Hybrid pixel direct detector for electron energy loss spectroscopy.

We characterize a hybrid pixel direct detector and demonstrate its suitability for electron energy loss spectroscopy (EELS). The detector has a large dynamic range, narrow point spread function, detective quantum efficiency ≥ 0.8 even without single electron arrival discrimination, and it is resilient to radiation damage. It is capable of detecting ~5 × 106 electrons/pixel/second, allowing it to accommodate up to 0.8 pA per pixel and hence >100 pA EELS zero-loss peak (ZLP) without saturation, if the ZLP is spread over >125 pixels (in the non-dispersion direction). At the same time, it can reliably detect isolated single electrons in the high loss region of the spectrum. The detector uses a selectable threshold to exclude low energy events, and this results in essentially zero dark current and readout noise. Its maximum frame readout rate at 16-bit digitization is 2250 full frames per second, allowing for fast spectrum imaging. We show applications including EELS of boron nitride in which an unsaturated zero loss peak is recorded at the same time as inner shell loss edges, elemental mapping of an STO/BTO/LMSO multilayer, and efficient parallel acquisition of angle-resolved EEL spectra (S(q, ω)) of boron nitride.

Phage capsid nanoparticles with defined ligand arrangement block influenza virus entry.

Multivalent interactions at biological interfaces occur frequently in nature and mediate recognition and interactions in essential physiological processes such as cell-to-cell adhesion. Multivalency is also a key principle that allows tight binding between pathogens and host cells during the initial stages of infection. One promising approach to prevent infection is the design of synthetic or semisynthetic multivalent binders that interfere with pathogen adhesion1-4. Here, we present a multivalent binder that is based on a spatially defined arrangement of ligands for the viral spike protein haemagglutinin of the influenza A virus. Complementary experimental and theoretical approaches demonstrate that bacteriophage capsids, which carry host cell haemagglutinin ligands in an arrangement matching the geometry of binding sites of the spike protein, can bind to viruses in a defined multivalent mode. These capsids cover the entire virus envelope, thus preventing its binding to the host cell as visualized by cryo-electron tomography. As a consequence, virus infection can be inhibited in vitro, ex vivo and in vivo. Such highly functionalized capsids present an alternative to strategies that target virus entry by spike-inhibiting antibodies5 and peptides6 or that address late steps of the viral replication cycle7.

Design guidelines for an electron diffractometer for structural chemistry and structural biology.

3D electron diffraction has reached a stage where the structures of chemical compounds can be solved productively. Instrumentation is lagging behind this development, and to date dedicated electron diffractometers for data collection based on the rotation method do not exist. Current studies use transmission electron microscopes as a workaround. These are optimized for imaging, which is not optimal for diffraction studies. The beam intensity is very high, it is difficult to create parallel beam illumination and the detectors used for imaging are of only limited use for diffraction studies. In this work, the combination of an EIGER hybrid pixel detector with a transmission electron microscope to construct a productive electron diffractometer is described. The construction not only refers to the combination of hardware but also to the calibration of the system, so that it provides rapid access to the experimental parameters that are necessary for processing diffraction data. Until fully integrated electron diffractometers become available, this describes a setup for productive and efficient operation in chemical crystallography.

Rapid Structure Determination of Microcrystalline Molecular Compounds Using Electron Diffraction.

Chemists of all fields currently publish about 50 000 crystal structures per year, the vast majority of which are X-ray structures. We determined two molecular structures by employing electron rather than X-ray diffraction. For this purpose, an EIGER hybrid pixel detector was fitted to a transmission electron microscope, yielding an electron diffractometer. The structure of a new methylene blue derivative was determined at 0.9 Šresolution from a crystal smaller than 1×2 µm2 . Several thousand active pharmaceutical ingredients (APIs) are only available as submicrocrystalline powders. To illustrate the potential of electron crystallography for the pharmaceutical industry, we also determined the structure of an API from its pill. We demonstrate that electron crystallography complements X-ray crystallography and is the technique of choice for all unsolved cases in which submicrometer-sized crystals were the limiting factor.

Single-particle cryo-EM using alignment by classification (ABC): the structure of Lumbricus terrestris haemoglobin.

Single-particle cryogenic electron microscopy (cryo-EM) can now yield near-atomic resolution structures of biological complexes. However, the reference-based alignment algorithms commonly used in cryo-EM suffer from reference bias, limiting their applicability (also known as the 'Einstein from random noise' problem). Low-dose cryo-EM therefore requires robust and objective approaches to reveal the structural information contained in the extremely noisy data, especially when dealing with small structures. A reference-free pipeline is presented for obtaining near-atomic resolution three-dimensional reconstructions from heterogeneous ('four-dimensional') cryo-EM data sets. The methodologies integrated in this pipeline include a posteriori camera correction, movie-based full-data-set contrast transfer function determination, movie-alignment algorithms, (Fourier-space) multivariate statistical data compression and unsupervised classification, 'random-startup' three-dimensional reconstructions, four-dimensional structural refinements and Fourier shell correlation criteria for evaluating anisotropic resolution. The procedures exclusively use information emerging from the data set itself, without external 'starting models'. Euler-angle assignments are performed by angular reconstitution rather than by the inherently slower projection-matching approaches. The comprehensive 'ABC-4D' pipeline is based on the two-dimensional reference-free 'alignment by classification' (ABC) approach, where similar images in similar orientations are grouped by unsupervised classification. Some fundamental differences between X-ray crystallography versus single-particle cryo-EM data collection and data processing are discussed. The structure of the giant haemoglobin from Lumbricus terrestris at a global resolution of ∼3.8 Šis presented as an example of the use of the ABC-4D procedure.

Cryo-EM structure of aerolysin variants reveals a novel protein fold and the pore-formation process.

Owing to their pathogenical role and unique ability to exist both as soluble proteins and transmembrane complexes, pore-forming toxins (PFTs) have been a focus of microbiologists and structural biologists for decades. PFTs are generally secreted as water-soluble monomers and subsequently bind the membrane of target cells. Then, they assemble into circular oligomers, which undergo conformational changes that allow membrane insertion leading to pore formation and potentially cell death. Aerolysin, produced by the human pathogen Aeromonas hydrophila, is the founding member of a major PFT family found throughout all kingdoms of life. We report cryo-electron microscopy structures of three conformational intermediates and of the final aerolysin pore, jointly providing insight into the conformational changes that allow pore formation. Moreover, the structures reveal a protein fold consisting of two concentric ß-barrels, tightly kept together by hydrophobic interactions. This fold suggests a basis for the prion-like ultrastability of aerolysin pore and its stoichiometry.

FEI's direct electron detector developments: Embarking on a revolution in cryo-TEM.

In early 2011 FEI Company launched the "Falcon", its first commercial direct electron detector product intended for application in 3-D electron microscopy in the life sciences. In this paper we discuss the principle of direct electron detection and its implementation in Falcon cameras. We describe the signal formation in the sensor and its impact on the detection quantum efficiency (DQE) of the sensor. Insights into the signal formation led us to improved camera designs. Three significant improvements are discussed. (1) Back thinning of the sensor. This is implemented in the second-generation Falcon (Falcon 2), where the sensor thickness is reduced to 50 µm, and in the latest generation Falcon 3 detector with further back-thinning down to 30 µm. (2) The introduction of electron counting, a signal processing technology implemented in Falcon 3. (3) Dose fractionation mode, which allows the user to access intermediate results during the illumination of the sample.

A posteriori correction of camera characteristics from large image data sets.

Large datasets are emerging in many fields of image processing including: electron microscopy, light microscopy, medical X-ray imaging, astronomy, etc. Novel computer-controlled instrumentation facilitates the collection of very large datasets containing thousands of individual digital images. In single-particle cryogenic electron microscopy ("cryo-EM"), for example, large datasets are required for achieving quasi-atomic resolution structures of biological complexes. Based on the collected data alone, large datasets allow us to precisely determine the statistical properties of the imaging sensor on a pixel-by-pixel basis, independent of any "a priori" normalization routinely applied to the raw image data during collection ("flat field correction"). Our straightforward "a posteriori" correction yields clean linear images as can be verified by Fourier Ring Correlation (FRC), illustrating the statistical independence of the corrected images over all spatial frequencies. The image sensor characteristics can also be measured continuously and used for correcting upcoming images.

Negative staining and cryo-negative staining: applications in biology and medicine.

Negative staining is widely applicable to isolated viruses, protein molecules, macromolecular assemblies and fibrils, subcellular membrane fractions, liposomes and artificial membranes, synthetic DNA arrays, and also to polymer solutions and a variety of nanotechnology samples. Techniques are provided for the preparation of the necessary support films (continuous carbon and holey/perforated carbon). The range of suitable negative stains is presented, with some emphasis on the benefit of using ammonium molybdate and of negative stain-trehalose combinations. Protocols are provided for the single droplet negative staining technique (on continuous and holey carbon support films), the floating and carbon sandwich techniques in addition to the negative staining-carbon film (NS-CF) technique for randomly dispersed fragile molecules, 2D crystallization of proteins and for cleavage of cells and organelles. Immuno-negative staining and negative staining of affinity labeled complexes (e.g., biotin-streptavidin) are presented in some detail. The formation of immune complexes in solution for droplet negative staining is given, as is the use of carbon-plastic support films as an adsorption surface on which to perform immunolabeling or affinity experiments, prior to negative staining. Dynamic biological systems can be investigated by negative staining, where the time period is in excess of a few minutes, but there are possibilities to greatly reduce the time by rapid stabilization of molecular systems with uranyl acetate or tannic acid. The more recently developed cryo-negative staining procedures are also included: first, the high concentration ammonium molybdate procedure on holey carbon films and second, the carbon sandwich procedure using uranyl formate. Several electron micrographs showing examples of applications of negative staining techniques are included and the chapter is thoroughly referenced.

Identification and characterization of ZapC, a stabilizer of the FtsZ ring in Escherichia coli.

In Escherichia coli, spatiotemporal control of cell division occurs at the level of the assembly/disassembly process of the essential cytoskeletal protein FtsZ. A number of regulators interact with FtsZ and modulate the dynamics of the assembled FtsZ ring at the midcell division site. In this article, we report the identification of an FtsZ stabilizer, ZapC (Z-associated protein C), in a protein localization screen conducted with E. coli. ZapC colocalizes with FtsZ at midcell and interacts directly with FtsZ, as determined by a protein-protein interaction assay in yeast. Cells lacking or overexpressing ZapC are slightly elongated and have aberrant FtsZ ring morphologies indicative of a role for ZapC in FtsZ regulation. We also demonstrate the ability of purified ZapC to promote lateral bundling of FtsZ in a sedimentation reaction visualized by transmission electron microscopy. While ZapC lacks sequence similarity with other nonessential FtsZ regulators, ZapA and ZapB, all three Zap proteins appear to play an important role in FtsZ regulation during rapid growth. Taken together, our results suggest a key role for lateral bundling of the midcell FtsZ polymers in maintaining FtsZ ring stability during division.

Negative staining and cryo-negative staining of macromolecules and viruses for TEM.

In this review we cover the technical background to negative staining of biomolecules and viruses, and then expand upon the different possibilities and limitations. Topics range from conventional air-dry negative staining of samples adsorbed to carbon support films, the variant termed the "negative staining-carbon film" technique and negative staining of samples spread across the holes of holey-carbon support films, to a consideration of dynamic/time-dependent negative staining. For each of these approaches examples of attainable data are given. The cryo-negative staining technique for the specimen preparation of frozen-hydrated/vitrified samples is also presented. A detailed protocol to successfully achieve cryo-negative staining with ammonium molybdate is given, as well as examples of data, which support the claim that cryo-negative staining provides a useful approach for the high-resolution study of macromolecular and viral structure.

Engagement of arginine finger to ATP triggers large conformational changes in NtrC1 AAA+ ATPase for remodeling bacterial RNA polymerase.

The NtrC-like AAA+ ATPases control virulence and other important bacterial activities through delivering mechanical work to σ54-RNA polymerase to activate transcription from σ54-dependent genes. We report the first crystal structure for such an ATPase, NtrC1 of Aquifex aeolicus, in which the catalytic arginine engages the γ-phosphate of ATP. Comparing the new structure with those previously known for apo and ADP-bound states supports a rigid-body displacement model that is consistent with large-scale conformational changes observed by low-resolution methods. First, the arginine finger induces rigid-body roll, extending surface loops above the plane of the ATPase ring to bind σ54. Second, ATP hydrolysis permits Pi release and retraction of the arginine with a reversed roll, remodeling σ54-RNAP. This model provides a fresh perspective on how ATPase subunits interact within the ring-ensemble to promote transcription, directing attention to structural changes on the arginine-finger side of an ATP-bound interface.

Cryonegative staining of macromolecular assemblies.

Cryoelectron microscopy (cryo-EM) combined with single-particle reconstruction methods is a powerful technique to study the structure of biological assemblies at molecular resolution (i.e., 3-10 Å). Since electron micrographs of frozen-hydrated biological particles are usually very noisy, improvement of the signal-to-noise ratio (SNR) is necessary and is usually achieved by image processing. We propose an alternative method to improve the contrast at the specimen preparation stage: cryonegative staining. Cryonegative staining aims to increase the SNR while preserving the biological samples in the frozen-hydrated state. Here, we present two alternative procedures to efficiently perform cryonegative staining on macromolecular assemblies. The first is very similar to conventional cryo-EM, the main difference being that the samples are observed in the presence of an additional contrasting agent, ammonium molybdate. The second is based on a carbon-sandwich method and is typically used with uranyl formate or acetate. Compared to air-dried negative staining at room temperature, the advantage of both cryonegative-staining procedures presented here is that the sample is kept hydrated at all steps and observed at liquid nitrogen temperature in the electron microscope. The advantage over conventional cryo-EM is that the SNR is improved by at least a factor of three. For each of these approaches, a few examples of attainable data are given. We cover the technical background to cryonegative staining of macromolecular assemblies, and then expand upon the different possibilities and limitations.

Molecular basis of transcription initiation in Archaea.

Compared with eukaryotes, the archaeal transcription initiation machinery-commonly known as the Pre-Initiation Complex-is relatively simple. The archaeal PIC consists of the TFIIB ortholog TFB, TBP, and an 11-subunit RNA polymerase (RNAP). The relatively small size of the entire archaeal PIC makes it amenable to structural analysis. Using purified RNAP, TFB, and TBP from the thermophile Pyrococcus furiosus, we assembled the biochemically active PIC at 65ºC. The intact archaeal PIC was isolated by implementing a cross-linking technique followed by size-exclusion chromatography, and the structure of this 440 kDa assembly was determined using electron microscopy and single-particle reconstruction techniques. Combining difference maps with crystal structure docking of various sub-domains, TBP and TFB were localized within the macromolecular PIC. TBP/TFB assemble near the large RpoB subunit and the RpoD/L "foot" domain behind the RNAP central cleft. This location mimics that of yeast TBP and TFIIB in complex with yeast RNAP II. Collectively, these results define the structural organization of the archaeal transcription machinery and suggest a conserved core PIC architecture.

Structure and regulatory mechanism of Aquifex aeolicus NtrC4: variability and evolution in bacterial transcriptional regulation.

Genetic changes lead gradually to altered protein function, making deduction of the molecular basis for activity from a sequence difficult. Comparative studies provide insights into the functional consequences of specific changes. Here we present structural and biochemical studies of NtrC4, a sigma-54 activator from Aquifex aeolicus, and compare it with NtrC1 (a paralog) and NtrC (a homolog from Salmonella enterica) to provide insight into how a substantial change in regulatory mechanism may have occurred. Activity assays show that assembly of NtrC4's active oligomer is repressed by the N-terminal receiver domain, and that BeF3- addition (mimicking phosphorylation) removes this repression. Observation of assembly without activation for NtrC4 indicates that it is much less strongly repressed than NtrC1. The crystal structure of the unactivated receiver-ATPase domain combination shows a partially disrupted interface. NMR structures of the regulatory domain show that its activation mechanism is very similar to that of NtrC1. The crystal structure of the NtrC4 DNA-binding domain shows that it is dimeric and more similar in structure to NtrC than NtrC1. Electron microscope images of the ATPase-DNA-binding domain combination show formation of oligomeric rings. Sequence alignments provide insights into the distribution of activation mechanisms in this family of proteins.

High-resolution single-particle 3D analysis on GroEL prepared by cryo-negative staining.

Cryo-negative staining was developed as a complementary technique to conventional cryo-electron microscopy on supramolecular complexes. It allows imaging biological samples in a comparable state of structural preservation to conventional cryo-EM but the staining produces better contrast in accessible areas and allows data recording at lower defocus values. Cryo-negative staining vitrifies biological particles in the presence of a concentrated ammonium molybdate solution at neutral pH. It was successfully used to study the structure and dynamics of several macromolecules, such as human transcription factors and RNA polymerases. Imaging macromolecular complexes with cryo-negative staining has been established previously to better than 2 nm detail. However, it has not been verified so far whether cryo-negative staining also visualizes secondary structure elements. Using the well known E. coli GroEL chaperonin, we could show that the structure is well preserved to approximately 10 A resolution. Secondary structure details are at least partially resolved in the electron density map.

ATP ground- and transition states of bacterial enhancer binding AAA+ ATPases support complex formation with their target protein, sigma54.

Transcription initiation by the sigma54 form of bacterial RNA polymerase requires hydrolysis of ATP by an enhancer binding protein (EBP). We present SAS-based solution structures of the ATPase domain of the EBP NtrC1 from Aquifex aeolicus in different nucleotide states. Structures of apo protein and that bound to AMPPNP or ADP-BeF(x) (ground-state mimics), ADP-AlF(x) (a transition-state mimic), or ADP (product) show substantial changes in the position of the GAFTGA loops that contact polymerase, particularly upon conversion from the apo state to the ADP-BeF(x) state, and from the ADP-AlF(x) state to the ADP state. Binding of the ATP analogs stabilizes the oligomeric form of the ATPase and its binding to sigma54, with ADP-AlF(x) having the largest effect. These data indicate that ATP binding promotes a conformational change that stabilizes complexes between EBPs and sigma54, while subsequent hydrolysis and phosphate release drive the conformational change needed to open the polymerase/promoter complex.

Molecular architecture and conformational flexibility of human RNA polymerase II.

Transcription by RNA polymerase II (RNAPII) is a central process in eukaryotic gene regulation. While atomic details exist for the yeast RNAPII, characterization of the human complex lags behind, mostly due to the inability to obtain large quantities of purified material. Although the complexes have the same protein composition and high sequence similarity, understanding of transcription and of transcription-coupled DNA repair (TCR) in humans will require the use of human proteins in structural studies. We have used cryo-electron microscopy, image reconstruction, and variance analysis to characterize the structure and dynamics of human RNAPII (hRNAPII). Our studies show that hRNAPII in solution parallels the conformational flexibility of the yeast structures crystallized in different states but also illustrate a more extensive conformational range with potential biological significance. This hRNAPII study will serve as a structural platform to build up higher-order transcription and TCR complexes and to gain information that may be unique to the human RNAPII system.

The structural basis for regulated assembly and function of the transcriptional activator NtrC.

In two-component signal transduction, an input triggers phosphorylation of receiver domains that regulate the status of output modules. One such module is the AAA+ ATPase domain in bacterial enhancer-binding proteins that remodel the sigma(54) form of RNA polymerase. We report X-ray solution scattering and electron microscopy structures of the activated, full-length nitrogen-regulatory protein C (NtrC) showing a novel mechanism for regulation of AAA+ ATPase assembly via the juxtaposition of the receiver domains and ATPase ring. Accompanying the hydrolysis cycle that is required for transcriptional activation, we observed major order-disorder changes in the GAFTGA loops involved in sigma(54) binding, as well as in the DNA-binding domains.