Revival of electron crystallography

Perspective: A toolbox for protein structure determination in physiological environment through oriented, 2D ordered, site specific immobilization

Revealing the structure of complex biological macromolecules, such as proteins, is an essential step for understanding the chemical mechanisms that determine the diversity of their functions. Synchrotron based X-ray crystallography and cryo-electron microscopy have made major contributions in determining thousands of protein structures even from micro-sized crystals. They suffer from some limitations that have not been overcome, such as radiation damage, the natural inability to crystallize a number of proteins, and experimental conditions for structure determination that are incompatible with the physiological environment. Today, the ultra-short and ultra-bright pulses of X-ray free-electron lasers have made attainable the dream to determine protein structures before radiation damage starts to destroy the samples. However, the signal-to-noise ratio remains a great challenge to obtain usable diffraction patterns from a single protein molecule. With the perspective to overcome these challenges, we describe here a new methodology that has the potential to overcome the signal-to-noise-ratio and protein crystallization limits. Using a multidisciplinary approach, we propose to create ordered, two dimensional protein arrays with defined orientation attached on a self-assembled-monolayer. We develop a literature-based flexible toolbox capable of assembling different kinds of proteins on a functionalized surface and consider using a graphene cover layer that will allow performing experiments with proteins in physiological conditions.

Purification of Human and Mammalian Membrane Proteins Expressed in Xenopus laevis Frog Oocytes for Structural Studies

This protocol describes the isolation of recombinant human and mammalian membrane proteins expressed in Xenopus laevis frog oocytes for structural studies. The cDNA-derived cRNA of the desired genes is injected into several hundreds of oocytes, which are incubated for several days to allow protein expression. Recombinant proteins are then purified via affinity chromatography. The novelty of this method comes from the design of a plasmid that produces multi-tagged proteins and, most importantly, the development of a protocol for efficiently discarding lipids, phospholipids, and lipoproteins from the oocyte egg yolk, which represent the major contaminants in protein purifications. Thus, the high protein purity and good yield obtained from this method allows protein structure determination by transmission electron microscopy of single detergent-solubilized protein particles and of 2D crystals of membrane protein embedded in lipid bilayers. Additionally, a radiotracer assay for functional analysis of the expressed target proteins in oocytes is described. Overall, this method is a valuable option for structural studies of mammalian and particularly human proteins, for which other expression systems often fail.

Selecting analytical tools for characterization of polymersomes in aqueous solution

We present 17 techniques to analyze polymersomes, in terms of their size, bilayer properties, elastic properties or surface charge.

High-resolution cryo-electron microscopy on macromolecular complexes and cell organelles

Cryo-electron microscopy techniques and computational 3-D reconstruction of macromolecular assemblies are tightly linked tools in modern structural biology. This symbiosis has produced vast amounts of detailed information on the structure and function of biological macromolecules. Typically, one of two fundamentally different strategies is used depending on the specimens and their environment. A: 3-D reconstruction based on repetitive and structurally identical unit cells that allow for averaging, and B: tomographic 3-D reconstructions where tilt-series between approximately ± 60 and ± 70° at small angular increments are collected from highly complex and flexible structures that are beyond averaging procedures, at least during the first round of 3-D reconstruction. Strategies of group A are averaging-based procedures and collect large number of 2-D projections at different angles that are computationally aligned, averaged together, and back-projected in 3-D space to reach a most complete 3-D dataset with high resolution, today often down to atomic detail. Evidently, success relies on structurally repetitive particles and an aligning procedure that unambiguously determines the angular relationship of all 2-D projections with respect to each other. The alignment procedure of small particles may rely on their packing into a regular array such as a 2-D crystal, an icosahedral (viral) particle, or a helical assembly. Critically important for cryo-methods, each particle will only be exposed once to the electron beam, making these procedures optimal for highest-resolution studies where beam-induced damage is a significant concern. In contrast, tomographic 3-D reconstruction procedures (group B) do not rely on averaging, but collect an entire dataset from the very same structure of interest. Data acquisition requires collecting a large series of tilted projections at angular increments of 1-2° or less and a tilt range of ± 60° or more. Accordingly, tomographic data collection exposes its specimens to a large electron dose, which is particularly problematic for frozen-hydrated samples. Currently, cryo-electron tomography is a rapidly emerging technology, on one end driven by the newest developments of hardware such as super-stabile microscopy stages as well as the latest generation of direct electron detectors and cameras. On the other end, success also strongly depends on new software developments on all kinds of fronts such as tilt-series alignment and back-projection procedures that are all adapted to the very low-dose and therefore very noisy primary data. Here, we will review the status quo of cryo-electron microscopy and discuss the future of cellular cryo-electron tomography from data collection to data analysis, CTF-correction of tilt-series, post-tomographic sub-volume averaging, and 3-D particle classification. We will also discuss the pros and cons of plunge freezing of cellular specimens to vitrified sectioning procedures and their suitability for post-tomographic volume averaging despite multiple artifacts that may distort specimens to some degree.

Membrane curvature in flaviviruses

Coordinated interplay between membrane proteins and the lipid bilayer is required for such processes as transporter function and the entrance of enveloped viruses into host cells. In this study, three-dimensional cryo-electron microscopy density maps of mature and immature flaviviruses were analyzed to assess the curvature of the membrane leaflets and its relation to membrane-bound viral glycoproteins. The overall morphology of the viral membrane is determined by the icosahedral scaffold composed of envelope (E) and membrane (M) proteins through interaction of the proteins' stem-anchor regions with the membrane. In localized regions, small membrane areas exhibit convex, concave, flat or saddle-shaped surfaces that are constrained by the specific protein organization within each membrane leaflet. These results suggest that the organization of membrane proteins in small enveloped viruses mediate the formation of membrane curvature.

Kinetic, energetic, and mechanical differences between dark-state rhodopsin and opsin

Rhodopsin, the photoreceptor pigment of the retina, initiates vision upon photon capture by its covalently linked chromophore 11-cis-retinal. In the absence of light, the chromophore serves as an inverse agonist locking the receptor in the inactive dark state. In the absence of chromophore, the apoprotein opsin shows low-level constitutive activity. Toward revealing insight into receptor properties controlled by the chromophore, we applied dynamic single-molecule force spectroscopy to quantify the kinetic, energetic, and mechanical differences between dark-state rhodopsin and opsin in native membranes from the retina of mice. Both rhodopsin and opsin are stabilized by ten structural segments. Compared to dark-state rhodopsin, the structural segments stabilizing opsin showed higher interaction strengths and mechanical rigidities and lower conformational variabilities, lifetimes, and free energies. These changes outline a common mechanism toward activating G-protein-coupled receptors. Additionally, we detected that opsin was more pliable and frequently stabilized alternate structural intermediates.

High-throughput methods for electron crystallography

Membrane proteins play a tremendously important role in cell physiology and serve as a target for an increasing number of drugs. Structural information is key to understanding their function and for developing new strategies for combating disease. However, the complex physical chemistry associated with membrane proteins has made them more difficult to study than their soluble cousins. Electron crystallography has historically been a successful method for solving membrane protein structures and has the advantage of providing a native lipid environment for these proteins. Specifically, when membrane proteins form two-dimensional arrays within a lipid bilayer, electron microscopy can be used to collect images and diffraction and the corresponding data can be combined to produce a three-dimensional reconstruction, which under favorable conditions can extend to atomic resolution. Like X-ray crystallography, the quality of the structures are very much dependent on the order and size of the crystals. However, unlike X-ray crystallography, high-throughput methods for screening crystallization trials for electron crystallography are not in general use. In this chapter, we describe two alternative methods for high-throughput screening of membrane protein crystallization within the lipid bilayer. The first method relies on the conventional use of dialysis for removing detergent and thus reconstituting the bilayer; an array of dialysis wells in the standard 96-well format allows the use of a liquid-handling robot and greatly increases throughput. The second method relies on titration of cyclodextrin as a chelating agent for detergent; a specialized pipetting robot has been designed not only to add cyclodextrin in a systematic way, but to use light scattering to monitor the reconstitution process. In addition, the use of liquid-handling robots for making negatively stained grids and methods for automatically imaging samples in the electron microscope are described.

Processing of electron diffraction patterns with the XDP program

Electron crystallography using two-dimensional crystals of membrane protein can provide high-resolution structure of a membrane protein within a lipid bilayer. With this technique, it is advantageous to use electron diffraction patterns to collect accurate intensities of the structure factors at high resolution. Here we describe how to process diffraction patterns using the XDP program and show what parameters are used and how they are determined in the process. Finally, the procedures for merging the intensity data will be described briefly.

High-density reconstitution of functional water channels into vesicular and planar block copolymer membranes

The exquisite selectivity and unique transport properties of membrane proteins can be harnessed for a variety of engineering and biomedical applications if suitable membranes can be produced. Amphiphilic block copolymers (BCPs), developed as stable lipid analogs, form membranes that functionally incorporate membrane proteins and are ideal for such applications. While high protein density and planar membrane morphology are most desirable, BCP–membrane protein aggregates have so far been limited to low protein densities in either vesicular or bilayer morphologies. Here, we used dialysis to reproducibly form planar and vesicular BCP membranes with a high density of reconstituted aquaporin-0 (AQP0) water channels. We show that AQP0 retains its biological activity when incorporated at high density in BCP membranes, and that the morphology of the BCP–protein aggregates can be controlled by adjusting the amount of incorporated AQP0. We also show that BCPs can be used to form two-dimensional crystals of AQP0.

Molecular driving forces defining lipid positions around aquaporin-0

Lipid-protein interactions play pivotal roles in biological membranes. Electron crystallographic studies of the lens-specific water channel aquaporin-0 (AQP0) revealed atomistic views of such interactions, by providing high-resolution structures of annular lipids surrounding AQP0. It remained unclear, however, whether these lipid structures are representative of the positions of unconstrained lipids surrounding an individual protein, and what molecular determinants define the lipid positions around AQP0. We addressed these questions by using molecular dynamics simulations and crystallographic refinement, and calculated time-averaged densities of dimyristoyl-phosphatidylcholine lipids around AQP0. Our simulations demonstrate that, although the experimentally determined crystallographic lipid positions are constrained by the crystal packing, they appropriately describe the behavior of unconstrained lipids around an individual AQP0 tetramer, and thus likely represent physiologically relevant lipid positions.While the acyl chains were well localized, the lipid head groups were not. Furthermore, in silico mutations showed that electrostatic interactions do not play a major role attracting these phospholipids towards AQP0. Instead, the mobility of the protein crucially modulates the lipid localization and explains the difference in lipid density between extracellular and cytoplasmic leaflets. Moreover, our simulations support a general mechanism in which membrane proteins laterally diffuse accompanied by several layers of localized lipids, with the positions of the annular lipids being influenced the most by the protein surface. We conclude that the acyl chains rather than the head groups define the positions of dimyristoyl-phosphatidylcholine lipids around AQP0. Lipid localization is largely determined by the mobility of the protein surface, whereas hydrogen bonds play an important but secondary role.

The use of trehalose in the preparation of specimens for molecular electron microscopy

Biological specimens have to be prepared for imaging in the electron microscope in a way that preserves their native structure. Two-dimensional (2D) protein crystals to be analyzed by electron crystallography are best preserved by sugar embedding. One of the sugars often used to embed 2D crystals is trehalose, a disaccharide used by many organisms for protection against stress conditions. Sugars such as trehalose can also be added to negative staining solutions used to prepare proteins and macromolecular complexes for structural studies by single-particle electron microscopy (EM). In this review, we describe trehalose and its characteristics that make it so well suited for preparation of EM specimens and we review specimen preparation methods with a focus on the use of trehalose.

Structure determination of channel and transport proteins by high-resolution microscopy techniques

High-resolution microscopy techniques provide a plethora of information on biological structures from the cellular level down to the molecular level. In this review, we present the unique capabilities of transmission electron and atomic force microscopy to assess the structure, oligomeric state, function and dynamics of channel and transport proteins in their native environment, the lipid bilayer. Most importantly, membrane proteins can be visualized in the frozen-hydrated state and in buffer solution by cryo-transmission electron and atomic force microscopy, respectively. We also illustrate the potential of the scintillation proximity assay to study substrate binding of detergent-solubilized transporters prior to crystallization and structural characterization.

Role of lipids in spheroidal high density lipoproteins

We study the structure and dynamics of spherical high density lipoprotein (HDL) particles through coarse-grained multi-microsecond molecular dynamics simulations. We simulate both a lipid droplet without the apolipoprotein A-I (apoA-I) and the full HDL particle including two apoA-I molecules surrounding the lipid compartment. The present models are the first ones among computational studies where the size and lipid composition of HDL are realistic, corresponding to human serum HDL. We focus on the role of lipids in HDL structure and dynamics. Particular attention is paid to the assembly of lipids and the influence of lipid-protein interactions on HDL properties. We find that the properties of lipids depend significantly on their location in the particle (core, intermediate region, surface). Unlike the hydrophobic core, the intermediate and surface regions are characterized by prominent conformational lipid order. Yet, not only the conformations but also the dynamics of lipids are found to be distinctly different in the different regions of HDL, highlighting the importance of dynamics in considering the functionalization of HDL. The structure of the lipid droplet close to the HDL-water interface is altered by the presence of apoA-Is, with most prominent changes being observed for cholesterol and polar lipids. For cholesterol, slow trafficking between the surface layer and the regimes underneath is observed. The lipid-protein interactions are strongest for cholesterol, in particular its interaction with hydrophobic residues of apoA-I. Our results reveal that not only hydrophobicity but also conformational entropy of the molecules are the driving forces in the formation of HDL structure. The results provide the first detailed structural model for HDL and its dynamics with and without apoA-I, and indicate how the interplay and competition between entropy and detailed interactions may be used in nanoparticle and drug design through self-assembly.

Bridging the gap between structural models of nicotinic receptor superfamily ion channels and their corresponding functional states

Aromatic-aromatic interactions are a prominent feature of the crystal structure of ELIC [Protein Data Bank (PDB) code 2VL0], a bacterial member of the nicotinic receptor superfamily of ion channels where five pore-facing phenylalanines come together to form a structure akin to a narrow iris that occludes the transmembrane pore. To identify the functional state of the channel that this structure represents, we engineered phenylalanines at various pore-facing positions of the muscle acetylcholine (ACh) receptor (one position at a time), including the position that aligns with the native phenylalanine 246 of ELIC, and assessed the consequences of such mutations using electrophysiological and toxin-binding assays. From our experiments, we conclude that the interaction among the side chains of pore-facing phenylalanines, rather than the accumulation of their independent effects, leads to the formation of a nonconductive conformation that is unresponsive to the application of ACh and is highly stable even in the absence of ligand. Moreover, electrophysiological recordings from a GLIC channel (another bacterial member of the superfamily) engineered to have a ring of phenylalanines at the corresponding pore-facing position suggest that this novel refractory state is distinct from the well-known desensitized state. It seems reasonable to propose then that it is in this peculiar nonconductive conformation that the ELIC channel was crystallized. It seems also reasonable to propose that, in the absence of rings of pore-facing aromatic side chains, such stable conformation may never be attained by the ACh receptor. Incidentally, we also noticed that the response of the proton-gated wild-type GLIC channel to a fast change in pH from pH 7.4 to pH 4.5 (on the extracellular side) is only transient, with the evoked current fading completely in a matter of  seconds. This raises the possibility that the crystal structures of GLIC obtained at pH 4.0 (PDB code 3EHZ) and pH 4.6 (PDB code 3EAM) correspond to the to the (well-known) desensitized state.

Electron cryomicroscopy of membrane proteins: specimen preparation for two-dimensional crystals and single particles

Membrane protein structure and function can be studied by two powerful and highly complementary electron cryomicroscopy (cryo-EM) methods: electron crystallography of two-dimensional (2D) crystals and single particle analysis of detergent-solubilized protein complexes. To obtain the highest-possible resolution data from membrane proteins, whether prepared as 2D crystals or single particles, cryo-EM samples must be vitrified with great care. Grid preparation for cryo-EM of 2D crystals is possible by back-injection, the carbon sandwich technique, drying in sugars before cooling in the electron microscope, or plunge-freezing. Specimen grids for single particle cryo-EM studies of membrane proteins are usually produced by plunge-freezing protein solutions, supported either by perforated or a continuous carbon film substrate. This review outlines the different techniques available and the suitability of each method for particular samples and studies. Experimental considerations in sample preparation and preservation include the protein itself and the presence of lipid or detergent. The appearance of cryo-EM samples in different conditions is also discussed.

Automated electron microscopy for evaluating two-dimensional crystallization of membrane proteins

Membrane proteins fulfill many important roles in the cell and represent the target for a large number of therapeutic drugs. Although structure determination of membrane proteins has become a major priority, it has proven to be technically challenging. Electron microscopy of two-dimensional (2D) crystals has the advantage of visualizing membrane proteins in their natural lipidic environment, but has been underutilized in recent structural genomics efforts. To improve the general applicability of electron crystallography, high-throughput methods are needed for screening large numbers of conditions for 2D crystallization, thereby increasing the chances of obtaining well ordered crystals and thus achieving atomic resolution. Previous reports describe devices for growing 2D crystals on a 96-well format. The current report describes a system for automated imaging of these screens with an electron microscope. Samples are inserted with a two-part robot: a SCARA robot for loading samples into the microscope holder, and a Cartesian robot for placing the holder into the electron microscope. A standard JEOL 1230 electron microscope was used, though a new tip was designed for the holder and a toggle switch controlling the airlock was rewired to allow robot control. A computer program for controlling the robots was integrated with the Leginon program, which provides a module for automated imaging of individual samples. The resulting images are uploaded into the Sesame laboratory information management system database where they are associated with other data relevant to the crystallization screen.

Electron crystallography and aquaporins

Electron crystallography of two-dimensional (2D) crystals can provide information on the structure of membrane proteins at near-atomic resolution. Originally developed and used to determine the structure of bacteriorhodopsin (bR), electron crystallography has recently been applied to elucidate the structure of aquaporins (AQPs), a family of membrane proteins that form pores mostly for water but also other solutes. While electron crystallography has made major contributions to our understanding of the structure and function of AQPs, structural studies on AQPs, in turn, have fostered a number of technical developments in electron crystallography. In this contribution, we summarize the insights electron crystallography has provided into the biology of AQPs, and describe technical advancements in electron crystallography that were driven by structural studies on AQP 2D crystals. In addition, we discuss some of the lessons that were learned from electron crystallographic work on AQPs.

Projection structure of DtpD (YbgH), a prokaryotic member of the peptide transporter family

Cellular uptake of di- and tripeptides has been characterized in numerous organisms, and various transporters have been identified. In contrast, structural information on peptide transporters is very sparse. Here, we have cloned, overexpressed, purified, and biochemically characterized DtpD (YbgH) from Escherichia coli, a prokaryotic member of the peptide transporter family. Its homologues in mammals, PEPT1 (SLC15A1) and PEPT2 (SLC15A2), not only transport peptides but also are of relevance for uptake of drugs as they accept a large spectrum of peptidomimetics such as beta-lactam antibiotics, antivirals, peptidase inhibitors, and others as substrates. Uptake experiments indicated that DtpD functions as a canonical peptide transporter and is, therefore, a valid model for structural studies of this family of proteins. Blue native polyacrylamide gel electrophoresis, gel filtration, and transmission electron microscopy of single-DtpD particles suggest that the transporter exists in a monomeric form when solubilized in detergent. Two-dimensional crystallization of DtpD yielded first tubular crystals that allowed the determination of a projection structure at better than 19 A resolution. This structure of DtpD represents the first structural view of a member of the peptide transporter family.

Electron crystallography as a technique to study the structure on membrane proteins in a lipidic environment

The native environment of integral membrane proteins is a lipid bilayer. The structure of a membrane protein is thus ideally studied in a lipidic environment. In the first part of this review we describe some membrane protein structures that revealed the surrounding lipids and provide a brief overview of the techniques that can be used to study membrane proteins in a lipidic environment. In the second part of this review we focus on electron crystallography of two-dimensional crystals as potentially the most suitable technique for such studies. We describe the individual steps involved in the electron crystallographic determination of a membrane protein structure and discuss current challenges that need to be overcome to transform electron crystallography into a technique that can be routinely used to analyze the structure of membrane proteins embedded in a lipid bilayer.

Electron crystallography of proteins in membranes

Electron crystallography has played a vital role in advancing our understanding of proteins in membranes since the 'fluid mosaic model' was proposed in 1972. It is now an established technique to reveal the structures of proteins in their natural bilayer environment and makes possible the study of biological mechanisms through freeze-trapping of transitional states. Thus, images and diffraction patterns of well-ordered, planar and tubular protein-lipid crystals are yielding atomic models, which tell us how the proteins in situ are designed and carry out their membrane-specific tasks. Recent methodological advances and the inclusion of tomographic and cryo-sectioning techniques are enabling detailed information to be obtained from increasingly smaller and more disordered membrane assemblies, extending the potential of this approach.

The influence of nano MgO and BaSO4 particle size additives on properties of PMMA bone cement

A common technique to aid in implant fixation into surrounding bone is to inject bone cement into the space between the implant and surrounding bone. The most common bone cement material used clinically today is poly(methyl methacrylate), or PMMA. Although promising, there are numerous disadvantages of using PMMA in bone fixation applications which has limited its wide spread use. Specifically, the PMMA polymerization reaction is highly exothermic in situ, thus, damaging surrounding bone tissue while curing. In addition, PMMA by itself is not visible using typical medical imaging techniques (such as X-rays required to assess new bone formation surrounding the implant). Lastly, although PMMA does support new bone growth, studies have highlighted decreased osteoblast (bone forming cell) functions on PMMA compared to other common orthopedic coating materials, such as calcium phosphates and hydroxyapatite. For these reasons, the goal of this study was to begin to investigate novel additives to PMMA which can enhance its cytocompatibility properties with osteoblasts, decrease its exothermic reaction when curing, and increase its radiopacity. Results of this study demonstrated that compared to conventional (or micron) equivalents, PMMA with nanoparticles of MgO and BaSO4 reduced harmful exothermic reactions of PMMA during solidification and increased radiopacity, respectively. Moreover, osteoblast adhesion increased on PMMA with nanoparticles of MgO and BaSO4 compared with PMMA alone. This study, thus, suggests that nanoparticles of MgO and BaSO4 should be further studied for improving properties of PMMA for orthopedic applications.

TRP channels entering the structural era

Transient receptor potential (TRP) channels are important in many neuronal and non-neuronal physiological processes. The past 2 years have seen much progress in the use of structural biology techniques to elucidate molecular mechanisms of TRP channel gating and regulation. Two approaches have proven fruitful: (i) a divide-and-conquer strategy has provided high-resolution structural details of TRP channel fragments although it fails to explain how these fragments are integrated in the full channel; and (ii) electron microscopy of entire TRP channels has yielded low-resolution images that provide a basis for testable models of TRP channel architecture. The results of each approach, summarized in this review, provide a preview of what the future holds in TRP channel structural biology.

Molecular electron microscopy: state of the art and current challenges

The objective of molecular electron microscopy (EM) is to use electron microscopes to visualize the structure of biological molecules. This Review provides a brief overview of the methods used in molecular EM, their respective strengths and successes, and current developments that promise an even more exciting future for molecular EM in the structural investigation of proteins and macromolecular complexes, studied in isolation or in the context of cells and tissues.

Junction-forming aquaporins

Aquaporins (AQPs) are a family of ubiquitous membrane channels that conduct water and solutes across membranes. This review focuses on AQP0 and AQP4, which in addition to forming water channels also appear to play a role in cell adhesion. We discuss the recently determined structures of the membrane junctions mediated by these two AQPs, the mechanisms that regulate junction formation, and evidence that supports a role for AQP0 and AQP4 in cell adhesion.

Interactions of lipids with aquaporin-0 and other membrane proteins

The structure of aquaporin-0 (AQP0) has recently been determined by electron crystallography of two-dimensional (2D) crystals and by X-ray crystallography of three-dimensional (3D) crystals. The electron crystallographic structure revealed nine lipids per AQP0 monomer, which form an almost complete bilayer. The lipids adopt a wide variety of conformations and tightly fill the space between adjacent AQP0 tetramers. The conformations of the lipid acyl chains appear to be determined not only by the protein surface but also by the acyl chains of adjacent lipid molecules. In the X-ray structure, the hydrophobic region of the protein is surrounded by a detergent micelle, with two ordered detergent molecules per AQP0 monomer. Despite the different environments, the electron crystallographic and X-ray structures of AQP0 are virtually identical, but they differ in the temperature factors of the atoms that either contact the lipids in the 2D crystals or are exposed to detergents in the 3D crystals. The temperature factors are higher in the X-ray structure, suggesting that the detergent-exposed AQP0 residues are less ordered than the corresponding ones contacting lipids in the 2D crystals. An examination of ordered detergent molecules in crystal structures of other aquaporins and of lipid molecules in 2D and 3D crystals of bacteriorhodopsin suggests that the increased conformational variability of detergent-exposed residues compared to lipid-contacting residues is a general feature.