Crystallization screens: compatibility with the lipidic cubic phase for in meso crystallization of membrane proteins

Lipidic Sponge Phase Crystallization of Membrane Proteins

Bicontinuous lipidic cubic phases can be used as a host for growing crystals of membrane proteins. Since the cubic phase is stiff, handling is difficult and time-consuming. Moreover, the conventional cubic phase may interfere with the hydrophilic domains of membrane proteins due to the limited size of the aqueous pores. Here, we introduce a new crystallization method that makes use of a liquid analogue of the cubic phase, the sponge phase. This phase facilitates a considerable increase in the allowed size of aqueous domains of membrane proteins, and is easily generalised to a conventional vapour diffusion crystallisation experiment, including the use of nanoliter drop crystallization robots. The appearance of the sponge phase was confirmed by visual inspection, small-angle X-ray scattering and NMR spectroscopy. Crystals of the reaction centre from Rhodobacter sphaeroides were obtained by a conventional hanging-drop experiment, were harvested directly without the addition of lipase or cryoprotectant, and the structure was refined to 2.2 Angstroms resolution. In contrast to our earlier lipidic cubic phase reaction centre structure, the mobile ubiquinone could be built and refined. The practical advantages of the sponge phase make it a potent tool for crystallization of membrane proteins.

Structural transitions of the mono-olein bicontinuous cubic phase induced by inclusion of protein lysozyme solutions

Inclusion of protein lysozyme molecules in the lipidic mono-olein cubic phase induces a transition from a Pn3m structure to an Im3m one. The small-angle x-ray scattering method with high-intensity synchrotron radiation enabled us to follow closely the transition depending on the conditions of lysozyme solutions. We show that concentrated lysozyme solutions induced the appearance of the Im3m structure coexisting with the Pn3m structure. From the relation between the lattice parameters of these two structures it is shown that they are related by the Bonnet transformation of the underlying triply periodic minimal surfaces. We found that the transition also occurred at lower lysozyme concentration when NaCl induced an attraction between lysozyme molecules. The origin of the transition was considered as a frustration in the cubic phase where lysozyme molecules were highly confined. A simple estimation of the frustration was given, which took into account the translational entropy of lysozyme molecules. At the highest concentration of lysozyme and NaCl the Im3m structure was found to disappear and left only the Pn3m structure. This was probably either due to the crystallization or phase separation of lysozyme solutions ongoing microscopically, which absorbed lysozyme molecules from channels of the cubic phase and thus removed the frustration.

Cubic phase nanoparticles (Cubosome): principles for controlling size, structure, and stability

Methods and compositions for producing lipid-based cubic phase nanoparticles were first discovered in the 1990s. Since then a number of studies have been presented, but little is known about how to control key properties such as particle size, morphology, and stability of cubic phase dispersions. In the present work we give examples of how these properties can be tuned by composition and processing conditions. Importantly we show that stable particle dispersions with consistent size and structure can be produced by a simple processing scheme comprising a homogenization and heat treatment step.

Cubic Phases and Cubic Phase Dispersions in a Phospholipid-Based System

A cubic liquid crystalline phase forming system based on the phospholipid dioleoylphosphatidylethanolamine (DOPE) which is fortified with small amounts of PEGylated (poly(ethylene) glycol) glycerol monooleate (PEG(660)-GMO) is characterized. The cubic phase formed by the DOPE/PEG(660)-GMO/water system coexists with water in the dilute part of the phase diagram and can be fragmented into colloidal size particles with retained cubic phase structure.

Lipidic cubic phases as matrices for membrane protein crystallization

This review provides detailed procedures for the crystallization of membrane proteins via the lipidic cubic phase method. Bacteriorhodopsin-specific, hands-on protocols are given for (i) the preparation of bacteriohordopsin from purple membrane by monomerization in octylglucoside and gel filtration chromatography or by selective extraction after pre-treatment with dodecyl-trimethylammonium bromide, (ii) the incorporation of bacteriorhodopsin into lipidic cubic phases by mixing in vials or within coupled syringes and, (iii) the crystallization of bacteriorhodopsin in the lipidic matrix by adding a solid salt or an overlaying with a solution. References for further useful procedures and materials are listed in order to provide biochemists and crystallographers with all information that is necessary to grow crystals of the membrane protein bacteriorhodopsin.

The effect of increasing concentrations of precipitating salts used to crystallize proteins on the structure of the lipidic Q224 cubic phase

A major obstacle to elucidating the structure of membrane proteins at high resolution is the difficulty of preparing these proteins as well as to grow well-ordered crystals. During the last few years several groups have considered the use of three-dimensional bicontinuous lipidic cubic phases as a possible crystallization matrix for such molecules. In a few cases these studies have been successfully approached by other laboratories, however, a number of questions remain, particularly in regard to the effects of solutes on the phase diagrams of lipid-water systems. In the present work we report the structural behavior of the lipidic Q224 (Pn3m), Q230 (Ia3d) and HII phases systematically studied in the presence of a range of concentrations of various salts and precipitating agents at various pH values. Some of the results reported here have been presented elsewhere Vargas et al. (2000) [Strategies in membrane protein crystallization. Chemical Prospectives in Crystallography of Molecular Biology. International School of Crystallography, NATO-ASI course, Erice (Italy)].

Membrane protein crystallization

The need for high-resolution structure information on membrane proteins is immediate and growing. Currently, the only reliable way to get it is crystallographically. The rate-limiting step from protein to structure is crystal production. An overview of the current ideas and experimental approaches prevailing in the area of membrane protein crystallization is presented. The long-established surfactant-based method has been reviewed extensively and is not examined in detail here. The focus instead is on the latest methods, all of which exploit the spontaneous self-assembling properties of lipids and detergent as vesicles (vesicle-fusion method), discoidal micelles (bicelle method), and liquid crystals or mesophases (in meso or cubic-phase method). In the belief that a knowledge of the underlying phase science is integral to understanding the molecular basis of these assorted crystallization strategies, the article begins with a brief primer on lipids, mesophases, and phase science, and the related issue of form and function as applied to lipids is addressed. The experimental challenges associated with and the solutions for procuring adequate amounts of homogeneous membrane proteins, or parts thereof, are examined. The cubic-phase method is described from the following perspectives: how it is done in practice, its general applicability and successes to date, and the nature of the mesophases integral to the process. Practical aspects of the method are examined with regard to salt, detergent, and screen solution effects; crystallization at low temperatures; tailoring the cubic phase to suit the target protein; different cubic-phase types; dealing with low-protein samples, colorless proteins, microcrystals, and radiation damage; transport within the cubic phase for drug design, cofactor retention, and phasing; using spectroscopy for quality control; harvesting crystals; and miniaturization and robotization for high-throughput screening. The section ends with a hypothesis for nucleation and growth of membrane protein crystals in meso. Thus far, the bicelle and vesicle-fusion methods have produced crystals of one membrane protein, bacteriorhodopsin. The experimental details of both methods are reviewed and their general applicability in the future is commented on. The three new methods are rationalized by analogy to crystallization in microgravity and with respect to epitaxy. A list of Web resources in the area of membrane protein crystallogenesis is included.

Detergents destabilize the cubic phase of monoolein: implications for membrane protein crystallization

The in meso method for membrane protein crystallization uses a lipidic cubic phase as the hosting medium. The cubic phase provides a lipid bilayer into which the protein presumably reconstitutes and from which protein crystals nucleate and grow. The solutions used to spontaneously form the protein-enriched cubic phase often contain significant amounts of detergents that were employed initially to purify and to solubilize the membrane protein. By virtue of their surface activity, detergents have the potential to impact on the phase properties of the in meso system and, by extension, the outcome of the crystallization process. The purpose of this study was to quantify the effects that a popular series of nonionic detergents, the n-alkyl-beta-D-glucopyranosides, have on the phase behavior of hydrated monoolein, the lipid upon which the in meso method is based. Phase identity and phase microstructure were characterized by small-angle x-ray diffraction on samples prepared to mimic in meso crystallization conditions. Measurements were made in the 0-40 degrees C range. Samples prepared in the cooling direction allow for the expression of metastability, a feature of liquid crystalline phases that might be exploited in low-temperature crystallization. The results show that the cubic phase is relatively insensitive to small amounts of alkyl glucosides. However, at higher levels the detergents trigger a transition to the lamellar phase in a temperature- and salt concentration-dependent manner. These effects have important implications for in meso crystallization. A diffraction-based method for assaying detergents is presented.

Crystallization of membrane proteins from media composed of connected-bilayer gels

The use of hydrated-lipid gels in which the bilayer is an infinitely periodic (or at least continuous), three-dimensional structure offers a relatively new approach for the crystallization of membrane proteins. While excellent crystals of the Halobacterial rhodopsins have been obtained with such media, success remains poor in extending their use to other membrane proteins. Experience with crystallization of bacteriorhodopsin has led us to recognize a number of improvements that can be made in the use of such hydrated-gel media, which may now prove to be of general value for the crystallization of other membrane proteins.

Towards redox active liquid crystalline phases of lipids: a monoolein/water system with entrapped derivatives of ferrocene

The phase and electrochemical behavior of the aqueous mixtures of monoolein (MO) and synthetic ferrocene (Fc) derivatives containing long alkyl chains-(Z)-octadec-9-enoylferrocene (1), (Z)-octadecen-9-ylferrocene (2), and ferrocenylmethyl (Z)-octadec-9-enoate (3)-were studied. At low hydration, the reversed micelles (L(2) phase) and cubic Q(230) phase of MO can accommodate relatively high amounts (>6 wt.%) of the Fc-derivative 2, whereas at high hydration, the pseudoternary cubic phase Q(224) is destabilized even at about 2 wt.% of this Fc. Increasing the Fc-derivative content induces L(alpha)-->L(2) and L(alpha)-->reversed bicontinuous cubic phase (Q(II))-->H(II) transitions depending upon hydration. A rough study of the MO system containing compounds 1 and 3 indicates very similar phase behavior to that of the MO/2/H(2)O system. Compound 2 apparently has no effect on the lipid monolayer thickness in the pseudoternary L(alpha), H(II) and Q(II) liquid crystalline phases of MO. Within a 3D-structure of the Q(224) phase, derivatives 1-3 exhibit electrochemical activity on the gold electrode. The one-electron redox conversion processes are electrochemically quasi-reversible and controlled by diffusion. The values of apparent diffusion coefficient (D(app)) and heterogeneous electron-transfer rate constant (k(s)) of Fcs are significantly lower in the cubic phase matrix when compared to the acetonitrile solution. By contrast, the MO H(II) phase with entrapped Fc-derivatives does not exhibit electrochemical activity on the electrode surface. It is suggested that the diffusional anisotropy and/or localized aggregation of compounds 1-3 within a 2D-structure of the H(II) phase account(s) for the latter observation.

Thermal behaviour of cubic phases rich in 1-monooleoyl-rac-glycerol in the ternary system. 1-monooleoyl-rac-glycerol/n-octyl-beta-D-glucoside/water

Using synchrotron X-ray diffraction the thermal behaviour was studied of the cubic phases in the 1-monooleoyl-rac-glycerol (MO)/n-octyl-beta-d-glucopyranoside (OG)/2H2O system with 58 or 45 wt % MO concentration and varying OG/2H2O contents. These MO contents correspond to a Pn3m cubic single-phase or a Pn3m cubic phase in excess water on the binary MO/water axis of the ternary phase diagram. The cubic liquid crystalline phases are stable with small fractions of OG, while higher OG concentrations trigger a cubic-to-lamellar phase transition. Moreover, with increasing OG concentration the initial Pn3m structure is completely converted to an Ia3d structure prior to the Lalpha phase being formed. Upon heating this effect is reversed, resulting in an Ia3d-to-Pn3m phase transition. For some samples additional peaks were observed in the diffractograms upon heating, resulting from the metastability notoriously shown by bicontinuous cubic phases. This judgement is supported by the fact that upon cooling these peaks were absent. Remarkably, both the Ia3d and the Pn3m cubic structures could be in equilibrium with excess water in this ternary system. A comparison is made with previous results on n-dodecyl-beta-d-maltoside (DM), showing that cubic phases with OG have higher thermal and compositional stability than with DM.

Membrane protein crystallization in meso: lipid type-tailoring of the cubic phase

Hydrated monoolein forms the cubic-Pn3m mesophase that has been used for in meso crystallization of membrane proteins. The crystals have subsequently provided high-resolution structures by crystallographic means. It is possible that the hosting cubic phase created by monoolein alone, which itself is not a common membrane component, will limit the range of membrane proteins crystallizable by the in meso method. With a view to expanding the range of applicability of the method, we investigated by x-ray diffraction the degree to which the reference cubic-Pn3m phase formed by hydrated monoolein could be modified by other lipid types. These included phosphatidylcholine (PC), phosphatidylethanolamine, phosphatidylserine, cardiolipin, lyso-PC, a polyethylene glycol-lipid, 2-monoolein, oleamide, and cholesterol. The results show that all nine lipids were accommodated in the cubic phase to some extent without altering phase identity. The positional isomer, 2-monoolein, was tolerated to the highest level. The least well tolerated were the anionic lipids, followed by lyso-PC. The others were accommodated to the extent of 20-25 mol %. Beyond a certain concentration limit, the lipid additives either triggered one or a series of phase transitions or saturated the phase and separated out as crystals, as seen with oleamide and cholesterol. The series of phases observed and their order of appearance were consistent with expectations in terms of interfacial curvature changes. The changes in phase type and microstructure have been rationalized on the basis of lipid molecular shape, interfacial curvature, and chain packing energy. The data should prove useful in the rational design of cubic phase crystallization matrices with different lipid profiles that match the needs of a greater range of membrane proteins.