Small Bicelles Research Articles

The Interaction of the Transmembrane Domain of SARS-CoV-2 E-Protein with Glycyrrhizic Acid in Lipid Bilayer

The interaction of the transmembrane domain of SARS-CoV-2 E-protein with glycyrrhizic acid in a model lipid bilayer (small isotropic bicelles) is demonstrated using various NMR techniques. Glycyrrhizic acid (GA) is the main active component of licorice root, and it shows antiviral activity against various enveloped viruses, including coronavirus. It is suggested that GA can influence the stage of fusion between the viral particle and the host cell by incorporating into the membrane. Using NMR spectroscopy, it was shown that the GA molecule penetrates into the lipid bilayer in a protonated state, but localizes on the bilayer surface in a deprotonated state. The transmembrane domain of SARS-CoV-2 E-protein facilitates deeper GA penetration into the hydrophobic region of bicelles at both acidic and neutral pH and promotes the self-association of GA at neutral pH. Phenylalanine residues of the E-protein interact with GA molecules inside the lipid bilayer at neutral pH. Furthermore, GA influences the mobility of the transmembrane domain of SARS-CoV-2 E-protein in the bilayer. These data provide deeper insight into the molecular mechanism of antiviral activity of glycyrrhizic acid.

Behavior of Most Widely Spread Lipids in Isotropic Bicelles.

Isotropic bicelles are a widely used membrane mimetic for structural studies of membrane proteins and their transmembrane domains. Simple and cheap in preparation, they contain a patch of lipid bilayer that reproduces the native environment of membrane proteins. Despite the obvious power of bicelles in reproducing the various kinds of environments, the vast majority of structural studies employ the single lipid/detergent system. On the other hand, even if the alternative bicelle composition is used, the properties of mixtures are not characterized, and the mere presence of lipid bilayer and discoidal shape of bicelle particles is not confirmed. Here we present an extensive investigation of various bicellar mixtures and describe the behavior of bicelles with lipids other than classical DMPC, namely sphingomyelins (SM), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylserines (PS), and cholesterol. These lipids are rarely used in modern structural biology, but can help a lot in understanding the influence of the membrane composition on the properties of both integral and peripheral membrane proteins. Additionally, the ability of diheptanoylphosphatidylcholine (DH7PC) to serve as a rim-forming agent was investigated. We followed the phase transitions as revealed by 31P NMR and size of particles measured by 1H NMR diffusion as the criteria of the proper morphology and structure of bicelles. As an outcome, we state that SM exclusively, and PG/PS in mixtures with zwitterionic lipids can form small isotropic bicelles, which reproduce the key features of lipid behavior in bilayers. Mixtures, containing exclusively the anionic lipids, fail to reveal the lipid phase transition and do not follow the size predicted for the ideal bicelle particles. PE and DH7PC are the unwanted components of bicellar mixtures, and cholesterol can be added to bicelles, however, with certain precautions. In combination with our several most recent works, this study provides a practical guide for the preparation of small isotropic bicelles.

Phase Transitions in Small Isotropic Bicelles.

Isotropic phospholipid bicelles are one of the most prospective membrane mimetics for the structural studies of membrane proteins in solution. Recent works provided an almost full set of data regarding the properties of isotropic bicelles; however, one major aspect of their behavior is still under consideration: the possible mixing between the lipid and detergent in the bilayer area. This problem may be resolved by studying the lipid phase transitions in bicelle particles. In the present work, we investigate two effects: phase transitions of bilayer lipids and temperature-induced growth of isotropic bicelles using the NMR spectroscopy. We propose an approach to study the phase transitions in isotropic bicelles based on the properties of 31P NMR spectra of bilayer-forming lipids. We show that phase transitions in small bicelles are "fractional", particles with the liquid-crystalline and gel bilayers coexist in solution at certain temperatures. We study the effects of lipid fatty chain type and demonstrate that the behavior of various lipids in bilayers is reproduced in the isotropic bicelles. We show that the temperature-induced growth of isotropic bicelles is not related directly to the phase transition but is the result of the reversible fusion of bicelle particles. In accordance with our data, rim detergents also have an impact on phase transitions: detergents that resist the temperature-induced growth provide the narrowest and most expressed transitions at higher temperatures. We demonstrate clearly that phase transitions take place even in the smallest bicelles that are applicable for structural studies of membrane proteins by solution NMR spectroscopy. This last finding, together with other data draws a thick line under the long-lasting argument about the relevance of small isotropic bicelles. We show with certainty that the small bicelles can reproduce the most fundamental property of lipid membranes: the ability to undergo phase transition.

Immersion Depths of Lipid Carbons in Bicelles Measured by Paramagnetic Relaxation Enhancement.

Myriads of biological processes occur in or at cellular lipid membranes. Knowledge about the localization of proteins, lipids, and other molecules within biological membranes is thus crucial for the understanding of such processes. Here, we present a method to determine the immersion depths of lipid carbon atoms in membranes by paramagnetic relaxation enhancement (PRE) caused by the presence of doxylated lipids. As membrane mimetics, we employ small isotropic bicelles made of synthetic lipids and of natural Escherichia coli phospholipid extract. Bicelles are particularly suitable for solution state NMR since they maintain a lipid bilayer while they are at the same time amenable to solution state NMR experiments. PREs were measured in the presence of different doxylated lipids with the nitroxide radical located in the headgroup and at various positions in the acyl chain. Theoretical PREs were calculated assuming a simple bicelle model using the Solomon-Bloembergen equations. Immersion depths of the lipid carbon atoms were obtained by a least-squares fit of the theoretical to the experimental PREs. The carbon immersion depths correspond well to results obtained by other methods and differences do not exceed 3-5 Å. This means that the method presented here provides sufficient resolution to distinguish the localization of carbons in different regions of the lipid bilayer, despite considerable simplifications of the underlying theory. These simplifications include a simple form of the spectral density function, which we find is sufficient to reliably determine immersion depths. A more complicated spectral density function that includes bicelle, lipid, and local motions may only improve the results if its parametrization is good enough. The approach presented here may be extended to the determination of protein localization in membranes employing realistic membrane mimetics like the bicelles made of E. coli phospholipid extract used here.

How a short pore forming peptide spans the lipid membrane.

Many antimicrobial peptides function by forming pores in the plasma membrane of the target cells. Intriguingly, some of these peptides are very short, and thus, it is not known how they can span the membrane, or whether other mechanisms of cell disruption are dominant. Here, the conformation and orientation of the 14-residue peptaibol SPF-5506-A4 (SPF) are investigated in lipid environments by atomistic and coarse grained molecular dynamics (MD) simulations, circular dichroism, and nuclear magnetic resonance (NMR) experiments. The MD simulations show that SPF is inserted spontaneously in a transmembrane orientation in both 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers resulting in thinning of the bilayers near the peptides, which drives the peptide aggregation. Furthermore, the backbone conformation of the peptide in the bilayer bound state is different from that of the NMR model solved in small bicelles. These results demonstrate that mutual adaption between the peptides and the membrane is likely to be important for pore formation.

Façade detergents as bicelle rim-forming agents for solution NMR spectroscopy

AbstractOut of all membrane mimetics available for solution nuclear magnetic resonance (NMR) spectroscopy, phospholipid bicelles are the most prospective. Unlike lipid-protein nanodiscs their size can be easily controlled over a wide range, and the exchange of matter between the particles can take place. However, recent studies revealed several major drawbacks of conventional 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and DMPC/3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) bicelles. First, size of such bicelles can increase dramatically upon heating, and, second, rim-forming detergents of bicelles can cause improper folding of the water-soluble globular domains of membrane proteins. In order to avoid these effects, we tested the Façade detergents as possible alternative rim-forming agents for small isotropic bicelles. In the present work we characterized the size of bicelles formed by 3α-hydroxy-7α,12α-di-((O-β-D-maltosyl)-2-hydroxyethoxy)-cholane (Façade-EM) and 3α-hydroxy-7α,12α-di-(((2-(trimethylamino)ethyl)phosphoryl)ethyloxy)-cholane Façade-EPC as a function of temperature and lipid/detergent ratio by 1H NMR diffusion spectroscopy. Additionally, the denaturing effects of these two rim-forming agents were investigated using the junction of the transmembrane and intracellular domains of the p75 neurotrophin receptor (p75NTR) as a model object. We show that the use of Façades allows decreasing the temperature-dependent growth of bicelles. The ability of Façade-EM-based bicelles to support the native structure and soluble state of the p75NTR intracellular domain was also revealed.

From Nanodiscs to Isotropic Bicelles: A Procedure for Solution Nuclear Magnetic Resonance Studies of Detergent-Sensitive Integral Membrane Proteins

Nanodiscs and isotropic bicelles are promising membrane mimetics in the field of solution nuclear magnetic resonance (NMR) spectroscopy of integral membrane proteins (IMPs). Despite varied challenges to solution NMR studies of IMPs, we attribute the paucity of solution NMR structures in these environments to the inability of diverse IMPs to withstand detergent treatment during standard nanodisc and bicelle preparations. Here, we present a strategy that creates small isotropic bicelles from IMPs co-translationally embedded in large nanodiscs using cell-free expression. Our results demonstrate appreciable gains in NMR spectral quality while preserving lipid-IMP contacts. We validate the approach on the detergent-sensitive LspA, which finally allowed us to perform high-quality triple-resonance NMR experiments for structural studies. Our strategy of producing bicelles from nanodiscs comprehensively avoids detergent during expression and preparation and is suitable for solution NMR spectroscopy of lipid-IMP complexes.

Characterization of Small Isotropic Bicelles with Various Compositions.

Structural studies of membrane proteins are of great importance and interest, with solution and solid state NMR spectroscopy being very promising tools for that task. However, such investigations are hindered by a number of obstacles, and in the first place by the fact that membrane proteins need an adequate environment that models the cell membrane. One of the most widely used and prospective membrane mimetics is isotropic bicelles. While large anisotropic bicelles are well-studied, the field of small bicelles contains a lot of "white spots". The present work reports the radii of particles and concentration of the detergents in the monomeric state in solutions of isotropic bicelles, formed by 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), and sodium cholate, as a function of lipid/detergent ratio and temperature. These parameters were measured using (1)H NMR diffusion spectroscopy for the bicelles composed of lipids with saturated fatty chains of different length and lipids, containing unsaturated fatty acid residue. The influence of a model transmembrane protein (membrane domain of rat TrkA) on the properties of bicelles and the effect of the bicelle size and composition on the properties of the transmembrane protein were investigated with heteronuclear NMR and nuclear Overhauser effect spectroscopy. We show that isotropic bicelles that are applicable for solution NMR spectroscopy behave as predicted by the theoretical models and are likely to be bicelles rather than mixed micelles. Using the obtained data, we propose a simple approach to control the size of bicelles at low concentrations. On the basis of our results, we compared different rim-forming agents and selected CHAPS as a detergent of choice for structural studies in bicelles, if the deuteration of the detergent is not required.

Characterization of fast-tumbling isotropic bicelles by PFG diffusion NMR.

Small isotropic bicelles are versatile membrane mimetics, which, in contrast to micelles, provide a lipid bilayer and are at the same time suitable for solution-state NMR studies. The lipid composition of the bilayer is flexible allowing for incorporation of various head groups and acyl chain types. In bicelles, lipids are solubilized by detergents, which are localized in the rim of the disk-shaped lipid bilayer. Bicelles have been characterized by a broad array of biophysical methods, pulsed-field gradient NMR (PFG NMR) being one of them. PFG NMR can readily be used to measure diffusion coefficients of macromolecules. It is thus employed to characterize bicelle size and morphology. Even more importantly, PFG NMR can be used to study the degree of protein association to membranes. Here, we present the advances that have been made in producing small, fast-tumbling isotropic bicelles from a variety of lipids and detergents, together with insights on the morphology of such mixtures gained from PFG NMR. Furthermore, we review approaches to study protein-membrane interaction by PFG NMR. Copyright © 2015 John Wiley & Sons, Ltd.

Membrane interactions in small fast-tumbling bicelles as studied by 31P NMR

Small fast-tumbling bicelles are ideal for studies of membrane interactions at molecular level; they allow analysis of lipid properties using solution-state NMR. In the present study we used 31P NMR relaxation to obtain detailed information on lipid head-group dynamics. We explored the effect of two topologically different membrane-interacting peptides on bicelles containing either dimyristoylphosphocholine (DMPC), or a mixture of DMPC and dimyristoylphosphoglycerol (DMPG), and dihexanoylphosphocholine (DHPC). KALP21 is a model transmembrane peptide, designed to span a DMPC bilayer and dynorphin B is a membrane surface active neuropeptide. KALP21 causes significant increase in bicelle size, as evidenced by both dynamic light scattering and 31P T2 relaxation measurements. The effect of dynorphin B on bicelle size is more modest, although significant effects on T2 relaxation are observed at higher temperatures. A comparison of 31P T1 values for the lipids with and without the peptides showed that dynorphin B has a greater effect on lipid head-group dynamics than KALP21, especially at elevated temperatures. From the field-dependence of T1 relaxation data, a correlation time describing the overall lipid motion was derived. Results indicate that the positively charged dynorphin B decreases the mobility of the lipid molecules – in particular for the negatively charged DMPG – while KALP21 has a more modest influence. Our results demonstrate that while a transmembrane peptide has severe effects on overall bilayer properties, the surface bound peptide has a more dramatic effect in reducing lipid head-group mobility. These observations may be of general importance for understanding peptide–membrane interactions.

Conformational dynamics of oligosaccharides characterized by paramagnetism-assisted NMR spectroscopy in conjunction with molecular dynamics simulation.

Oligosaccharides play pivotal roles in physiological and pathological contexts primarily through their interactions with proteins on cell surfaces and in intracellular environments. Although crystallographic approaches provide cumulative information about the atomic details of oligosaccharides complexed with proteins, quantitative characterization of the dynamic conformation of uncomplexed oligosaccharides is essential for better understanding of the energetics of carbohydrate–protein interactions. Nuclear magnetic resonance (NMR) spectroscopy is a potentially powerful tool for describing the conformational dynamics of oligosaccharides in solutions at an atomic resolution. However, methodological improvements are needed in applying NMR techniques to the analyses of the dynamic conformations of oligosaccharides during sample preparation, spectral observation, and data interpretation. This presentation outlines our recently developed method of dealing with dynamic conformational ensembles of oligosaccharides using paramagnetism-assisted NMR spectroscopy in conjunction with molecular dynamics (MD) simulation. A key to this approach is the introduction of a paramagnetic lanthanide ion to the reducing end of oligosaccharides as the source of the atomic long-distance information. We successfully applied this method to the validation of MD-derived conformational spaces occupied by a series of sialyl oligosaccharide moieties of GM1, GM2, and GM3 gangliosides. The applicability of NMR is also revealed for characterizing the dynamic interactions of ganglioside clusters with intrinsically disordered proteins associated with neurodegenerative disorders using ganglioside-embedding small bicelles as nanoscale standardized membrane mimics.

Characterization of the morphology of fast-tumbling bicelles with varying composition.

Small, fast-tumbling bicelles are frequently used in solution NMR studies of protein-lipid interactions. For this purpose it is critical to have information about the organization of the lipids within the bicelle structure. We have studied the morphology of small, fast-tumbling bicelles containing DMPC and DHPC as a function of temperature, lipid concentration, and the relative ratio (q value) of lipid (DMPC) to detergent (DHPC) amounts. Dynamic light scattering and cryo-transmission electron microscopy techniques were used to measure the size of the bicelles and to monitor the shape and dispersity of the particles in the samples. The stability and size of DMPC-containing bicelle mixtures were found to be highly dependent on temperature and the total lipid concentration for mixtures with q = 1 and q = 1.5. Stable DMPC/DHPC bicelles are only formed at low q values (0.5). Bicelle mixtures with q > 0.5 appear to be multidisperse containing more than one component, one with r(H) around 2.5 nm and one with r(H) of 6-8 nm. This is interpreted as a coexistence of small (possibly mixed micelles) bicelles and much larger bicelles. Incubating the sample at 37 °C increases the phase separation. Moreover, low total amphiphile concentrations and low q values lead to the formation of a temperature-independent morphology, interpreted as the formation of small particles in which the DHPC and DMPC are more mixed. On the basis of these results, we propose the existence of a critical bicelle concentration, a parameter that determines the existence of bilayered bicelles, which varies with q value. This polymorphism was not observed at any concentrations for q = 0.5 bicelles, for which a small but detectable temperature dependence was observed at high concentrations. The results demonstrate that q = 0.5 mixtures predominantly form "classical" bicelles, but that caution is needed when using fast-tumbling mixtures with q values higher than 0.5.

脂質膜における構造および相互作用解析：バイセルを用いた検討

Although many non-peptidic drugs target biological membrane and membrane proteins, it is still difficult to determine the membrane-bound conformation of the drugs. To solve those problems, we have utilized bicelles as a membrane model, since the bicelles, which have planar lipid bilayer portions, are thought to be a more appropriate membrane model than micelles. Small-sized bicelles allow for liquid NMR measurements due to isotropic fast tumbling in solution. We have applied small bicelles to erythromycin A, salinomycin, and amphidinol 3, and determined their membrane-bound structures as well as their positions and orientations in the membranes using coupling constants, NOEs, and paramagnetic relaxation methods. Recently, we found that sphingomyelin, a major lipid constituent of lipid rafts, also forms bicelles, and established its conformation in the bicelles. These studies show the general utility of small bicelles for detailed conformation and orientation analysis of membrane-associated drugs and lipid molecules. We are now developing a bicelle-based crystallization method for membrane proteins, which will facilitate the cocrystalization of membrane proteins and hydrophobic drugs.

The Role of Trp in Arg-Rich Paddle Domain-Lipid Interaction

Paddle domains in voltage-gated K+ channels exert their function to mediate voltage gating by altering their positions depending on membrane potential. Two Arg-rich paddle domains, KvAPp from the Kv channel from Aeropyrum pernix and HsapBKp from the BK channel from Homo sapiens, have been shown to be able to induce a thinning of a bilayer which may be the key of gating mechanism. However, differences between the two paddle domains were also observed, e.g., calcein leakage in large unilamellar vesicles induced by HsapBKp is more pronounced and HsapBKp can probably undergo structural transitions indicated by other spectroscopic results. The presence of a Trp in HsapBKp but not in KvAPp may be the reason for the different direct membrane effects. Therefore, in this study we have investigated the role of the Trp in the human BK channel. We have used a range of spectroscopic techniques to elucidate the role of the Trp residue by studying HsapBKp and a variant where Trp was replaced by Ala, and KvAPp and the variant where Ala was replaced by Trp. As a comparison, we also investigated a model transmembrane peptide, KALP21, and a variant of this peptide where an Ala residue in the middle of the hydrophobic region was substituted by Trp. The results show that a Trp in the middle of the sequence alters the structure, and changes the way that the motifs interact with lipids. A Trp promotes disruption of fast-tumbling small bicelles as well as magnetically aligned bicelles. The presence of a Trp residue explains the differences by which the Arg-rich motifs from the Kv and BK interact with the membrane.

New NMR Tools for Characterizing the Dynamic Conformations and Interactions of Oligosaccharides

Abstract Oligosaccharides play pivotal roles in various molecular recognition events on cell surfaces and in intracellular environments. Although information about the three-dimensional structure of oligosaccharides is crucial for understanding the mechanisms underlying their biological functions and for designing drugs targeting carbohydrate recognition systems, their inherent flexibility hampers detailed conformational analysis. NMR spectroscopy has immense potential for providing atomic details of oligosaccharide structures in solution as well as in complexes with other biomolecules. However, the traditional NMR approach based on local conformational information provided by the nuclear Overhauser effect (NOE) is often precluded by insufficient information about long interatomic distances and hydrogen bonds involving hydroxy groups. To address these issues, new NMR techniques have recently been developed, exploiting the effects of introduced paramagnetic probes and stable isotopes to target oligosaccharides. Lanthanoid tagging and the spin labeling of oligosaccharides induce paramagnetic effects such as pseudocontact shift and paramagnetic relaxation enhancement, offering NOE-independent sources of long-distance information. Deuterium-induced isotope shifts of neighboring 13C resonances provide an experimental tool for the identification of hydrogen bonds involving oligosaccharide hydroxy groups. The uniform and position-selective 13C enrichment of oligosaccharides can be achieved by metabolic labeling using genetically engineered yeast cells. Furthermore, small ganglioside-embedding bicelles have been used for detailed NMR analyses of intermolecular interactions involving glycolipids in membrane environments. These NMR approaches, in conjunction with computational simulation, have opened up new vistas for the analysis of oligosaccharide conformations and interactions at atomic level.

Ganglioside-embedding small bicelles for probing membrane-landing processes of intrinsically disordered proteins

We successfully displayed a series of gangliosides on small bicelles with a uniform confined size, offering nanoscale standardized membrane mimics for spectroscopic characterization of weak encounter complexes formed between ganglioside clusters and amyloidogenic proteins. This enabled probing of initial membrane-landing processes of α-synuclein as a therapeutic target by NMR spectroscopy.

Kinetic Process of Formation and Reconstruction of Small Unilamellar Vesicles Consisting of Long- and Short-Chain Lipids

Whereas the phase separation of normal phospholipids induces formation of microdomains on the surface of spherical vesicles, the separation of a long- and short-chain lipids can induce perforation of small unilamellar vesicles (SUVs) and transformation into bilayered micelles (bicelles) because the edges of the bilayers are stabilized by the short-chain lipid microdomain. In this study, the effect of the phase separation of lipids on the transformation behavior of SUVs consisting of a mixture of long- and short-chain lipids was investigated using small-angle neutron scattering. At the temperature jump from below to above the chain melting temperature of the long-chain lipid, T(c), bicelles fused together and transformed into SUVs when their size reached a critical radius. In contrast, a sequential transformation of small SUVs to small bicelles, small bicelles to large bicelles, and large bicelles to large SUVs occurred when the temperature jumped from a value far above T(c) to one slightly above T(c). To the best of my knowledge, this is the first report of such reconstruction of vesicles. By considering the bending energy of the membrane, the line tension of the rim, and the perforation due to the phase separation, the mechanism of the transformation processes was clarified.

Using Fluorine Nuclear Magnetic Resonance To Probe the Interaction of Membrane-Active Peptides with the Lipid Bilayer

A variety of biologically active peptides exert their function through direct interactions with the lipid membrane of the cell. These surface interactions are generally transient and highly dynamic, making them hard to study. Here we have examined the feasibility of using solution phase (19)F nuclear magnetic resonance (NMR) to study peptide-membrane interactions. Using the antimicrobial peptide MSI-78 as a model system, we demonstrate that peptide binding to either small unilamellar vesicles (SUVs) or bicelles can readily be detected by simple one-dimensional (19)F NMR experiments with peptides labeled with l-4,4,4-trifluoroethylglycine. The (19)F chemical shift associated with the peptide-membrane complex is sensitive both to the position of the trifluoromethyl reporter group (whether in the hydrophobic face or positively charged face of the amphipathic peptide) and to the curvature of the lipid bilayer (whether the peptide is bound to SUVs or bicelles). (19)F spin echo experiments using the Carr-Purcell-Meiboom-Gill pulse sequence were used to measure the transverse relaxation (T(2)) of the nucleus and thereby examine the local mobility of the MSI-78 analogues bound to bicelles. The fluorine probe positioned in the hydrophobic face of the peptide relaxes at a rate that correlates with the tumbling of the bicelle, suggesting that it is relatively immobile, whereas the probe at the positively charged face relaxes more slowly, indicating this position is much more dynamic. These results are in accord with structural models of MSI-78 bound to lipids and point to the feasibility of using fluorine-labeled peptides to monitor peptide-membrane interactions in living cells.

Structure determination of the seven-helix transmembrane receptor sensory rhodopsin II by solution NMR spectroscopy

Seven-helical membrane proteins represent a challenge for structural biology. Here, we report the first NMR structure determination of a detergent-solubilized seven-helical transmembrane (7TM) protein, the phototaxis receptor sensory rhodopsin II (pSRII) from Natronomonas pharaonis, as a proof of principle. The overall quality of the structure ensemble is extremely good (backbone root mean squared deviation of 0.48 Å) and agrees well with previously determined X-ray structures. Furthermore, measurements in more native-like small phospholipid bicelles indicate that the protein structure is the same as in detergent micelles, suggesting that environment specific effects are minimal when using mild detergents. We use our case study as a platform to discuss the feasibility of similar solution NMR studies for other 7TM proteins including members of the family of G protein-coupled receptors (GPCRs).

Assessing the size, stability, and utility of isotropically tumbling bicelle systems for structural biology

Aqueous phospholipid mixtures that form bilayered micelles (bicelles) have gained wide use by molecular biophysicists during the past 20 years for spectroscopic studies of membrane-bound peptides and structural refinement of soluble protein structures. Nonetheless, the utility of bicelle systems may be compromised by considerations of cost, chemical stability, and preservation of the bicelle aggregate organization under a broad range of temperature, concentration, pH, and ionic strength conditions. In the current work, 31P nuclear magnetic resonance (NMR) and atomic force microscopy (AFM) have been used to monitor the size and morphology of isotropically tumbling small bicelles formed by mixtures of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) or 1,2-di-O-tetradecyl- sn-glycero-3-phosphocholine (DIOMPC) with either 1,2-dihexanoyl- sn-glycero-3-phosphocholine (DHPC) or 1,2-di-O-hexyl- sn-glycero-3-phosphocholine (DIOHPC), testing their tolerance of variations in commonly used experimental conditions. 1H- 15N 2D NMR has been used to demonstrate the usefulness of the robust DMPC–DIOHPC system for conformational studies of a fatty acid-binding protein that shuttles small ligands to and from biological membranes.