Lipid dynamics in fast-tumbling bicelles with varying bilayer thickness: Effect of model transmembrane peptides

Co-Encapsulation of Curcumin and α-Tocopherol in Bicosome Systems: Physicochemical Properties and Biological Activity

A novel co-encapsulation system called bicosomes (bicelles within liposomes) has been developed to overcome the limitations associated with the topical application of curcumin (cur) and α-tocopherol (α-toc). The physicochemical properties and biological activity in vitro of bicosome systems were evaluated. Bicelles were prepared with DPPC, DHPC, cur, and α-toc (cur/α-toc-bicelles). Liposomal vesicles loading cur/α-toc-bicelles were prepared with Lipoid P-100 and cholesterol-forming cur/α-toc-bicosomes. Three cur/α-toc-bicosomes were evaluated using different total lipid percentages (12, 16, and 20% w/v). The results indicated that formulations manage to solubilize cur and α-toc in homogeneous bicelles < 20 nm, while the bicosomes reaches 303-420 nm depending on the total lipid percentage in the systems. Bicosomes demonstrated high-encapsulation efficiency (EE) for cur (56-77%) and α-toc (51-65%). The loading capacity (LC) for both antioxidant compounds was 52-67%. In addition, cur/α-toc-bicosomes decreased the lipid oxidation by 52% and increased the antioxidant activity by 60% compared to unloaded bicosomes. The cell viability of these cur/α-toc-bicosomes was >85% in fibroblasts (3T3L1/CL-173™) and ≥65% in keratinocytes (Ha-CaT) and proved to be hematologically compatible. The cur/α-toc-bicelles and cur/α-toc-bicosomes inhibited the growth of C. albicans in a range between 33 and 76%. Our results propose bicosome systems as a novel carrier able to co-encapsulate, solubilize, protect, and improve the delivery performance of antioxidant molecules. The relevance of these findings is based on the synergistic antioxidant effect of its components, its biocompatibility, and its efficacy for dermal tissue treatment damaged by oxidative stress or by the presence of C. albicans. However, further studies are needed to assess the efficacy and safety of cur/α-toc bicosomes in vitro and in vivo.

Scaling Protein-Water Interactions in the Martini 3 Coarse-Grained Force Field to Simulate Transmembrane Helix Dimers in Different Lipid Environments

Martini 3, the latest version of the widely used Martini force field for coarse-grained molecular dynamics simulations, is a promising tool to investigate proteins in phospholipid bilayers. However, simulating other lipid environments, such as detergent micelles, presents challenges due to the absence of validated parameters for their constituent molecules. Here, we propose parameters for the micelle-forming surfactant, dodecylphosphocholine (DPC). These result in micelle assembly with aggregation numbers in agreement with the experimental values. However, we identified a lack of hydrophobic interactions between transmembrane helix protein dimers and the tails of DPC molecules, preventing insertion and stabilization of the protein in the micelles. This problem was also observed for protein insertion by self-assembling 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or dipalmitoylphosphatidylcholine (DPPC) bilayers. We propose the reduction of the nonbonded interactions between protein and water beads by 10% as a simple and effective solution to this problem that enables protein encapsulation in phospholipid micelles and bilayers without altering protein dimerization or the bilayer structure.

Stable Discoidal Bicelles: Formulation, Characterization, and Functions

Bicellar mixtures have been used as alignable membrane substrates under a magnetic field applicable for the structural characterization of membrane-associated proteins. Recently, it has shown that bicelles can serve as nanocarriers to effectively deliver hydrophobic therapeutic molecules to cancer cells with a three- to ten-fold enhancement compared to that of liposomes of a chemically identical composition. In this chapter, detailed preparation protocol, common structural characterization methods, the structural stability, the cellular uptake and a few unique functions of bicellar nanodiscs are discussed.

Evaluation of the Rotating-Frame Relaxation (T1ρ) Filter and Its Application in Metabolomics as an Alternative to the Transverse Relaxation (T2) Filter

Nuclear magnetic resonance (NMR)-based metabolomic studies commonly involve the use of T2 filter pulse sequences to eliminate or attenuate the broad signals from large molecules and improve spectral resolution. In this paper, we demonstrate that the T1ρ filter-based pulse sequence represents an interesting alternative because it allows the stability and the reproducibility needed for statistical analysis. The integrity of the samples and the stability of the instruments were assessed for different filter durations and amplitudes. We showed that the T1ρ filter pulse sequence did not induce sample overheating for a filter duration of up to 500 ms. The reproducibility was evaluated and compared with the T2 filter in serum and liver samples. The implementation is relatively simple and provides the same statistical and analytical results as those obtained with the standard filters. Regarding tissues analysis, because the duration of the filter is the same as that of the spin-lock, the synchronization of the echo delays with the magic angle spinning (MAS) rate is no longer necessary as for T2 filter-based sequences. The results presented in this article aim at establishing a new protocol to improve metabolomic studies and pave the way for future developments on T1ρ alternative filters, in liquid and HR-MAS NMR experiments.

Refining internal bilayer structure of bicelles resolved by extended-q small angle X-ray scattering

The internal profile across the bilayer reveals important structural information regarding the crystallinity of acyl chains or the positions of encapsulated species. Here, we demonstrate that a simple five-layer-core-shell discoidal model can be employed to best fit the extended-q small angle X-ray scattering (SAXS) data and resolve the bilayer internal structure (with sub-nanometer resolution) of a nanoscale discoidal system comprised of a mixture of long- and short- chain lipids (known as "bicelles"). In contrast to the traditional core-shell discoidal model, the detailed structure in the hydrophobic core such as the methylene and methyl groups can be distinguished via this model. The refined model is validated by the SAXS data of bicelles whose electron scattering length density of the hydrophobic core is adjusted by the addition of a long-chain lipid with a fluorine-end group. The higher resolution of the bilayer internal structure can be employed to advance our understanding of the interaction and conformation of the membrane and associated molecules, such as membrane-associated proteins and locations of entrapped species in the lipid nanoparticles.

Targeted Delivery of Adamantylated Peptidoglycan Immunomodulators in Lipid Nanocarriers: NMR Shows That Cargo Fragments Are Available on the Surface

We present an in-depth investigation of the membrane interactions of peptidoglycan (PGN)-based immune adjuvants designed for lipid-based delivery systems using NMR spectroscopy. The derivatives contain a cargo peptidoglycan (PGN) dipeptide fragment and an adamantyl group, which serves as an anchor to the lipid bilayer. Furthermore, derivatives with a mannose group that can actively target cell surface receptors on immune cells are also studied. We showed that the targeting mannose group and the cargo PGN fragment are both available on the lipid bilayer surface, thereby enabling interactions with cognate receptors. We found that the nonmannosylated compounds are incorporated stronger into the lipid assemblies than the mannosylated ones, but the latter compounds penetrate deeper in the bilayer. This might be explained by stronger electrostatic interactions available for zwitterionic nonmannosylated derivatives as opposed to the compounds in which the charged N-terminus is capped by mannose groups. The higher incorporation efficiency of the nonmannosylated compounds correlated with a larger relative enhancement in immune stimulation activities upon lipid incorporation compared to that of the derivatives with the mannose group. The chirality of the adamantyl group also influenced the incorporation efficiency, which in turn correlated with membrane-associated conformations that affect possible intermolecular interactions with lipid molecules. These findings will help in improving the development of PGN-based immune adjuvants suitable for delivery in lipid nanoparticles.

Peptide-bicelle interaction: Following variations in size and morphology by a combined NMR-SAXS approach

Changes in membrane properties occurring upon protein interaction are key questions in understanding membrane protein function. To report on the occurring size and shape variation we present here a combined NMR-SAXS method performed under physiological conditions using the same samples, enabling determination of a global parameter, the hydration radius (r_H) and estimating the bicelle shape. We use zwitterionic (DMPC/DHPC) and negatively charged (DMPC/DHPC/DMPG) bicelles and investigate the interaction with model transmembrane and surface active peptides (KALP23 and melittin). ¹H NMR measurements based mostly on the translational diffusion coefficient D determination are used to characterize cmc values of DHPC micelles under the investigated conditions, to describe DHPC distribution with exact determination of the q (long chain/short chain) lipid ratio, to estimate aggregation numbers and effective r_H values. The scattering curve is used to fit a lenticular core-shell model enabling us to describe the bicelle shape in terms of ellipsoidal axis length parameters. For all studied systems formation of oblate ellipsoids is found. Even though the r_G/r_H ratio would be an elegant way to characterize shape variations, we show that changes occurring upon peptide-bicelle interaction in the "effective" size and in the measure on the anisometry - morphology - of the objects can be described by using r_H and the simplistic ellipsoidal core-shell model. While the influence of the transmembrane KALP peptide is significant, effects upon addition of surface active melittin peptide seem negligible. This synergy of techniques under controlled conditions can provide information about bicellar shape modulation occurring during peptide-bicelle interactions.

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.

Stable Discoidal Bicelles: A Platform of Lipid Nanocarriers for Cellular Delivery

Bicellar mixtures have been used as alignable membrane substrates for the structural characterization of membrane-associated proteins. Most recently, it has been shown that bicelles can serve as nanocarriers to effectively deliver hydrophobic molecules to cancer cells with a 3- to 10-fold enhancement compared to that of chemically identical liposomes. In this chapter, a detailed preparation protocol, common structural characterization methods, the structural stability and the cellular uptake of bicellar nanodisks are discussed.

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.

Bicelles and Other Membrane Mimics: Comparison of Structure, Properties, and Dynamics from MD Simulations

The increased interest in studying membrane proteins has led to the development of new membrane mimics such as bicelles and nanodiscs. However, only limited knowledge is available of how these membrane mimics are affected by embedded proteins and how well they mimic a lipid bilayer. Herein, we present molecular dynamics simulations to elucidate structural and dynamic properties of small bicelles and compare them to a large alignable bicelle, a small nanodisc, and a lipid bilayer. Properties such as lipid packing and properties related to embedding both an α-helical peptide and a transmembrane protein are investigated. The small bicelles are found to be very dynamic and mainly assume a prolate shape substantiating that small bicelles cannot be regarded as well-defined disclike structures. However, addition of a peptide results in an increased tendency to form disc-shaped bicelles. The small bicelles and the nanodiscs show increased peptide solvation and difference in peptide orientation compared to embedding in a bilayer. The large bicelle imitated a bilayer well with respect to both curvature and peptide solvation, although peripheral binding of short tailed lipids to the embedded proteins is observed, which could hinder ligand binding or multimer formation.

Modulation of plasma membrane Ca2+-ATPase by neutral phospholipids: effect of the micelle-vesicle transition and the bilayer thickness

The effects of lipids on membrane proteins are likely to be complex and unique for each membrane protein. Here we studied different detergent/phosphatidylcholine reconstitution media and tested their effects on plasma membrane Ca(2+) pump (PMCA). We found that Ca(2+)-ATPase activity shows a biphasic behavior with respect to the detergent/phosphatidylcholine ratio. Moreover, the maximal Ca(2+)-ATPase activity largely depends on the length and the unsaturation degree of the hydrocarbon chain. Using static light scattering and fluorescence correlation spectroscopy, we monitored the changes in hydrodynamic radius of detergent/phosphatidylcholine particles during the micelle-vesicle transition. We found that, when PMCA is reconstituted in mixed micelles, neutral phospholipids increase the enzyme turnover. The biophysical changes associated with the transition from mixed micelles to bicelles increase the time of residence of the phosphorylated intermediate (EP), decreasing the enzyme turnover. Molecular dynamics simulations analysis of the interactions between PMCA and the phospholipid bilayer in which it is embedded show that in the 1,2-dioleoyl-sn-glycero-3-phosphocholine bilayer, charged residues of the protein are trapped in the hydrophobic core. Conversely, in the 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer, the overall hydrophobic-hydrophilic requirements of the protein surface are fulfilled the best, reducing the thermodynamic cost of exposing charged residues to the hydrophobic core. The apparent mismatch produced by a 1,2-dioleoyl-sn-glycero-3-phosphocholine thicker bilayer could be a structural foundation to explain its functional effect on PMCA.

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.

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.

EmrE dimerization depends on membrane environment

The small multi-drug resistant (SMR) transporter EmrE functions as a homodimer. Although the small size of EmrE would seem to make it an ideal model system, it can also make it challenging to work with. As a result, a great deal of controversy has surrounded even such basic questions as the oligomeric state. Here we show that the purified protein is a homodimer in isotropic bicelles with a monomer-dimer equilibrium constant (KMD(2D)) of 0.002-0.009mol% for both the substrate-free and substrate-bound states. Thus, the dimer is stabilized in bicelles relative to detergent micelles where the KMD(2D) is only 0.8-0.95mol% (Butler et al. 2004). In dilauroylphosphatidylcholine (DLPC) liposomes KMD(2D) is 0.0005-0.0008mol% based on Förster resonance energy transfer (FRET) measurements, slightly tighter than bicelles. These results emphasize the importance of the lipid membrane in influencing dimer affinity.

Solution NMR studies of cell-penetrating peptides in model membrane systems

Cell-penetrating peptides (CPPs) are a class of short, often cationic peptides that have the capability to translocate across cellular membranes, and although the translocation most likely involves several pathways, they interact directly with membranes, as well as with model bilayers. Most CPPs attain a three-dimensional structure when interacting with bilayers, while they are more or less unstructured in aqueous solution. To understand the relationship between structure and the effect that CPPs have on membranes it is of great importance to investigate CPPs at atomic resolution in a suitable membrane model. Moreover, the location in bilayers is likely to be correlated with the translocation mechanism. Solution-state NMR offers a unique possibility to investigate structure, dynamics and location of proteins and peptides in bilayers. This review focuses on solution NMR as a tool for investigating CPP-lipid interactions. Structural propensities and cell-penetrating capabilities can be derived from a combination of CPP solution structures and studies of the effect that the peptides have on bilayers and the localization in a bilayer.

Bicelles at low concentrations

Bilayered detergent-lipid assemblies known as bicelles have been widely used as model membranes in structural biological studies and are being explored for wider applications, including pharmaceutical use. Most studies to date have involved the use of concentrated bicelle mixtures, such that little is known about the capacity of bicellar mixtures to be diluted without unwanted transitions to nonisotropic phases. Here, different detergent/lipid mixtures have been explored, leading to the identification of two different families of bicelles for which it is possible to lower the total amphiphile (detergent + lipid) concentration to <1% (w/v) while retaining isotropic assemblies. These include a novel family of bicelles based on mixtures of 6-cyclohexyl-1-hexylphosphocholine (Cyclofos-6) and the lipid dimyristoylphosphatidylcholine (DMPC). Bicelles formed by these mixtures can be diluted to <0.5% and also have attractive biochemical properties. However, a caveat of our results is that the diffusion coefficients measured for the lipid component of the different bicelles tested were seen to be dependent on sample history, even though all samples were optically transparent. This suggests that the phase behavior of bicelles at low lipid-to-detergent ratios may be more complex than previously appreciated.

Crystallization of membrane proteins in bicelles

The structural biology of membrane proteins remains a challenging field, partly due to the difficulty in obtaining high-quality crystals. We developed the bicelle method as a tool to aid with the production of membrane protein crystals. Bicelles are bilayer discs that are formed by a mixture of a detergent and a lipid. They combine the ease of use of detergents with the benefits of a lipidic medium. Bicelles maintain membrane proteins in a bilayer milieu, which is more similar to their native environment than detergent micelles. At the same time, bicelles are liquid at certain temperatures and they can be integrated into standard crystallization techniques without the need for specialized equipment.

Structural Versatility of Bicellar Systems and Their Possibilities as Colloidal Carriers

Bicellar systems are lipid nanostructures formed by long- and short-chained phospholipids dispersed in aqueous solution. The morphological transitions of bicellar aggregates due to temperature, composition and time variations have been revised in this work. To this end, two bicellar systems have been considered; one formed by dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl- phosphatidylcholine (DHPC) and another formed by dipalmitoyl-phosphatidylcholine (DPPC) and DHPC. The relationship between the magnetic alignment, the morphology of the aggregates and the phase transition temperature (T_m) of lipids is discussed. In general terms, the non-alignable samples present rounded objects at temperature below the T_m. Above this temperature, an increase of viscosity is followed by the formation of large elongated aggregates. Alignable samples presented discoidal objects below the T_m. The best alignment was achieved above this temperature with large areas of lamellar stacked bilayers and some multilamellar vesicles. The effect of the inclusion of ceramides with different chain lengths in the structure of bicelles is also revised in the present article. A number of physical techniques show that the bicellar structures are affected by both the concentration and the type of ceramide. Systems are able to incorporate 10% mol of ceramides that probably are organized forming domains. The addition of 20% mol of ceramides promotes destabilization of bicelles, promoting the formation of mixed systems that include large structures. Bicellar systems have demonstrated to be morphologically stable with time, able to encapsulate different actives and to induce specific effects on the skin. These facts make bicellar systems good candidates as colloidal carriers for dermal delivery. However, water dilution induces structural changes and formation of vesicular structures in the systems; stabilization strategies have been been explored in recent works and are also updated here.

NMR studies of three-dimensional structure and positioning of CPPs in membrane model systems

CPPs are generally short cationic peptides that have the capability to interact directly with membranes. Most CPPs attain a three-dimensional structure when interacting with bilayers, while they are more or less unstructured in aqueous solution. To understand the relationship between structure and the effect that CPPs have on membranes, it is of great importance to investigate CPPs with atomic resolution in a suitable membrane model. Nuclear magnetic resonance (NMR) is an excellent technique both for studying solution structures of peptides as well as for investigating their location within a model bilayer. This chapter outlines protocols for producing model membrane systems for NMR investigations as well as the basic NMR tools for determining the three-dimensional structure of CPPs and for investigating the details in lipid-peptide interactions, i.e., the localization of the CPP in the bilayer.

Bicosomes: bicelles in dilute systems

Bicelles are discoidal phospholipid nanostructures at high lipid concentrations. Under dilute conditions, bicelles become larger and adopt a variety of morphologies. This work proposes a strategy to preserve the discoidal morphology of bicelles in environments with high water content. Bicelles were formed in concentrated conditions and subsequently encapsulated in liposomes. Later dilution of these new structures, called bicosomes, demonstrated that lipid vesicles were able to isolate and protect bicelles entrapped inside them from the medium. Characterization of systems before and after dilution by dynamic light-scattering spectroscopy and cryo-transmission electron microscopy showed that free bicelles changed in size and morphology, whereas encapsulated bicelles remained unaltered by the effect of dilution. Free and entrapped bicelles (containing the paramagnetic contrast agent gadodiamide) were injected into rat brain lateral ventricles. Coronal and sagittal visualization was performed by magnetic resonance imaging. Whereas rats injected with free bicelles did not survive the surgery, those injected with bicosomes did, and a hyperintensity effect due to gadodiamide was observed in the cerebrospinal fluid. These results indicate that bicosomes are a good means of preserving the morphology of bicelles under dilution conditions.

Left-handed dimer of EphA2 transmembrane domain: Helix packing diversity among receptor tyrosine kinases

The Eph receptor tyrosine kinases and their membrane-bound ephrin ligands control a diverse array of cell-cell interactions in the developing and adult organisms. During signal transduction across plasma membrane, Eph receptors, like other receptor tyrosine kinases, are involved in lateral dimerization and subsequent oligomerization presumably with proper assembly of their single-span transmembrane domains. Spatial structure of dimeric transmembrane domain of EphA2 receptor embedded into lipid bicelle was obtained by solution NMR, showing a left-handed parallel packing of the transmembrane helices (535-559)(2). The helices interact through the extended heptad repeat motif L(535)X(3)G(539)X(2)A(542)X(3)V(546)X(2)L(549) assisted by intermolecular stacking interactions of aromatic rings of (FF(557))(2), whereas the characteristic tandem GG4-like motif A(536)X(3)G(540)X(3)G(544) is not used, enabling another mode of helix-helix association. Importantly, a similar motif AX(3)GX(3)G as was found is responsible for right-handed dimerization of transmembrane domain of the EphA1 receptor. These findings serve as an instructive example of the diversity of transmembrane domain formation within the same family of protein kinases and seem to favor the assumption that the so-called rotation-coupled activation mechanism may take place during the Eph receptor signaling. A possible role of membrane lipid rafts in relation to Eph transmembrane domain oligomerization and Eph signal transduction was also discussed.

Structure elucidation of dimeric transmembrane domains of bitopic proteins

The interaction between transmembrane helices is of great interest because it directly determines biological activity of a membrane protein. Either destroying or enhancing such interactions can result in many diseases related to dysfunction of different tissues in human body. One much studied form of membrane proteins known as bitopic protein is a dimer containing two membrane-spanning helices associating laterally. Establishing structure-function relationship as well as rational design of new types of drugs targeting membrane proteins requires precise structural information about this class of objects. At present time, to investigate spatial structure and internal dynamics of such transmembrane helical dimers, several strategies were developed based mainly on a combination of NMR spectroscopy, optical spectroscopy, protein engineering and molecular modeling. These approaches were successfully applied to homo- and heterodimeric transmembrane fragments of several bitopic proteins, which play important roles in normal and in pathological conditions of human organism.

Solution structure of the HsapBK K+ channel voltage-sensor paddle sequence

Voltage-gated potassium channels open and close in response to changes in the membrane potential. In this study, we have determined the NMR solution structure of the putative S3b-S4 voltage-sensor paddle fragment, the part that moves to mediate voltage gating, of the HsapBK potassium channel in dodecylphosphocholine (DPC) micelles. This paper presents the first structure of the S3b-S4 fragment from a BK channel. Diffusion coefficients as determined from PFG NMR experiments showed that a well-defined complex between the peptide and DPC molecules was formed. The structure reveals a helix-turn-helix motif, which is in agreement with crystal structures of other voltage-gated potassium channels, thus indicating that it is feasible to study the isolated fragment. The paddle motifs generally contain several basic residues, implicated in the gating. The critical Arg residues in this structure all reside on the surface, which is in agreement with crystal structures of K(v) channels. Similarities in the structure of the S3b-S4 fragment in BK and K(v) channels as well as important differences are seen, which may be important for explaining the details in paddle movement within a bilayer.

Biomimetic approaches for studying membrane processes

This short review focuses on recent innovative systems and experimental approaches designed to investigate membrane processes and biomolecular interactions associated with membranes. Our emphasis is on "biomimetics" which reflects the significance and contributions of the chemistry/biology interface in addressing complex biological questions. We have not limited this review to discussion of new "sensors" or "assays"per se, but rather we tried to review new concepts employed for analysis of membrane processes.