Achievable resolution from images of biological specimens acquired from a 4k x 4k CCD camera in a 300-kV electron cryomicroscope

Cryo-electron tomography related radiation-damage parameters for individual-molecule 3D structure determination

To understand the dynamic structure-function relationship of soft- and biomolecules, the determination of the three-dimensional (3D) structure of each individual molecule (nonaveraged structure) in its native state is sought-after. Cryo-electron tomography (cryo-ET) is a unique tool for imaging an individual object from a series of tilted views. However, due to radiation damage from the incident electron beam, the tolerable electron dose limits image contrast and the signal-to-noise ratio (SNR) of the data, preventing the 3D structure determination of individual molecules, especially at high-resolution. Although recently developed technologies and techniques, such as the direct electron detector, phase plate, and computational algorithms, can partially improve image contrast/SNR at the same electron dose, the high-resolution structure, such as tertiary structure of individual molecules, has not yet been resolved. Here, we review the cryo-electron microscopy (cryo-EM) and cryo-ET experimental parameters to discuss how these parameters affect the extent of radiation damage. This discussion can guide us in optimizing the experimental strategy to increase the imaging dose or improve image SNR without increasing the radiation damage. With a higher dose, a higher image contrast/SNR can be achieved, which is crucial for individual-molecule 3D structure. With 3D structures determined from an ensemble of individual molecules in different conformations, the molecular mechanism through their biochemical reactions, such as self-folding or synthesis, can be elucidated in a straightforward manner.

Machining protein microcrystals for structure determination by electron diffraction

We demonstrate that ion-beam milling of frozen, hydrated protein crystals to thin lamella preserves the crystal lattice to near-atomic resolution. This provides a vehicle for protein structure determination, bridging the crystal size gap between the nanometer scale of conventional electron diffraction and micron scale of synchrotron microfocus beamlines. The demonstration that atomic information can be retained suggests that milling could provide such detail on sections cut from vitrified cells.

Cryo-electron Microscopy Analysis of Structurally Heterogeneous Macromolecular Complexes

Cryo-electron microscopy (cryo-EM) has for a long time been a technique of choice for determining structure of large and flexible macromolecular complexes that were difficult to study by other experimental techniques such as X-ray crystallography or nuclear magnetic resonance. However, a fast development of instruments and software for cryo-EM in the last decade has allowed that a large range of complexes can be studied by cryo-EM, and that their structures can be obtained at near-atomic resolution, including the structures of small complexes (e.g., membrane proteins) whose size was earlier an obstacle to cryo-EM. Image analysis to identify multiple coexisting structures in the same specimen (multiconformation reconstruction) is now routinely done both to solve structures at near-atomic resolution and to study conformational dynamics. Methods for multiconformation reconstruction and latest examples of their applications are the focus of this review.

CTER-rapid estimation of CTF parameters with error assessment

In structural electron microscopy, the accurate estimation of the Contrast Transfer Function (CTF) parameters, particularly defocus and astigmatism, is of utmost importance for both initial evaluation of micrograph quality and for subsequent structure determination. Due to increases in the rate of data collection on modern microscopes equipped with new generation cameras, it is also important that the CTF estimation can be done rapidly and with minimal user intervention. Finally, in order to minimize the necessity for manual screening of the micrographs by a user it is necessary to provide an assessment of the errors of fitted parameters values. In this work we introduce CTER, a CTF parameters estimation method distinguished by its computational efficiency. The efficiency of the method makes it suitable for high-throughput EM data collection, and enables the use of a statistical resampling technique, bootstrap, that yields standard deviations of estimated defocus and astigmatism amplitude and angle, thus facilitating the automation of the process of screening out inferior micrograph data. Furthermore, CTER also outputs the spatial frequency limit imposed by reciprocal space aliasing of the discrete form of the CTF and the finite window size. We demonstrate the efficiency and accuracy of CTER using a data set collected on a 300kV Tecnai Polara (FEI) using the K2 Summit DED camera in super-resolution counting mode. Using CTER we obtained a structure of the 80S ribosome whose large subunit had a resolution of 4.03Å without, and 3.85Å with, inclusion of astigmatism parameters.

Recording high-resolution images of two-dimensional crystals of membrane proteins

Principles underlying the recording of high-quality/resolution images of two-dimensional crystals of membrane proteins are discussed in the context of instrumental conditions and operational procedures. A detailed example of low-dose microscope settings is provided along with an overview of a program that implements a computer-aided data acquisition procedure.

Direct electron detection yields cryo-EM reconstructions at resolutions beyond 3/4 Nyquist frequency

One limitation in electron cryo-microscopy (cryo-EM) is the inability to recover high-resolution signal from the image-recording media at the full-resolution limit of the transmission electron microscope. Direct electron detection using CMOS-based sensors for digitally recording images has the potential to alleviate this shortcoming. Here, we report a practical performance evaluation of a Direct Detection Device (DDD®) for biological cryo-EM at two different microscope voltages: 200 and 300 kV. Our DDD images of amorphous and graphitized carbon show strong per-pixel contrast with image resolution near the theoretical sampling limit of the data. Single-particle reconstructions of two frozen-hydrated bacteriophages, P22 and ε15, establish that the DDD is capable of recording usable signal for 3D reconstructions at about 4/5 of the Nyquist frequency, which is a vast improvement over the performance of conventional imaging media. We anticipate the unparalleled performance of this digital recording device will dramatically benefit cryo-EM for routine tomographic and single-particle structural determination of biological specimens.

1.16 Cryo-Electron Microscopy and Tomography of Virus Particles

Human infectious disease is classified into five etiologies: bacterial, viral, parasitic, fungal, and prion. Viral infections are unique in that they recruit human cellular machinery to replicate themselves and spread infection. The number of viruses causing human disease is vast, and viruses can be broadly categorized by their structures. Many viruses, such as influenza, appear to be amorphous particles, whereas others, such as herpes simplex virus, rhinovirus, dengue virus, and adenovirus, have roughly symmetric structural components. Icosahedral viruses have been a target of electron microscopists for years, and they were some of the first objects to be reconstructed three-dimensionally from electron micrographs. The ease with which highly purified and conformationally uniform virus samples can be produced makes them an ideal target structural studies. Apart from their biological significance, these virus samples have played a pivotal role in the development of new methodologies in the field of molecular biology as well as in cryo-electron microscopy and cryo-electron tomography.

Practical performance evaluation of a 10k × 10k CCD for electron cryo-microscopy

Electron cryo-microscopy (cryo-EM) images are commonly collected using either charge-coupled devices (CCD) or photographic film. Both film and the current generation of 16 megapixel (4k × 4k) CCD cameras have yielded high-resolution structures. Yet, despite the many advantages of CCD cameras, more than two times as many structures of biological macromolecules have been published in recent years using photographic film. The continued preference to film, especially for subnanometer-resolution structures, may be partially influenced by the finer sampling and larger effective specimen imaging area offered by film. Large format digital cameras may finally allow them to overtake film as the preferred detector for cryo-EM. We have evaluated a 111-megapixel (10k × 10k) CCD camera with a 9 μm pixel size. The spectral signal-to-noise ratios of low dose images of carbon film indicate that this detector is capable of providing signal up to at least 2/5 Nyquist frequency potentially retrievable for 3D reconstructions of biological specimens, resulting in more than double the effective specimen imaging area of existing 4k × 4k CCD cameras. We verified our estimates using frozen-hydrated ε15 bacteriophage as a biological test specimen with previously determined structure, yielding a ∼7 Å resolution single particle reconstruction from only 80 CCD frames. Finally, we explored the limits of current CCD technology by comparing the performance of this detector to various CCD cameras used for recording data yielding subnanometer resolution cryo-EM structures submitted to the electron microscopy data bank (http://www.emdatabank.org/).

Radiation damage in single-particle cryo-electron microscopy: effects of dose and dose rate

Radiation damage is an important resolution limiting factor both in macromolecular X-ray crystallography and cryo-electron microscopy. Systematic studies in macromolecular X-ray crystallography greatly benefited from the use of dose, expressed as energy deposited per mass unit, which is derived from parameters including incident flux, beam energy, beam size, sample composition and sample size. In here, the use of dose is reintroduced for electron microscopy, accounting for the electron energy, incident flux and measured sample thickness and composition. Knowledge of the amount of energy deposited allowed us to compare doses with experimental limits in macromolecular X-ray crystallography, to obtain an upper estimate of radical concentrations that build up in the vitreous sample, and to translate heat-transfer simulations carried out for macromolecular X-ray crystallography to cryo-electron microscopy. Stroboscopic exposure series of 50-250 images were collected for different incident flux densities and integration times from Lumbricus terrestris extracellular hemoglobin. The images within each series were computationally aligned and analyzed with similarity metrics such as Fourier ring correlation, Fourier ring phase residual and figure of merit. Prior to gas bubble formation, the images become linearly brighter with dose, at a rate of approximately 0.1% per 10 MGy. The gradual decomposition of a vitrified hemoglobin sample could be visualized at a series of doses up to 5500 MGy, by which dose the sample was sublimed. Comparison of equal-dose series collected with different incident flux densities showed a dose-rate effect favoring lower flux densities. Heat simulations predict that sample heating will only become an issue for very large dose rates (50 e(-)Å(-2) s(-1) or higher) combined with poor thermal contact between the grid and cryo-holder. Secondary radiolytic effects are likely to play a role in dose-rate effects. Stroboscopic data collection combined with an improved understanding of the effects of dose and dose rate will aid single-particle cryo-electron microscopists to have better control of the outcome of their experiments.

Zernike phase contrast cryo-electron microscopy and tomography for structure determination at nanometer and subnanometer resolutions

Zernike phase contrast cryo-electron microscopy (ZPC-cryoEM) is an emerging technique that is capable of producing higher image contrast than conventional cryoEM. By combining this technique with advanced image processing methods, we achieved subnanometer resolution for two biological specimens: 2D bacteriorhodopsin crystal and epsilon15 bacteriophage. For an asymmetric reconstruction of epsilon15 bacteriophage, ZPC-cryoEM can reduce the required amount of data by a factor of approximately 3, compared with conventional cryoEM. The reconstruction was carried out to 13 A resolution without the need to correct the contrast transfer function. New structural features at the portal vertex of the epsilon15 bacteriophage are revealed in this reconstruction. Using ZPC cryo-electron tomography (ZPC-cryoET), a similar level of data reduction and higher resolution structures of epsilon15 bacteriophage can be obtained relative to conventional cryoET. These results show quantitatively the benefits of ZPC-cryoEM and ZPC-cryoET for structural determinations of macromolecular machines at nanometer and subnanometer resolutions.

4.6A Cryo-EM reconstruction of tobacco mosaic virus from images recorded at 300 keV on a 4k x 4k CCD camera

Tobacco mosaic virus (TMV) is a plant virus with a highly ordered organisation and has been described in three different structural states: As stacked disks without RNA (X-ray crystallography), as a helical form with RNA (X-ray fibre diffraction) and as a second distinct helical form with RNA (cryo-EM). Here we present a structural analysis of TMV as a test object to assess the quality of cryo-EM images recorded at 300 keV on a CCD camera. The 4.6 Å TMV structure obtained is consistent with the previous cryo-EM structure and confirms that there is a second helical form of TMV. The structure here also shows that with a similar number of TMV segments an equivalent resolution can be achieved with a 4k CCD camera at 300 keV.

JADAS: a customizable automated data acquisition system and its application to ice-embedded single particles

The JEOL Automated Data Acquisition System (JADAS) is a software system built for the latest generation of the JEOL Transmission Electron Microscopes. It is designed to partially or fully automate image acquisition for ice-embedded single particles under low dose conditions. Its built-in flexibility permits users to customize the order of various imaging operations. In this paper, we describe how JADAS is used to accurately locate and image suitable specimen areas on a grid of ice-embedded particles. We also demonstrate the utility of JADAS by imaging the epsilon 15 bacteriophage with the JEM3200FSC electron cryo-microscope, showing that sufficient images can be collected in a single 8h session to yield a subnanometer resolution structure which agrees with the previously determined structure.