Linear fitting of multi-threshold counting data with a pixel-array detector for spectral X-ray imaging.

Experiments and modeling are described to perform spectral fitting of multi-threshold counting measurements on a pixel-array detector. An analytical model was developed for describing the probability density function of detected voltage in X-ray photon-counting arrays, utilizing fractional photon counting to account for edge/corner effects from voltage plumes that spread across multiple pixels. Each pixel was mathematically calibrated by fitting the detected voltage distributions to the model at both 13.5 keV and 15.0 keV X-ray energies. The model and established pixel responses were then exploited to statistically recover images of X-ray intensity as a function of X-ray energy in a simulated multi-wavelength and multi-counting threshold experiment.

Towards protein-crystal centering using second-harmonic generation (SHG) microscopy.

The potential of second-harmonic generation (SHG) microscopy for automated crystal centering to guide synchrotron X-ray diffraction of protein crystals was explored. These studies included (i) comparison of microcrystal positions in cryoloops as determined by SHG imaging and by X-ray diffraction rastering and (ii) X-ray structure determinations of selected proteins to investigate the potential for laser-induced damage from SHG imaging. In studies using ß2 adrenergic receptor membrane-protein crystals prepared in lipidic mesophase, the crystal locations identified by SHG images obtained in transmission mode were found to correlate well with the crystal locations identified by raster scanning using an X-ray minibeam. SHG imaging was found to provide about 2 µm spatial resolution and shorter image-acquisition times. The general insensitivity of SHG images to optical scatter enabled the reliable identification of microcrystals within opaque cryocooled lipidic mesophases that were not identified by conventional bright-field imaging. The potential impact of extended exposure of protein crystals to five times a typical imaging dose from an ultrafast laser source was also assessed. Measurements of myoglobin and thaumatin crystals resulted in no statistically significant differences between structures obtained from diffraction data acquired from exposed and unexposed regions of single crystals. Practical constraints for integrating SHG imaging into an active beamline for routine automated crystal centering are discussed.

Global radiation damage: temperature dependence, time dependence and how to outrun it.

A series of studies that provide a consistent and illuminating picture of global radiation damage to protein crystals, especially at temperatures above ∼200 K, are described. The radiation sensitivity shows a transition near 200 K, above which it appears to be limited by solvent-coupled diffusive processes. Consistent with this interpretation, a component of global damage proceeds on timescales of several minutes at 180 K, decreasing to seconds near room temperature. As a result, data collection times of order 1 s allow up to half of global damage to be outrun at 260 K. Much larger damage reductions near room temperature should be feasible using larger dose rates delivered using microfocused beams, enabling a significant expansion of structural studies of proteins under more nearly native conditions.

Global radiation damage at 300 and 260 K with dose rates approaching 1†MGy†s⁻¹.

Global radiation damage to 19 thaumatin crystals has been measured using dose rates from 3 to 680 kGy s⁻¹. At room temperature damage per unit dose appears to be roughly independent of dose rate, suggesting that the timescales for important damage processes are less than ∼1 s. However, at T = 260 K approximately half of the global damage manifested at dose rates of ∼10 kGy s⁻¹ can be outrun by collecting data at 680 kGy s⁻¹. Appreciable sample-to-sample variability in global radiation sensitivity at fixed dose rate is observed. This variability cannot be accounted for by errors in dose calculation, crystal slippage or the size of the data sets in the assay.

Polo: an open-source graphical user interface for crystallization screening.

Polo is a Python-based graphical user interface designed to streamline viewing and analysis of images to monitor crystal growth, with a specific target to enable users of the High-Throughput Crystallization Screening Center at Hauptman-Woodward Medical Research Institute (HWI) to efficiently inspect their crystallization experiments. Polo aims to increase efficiency, reducing time spent manually reviewing crystallization images, and to improve the potential of identifying positive crystallization conditions. Polo provides a streamlined one-click graphical interface for the Machine Recognition of Crystallization Outcomes (MARCO) convolutional neural network for automated image classification, as well as powerful tools to view and score crystallization images, to compare crystallization conditions, and to facilitate collaborative review of crystallization screening results. Crystallization images need not have been captured at HWI to utilize Polo's basic functionality. Polo is free to use and modify for both academic and commercial use under the terms of the copyleft GNU General Public License v3.0.

Focusing, collimation and flux throughput at the IMCA-CAT bending-magnet beamline at the Advanced Photon Source.

The IMCA-CAT bending-magnet beamline was upgraded with a collimating mirror in order to achieve the energy resolution required to conduct high-quality multi- and single-wavelength anomalous diffraction (MAD/SAD) experiments without sacrificing beamline flux throughput. Following the upgrade, the bending-magnet beamline achieves a flux of 8 x 10(11) photons s(-1) at 1 A wavelength, at a beamline aperture of 1.5 mrad (horizontal) x 86 microrad (vertical), with energy resolution (limited mostly by the intrinsic resolution of the monochromator optics) deltaE/E = 1.5 x 10(-4) (at 10 kV). The beamline operates in a dynamic range of 7.5-17.5 keV and delivers to the sample focused beam of size (FWHM) 240 microm (horizontally) x 160 microm (vertically). The performance of the 17-BM beamline optics and its deviation from ideally shaped optics is evaluated in the context of the requirements imposed by the needs of protein crystallography experiments. An assessment of flux losses is given in relation to the (geometric) properties of major beamline components.

Accelerating pharmaceutical structure-guided drug design: a successful model.

The impact and value of structure-based drug design to pharmaceutical discovery across the industry are now undeniable, with many break-through therapies on the market that are structure based in nature. Enabling the structural research is the Industrial Macromolecular Crystallography Association-Collaborative Access Team (IMCA-CAT), formed over 25 years ago as a world-class research facility at the synchrotron at Argonne National Laboratory. What makes IMCA-CAT unique is the strategy of the founding consortium to comprehensively provide for the evolving needs of industry in one facility. This includes year-round high-quality data, capabilities that match target portfolios, throughput and capacity that are never limiting, and unfailing security. Here, we illuminate the unique capabilities offered by IMCA-CAT and instruct how all industrial organizations can access this facility.

Conservation of Structure and Immune Antagonist Functions of Filoviral VP35 Homologs Present in Microbat Genomes.

Non-retroviral integrated RNA viral sequences (NIRVs) potentially encoding ∼280 amino acid homologs to filovirus VP35 proteins are present across the Myotis genus of bats. These are estimated to have been maintained for ∼18 million years, indicating their co-option. To address the reasons for co-option, 16 Myotis VP35s were characterized in comparison to VP35s from the extant filoviruses Ebola virus and Marburg virus, in which VP35s play critical roles in immune evasion and RNA synthesis. The Myotis VP35s demonstrated a conserved suppression of innate immune signaling, albeit with reduced potency, in either human or Myotis cells. Their attenuation reflects a lack of dsRNA binding that in the filoviral VP35s correlates with potent suppression of interferon responses. Despite divergent function, evolution has preserved in Myotis the structure of the filoviral VP35s, indicating that this structure is critical for co-opted function, possibly as a regulator of innate immune signaling.

Spectral X-Ray Diffraction using a 6 Megapixel Photon Counting Array Detector.

Pixel-array array detectors allow single-photon counting to be performed on a massively parallel scale, with several million counting circuits and detectors in the array. Because the number of photoelectrons produced at the detector surface depends on the photon energy, these detectors offer the possibility of spectral imaging. In this work, a statistical model of the instrument response is used to calibrate the detector on a per-pixel basis. In turn, the calibrated sensor was used to perform separation of dual-energy diffraction measurements into two monochromatic images. Targeting applications include multi-wavelength diffraction to aid in protein structure determination and X-ray diffraction imaging.

Structure of the Neurospora SET domain protein DIM-5, a histone H3 lysine methyltransferase.

AdoMet-dependent methylation of histones is part of the "histone code" that can profoundly influence gene expression. We describe the crystal structure of Neurospora DIM-5, a histone H3 lysine 9 methyltranferase (HKMT), determined at 1.98 A resolution, as well as results of biochemical characterization and site-directed mutagenesis of key residues. This SET domain protein bears no structural similarity to previously characterized AdoMet-dependent methyltransferases but includes notable features such as a triangular Zn3Cys9 zinc cluster in the pre-SET domain and a AdoMet binding site in the SET domain essential for methyl transfer. The structure suggests a mechanism for the methylation reaction and provides the structural basis for functional characterization of the HKMT family and the SET domain.