ACE: automated CTF estimation

Assembly of anthrax toxin pore: lethal-factor complexes into lipid nanodiscs

We have devised a procedure to incorporate the anthrax protective antigen (PA) pore complexed with the N-terminal domain of anthrax lethal factor (LFN ) into lipid nanodiscs and analyzed the resulting complexes by negative-stain electron microscopy. Insertion into nanodiscs was performed without relying on primary and secondary detergent screens. The preparations were relatively pure, and the percentage of PA pore inserted into nanodiscs on EM grids was high (∼43%). Three-dimensional analysis of negatively stained single particles revealed the LFN -PA nanodisc complex mirroring the previous unliganded PA pore nanodisc structure, but with additional protein density consistent with multiple bound LFN molecules on the PA cap region. The assembly procedure will facilitate collection of higher resolution cryo-EM LFN -PA nanodisc structures and use of advanced automated particle selection methods.

Maskiton: Interactive, web-based classification of single-particle electron microscopy images

Electron microscopy (EM) is an important tool for determining the composition, arrangement and structure of biological macromolecules. When studying structurally heterogeneous samples using EM, classification is a critical step toward achieving higher resolution and identifying biologically significant conformations. We have developed an interactive, web-based tool, called Maskiton, for creating custom masks and performing 2D classifications on aligned single-particle EM images. The Maskiton interface makes it considerably easier and faster to explore the significance of heterogeneity in single-particle datasets. Maskiton features include: resumable uploads to facilitate transfer of large datasets to the server, custom mask creation in the browser, continual progress updates, and interactive viewing of classification results. To demonstrate the value of this tool, we provide examples of its use on several experimental datasets and include analyses of the independent terminus mobility within the Ltn1 E3 ubiquitin ligase, the in vitro assembly of 30S ribosomal subunits, and classification complexity reduction within Immunoglobulin M. This work also serves as a proof-of-concept for the development of future cross-platform, interactive user interfaces for electron microscopy data processing.

A pseudoatomic model of the COPII cage obtained from cryo-electron microscopy and mass spectrometry

COPII vesicles transport proteins from the endoplasmic reticulum to the Golgi apparatus. Previous COPII-cage cryo-EM structures lacked the resolution necessary to determine the residues of Sec13 and Sec31 that mediate assembly and flexibility of the COPII cage. Here we present a 12-Å structure of the human COPII cage, where the tertiary structure of Sec13 and Sec31 is clearly identifiable. We employ this structure and a homology model of the Sec13-Sec31 complex to create a reliable pseudoatomic model of the COPII cage. We combined this model with hydrogen/deuterium-exchange MS analysis to characterize four distinct contact regions at the vertices of the COPII cage. Furthermore, we found that the two-fold symmetry of the Sec31 dimeric region in Sec13-Sec31 is broken upon cage formation and that the resulting hinge is essential to form the proper edge geometry in COPII cages.

Atomic modeling of cryo-electron microscopy reconstructions--joint refinement of model and imaging parameters

When refining the fit of component atomic structures into electron microscopic reconstructions, use of a resolution-dependent atomic density function makes it possible to jointly optimize the atomic model and imaging parameters of the microscope. Atomic density is calculated by one-dimensional Fourier transform of atomic form factors convoluted with a microscope envelope correction and a low-pass filter, allowing refinement of imaging parameters such as resolution, by optimizing the agreement of calculated and experimental maps. A similar approach allows refinement of atomic displacement parameters, providing indications of molecular flexibility even at low resolution. A modest improvement in atomic coordinates is possible following optimization of these additional parameters. Methods have been implemented in a Python program that can be used in stand-alone mode for rigid-group refinement, or embedded in other optimizers for flexible refinement with stereochemical restraints. The approach is demonstrated with refinements of virus and chaperonin structures at resolutions of 9 through 4.5 Å, representing regimes where rigid-group and fully flexible parameterizations are appropriate. Through comparisons to known crystal structures, flexible fitting by RSRef is shown to be an improvement relative to other methods and to generate models with all-atom rms accuracies of 1.5-2.5 Å at resolutions of 4.5-6 Å.

Calmodulin-induced structural changes in endothelial nitric oxide synthase

We have derived structures of intact calmodulin (CaM)-free and CaM-bound endothelial nitric oxide synthase (eNOS) by reconstruction from cryo-electron micrographs. The CaM-free reconstruction is well fitted by the oxygenase domain dimer, but the reductase domains are not visible, suggesting they are mobile and thus delocalized. Additional protein is visible in the CaM-bound reconstruction, concentrated in volumes near two basic patches on each oxygenase domain. One of these corresponds with a presumptive docking site for the reductase domain FMN-binding module. The other is proposed to correspond with a docking site for CaM. A model is suggested in which CaM binding and docking position the reductase domains near the oxygenase domains and promote docking of the FMN-binding modules required for electron transfer.

FASTDEF: fast defocus and astigmatism estimation for high-throughput transmission electron microscopy

In this work we present a fast and automated algorithm for estimating the contrast transfer function (CTF) of a transmission electron microscope. The approach is very suitable for High Throughput work because: (a) it does not require any initial defocus estimation, (b) it is almost an order of magnitude faster than existing approaches, (c) it opens the way to well-defined extensions to the estimation of higher order aberrations, at the same time that provides defocus and astigmatism estimations comparable in accuracy to well established methods, such as Xmipp and CTFFIND3 approaches. The new algorithm is based on obtaining the wrapped modulating phase of the power spectra density pattern by the use of a quadrature filter. This phase is further unwrapped in order to obtain the continuous and smooth absolute phase map; then a Zernike polynomial fitting is performed and the defocus and astigmatism parameters are determined. While the method does not require an initial estimation of the defocus parameters or any non-linear optimization procedure, these approaches can be used if further refinement is desired. Results of the CTF estimation method are presented for standard negative stained images, cryo-electron microscopy images in the absence of carbon support, as well as micrographs with only ice. Additionally, we have also tested the proposed method with micrographs acquired from tilted and untilted samples, obtaining good results. The algorithm is freely available as a part of the Xmipp package [http://xmipp.cnb.csic.es].

A graph theory method for determination of cryo-EM image focuses

Accurate determination of micrograph focuses is essential for averaging multiple images to reach high-resolution 3-D reconstructions in electron cryo-microscopy (cryo-EM). Current methods use iterative fitting of focus-dependent simulated power spectra to the power spectra of experimental images, with the fitting performed independently for different images. Here we have developed a novel graph theory based method in which the rotational average focus and individual angular sector focuses of all images are determined simultaneously in closed form using the least square solution of overdetermined linear equations. The new method was shown to be fast, accurate, and robust in tests with large datasets of experimental low dose cryo-EM images. Its integration with three classic power spectra fitting methods also allows cross validation of the results by these vastly different methods. The new integrated focus determination method will improve reliability of automated focus determination for large-scale data processing that is increasingly common in the cryo-EM field.

Molecular architecture of human polycomb repressive complex 2

Polycomb Repressive Complex 2 (PRC2) is essential for gene silencing, establishing transcriptional repression of specific genes by tri-methylating Lysine 27 of histone H3, a process mediated by cofactors such as AEBP2. In spite of its biological importance, little is known about PRC2 architecture and subunit organization. Here, we present the first three-dimensional electron microscopy structure of the human PRC2 complex bound to its cofactor AEBP2. Using a novel internal protein tagging-method, in combination with isotopic chemical cross-linking and mass spectrometry, we have localized all the PRC2 subunits and their functional domains and generated a detailed map of interactions. The position and stabilization effect of AEBP2 suggests an allosteric role of this cofactor in regulating gene silencing. Regions in PRC2 that interact with modified histone tails are localized near the methyltransferase site, suggesting a molecular mechanism for the chromatin-based regulation of PRC2 activity.DOI:http://dx.doi.org/10.7554/eLife.00005.001.

Interaction of the mediator head module with RNA polymerase II

Mediator, a large (21 polypeptides, MW ∼1 MDa) complex conserved throughout eukaryotes, plays an essential role in control of gene expression by conveying regulatory signals that influence the activity of the preinitiation complex. However, the precise mode of interaction between Mediator and RNA polymerase II (RNAPII), and the mechanism of regulation by Mediator remain elusive. We used cryo-electron microscopy and reconstituted in vitro transcription assays to characterize a transcriptionally-active complex including the Mediator Head module and components of a minimum preinitiation complex (RNAPII, TFIIF, TFIIB, TBP, and promoter DNA). Our results reveal how the Head interacts with RNAPII, affecting its conformation and function.

Engineered mutations change the structure and stability of a virus-like particle

The single-coat protein (CP) of bacteriophage Qβ self-assembles into T = 3 icosahedral virus-like particles (VLPs), of interest for a wide range of applications. These VLPs are very stable, but identification of the specific molecular determinants of this stability is lacking. To investigate these determinants along with manipulations that confer more capabilities to our VLP material, we manipulated the CP primary structure to test the importance of various putative stabilizing interactions. Optimization of a procedure to incorporate fused CP subunits allowed for good control over the average number of covalent dimers in each VLP. We confirmed that the disulfide linkages are the most important stabilizing elements for the capsid and that acidic conditions significantly enhance the resistance of VLPs to thermal degradation. Interdimer interactions were found to be less important for VLP assembly than intradimer interactions. Finally, a single point mutation in the CP resulted in a population of smaller VLPs in three distinct structural forms.

Structure of AAV-DJ, a retargeted gene therapy vector: cryo-electron microscopy at 4.5 Å resolution

AAV-DJ, a leading candidate vector for liver gene therapy, was created through random homologous recombination followed by directed evolution, selecting for in vivo liver tropism and resistance to in vitro immune neutralization. Here, the 4.5 Å resolution cryo-EM structure is determined for the engineered AAV vector, revealing structural features that illuminate its phenotype. The heparan sulfate receptor-binding site is little changed from AAV-2, and heparin-binding affinity is similar. A loop that is antigenic in other serotypes has a unique conformation in AAV-DJ that would conflict with the binding of an AAV-2 neutralizing monoclonal antibody. This is consistent with increased resistance to neutralization by human polyclonal sera, raising the possibility that changed tropism may be a secondary effect of altered immune interactions. The reconstruction exemplifies analysis of fine structural changes and the potential of cryo-EM, in favorable cases, to characterize mutant or ligand-bound complexes.

Structure of adeno-associated virus-2 in complex with neutralizing monoclonal antibody A20

The use of adeno-associated virus (AAV) as a gene therapy vector is limited by the host neutralizing immune response. The cryo-electron microscopy (EM) structure at 8.5Å resolution is determined for a complex of AAV-2 with the Fab' fragment of monoclonal antibody (MAb) A20, the most extensively characterized AAV MAb. The binding footprint is determined through fitting the cryo-EM reconstruction with a homology model following sequencing of the variable domain, and provides a structural basis for integrating diverse prior epitope mappings. The footprint extends from the previously implicated plateau to the side of the spike, and into the conserved canyon, covering a larger area than anticipated. Comparison with structures of binding and non-binding serotypes indicates that recognition depends on a combination of subtle serotype-specific features. Separation of the neutralizing epitope from the heparan sulfate cell attachment site encourages attempts to develop immune-resistant vectors that can still bind to target cells.

Structural basis of membrane bending by the N-BAR protein endophilin

Functioning as key players in cellular regulation of membrane curvature, BAR domain proteins bend bilayers and recruit interaction partners through poorly understood mechanisms. Using electron cryomicroscopy, we present reconstructions of full-length endophilin and its N-terminal N-BAR domain in their membrane-bound state. Endophilin lattices expose large areas of membrane surface and are held together by promiscuous interactions between endophilin's amphipathic N-terminal helices. Coarse-grained molecular dynamics simulations reveal that endophilin lattices are highly dynamic and that the N-terminal helices are required for formation of a stable and regular scaffold. Furthermore, endophilin accommodates different curvatures through a quantized addition or removal of endophilin dimers, which in some cases causes dimerization of endophilin's SH3 domains, suggesting that the spatial presentation of SH3 domains, rather than affinity, governs the recruitment of downstream interaction partners.

The structure of the Sec13/31 COPII cage bound to Sec23

Structural studies have revealed some of the organizing principles and mechanisms involved in the assembly of the COPII coat including the location of the Sec23/24 adapter layer. Previous studies, however, were unable to unambiguously determine the positions of Sec23 and Sec24 in the coat. Here, we have determined a cryogenic electron microscopic structure of Sec13/31 together with Sec23. Electron tomography revealed that the binding of Sec23 induces Sec13/31 to form a variety of different geometries including a cuboctahedron, as was previously characterized for Sec13/31 alone. Single-particle reconstruction of the Sec13/31-23 cuboctahedra revealed that the binding of Sec23 induces a conformational change in Sec13/31, resulting in a more extended conformation. Docking Sec23 crystal structures into the electron microscopy map suggested that Sec24 projects its cargo binding surface out into the large open faces of the coat. These results have implications for the mechanisms by which COPII transports large cargos, cargos with large intracellular domains, and for tethering complexes that must project out of the coat in order to interact with their binding partners. Furthermore, Sec23 binds Sec13/31 at two unique sites in the coat, which suggests that each site may have unique roles in the mechanisms of COPII vesiculation.

Toolbox for non-intrusive structural and functional analysis of recombinant VLP based vaccines: a case study with hepatitis B vaccine

Background

Fundamental to vaccine development, manufacturing consistency, and product stability is an understanding of the vaccine structure-activity relationship. With the virus-like particle (VLP) approach for recombinant vaccines gaining popularity, there is growing demand for tools that define their key characteristics. We assessed a suite of non-intrusive VLP epitope structure and function characterization tools by application to the Hepatitis B surface antigen (rHBsAg) VLP-based vaccine.

Methodology

The epitope-specific immune reactivity of rHBsAg epitopes to a given monoclonal antibody was monitored by surface plasmon resonance (SPR) and quantitatively analyzed on rHBsAg VLPs in-solution or bound to adjuvant with a competitive enzyme-linked immunosorbent assay (ELISA). The structure of recombinant rHBsAg particles was examined by cryo transmission electron microscopy (cryoTEM) and in-solution atomic force microscopy (AFM).

Principal Findings

SPR and competitive ELISA determined relative antigenicity in solution, in real time, with rapid turn-around, and without the need of dissolving the particulate aluminum based adjuvant. These methods demonstrated the nature of the clinically relevant epitopes of HBsAg as being responsive to heat and/or redox treatment. In-solution AFM and cryoTEM determined vaccine particle size distribution, shape, and morphology. Redox-treated rHBsAg enabled 3D reconstruction from CryoTEM images – confirming the previously proposed octahedral structure and the established lipid-to-protein ratio of HBsAg particles. Results from these non-intrusive biophysical and immunochemical analyses coalesced into a comprehensive understanding of rHBsAg vaccine epitope structure and function that was important for assuring the desired epitope formation, determinants for vaccine potency, and particle stability during vaccine design, development, and manufacturing.

Significance

Together, the methods presented here comprise a novel suite of non-intrusive VLP structural and functional characterization tools for recombinant vaccines. Key VLP structural features were defined and epitope-specific antigenicity was quantified while preserving epitope integrity and particle morphology. These tools should facilitate the development of other VLP-based vaccines.

Complete subunit architecture of the proteasome regulatory particle

The proteasome is the major ATP-dependent protease in eukaryotic cells, but limited structural information strongly restricts a mechanistic understanding of its activities. The proteasome regulatory particle, consisting of the lid and base subcomplexes, recognizes and processes poly-ubiquitinated substrates. We used electron microscopy and a newly-developed heterologous expression system for the lid to delineate the complete subunit architecture of the regulatory particle. Our studies reveal the spatial arrangement of ubiquitin receptors, deubiquitinating enzymes, and the protein unfolding machinery at subnanometer resolution, outlining the substrate’s path to degradation. Unexpectedly, the ATPase subunits within the base unfoldase are arranged in a spiral staircase, providing insight into potential mechanisms for substrate translocation through the central pore. Large conformational rearrangements of the lid upon holoenzyme formation suggest allosteric regulation of deubiquitination. We provide a structural basis for the ability of the proteasome to degrade a diverse set of substrates and thus regulate vital cellular processes.

A helical processing pipeline for EM structure determination of membrane proteins

Electron crystallography plays a key role in the structural biology of integral membrane proteins (IMPs) by offering one of the most direct means of providing insight into the functional state of these molecular machines in their lipid-associated forms, and also has the potential to facilitate examination of physiologically relevant transitional states and complexes. Helical or tubular crystals, which are the natural product of proteins crystallizing on the surface of a cylindrical vesicle, offer some unique advantages, such as three-dimensional (3D) information from a single view, compared to other crystalline forms. While a number of software packages are available for processing images of helical crystals to produce 3D electron density maps, widespread exploitation of helical image reconstruction is limited by a lack of standardized approaches and the initial effort and specialized expertise required. Our goal is to develop an integrated pipeline to enable structure determination by transmission electron microscopy (TEM) of IMPs in the form of tubular crystals. We describe here the integration of standard Fourier-Bessel helical analysis techniques into Appion, an integrated, database-driven pipeline.

A pseudoatomic model of the dynamin polymer identifies a hydrolysis-dependent powerstroke

The GTPase dynamin catalyzes membrane fission by forming a collar around the necks of clathrin-coated pits, but the specific structural interactions and conformational changes that drive this process remain a mystery. We present the GMPPCP-bound structures of the truncated human dynamin 1 helical polymer at 12.2 Å and a fusion protein, GG, linking human dynamin 1's catalytic G domain to its GTPase effector domain (GED) at 2.2 Å. The structures reveal the position and connectivity of dynamin fragments in the assembled structure, showing that G domain dimers only form between tetramers in sequential rungs of the dynamin helix. Using chemical crosslinking, we demonstrate that dynamin tetramers are made of two dimers, in which the G domain of one molecule interacts in trans with the GED of another. Structural comparison of GG(GMPPCP) to the GG transition-state complex identifies a hydrolysis-dependent powerstroke that may play a role in membrane-remodeling events necessary for fission.

Defocus estimation from stroboscopic cryo-electron microscopy data

Defocus estimation is an important step for improving the resolution of single particle reconstructions. It can be troublesome to estimate the defocus from low-dose cryo-electron microscopy (cryo-EM) data, particularly if there is not sufficient contrast present in the Fourier transform of the micrograph. Most existing approaches estimate the defocus from the presence of Thon rings within the power spectrum, employing image enhancement techniques to highlight these rings. In this paper, an approach to estimating the defocus from a stroboscopic image series is described. The image series is used to obtain two statistical metrics: figure of merit (FOM) and Q-factor. These metrics have been used to estimate the defoci from low-dose stroboscopic cryo-EM data consisting of a variable number of images.

Subunits fold at position-dependent rates during maturation of a eukaryotic RNA virus

Effective antiviral agents are difficult to develop because of the close relationship between the cell biology of the virus and host. However, viral capsid maturation, the in vivo process where the particle transitions from a noninfectious provirion to an infectious virion, is an ideal process to interrupt because the provirion is usually fragile and the conversion to the virion often involves large conformational changes and autocatalytic chemistry that can be hampered by small molecules. The Nudaurelia capensis omega virus (N omegaV) is one of the few eukaryotic viruses where this process can be investigated in vitro with a variety of biophysical methods, allowing fundamental chemical and structural principles of the maturation to be established. It has a T = 4 quasi-equivalent capsid with a dramatic maturation pathway that includes a particle size reduction of 100 A and an autocatalytic cleavage. Here we use cryo-EM and difference maps, computed at three time points following maturation initiation, to show that regions of N omegaV subunit folding are maturation dependent and occur at rates determined by their quasi-equivalent position in the capsid, explaining the unusual kinetics of the maturation cleavage. This study shows that folding is rapid and peptide chain self-cleavage occurs early for subunits adjacent to 3-fold and 5-fold icosahedral symmetry elements and that folding is slower in regions where molecular switches are required for the formation of the proper interfacial contacts. The results connect viral maturation to the well-studied assembly-dependent folding that occurs in the formation of cellular complexes.

The architecture and chemical stability of the archaeal Sulfolobus turreted icosahedral virus

Viruses utilize a diverse array of mechanisms to deliver their genomes into hosts. While great strides have been made in understanding the genome delivery of eukaryotic and prokaryotic viruses, little is known about archaeal virus genome delivery and the associated particle changes. The Sulfolobus turreted icosahedral virus (STIV) is a double-stranded DNA (dsDNA) archaeal virus that contains a host-derived membrane sandwiched between the genome and the proteinaceous capsid shell. Using cryo-electron microscopy (cryo-EM) and different biochemical treatments, we identified three viral morphologies that may correspond to biochemical disassembly states of STIV. One of these morphologies was subtly different from the previously published 27-A-resolution electron density that was interpreted with the crystal structure of the major capsid protein (MCP). However, these particles could be analyzed at 12.5-A resolution by cryo-EM. Comparing these two structures, we identified the location of multiple proteins forming the large turret-like appendages at the icosahedral vertices, observed heterogeneous glycosylation of the capsid shell, and identified mobile MCP C-terminal arms responsible for tethering and releasing the underlying viral membrane to and from the capsid shell. Collectively, our studies allow us to propose a fusogenic mechanism of genome delivery by STIV, in which the dismantled capsid shell allows for the fusion of the viral and host membranes and the internalization of the viral genome.

Zernike phase plate cryoelectron microscopy facilitates single particle analysis of unstained asymmetric protein complexes

Single particle reconstruction from cryoelectron microscopy images, though emerging as a powerful means in structural biology, is faced with challenges as applied to asymmetric proteins smaller than megadaltons due to low contrast. Zernike phase plate can improve the contrast by restoring the microscope contrast transfer function. Here, by exploiting simulated Zernike and conventional defocused cryoelectron microscope images with noise characteristics comparable to those of experimental data, we quantified the efficiencies of the steps in single particle analysis of ice-embedded RNA polymerase II (500 kDa), transferrin receptor complex (290 kDa), and T7 RNA polymerase lysozyme (100 kDa). Our results show Zernike phase plate imaging is more effective as to particle identification and also sorting of orientations, conformations, and compositions. Moreover, our analysis on image alignment indicates that Zernike phase plate can, in principle, reduce the number of particles required to attain near atomic resolution by 10-100 fold for proteins between 100 kDa and 500 kDa.

High-resolution cryo-electron microscopy structures of murine norovirus 1 and rabbit hemorrhagic disease virus reveal marked flexibility in the receptor binding domains

Our previous structural studies on intact, infectious murine norovirus 1 (MNV-1) virions demonstrated that the receptor binding protruding (P) domains are lifted off the inner shell of the virus. Here, the three-dimensional (3D) reconstructions of recombinant rabbit hemorrhagic disease virus (rRHDV) virus-like particles (VLPs) and intact MNV-1 were determined to approximately 8-A resolution. rRHDV also has a raised P domain, and therefore, this conformation is independent of infectivity and genus. The atomic structure of the MNV-1 P domain was used to interpret the MNV-1 reconstruction. Connections between the P and shell domains and between the floating P domains were modeled. This observed P-domain flexibility likely facilitates virus-host receptor interactions.

Three-dimensional structure of the anthrax toxin pore inserted into lipid nanodiscs and lipid vesicles

A major goal in understanding the pathogenesis of the anthrax bacillus is to determine how the protective antigen (PA) pore mediates translocation of the enzymatic components of anthrax toxin across membranes. To obtain structural insights into this mechanism, we constructed PA-pore membrane complexes and visualized them by using negative-stain electron microscopy. Two populations of PA pores were visualized in membranes, vesicle-inserted and nanodisc-inserted, allowing us to reconstruct two virtually identical PA-pore structures at 22-A resolution. Reconstruction of a domain 4-truncated PA pore inserted into nanodiscs showed that this domain does not significantly influence pore structure. Normal mode flexible fitting of the x-ray crystallographic coordinates of the PA prepore indicated that a prominent flange observed within the pore lumen is formed by the convergence of mobile loops carrying Phe427, a residue known to catalyze protein translocation. Our results have identified the location of a crucial functional element of the PA pore and documented the value of combining nanodisc technology with electron microscopy to examine the structures of membrane-interactive proteins.

A novel approximation method of CTF amplitude correction for 3D single particle reconstruction

The typical resolution of three-dimensional reconstruction by cryo-EM single particle analysis is now being pushed up to and beyond the nanometer scale. Correction of the contrast transfer function (CTF) of electron microscopic images is essential for achieving such a high resolution. Various correction methods exist and are employed in popular reconstruction software packages. Here, we present a novel approximation method that corrects the amplitude modulation introduced by the contrast transfer function by convoluting the images with a piecewise continuous function. Our new approach can easily be implemented and incorporated into other packages. The implemented method yielded higher resolution reconstructions with data sets from both highly symmetric and asymmetric structures. It is an efficient alternative correction method that allows quick convergence of the 3D reconstruction and has a high tolerance for noisy images, thus easing a bottleneck in practical reconstruction of macromolecules.

Structure of the human Dicer-TRBP complex by electron microscopy

Dicer is a specialized ribonuclease that initiates RNA interference (RNAi) by cleaving double-stranded RNA (dsRNA) into small RNA fragments about 22 nucleotides long. Here, we present the three-dimensional structure of human Dicer bound to the protein TRBP at approximately 20 A resolution determined by negative-stain electron microscopy (EM) and single-particle analysis. Our analysis reveals that the Dicer-TRBP complex is an L-shaped molecule with a long edge of 150 A and a 100 A extension on one end. A surface trench runs the length of the long edge of the molecule, defining a putative dsRNA-binding site. Docking the crystal structure of Giardia Dicer, which represents the nuclease core of human Dicer, into the EM map suggests two possible overall molecular architectures for human Dicer. These results offer insights into the structure of Dicer proteins found in multicellular organisms and provide a conceptual framework for understanding the initiation of RNAi.

Automation in single-particle electron microscopy connecting the pieces

Throughout the history of single-particle electron microscopy (EM), automated technologies have seen varying degrees of emphasis and development, usually depending upon the contemporary demands of the field. We are currently faced with increasingly sophisticated devices for specimen preparation, vast increases in the size of collected data sets, comprehensive algorithms for image processing, sophisticated tools for quality assessment, and an influx of interested scientists from outside the field who might lack the skills of experienced microscopists. This situation places automated techniques in high demand. In this chapter, we provide a generic definition of and discuss some of the most important advances in automated approaches to specimen preparation, grid handling, robotic screening, microscope calibrations, data acquisition, image processing, and computational infrastructure. Each section describes the general problem and then provides examples of how that problem has been addressed through automation, highlighting available processing packages, and sometimes describing the particular approach at the National Resource for Automated Molecular Microscopy (NRAMM). We contrast the more familiar manual procedures with automated approaches, emphasizing breakthroughs as well as current limitations. Finally, we speculate on future directions and improvements in automated technologies. Our overall goal is to present automation as more than simply a tool to save time. Rather, we aim to illustrate that automation is a comprehensive and versatile strategy that can deliver biological information on an unprecedented scale beyond the scope available with classical manual approaches.

Fourier-Bessel reconstruction of helical assemblies

Helical symmetry is commonly used for building macromolecular assemblies. Helical symmetry is naturally present in viruses and cytoskeletal filaments and also occurs during crystallization of isolated proteins, such as Ca-ATPase and the nicotinic acetyl choline receptor. Structure determination of helical assemblies by electron microscopy has a long history dating back to the original work on three-dimensional (3D) reconstruction. A helix offers distinct advantages for structure determination. Not only can one improve resolution by averaging across the constituent subunits, but each helical assembly provides multiple views of these subunits and thus provides a complete 3D data set. This review focuses on Fourier methods of helical reconstruction, covering the theoretical background, a step-by-step guide to the process, and a practical example based on previous work with Ca-ATPase. Given recent results from helical reconstructions at atomic resolution and the development of graphical user interfaces to aid in the process, these methods are likely to continue to make an important contribution to the field of structural biology.

Image restoration in cryo-electron microscopy

Image restoration techniques are used to obtain, given experimental measurements, the best possible approximation of the original object within the limits imposed by instrumental conditions and noise level in the data. In molecular electron microscopy (EM), we are mainly interested in linear methods that preserve the respective relationships between mass densities within the restored map. Here, we describe the methodology of image restoration in structural EM, and more specifically, we will focus on the problem of the optimum recovery of Fourier amplitudes given electron microscope data collected under various defocus settings. We discuss in detail two classes of commonly used linear methods, the first of which consists of methods based on pseudoinverse restoration, and which is further subdivided into mean-square error, chi-square error, and constrained based restorations, where the methods in the latter two subclasses explicitly incorporates non-white distribution of noise in the data. The second class of methods is based on the Wiener filtration approach. We show that the Wiener filter-based methodology can be used to obtain a solution to the problem of amplitude correction (or "sharpening") of the EM map that makes it visually comparable to maps determined by X-ray crystallography, and thus amenable to comparative interpretation. Finally, we present a semiheuristic Wiener filter-based solution to the problem of image restoration given sets of heterogeneous solutions. We conclude the chapter with a discussion of image restoration protocols implemented in commonly used single particle software packages.

A toolbox for ab initio 3-D reconstructions in single-particle electron microscopy

Structure determination of a novel macromolecular complex via single-particle electron microscopy depends upon overcoming the challenge of establishing a reliable 3-D reconstruction using only 2-D images. There are a variety of strategies that deal with this issue, but not all of them are readily accessible and straightforward to use. We have developed a "toolbox" of ab initio reconstruction techniques that provide several options for calculating 3-D volumes in an easily managed and tightly controlled work-flow that adheres to standard conventions and formats. This toolbox is designed to streamline the reconstruction process by removing the necessity for bookkeeping, while facilitating transparent data transfer between different software packages. It currently includes procedures for calculating ab initio reconstructions via random or orthogonal tilt geometry, tomograms, and common lines, all of which have been tested using the 50S ribosomal subunit. Our goal is that the accessibility of multiple independent reconstruction algorithms via this toolbox will improve the ease with which models can be generated, and provide a means of evaluating the confidence and reliability of the final reconstructed map.

Three-dimensional EM structure of an intact activator-dependent transcription initiation complex

We present the experimentally determined 3D structure of an intact activator-dependent transcription initiation complex comprising the Escherichia coli catabolite activator protein (CAP), RNA polymerase holoenzyme (RNAP), and a DNA fragment containing positions -78 to +20 of a Class I CAP-dependent promoter with a CAP site at position -61.5 and a premelted transcription bubble. A 20-A electron microscopy reconstruction was obtained by iterative projection-based matching of single particles visualized in carbon-sandwich negative stain and was fitted using atomic coordinate sets for CAP, RNAP, and DNA. The structure defines the organization of a Class I CAP-RNAP-promoter complex and supports previously proposed interactions of CAP with RNAP alpha subunit C-terminal domain (alphaCTD), interactions of alphaCTD with sigma(70) region 4, interactions of CAP and RNAP with promoter DNA, and phased-DNA-bend-dependent partial wrapping of DNA around the complex. The structure also reveals the positions and shapes of species-specific domains within the RNAP beta', beta, and sigma(70) subunits.

The P22 tail machine at subnanometer resolution reveals the architecture of an infection conduit

The portal channel is a key component in the life cycle of bacteriophages and herpesviruses. The bacteriophage P22 portal is a 1 megadalton dodecameric oligomer of gp1 that plays key roles in capsid assembly, DNA packaging, assembly of the infection machinery, and DNA ejection. The portal is the nucleation site for the assembly of 39 additional subunits generated from multiple copies of four gene products (gp4, gp10, gp9, and gp26), which together form the multifunctional tail machine. These components are organized with a combination of 12-fold (gp1, gp4), 6-fold (gp10, trimers of gp9), and 3-fold (gp26, gp9) symmetry. Here we present the 3-dimensional structures of the P22 assembly-naive portal formed from expressed subunits (gp1) and the intact tail machine purified from infectious virions. The assembly-naive portal structure exhibits a striking structural similarity to the structures of the portal proteins of SPP1 and phi29 derived from X-ray crystallography.

CTF determination and correction for low dose tomographic tilt series

The resolution of cryo-electron tomography can be limited by the first zero of the microscope's contrast transfer function (CTF). To achieve higher resolution, it is critical to determine the CTF and correct its phase inversions. However, the extremely low signal-to-noise ratio (SNR) and the defocus gradient in the projections of tilted specimens make this process challenging. Two programs, CTFPLOTTER and CTFPHASEFLIP, have been developed to address these issues. CTFPLOTTER obtains a 1D power spectrum by periodogram averaging and rotational averaging and it estimates the noise background with a novel approach, which uses images taken with no specimen. The background-subtracted 1D power spectra from image regions at different defocus values are then shifted to align their first zeros and averaged together. This averaging improves the SNR sufficiently that it becomes possible to determine the defocus for subsets of the tilt series rather than just the entire series. CTFPHASEFLIP corrects images line-by-line by inverting phases appropriately in thin strips of the image at nearly constant defocus. CTF correction by these methods is shown to improve the resolution of aligned, averaged particles extracted from tomograms. However, some restoration of Fourier amplitudes at high frequencies is important for seeing the benefits from CTF correction.

Appion: an integrated, database-driven pipeline to facilitate EM image processing

The use of cryoEM and three-dimensional image reconstruction is becoming increasingly common. Our vision for this technique is to provide a straightforward manner in which users can proceed from raw data to a reliable 3D reconstruction through a pipeline that both facilitates management of the processing steps and makes the results at each step more transparent. Tightly integrated with a relational SQL database, Appion is a modular and transparent pipeline that extends existing software applications and procedures. The user manages and controls the software modules via web-based forms, and all results are similarly available using web-based viewers directly linked to the underlying database, enabling even naive users to quickly deduce the quality of their results. The Appion API was designed with the principle that applications should be compatible with a broad range of specimens and that libraries and routines are modular and extensible. Presented here is a description of the design and architecture of the working Appion pipeline prototype and some results of its use.

Drosophila A virus is an unusual RNA virus with a T=3 icosahedral core and permuted RNA-dependent RNA polymerase

The vinegar fly, Drosophila melanogaster, is a popular model for the study of invertebrate antiviral immune responses. Several picorna-like viruses are commonly found in both wild and laboratory populations of D. melanogaster. The best-studied and most pathogenic of these is the dicistrovirus Drosophila C virus. Among the uncharacterized small RNA viruses of D. melanogaster, Drosophila A virus (DAV) is the least pathogenic. Historically, DAV has been labelled as a picorna-like virus based on its particle size and the content of its RNA genome. Here, we describe the characterization of both the genome and the virion structure of DAV. Unexpectedly, the DAV genome was shown to encode a circular permutation in the palm-domain motifs of the RNA-dependent RNA polymerase. This arrangement has only been described previously for a subset of viruses from the double-stranded RNA virus family Birnaviridae and the T=4 single-stranded RNA virus family Tetraviridae. The 8 A (0.8 nm) DAV virion structure computed from cryo-electron microscopy and image reconstruction indicates that the virus structural protein forms two discrete domains within the capsid. The inner domain is formed from a clear T=3 lattice with similarity to the beta-sandwich domain of tomato bushy stunt virus, whilst the outer domain is not ordered icosahedrally, but forms a cage-like structure that surrounds the core domain. Taken together, this indicates that DAV is highly divergent from previously described viruses.

Error analysis in the determination of the electron microscopical contrast transfer function parameters from experimental power Spectra

BACKGROUND

The transmission electron microscope is used to acquire structural information of macromolecular complexes. However, as any other imaging device, it introduces optical aberrations that must be corrected if high-resolution structural information is to be obtained. The set of all aberrations are usually modeled in Fourier space by the so-called Contrast Transfer Function (CTF). Before correcting for the CTF, we must first estimate it from the electron micrographs. This is usually done by estimating a number of parameters specifying a theoretical model of the CTF. This estimation is performed by minimizing some error measure between the theoretical Power Spectrum Density (PSD) and the experimentally observed PSD. The high noise present in the micrographs, the possible local minima of the error function for estimating the CTF parameters, and the cross-talking between CTF parameters may cause errors in the estimated CTF parameters.

RESULTS

In this paper, we explore the effect of these estimation errors on the theoretical CTF. For the CTF model proposed in 1 we show which are the most sensitive CTF parameters as well as the most sensitive background parameters. Moreover, we provide a methodology to reveal the internal structure of the CTF model (which parameters influence in which parameters) and to estimate the accuracy of each model parameter. Finally, we explore the effect of the variability in the detection of the CTF for CTF phase and amplitude correction.

CONCLUSION

We show that the estimation errors for the CTF detection methodology proposed in 1 does not show a significant deterioration of the CTF correction capabilities of subsequent algorithms. All together, the methodology described in this paper constitutes a powerful tool for the quantitative analysis of CTF models that can be applied to other models different from the one analyzed here.

Estimating contrast transfer function and associated parameters by constrained non-linear optimization

The three-dimensional reconstruction of macromolecules from two-dimensional single-particle electron images requires determination and correction of the contrast transfer function (CTF) and envelope function. A computational algorithm based on constrained non-linear optimization is developed to estimate the essential parameters in the CTF and envelope function model simultaneously and automatically. The application of this estimation method is demonstrated with focal series images of amorphous carbon film as well as images of ice-embedded icosahedral virus particles suspended across holes.

Bacteriophage lambda stabilization by auxiliary protein gpD: timing, location, and mechanism of attachment determined by cryo-EM

We report the cryo-EM structure of bacteriophage lambda and the mechanism for stabilizing the 20-A-thick capsid containing the dsDNA genome. The crystal structure of the HK97 bacteriophage capsid fits most of the T = 7 lambda particle density with only minor adjustment. A prominent surface feature at the 3-fold axes corresponds to the cementing protein gpD, which is necessary for stabilization of the capsid shell. Its position coincides with the location of the covalent cross-link formed in the docked HK97 crystal structure, suggesting an evolutionary replacement of this gene product in lambda by autocatalytic chemistry in HK97. The crystal structure of the trimeric gpD, in which the 14 N-terminal residues required for capsid binding are disordered, fits precisely into the corresponding EM density. The N-terminal residues of gpD are well ordered in the cryo-EM density, adding a strand to a beta-sheet formed by the capsid proteins and explaining the mechanism of particle stabilization.

Characterization of large conformational changes and autoproteolysis in the maturation of a T=4 virus capsid

Nudaurelia capensis omega virus-like particles have been characterized as a 480-A procapsid and a 410-A capsid, both with T=4 quasisymmetry. Procapsids transition to capsids when pH is lowered from 7.6 to 5.0. Capsids undergo autoproteolysis at residue 570, generating the 74-residue C-terminal polypeptide that remains with the particle. Here we show that the particle size becomes smaller under conditions between pH 6.8 and 6.0 without activating cleavage and that the particle remains at an intermediate size when the pH is carefully maintained. At pH 5.8, cleavage is very slow, becoming detectable only after 9 h. The optimum pH for cleavage is 5.0 (half-life, approximately 30 min), with a significant reduction in the cleavage rate at pH values below 5. We also show that lowering the pH is required only to make the virus particles compact and to presumably form the active site for autoproteolysis but not for the chemistry of cleavage. The cleavage reaction proceeds at pH 7.0 after approximately 10% of the subunits cleave at pH 5.0. Employing the virion crystal structure for reference, we investigated the role of electrostatic repulsion of acidic residues in the pH-dependent large conformational changes. Three mutations of Glu to Gln that formed procapsids showed three different phenotypes on maturation. One, close to the threefold and quasithreefold symmetry axes and far from the cleavage site, did not mature at pH 5, and electron cryomicroscopy reconstruction showed that it was intermediate in size between those of the procapsid and capsid; one near the cleavage site exhibited a wild-type phenotype; and a third, far from the cleavage site, resulted in cleavage of 50% of the subunits after 4 h, suggesting quasiequivalent specificity of the mutation.

Structural basis for cargo regulation of COPII coat assembly

Using cryo-electron microscopy, we have solved the structure of an icosidodecahedral COPII coat involved in cargo export from the endoplasmic reticulum (ER) coassembled from purified cargo adaptor Sec23-24 and Sec13-31 lattice-forming complexes. The coat structure shows a tetrameric assembly of the Sec23-24 adaptor layer that is well positioned beneath the vertices and edges of the Sec13-31 lattice. Fitting the known crystal structures of the COPII proteins into the density map reveals a flexible hinge region stemming from interactions between WD40 beta-propeller domains present in Sec13 and Sec31 at the vertices. The structure shows that the hinge region can direct geometric cage expansion to accommodate a wide range of bulky cargo, including procollagen and chylomicrons, that is sensitive to adaptor function in inherited disease. The COPII coat structure leads us to propose a mechanism by which cargo drives cage assembly and membrane curvature for budding from the ER.

Polyvalent display of heme on hepatitis B virus capsid protein through coordination to hexahistidine tags

The addition of a hexahistidine tag to the N terminus of the hepatitis B capsid protein gives rise to a self-assembled particle with 80 sites of high local density of histidine side chains. Iron protoporphyrin IX has been found to bind tightly at each of these sites, making a polyvalent system of well-defined spacing between metalloporphyrin complexes. The spectroscopic and redox properties of the resulting particle are consistent with the presence of 80 site-isolated bis(histidine)-bound heme centers, comprising a polyvalent b-type cytochrome mimic.

Radial averages of astigmatic TEM images

The Contrast Transfer Function (CTF) of an image, which modulates images taken from a Transmission Electron Microscope (TEM), is usually determined from the radial average of the power spectrum of the image (Frank, J., Three-dimensional Electron Microscopy of Macromolecular Assemblies, Oxford University Press, Oxford, 2006). The CTF is primarily defined by the defocus. If the defocus estimate is accurate enough then it is possible to demodulate the image, which is popularly known as the CTF correction. However, it is known that the radial average is somewhat attenuated if the image is astigmatic (see Fernando, K.V., Fuller, S.D., 2007. Determination of astigmatism in TEM images. Journal of Structural Biology 157, 189-200) but this distortion due to astigmatism has not been fully studied or understood up to now. We have discovered the exact mathematical relationship between the radial averages of TEM images with and without astigmatism. This relationship is determined by a zeroth order Bessel function of the first kind and hence we can exactly quantify this distortion in the radial averages of signal and power spectra of astigmatic images. The argument to this Bessel function is similar to an aberration function (without the spherical aberration term) except that the defocus parameter is replaced by the differences of the defoci in the major and minor axes of astigmatism. The ill effects due this Bessel function are twofold. Since the zeroth order Bessel function is a decaying oscillatory function, it introduces additional zeros to the radial average and it also attenuates the CTF signal in the radial averages. Using our analysis, it is possible to simulate the effects of astigmatism in radial averages by imposing Bessel functions on idealized radial averages of images which are not astigmatic. We validate our theory using astigmatic TEM images.

A test-bed for optimizing high-resolution single particle reconstructions

It is becoming routine for cryoEM single particle reconstructions to result in 3D electron density maps with resolutions of approximately 10A, but maps with resolutions of 5A or better are still celebrated events. The electron microscope has a resolving power to better than 2A, and thus should not be a limiting factor; instead the practical limitations in resolution most likely arise from a combination of specimen preparation methods, data collection parameters, and data analysis procedures. With the aid of a highly automated system for acquiring images, coupled to a relational database to keep track of all processing parameters, we have taken a systematic approach to optimizing parameters affecting the resolution of single particle reconstructions. Using GroEL as a test-bed, we performed a series of 3D reconstructions where we systematically varied the number of particles used in computing the map, the accelerating voltage of the microscope, and the electron dose used to acquire the images. We also investigated methods for excluding unacceptable or "bad" particles from contributing to the final 3D map. Using relatively standard instrumentation (Tecnai F20, 4K x 4K CCD, side entry cold stage) and a completely automated approach, these approaches resulted in a map with a nominal resolution of 5.4A (FSC(0.5)) in which secondary structure is clearly discernable and the handedness of some of the alpha-helices in the GroEL structure can be determined.

Structure of the chloroplast ribosome: novel domains for translation regulation

Gene expression in chloroplasts is controlled primarily through the regulation of translation. This regulation allows coordinate expression between the plastid and nuclear genomes, and is responsive to environmental conditions. Despite common ancestry with bacterial translation, chloroplast translation is more complex and involves positive regulatory mRNA elements and a host of requisite protein translation factors that do not have counterparts in bacteria. Previous proteomic analyses of the chloroplast ribosome identified a significant number of chloroplast-unique ribosomal proteins that expand upon a basic bacterial 70S-like composition. In this study, cryo-electron microscopy and single-particle reconstruction were used to calculate the structure of the chloroplast ribosome to a resolution of 15.5 Å. Chloroplast-unique proteins are visualized as novel structural additions to a basic bacterial ribosome core. These structures are located at optimal positions on the chloroplast ribosome for interaction with mRNAs during translation initiation. Visualization of these chloroplast-unique structures on the ribosome, combined with mRNA cross-linking, allows us to propose a model for translation initiation in chloroplasts in which chloroplast-unique ribosomal proteins interact with plastid-specific translation factors and RNA elements to facilitate regulated translation of chloroplast mRNAs.

Translation of mRNA into protein is the main step for the regulation of gene expression in the chloroplast, the photosynthetic organelle of plant cells. Translation is conducted by the ribosome, a large macromolecular machine composed of RNA and protein. Studies have shown that the composition of the chloroplast ribosome is similar to that of bacterial ribosomes, but also that chloroplast ribosomes contain a number of unique proteins. We present the three-dimensional structure of the chloroplast ribosome, as calculated using cryo-electron microscopy and single-particle reconstruction. Chloroplast-unique structures are clearly visible on our ribosome map, and expand upon a basic bacterial ribosome-like core. The role of these chloroplast-unique ribosomal proteins in regulating translation of chloroplast mRNAs, including light-regulated translation, is suggested by the location of these structures on the ribosome. Biochemical data confirm a predicted function in chloroplast translation for some of the unique proteins. Our model for translation in the chloroplast incorporates decades of biochemical and genetic studies with the structure presented here, and should help guide future studies to understand the molecular mechanisms of translation regulation in the chloroplast.

Cryo-electron microscopy and single-particle reconstruction were used to calculate the structure of the chloroplast ribosome. Chloroplast-unique proteins are visualized as novel structural additions to a basic bacterial ribosome core.

Unnatural amino acid incorporation into virus-like particles

Virus-like particles composed of hepatitis B virus (HBV) or bacteriophage Qbeta capsid proteins have been labeled with azide- or alkyne-containing unnatural amino acids by expression in a methionine auxotrophic strain of E. coli. The substitution does not affect the ability of the particles to self-assemble into icosahedral structures indistinguishable from native forms. The azide and alkyne groups were addressed by Cu(I)-catalyzed [3 + 2] cycloaddition: HBV particles were decomposed by the formation of more than 120 triazole linkages per capsid in a location-dependent manner, whereas Qbeta suffered no such instability. The marriage of these well-known techniques of sense-codon reassignment and bioorthogonal chemical coupling provides the capability to construct polyvalent particles displaying a wide variety of functional groups with near-perfect control of spacing.