probes trans-parinaric acid (tPA), diphenylhexatriene (DPH), and 1-oleoyl-2-propionyl[DPH]-sn-glycero-3-phosphocholine (O-DPH-PC) affect the membrane properties of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) bilayers using ²H and ³¹P NMR spectroscopy in the solid state. In addition, using 2D ¹H magic-angle spinning (MAS) nuclear Overhauser enhancement spectroscopy (NOESY) NMR, we have determined the distribution of the probe moieties in the POPC membrane parallel to the membrane normal. We found that the different probes exhibit distinct membrane localizations and distributions, e.g. tPA is located parallel to the membrane normal while DPH predominantly exist in two orientations. Further, tPA was conjugated to sphingomyelin (tPA-SM) as a substitute for the acyl chain in the SM. ¹H NOESY NMR was used to probe the interaction of the tPA-SM with cholesterol as dominant in liquid ordered membrane domains in comparison to POPC-cholesterol interaction in membranes composed of ternary lipid mixtures. We could show that tPA-SM exhibited a strong favorable and very temperature-dependent interaction with cholesterol in comparison to POPC. In conclusion, the NMR techniques can explain probe behavior but also be used to measure lipid-specific affinities between different lipid segments and individual molecules in complex bilayers, relevant to understanding nanodomain formation in biological membranes.

Interaction of statins with phospholipid bilayers studied by solid-state NMR spectroscopy

Statins are drugs that specifically inhibit the enzyme HMG-CoA reductase and thereby reduce the concentration of low-density lipoprotein cholesterol, which represents a well-established risk factor for the development of atherosclerosis. The results of several clinical trials have shown that there are important intermolecular differences responsible for the broader pharmacologic actions of statins, even beyond HMG-CoA reductase inhibition. According to one hypothesis, the biological effects exerted by these compounds depend on their localization in the cellular membrane. The aim of the current work was to study the interactions of different statins with phospholipid membranes and to investigate their influence on the membrane structure and dynamics using various solid-state NMR techniques. Using ¹H NOESY MAS NMR, it was shown that atorvastatin, cerivastatin, fluvastatin, rosuvastatin, and some percentage of pravastatin intercalate the lipid-water interface of POPC membranes to different degrees. Based on cross-relaxation rates, the different average distribution of the individual statins in the bilayer was determined quantitatively. Investigation of the influence of the investigated statins on membrane structure revealed that lovastatin had the least effect on lipid packing and chain order, pravastatin significantly lowered lipid chain order, while the other statins slightly decreased lipid chain order parameters mostly in the middle segments of the phospholipid chains.

Expression, Functional Characterization, and Solid-State NMR Investigation of the G Protein-Coupled GHS Receptor in Bilayer Membranes

The expression, functional reconstitution and first NMR characterization of the human growth hormone secretagogue (GHS) receptor reconstituted into either DMPC or POPC membranes is described. The receptor was expressed in E. coli. refolded, and reconstituted into bilayer membranes. The molecule was characterized by ¹⁵N and ¹³C solid-state NMR spectroscopy in the absence and in the presence of its natural agonist ghrelin or an inverse agonist. Static ¹⁵N NMR spectra of the uniformly labeled receptor are indicative of axially symmetric rotational diffusion of the G protein-coupled receptor in the membrane. In addition, about 25% of the ¹⁵N sites undergo large amplitude motions giving rise to very narrow spectral components. For an initial quantitative assessment of the receptor mobility, ¹H-¹³C dipolar coupling values, which are scaled by molecular motions, were determined quantitatively. From these values, average order parameters, reporting the motional amplitudes of the individual receptor segments can be derived. Average backbone order parameters were determined with values between 0.56 and 0.69, corresponding to average motional amplitudes of 40–50° of these segments. Differences between the receptor dynamics in DMPC or POPC membranes were within experimental error. Furthermore, agonist or inverse agonist binding only insignificantly influenced the average molecular dynamics of the receptor.

Juxta-terminal Helix Unwinding as a Stabilizing Factor to Modulate the Dynamics of Transmembrane Helices

Transmembrane helices of integral membrane proteins often are flanked by interfacial aromatic residues that can serve as anchors to aid the stabilization of a tilted transmembrane orientation. Yet, physical factors that govern the orientation or dynamic averaging of individual transmembrane helices are not well understood and have not been adequately explained. Using solid-state (2) H NMR spectroscopy to examine lipid bilayer-incorporated model peptides of the GWALP23 (acetyl-GGALW(LA)6 LWLAGA-amide) family, we observed substantial unwinding at the terminals of several tilted helices spanning the membranes of DLPC, DMPC, or DOPC lipid bilayers. The fraying of helix ends might be vital for defining the dynamics and orientations of transmembrane helices in lipid bilayer membranes.

17β-estradiol (E2) in membranes: Orientation and dynamic properties

Non-genomic membrane effects of estrogens are of great interest because of the diverse biological activities they may elicit. To further our understanding of the molecular features of the interaction between estrogenic hormones and membrane bilayers, we have determined the preferred orientation, location, and dynamic properties of 17β-estradiol (E2) in two different phospholipid membrane environments using (2)H-NMR and 2D (1)H-(13)C HSQC in conjunction with molecular dynamics simulations. Unequivocal spectral assignments to specific (2)H labels were made possible by synthesizing six selectively deuterated E2 molecules. The data allow us to conclude that the E2 molecule adopts a nearly "horizontal" orientation in the membrane bilayer with its long axis essentially perpendicular to the lipid acyl-chains. All four rings of the E2 molecule are located near the membrane interface, allowing both the E2 3-OH and the 17β-OH groups to engage in hydrogen bonding and electrostatic interactions with polar phospholipid groups. The findings augment our knowledge of the molecular interactions between E2 and membrane bilayer and highlight the asymmetric nature of the dynamic motions of the rigid E2 molecule in a membrane environment.

Interaction of Spin-Labeled Lipid Membranes with Transition Metal Ions

The large values of spin relaxation enhancement (RE) for PC spin-labels in the phospholipid membrane induced by paramagnetic metal salts dissolved in the aqueous phase can be explained by Heisenberg spin exchange due to conformational fluctuations of the nitroxide group as a result of membrane fluidity, flexibility of lipid chains, and, possibly, amphiphilic nature of the nitroxide label. Whether the magnetic interaction occurs predominantly via Heisenberg spin exchange (Ni) or by the dipole–dipole (Gd) mechanism, it is essential for the paramagnetic ion to get into close proximity to the nitroxide moiety for efficient RE. For different salts of Ni the RE in phosphatidylcholine membranes follows the anionic Hofmeister series and reflects anion adsorption followed by anion-driven attraction of paramagnetic cations on the choline groups. This adsorption is higher for chaotropic ions, e.g., perchlorate. (A chaotropic agent is a molecule in water solution that can disrupt the hydrogen bonding network between water molecules.) However, there is no anionic dependence of RE for model membranes made from negatively charged lipids devoid of choline groups. We used Ni-induced RE to study the thermodynamics and electrostatics of ion/membrane interactions. We also studied the effect of membrane composition and the phase state on the RE values. In membranes with cholesterol a significant difference is observed between PC labels with nitroxide tethers long enough vs not long enough to reach deep into the membrane hydrophobic core behind the area of fused cholesterol rings. This study indicates one must be cautious in interpreting data obtained by PC labels in fluid membranes in terms of probing membrane properties at different immersion depths when it can be affected by paramagnetic species at the membrane surface.

Coexistence of probe conformations in lipid phases-a polarized fluorescence microspectroscopy study

Several well-established fluorescence methods depend on environment-sensitive probes that report about molecular properties of their local environment. For reliable interpretation of experiments, careful characterization of probes' behavior is required. In this study, bleaching-corrected polarized fluorescence microspectroscopy with nanometer spectral peak position resolution was applied to characterize conformations of two alkyl chain-labeled 7-nitro-2-1,3-benzoxadiazol-4-yl phospholipids in three model membranes, representing the three main lipid phases. The combination of polarized and spectral detection revealed two main probe conformations with their preferential fluorophore dipole orientations roughly parallel and perpendicular to membrane normal. Their peak positions were separated by 2-6 nm because of different local polarities and depended on lipid environment. The relative populations of conformations, estimated by a numerical model, indicated a specific sensitivity of the two probes to molecular packing with cholesterol. The coexistence of probe conformations could be further exploited to investigate membrane organization below microscopy spatial resolution, such as lipid rafts. With the addition of polarized excitation or detection to any environment-sensitive fluorescence imaging technique, the conformational analysis can be directly applied to explore local membrane complexity.

Measuring membrane penetration with depth-dependent fluorescence quenching: distribution analysis is coming of age

Depth-dependent fluorescence quenching by lipid-attached quenchers (e.g., bromine atoms and doxyl groups) is an important tool for determining the penetration of proteins and peptides into lipid bilayers. Extracting quantitative information and accurate calculations of the depth of the fluorophore are complicated by thermal disorder, resulting in broad distributions of the transverse positions of both quenchers and fluorophores. Twenty-one years ago a methodology called distribution analysis (DA) was introduced, based on the emerging view of the complexity of the transverse organization of lipid bilayer structure. The method is aimed at extracting quantitative information on membrane penetration, such as position and width of fluorophore's distribution along the depth coordinate and its exposure to the lipid phase. Here we review recent progress in refining the DA method and illustrate its applications to protein-membrane interactions. We demonstrate how basic assumptions of the DA approach can be validated using molecular dynamics simulations and how the precision of depth determination is improved by applying a new protocol based on a combination of steady-state and time-resolved fluorescence quenching. Using the example of the MPER fragment of the membrane-spanning domain of the HIV-1 gp41 fusion protein, we illustrate how DA applications and computer simulations can be used together to reveal the molecular organization of a protein-membrane complex. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins

Elucidating structure and topology of membrane proteins (MPs) is essential for unveiling functionality of these important biological constituents. Oriented-sample solid-state NMR (OS-NMR) is capable of providing such information on MPs under nearly physiological conditions. However, two dimensional OS-NMR experiments can take several days to complete due to long longitudinal relaxation times combined with the large number of scans to achieve sufficient signal sensitivity in biological samples. Here, free radicals 5-DOXYL stearic acid, TEMPOL, and CAT-1 were added to uniformly (15)N-labeled Pf1 coat protein reconstituted in DMPC/DHPC bicelles, and their effect on the longitudinal relaxation times (T1Z) was investigated. The dramatically shortened T1Z's allowed for the signal gain per unit time to be used for either: (i) up to a threefold reduction of the total experimental time at 99% magnetization recovery or (ii) obtaining up to 74% signal enhancement between the control and radical samples during constant experimental time at "optimal" relaxation delays. In addition, through OS-NMR and high-field EPR studies, free radicals were able to provide positional constraints in the bicelle system, which provide a description of the location of each residue in Pf1 coat protein within the bicellar membranes. This information can be useful in the determination of oligomerization states and immersion depths of larger membrane proteins.

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.

FERM domain of moesin desorbs the basic-rich cytoplasmic domain of l-selectin from the anionic membrane surface

Moesin and calmodulin (CaM) jointly associate with the cytoplasmic domain of l-selectin in the cell to modulate the function and ectodomain shedding of l-selectin. Using fluorescence spectroscopy, we have examined the association of moesin FERM domain with the recombinant transmembrane and cytoplasmic domains of l-selectin (CLS) reconstituted in model phospholipid liposomes. The dissociation constant of moesin FERM domain to CLS in the phosphatidylcholine liposome is about 300nM. In contrast to disrupting the CaM association with CLS, inclusion of anionic phosphatidylserine lipids in the phosphatidylcholine liposome increased the apparent binding affinity of moesin FERM domain for CLS. Using the environmentally sensitive fluorescent probe attached to the cytoplasmic domain of CLS and the nitroxide quencher attached to the lipid bilayer, we showed that the association of moesin FERM domain induced the desorption of the basic-rich cytoplasmic domain of CLS from the anionic membrane surface, which enabled subsequent association of CaM to the cytoplasmic domain of CLS. These results have elucidated the molecular basis for the moesin/l-selectin/CaM ternary complex and suggested an important role of phospholipids in modulating l-selectin function and shedding.

Molecular dynamics simulations of depth distribution of spin-labeled phospholipids within lipid bilayer

Spin-labeled lipids are commonly used as fluorescence quenchers in studies of membrane penetration of dye-labeled proteins and peptides using depth-dependent quenching. Accurate calculations of depth of the fluorophore rely on the use of several spin labels placed in the membrane at various positions. The depth of the quenchers (spin probes) has to be determined independently; however, experimental determination of transverse distributions of spin probe depths is difficult. In this Article, we use molecular dynamics (MD) simulations to study the membrane behavior and depth distributions of spin-labeled phospholipids in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. To probe different depths within the bilayer, a series containing five Doxyl-labeled lipids (n-Doxyl PC) has been studied, in which a spin moiety was covalently attached to nth carbon atoms (where n = 5, 7, 10, 12, and 14) of the sn-2 stearoyl chain of the host phospholipid. Our results demonstrate that the chain-attached spin labels are broadly distributed across the model membrane and their environment is characterized by a high degree of mobility and structural heterogeneity. Despite the high thermal disorder, the depth distributions of the Doxyl labels were found to correlate well with their attachment positions, indicating that the distribution of the spin label within the model membrane is dictated by the depth of the nth lipid carbon atom and not by intrinsic properties of the label. In contrast, a much broader and heterogeneous distribution was observed for a headgroup-attached Tempo spin label of Tempo-PC lipids. MD simulations reveal that, due to the hydrophobic nature, a Tempo moiety favors partitioning from the headgroup region deeper into the membrane. Depending on the concentration of Tempo-PC lipids, the probable depth of the Tempo moiety could span a range from 14.4 to 18.2 Å from the membrane center. Comparison of the MD-estimated immersion depths of Tempo and n-Doxyl labels with their suggested experimental depth positions allows us to review critically the possible sources of error in depth-dependent fluorescence quenching studies.

Jahn-Teller dynamics in a series of high-symmetry Co(II) chelates determine paramagnetic relaxation enhancements

NMR paramagnetic relaxation enhancements (PREs) of a series of structurally characterized, trigonal bis-trispyrazolylborate (Tp) chelates of high-spin Co(II), spanning 100-850 MHz in field, are reported. Prior knowledge of the metal-nucleus distances allows numerical extraction of position-dependent electron spin relaxation rates (τ(c)(-1)) from direct measurement of the individual PREs of the four symmetry distinct protons in Co(Tp)(2), using available closed-form expressions. The data for this electronically complex system where spin-orbit coupling defines the ground state electronic structure are analyzed in terms of the Solomon-Bloembergen-Morgan (SBM) relations, as well as available zero-field splitting limit theories. A simple angular correction is shown to be sufficient to reconcile the individual τ(c)(T) data for the four classes of protons. The data identify a previously unrecognized dynamic Jahn-Teller effect in these historically important complexes, with a barrier of ~230 cm(-1), pointing to a level of dynamics in trispyrazolylborate chemistry that has not been described before, and further show that it is the Jahn-Teller that is responsible for the PREs in fluid solution. A field-dependent component is also identified for the two protons nearest g(//), which is suggested to arise due to Zeeman mixing of excited state character into the ground level.

Conformational distributions and hydrogen bonding in gel and frozen lipid bilayers: a high frequency spin-label ESR study

The ESR parameters of PC spin labels in frozen membranes do not simply represent the membrane polarity or water penetration profile. Instead, they show a distribution between hydrogen-bonded (HB) and non-hydrogen-bonded (non-HB) states, which is affected by a number of factors in the membrane composition. Similar to the exclusion of solutes from crystallizing solvents, the pure bulk gel phase excludes nitroxides, forcing acyl chains to take bent conformations. In these conformations, the nitroxide is hydrogen-bonded. Furthermore, upon gradual cooling in the supercooled gel, PC labels undergo slow lateral aggregation, resulting in a broad background signal. However, if the sample is instantly frozen, this background is replaced by the HB component. In membranes with cholesterol, the observed HB/non-HB ratio can best be described by a partition-like equilibrium between nitroxides located in defects of lipid structure within the hydrophobic core and those close to the membrane surface.

Metalloporphyrin intercalation in liposome membranes: ESR study

Liposomes characterized by membranes featuring diverse fluidity (liquid-crystalline and/or gel phase), prepared from egg yolk lecithin (EYL) and dipalmitoylphosphatidylcholine (DPPC), were doped with selected metalloporphyrins and the time-related structural and dynamic changes within the lipid double layer were investigated. Porphyrin complexes of Mg(II), Mn(III), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and the metal-free base were embedded into the particular liposome systems and tested for 350 h at 24°C using the electron spin resonance (ESR) spin probe technique. 5-DOXYL, 12-DOXYL, and 16-DOXYL stearic acid methyl ester spin labels were applied to explore the interior of the lipid bilayer. Only the 16-DOXYL spin probe detected evident structural changes inside the lipid system due to porphyrin intercalation. Fluidity of the lipid system and the type of the porphyrin complex introduced significantly affected the intermolecular interactions, which in certain cases may result in self-assembly of metalloporphyrin molecules within the liposome membrane, reflected in the presence of new lines in the relevant ESR spectra. The most pronounced time-related effects were demonstrated by the EYL liposomes (liquid-crystalline phase) when doped with Mg and Co porphyrins, whereas practically no spectral changes were revealed for the metal-free base and both the Ni and Zn dopants. ESR spectra of the porphyrin-doped gel phase of DPPC liposomes did not show any extra lines; however, they indicated the formation of a more rigid lipid medium. Electronic configuration of the porphyrin's metal center appeared crucial to the degree of molecular reorganization within the phospholipid bilayer system.

Solid-state NMR paramagnetic relaxation enhancement immersion depth studies in phospholipid bilayers

A new approach for determining the membrane immersion depth of a spin-labeled probe has been developed using paramagnetic relaxation enhancement (PRE) in solid-state NMR spectroscopy. A DOXYL spin label was placed at different sites of 1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine (PSPC) phospholipid bilayers as paramagnetic moieties and the resulting enhancements of the longitudinal relaxation (T₁) times of ³¹P nuclei on the surface of the bilayers were measured by a standard inversion recovery pulse sequence. The ³¹P NMR spin-lattice relaxation times decrease steadily as the DOXYL spin label moves closer to the surface as well as the concentration of the spin-labeled lipids increase. The enhanced relaxation vs. the position and concentration of spin-labels indicate that PRE induced by the DOXYL spin label are significant to determine longer distances over the whole range of the membrane depths. When these data were combined with estimated correlation times τ(c), the r⁻⁶-weighted, time-averaged distances between the spin-labels and the ³¹P nuclei on the membrane surface were estimated. The application of using this solid-state NMR PRE approach coupled with site-directed spin labeling (SDSL) may be a powerful method for measuring membrane protein immersion depth.

Solution structure and phospholipid interactions of the isolated voltage-sensor domain from KvAP

Voltage-sensor domains (VSDs) are specialized transmembrane segments that confer voltage sensitivity to many proteins such as ion channels and enzymes. The activities of these domains are highly dependent on both the chemical properties and the physical properties of the surrounding membrane environment. To learn about VSD-lipid interactions, we used nuclear magnetic resonance spectroscopy to determine the structure and phospholipid interface of the VSD from the voltage-dependent K(+) channel KvAP (prokaryotic Kv from Aeropyrum pernix). The solution structure of the KvAP VSD solubilized within phospholipid micelles is similar to a previously determined crystal structure solubilized by a nonionic detergent and complexed with an antibody fragment. The differences observed include a previously unidentified short amphipathic α-helix that precedes the first transmembrane helix and a subtle rigid-body repositioning of the S3-S4 voltage-sensor paddle. Using (15)N relaxation experiments, we show that much of the VSD, including the pronounced kink in S3 and the S3-S4 paddle, is relatively rigid on the picosecond-to-nanosecond timescale. In contrast, the kink in S3 is mobile on the microsecond-to-millisecond timescale and may act as a hinge in the movement of the paddle during channel gating. We characterized the VSD-phospholipid micelle interactions using nuclear Overhauser effect spectroscopy and showed that the micelle uniformly coats the KvAP VSD and approximates the chemical environment of a phospholipid bilayer. Using paramagnetically labeled phospholipids, we show that bilayer-forming lipids interact with the S3 and S4 helices more strongly than with S1 and S2.

Effects of the endosomal lipid bis(monoacylglycero)phosphate on the thermotropic properties of DPPC: A 2H NMR and spin label EPR study

Bis(monoacylglycero)phosphate (BMP) is an endosomal lipid with a unique structure that is implicated in the formation of intraendosomal vesicular bodies. Here we have characterized the effects of dioleoyl-BMP (BMP(18:1)) at concentrations of 5, 10, 15 and 20mol% on the thermotropic behavior of dipalmitoyl phosphatidylcholine (DPPC) vesicles, and compared them to those of equimolar concentrations of dioleoyl phosphatidylglycerol (DOPG), a structural isoform of BMP(18:1). Because BMP is found in the acidic environments of the late endosome and intralysosomal vesicles, samples were prepared at pH 4.2 to mimic the pH of the lysosome. Both (2)H NMR of perdeuterated DPPC and spin-labeled EPR with 16-doxyl phosphatidylcholine were utilized in these investigations. NMR and EPR results show that BMP(18:1) induces a lowering in the main phase transition temperature of DPPC similar to that of DOPG. The EPR studies reveal that BMP(18:1) induced more disorder in the L(beta) phase when compared to equimolar concentrations of DOPG. Analysis from dePaked (2)H NMR spectra in the L(alpha) phase reveals that BMP(18:1) induces less disorder than equal concentrations of DOPG. Additionally, the results demonstrate that BMP mixes with other phospholipids as a phospholipid and not as a detergent molecule as once speculated.

Localisation of BODIPY-labelled phosphatidylcholines in lipid bilayers

A series of sn-2 acyl-labelled phosphatidyl-cholines (PC), bearing 4,4-difluoro-1-3-5-7-tetra-methyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me(4)-BODIPY) at the end of the C(n)-acyl chains were solubilised in unilamellar vesicles and studied with respect to the order and location of the Me(4)-BODIPY (denoted: B) group. The obtained results are based on time-resolved electronic energy transfer from donors (2-(9-anthroyloxy)-stearic acid) localised in the lipid-water interface to acceptors BnPC (n = 3, 5, 7, 9, 11, 13, 15), as well as the energy migration among the Me(4)-BODIPY groups of BnPC:s. The donor-acceptor and the donor-donor experiments strongly suggest that the Me(4)-BODIPY group in BnPC tends to loop back close to the lipid-water interface. The Me(4)-BODIPY groups, residing in the two bilayer leaflets, are located at approximately the same depth, and transversally separated by ca. 27 A for all n-values. Close to the interface, the optimal transversal distribution widens somewhat with increasing length of the sn-2 acyl chain. The obtained order parameter profile of the BnPC:s is also compatible with such a location.

Orientation and dynamics of transmembrane peptides: the power of simple models

In this review we discuss recent insights obtained from well-characterized model systems into the factors that determine the orientation and tilt angles of transmembrane peptides in lipid bilayers. We will compare tilt angles of synthetic peptides with those of natural peptides and proteins, and we will discuss how tilt can be modulated by hydrophobic mismatch between the thickness of the bilayer and the length of the membrane spanning part of the peptide or protein. In particular, we will focus on results obtained on tryptophan-flanked model peptides (WALP peptides) as a case study to illustrate possible consequences of hydrophobic mismatch in molecular detail and to highlight the importance of peptide dynamics for the experimental determination of tilt angles. We will conclude with discussing some future prospects and challenges concerning the use of simple peptide/lipid model systems as a tool to understand membrane structure and function.

The interaction of neuropeptide Y with negatively charged and zwitterionic phospholipid membranes

The interaction of the 36 amino acid neuropeptide Y (NPY) with liposomes was studied using the intrinsic tyrosine fluorescence of NPY and an NPY fragment comprising amino acids 18-36. The vesicular membranes were composed of phosphatidylcholine and phosphatidylserine at varying mixing ratios. From the experimentally measured binding curves, the standard Gibbs free energy for the peptide transfer from aqueous solution to the lipid membrane was calculated to be around -30 kJ/mol for membrane mixtures containing physiological amounts of acidic lipids at pH 5. The effective charge of the peptide depends on the pH of the buffer and is about half of its theoretical net charge. The results were confirmed using the fluorescence of the NPY analogue [Trp(32)]-NPY. Further, the position of NPY's alpha-helix in the membrane was estimated from the intrinsic tyrosine fluorescence of NPY, from quenching experiments with spin-labelled phospholipids using [Trp(32)]-NPY, and from (1)H magic-angle spinning NMR relaxation measurements using spin-labelled [Ala(31), TOAC(32)]-NPY. The results suggest that the immersion depth of NPY into the membrane is triggered by the membrane composition. The alpha-helix of NPY is located in the upper chain region of zwitterionic membranes but its position is shifted to the glycerol region in negatively charged membranes. For membranes composed of phosphatidylcholine and phosphatidylserine, an intermediate position of the alpha-helix is observed.

Biophysical characterization of a new phospholipid analogue with a spin-labeled unsaturated fatty acyl chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Membrane binding of lipidated Ras peptides and proteins--the structural point of view

Biological membranes are interesting interfaces, at which important biological processes occur. In addition to integral membrane proteins, a number of proteins bind to the membrane surface and associate with it. Posttranslational lipid modification is one important mechanism, by which soluble molecules develop a propensity towards the membrane and reversibly bind to it. Membrane binding by insertion of hydrophobic lipid moieties is relevant for up to 10% of all cellular proteins. A particular interesting lipid-modified protein is the small GTPase Ras, which plays a key role in cellular signal transduction. Until recently, the structural basis for membrane binding of Ras was not well-defined. However, with the advent of new synthesis techniques and the advancement of several biophysical methods, a number of structural and dynamical features about membrane binding of Ras proteins have been revealed. This review will summarize the chemical biology of Ras and discuss in more detail the biophysical and structural features of the membrane bound C-terminus of the protein.

Amplification of diacylglycerol activation of protein kinase C by cholesterol

The combined effects of cholesterol, a major cell membrane component, and the lipid second messenger diacylglycerol on the activity of protein kinase C (PK-C) and the structure of phosphatidylcholine/phosphatidylserine bilayers were investigated using specific PK-C assays and (2)H NMR. Whereas the classical activation of PK-C was observed as an effect of diacylglycerol, in the absence of this second messenger, cholesterol did not affect PK-C activity. A novel effect of amplified PK-C activation was observed in the presence of both cholesterol and diacylglycerol concentrations within the physiological range of each of these components. (2)H NMR results suggest that this phenomenon is due to cholesterol- and diacylglycerol-induced increased propensity of the lipids to adopt nonbilayer phases, effectively destabilizing the bilayer structure. The magnitude of the effect was a function of cholesterol concentration, implying that laterally separated cell membrane domains with distinct cholesterol concentrations have the capacity to differ in their sensitivity to extracellular stimuli.