Optimization of data collection taking radiation damage into account

Doses for X-ray and electron diffraction: New features in RADDOSE-3D including intensity decay models

New features in the dose estimation program RADDOSE-3D are summarised. They include the facility to enter a diffraction intensity decay model which modifies the "Diffraction Weighted Dose" output from a "Fluence Weighted Dose" to a "Diffraction-Decay Weighted Dose", a description of RADDOSE-ED for use in electron diffraction experiments, where dose is historically quoted in electrons/Å2 rather than in gray (Gy), and finally the development of a RADDOSE-3D GUI, enabling easy access to all the options available in the program.

Conserved Signal Transduction Mechanisms and Dark Recovery Kinetic Tuning in the Pseudomonadaceae Short Light, Oxygen, Voltage (LOV) Protein Family

Light-Oxygen-Voltage (LOV) flavoproteins transduce a light signal into variable signaling outputs via a structural rearrangement in the sensory core domain, which is then relayed to fused effector domains via α-helical linker elements. Short LOV proteins from Pseudomonadaceae consist of a LOV sensory core and N- and C-terminal α-helices of variable length, providing a simple model system to study the molecular mechanism of allosteric activation. Here we report the crystal structures of two LOV proteins from Pseudomonas fluorescens - SBW25-LOV in the fully light-adapted state and Pf5-LOV in the dark-state. In a comparative analysis of the Pseudomonadaceae short LOVs, the structures demonstrate light-induced rotation of the core domains and splaying of the proximal A'α and Jα helices in the N and C-termini, highlighting evidence for a conserved signal transduction mechanism. Another distinguishing feature of the Pseudomonadaceae short LOV protein family is their highly variable dark recovery, ranging from seconds to days. Understanding this variability is crucial for tuning the signaling behavior of LOV-based optogenetic tools. At 37 °C, SBW25-LOV and Pf5-LOV exhibit adduct state lifetimes of 1470 min and 3.6 min, respectively. To investigate this remarkable difference in dark recovery rates, we targeted three residues lining the solvent channel entrance to the chromophore pocket where we introduced mutations by exchanging the non-conserved amino acids from SBW25-LOV into Pf5-LOV and vice versa. Dark recovery kinetics of the resulting mutants, as well as MD simulations and solvent cavity calculations on the crystal structures suggest a correlation between solvent accessibility and adduct lifetime.

Lattice response to the radiation damage of molecular crystals: radiation-induced versus thermal expansivity

The interaction of intense synchrotron radiation with molecular crystals frequently modifies the crystal structure by breaking bonds, producing fragments and, hence, inducing disorder. Here, a second-rank tensor of radiation-induced lattice strain is proposed to characterize the structural susceptibility to radiation. Quantitative estimates are derived using a linear response approximation from experimental data collected on three materials Hg(NO3)2(PPh3)2, Hg(CN)2(PPh3)2 and BiPh3 [PPh3 = triphenylphosphine, P(C6H5)3; Ph = phenyl, C6H5], and are compared with the corresponding thermal expansivities. The associated eigenvalues and eigenvectors show that the two tensors are not the same and therefore probe truly different structural responses. The tensor of radiative expansion serves as a measure of the susceptibility of crystal structures to radiation damage.

Radiation damage as a source of information

The structural strain induced by temperature (`phonon pressure') and radiation damage (`defect pressure') is not necessarily correlated because of different underlying structural mechanisms. Here synchrotron experiments may provide new and yet unexplored opportunities. A recent publication by McMonagle et al. [(2024), Acta Cryst. B80, 13-18] is an excellent illustration of this.

DNA-guided transcription factor cooperativity shapes face and limb mesenchyme

Transcription factors (TFs) can define distinct cellular identities despite nearly identical DNA-binding specificities. One mechanism for achieving regulatory specificity is DNA-guided TF cooperativity. Although in vitro studies suggest that it may be common, examples of such cooperativity remain scarce in cellular contexts. Here, we demonstrate how "Coordinator," a long DNA motif composed of common motifs bound by many basic helix-loop-helix (bHLH) and homeodomain (HD) TFs, uniquely defines the regulatory regions of embryonic face and limb mesenchyme. Coordinator guides cooperative and selective binding between the bHLH family mesenchymal regulator TWIST1 and a collective of HD factors associated with regional identities in the face and limb. TWIST1 is required for HD binding and open chromatin at Coordinator sites, whereas HD factors stabilize TWIST1 occupancy at Coordinator and titrate it away from HD-independent sites. This cooperativity results in the shared regulation of genes involved in cell-type and positional identities and ultimately shapes facial morphology and evolution.

DNA-guided transcription factor cooperativity shapes face and limb mesenchyme

Transcription factors (TFs) can define distinct cellular identities despite nearly identical DNA-binding specificities. One mechanism for achieving regulatory specificity is DNA-guided TF cooperativity. Although in vitro studies suggest it may be common, examples of such cooperativity remain scarce in cellular contexts. Here, we demonstrate how 'Coordinator', a long DNA motif comprised of common motifs bound by many basic helix-loop-helix (bHLH) and homeodomain (HD) TFs, uniquely defines regulatory regions of embryonic face and limb mesenchyme. Coordinator guides cooperative and selective binding between the bHLH family mesenchymal regulator TWIST1 and a collective of HD factors associated with regional identities in the face and limb. TWIST1 is required for HD binding and open chromatin at Coordinator sites, while HD factors stabilize TWIST1 occupancy at Coordinator and titrate it away from HD-independent sites. This cooperativity results in shared regulation of genes involved in cell-type and positional identities, and ultimately shapes facial morphology and evolution.

Stabilization of Cereibacter sphaeroides Photosynthetic Reaction Center by the Introduction of Disulfide Bonds

The photosynthetic reaction center of the purple nonsulfur bacterium Cereibacter sphaeroides is a useful model for the study of mechanisms of photoinduced electron transfer and a promising component for photo-bio-electrocatalytic systems. The basic research and technological applications of this membrane pigment-protein complex require effective approaches to increase its structural stability. In this work, a rational design approach to genetically modify the reaction centers by introducing disulfide bonds is used. This resulted in significantly increasing the thermal stability of some of the mutant pigment-protein complexes. The formation of the S-S bonds was confirmed by X-ray crystallography as well as SDS-PAGE, and the optical properties of the reaction centers were studied. The genetically modified reaction centers presented here preserved their ability for photochemical charge separation and could be of interest for basic science and biotechnology.

Structural insights into the effects of glycerol on ligand binding to cytochrome P450

New antitubercular drugs are vital due to the spread of resistant strains. Carbethoxyhexyl imidazole (CHImi) inhibits cytochrome P450 CYP124, which is a steroid-metabolizing enzyme that is important for the survival of Mycobacterium tuberculosis in macrophages. The available crystal structure of the CYP124-CHImi complex reveals two glycerol molecules in the active site. A 1.15 Å resolution crystal structure of the glycerol-free CYP124-CHimi complex reported here shows multiple conformations of CHImi and the CYP124 active site which were previously restricted by glycerol. Complementary molecular dynamics simulations show coherence of the ligand and enzyme conformations. Spectrophotometric titration confirmed the influence of glycerol on CHImi binding: the affinity decreases more than tenfold in glycerol-containing buffer. In addition, it also showed that glycerol has a similar effect on other azole and triazole CYP124 ligands. Together, these data show that glycerol may compromise structural-functional studies and impede rational drug-design campaigns.

Residue alterations within a conserved hydrophobic pocket influence light, oxygen, voltage photoreceptor dark recovery

Light, oxygen, voltage (LOV) photoreceptors are widely distributed throughout all kingdoms of life, and have in recent years, due to their modular nature, been broadly used as sensor domains for the construction of optogenetic tools. For understanding photoreceptor function as well as for optogenetic tool design and fine-tuning, a detailed knowledge of the photophysics, photochemistry, and structural changes underlying the LOV signaling paradigm is instrumental. Mutations that alter the lifetime of the photo-adduct signaling state represent a convenient handle to tune LOV sensor on/off kinetics and, thus, steady-state on/off equilibria of the photoreceptor (or optogenetic switch). Such mutations, however, should ideally only influence sensor kinetics, while being benign with regard to the nature of the structural changes that are induced by illumination, i.e., they should not result in a disruption of signal transduction. In the present study, we identify a conserved hydrophobic pocket for which mutations have a strong impact on the adduct-state lifetime across different LOV photoreceptor families. Using the slow cycling bacterial short LOV photoreceptor PpSB1-LOV, we show that the I48T mutation within this pocket, which accelerates adduct rupture, is otherwise structurally and mechanistically benign, i.e., light-induced structural changes, as probed by NMR spectroscopy and X-ray crystallography, are not altered in the variant. Additional mutations within the pocket of PpSB1-LOV and the introduction of homologous mutations in the LOV photoreceptor YtvA of Bacillus subtilis and the Avena sativa LOV2 domain result in similarly altered kinetics. Given the conserved nature of the corresponding structural region, the here identified mutations should find application in dark-recovery tuning of optogenetic tools and LOV photoreceptors, alike.

Graphical abstract

Development of Noncovalent Small-Molecule Keap1-Nrf2 Inhibitors by Fragment-Based Drug Discovery

Targeting the protein-protein interaction (PPI) between the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and its repressor, Kelch-like ECH-associated protein 1 (Keap1), constitutes a promising strategy for treating diseases involving oxidative stress and inflammation. Here, a fragment-based drug discovery (FBDD) campaign resulted in novel, high-affinity (K_i = 280 nM), and cell-active noncovalent small-molecule Keap1-Nrf2 PPI inhibitors. We screened 2500 fragments using orthogonal assays─fluorescence polarization (FP), thermal shift assay (TSA), and surface plasmon resonance (SPR)─and validated the hits by saturation transfer difference (STD) NMR, leading to 28 high-priority hits. Thirteen co-structures showed fragments binding mainly in the P4 and P5 subpockets of Keap1's Kelch domain, and three fluorenone-based fragments featuring a novel binding mode were optimized by structure-based drug discovery. We thereby disclose several fragment hits, including their binding modes, and show how FBDD can be performed to find new small-molecule Keap1-Nrf2 PPI inhibitors.

High-pressure crystallography shows noble gas intervention into protein-lipid interaction and suggests a model for anaesthetic action

In this work we examine how small hydrophobic molecules such as inert gases interact with membrane proteins (MPs) at a molecular level. High pressure atmospheres of argon and krypton were used to produce noble gas derivatives of crystals of three well studied MPs (two different proton pumps and a sodium light-driven ion pump). The structures obtained using X-ray crystallography showed that the vast majority of argon and krypton binding sites were located on the outer hydrophobic surface of the MPs – a surface usually accommodating hydrophobic chains of annular lipids (which are known structural and functional determinants for MPs). In conformity with these results, supplementary in silico molecular dynamics (MD) analysis predicted even greater numbers of argon and krypton binding positions on MP surface within the bilayer. These results indicate a potential importance of such interactions, particularly as related to the phenomenon of noble gas-induced anaesthesia.

Noble gases are known to interact with proteins and can be good anaesthetics in hyperbaric conditions. This study identifies argon and krypton binding sites on membrane proteins and proposes as a hypothesis that noble gases, by altering protein/lipid contacts, may affect protein function.

Quantifying and comparing radiation damage in the Protein Data Bank

Radiation damage remains one of the major bottlenecks to accurate structure solution in protein crystallography. It can induce structural and chemical changes in protein crystals, and is hence an important consideration when assessing the quality and biological veracity of crystal structures in repositories like the Protein Data Bank (PDB). However, detection of radiation damage artefacts has traditionally proved very challenging. To address this, here we introduce the Bnet metric. Bnet summarises in a single value the extent of damage suffered by a crystal structure by comparing the B-factor values of damage-prone and non-damage-prone atoms in a similar local environment. After validating that Bnet successfully detects damage in 23 different crystal structures previously characterised as damaged, we calculate Bnet values for 93,978 PDB crystal structures. Our metric highlights a range of damage features, many of which would remain unidentified by the other summary statistics typically calculated for PDB structures.

Versatile microporous polymer-based supports for serial macromolecular crystallography

Serial data collection has emerged as a major tool for data collection at state-of-the-art light sources, such as microfocus beamlines at synchrotrons and X-ray free-electron lasers. Challenging targets, characterized by small crystal sizes, weak diffraction and stringent dose limits, benefit most from these methods. Here, the use of a thin support made of a polymer-based membrane for performing serial data collection or screening experiments is demonstrated. It is shown that these supports are suitable for a wide range of protein crystals suspended in liquids. The supports have also proved to be applicable to challenging cases such as membrane proteins growing in the sponge phase. The sample-deposition method is simple and robust, as well as flexible and adaptable to a variety of cases. It results in an optimally thin specimen providing low background while maintaining minute amounts of mother liquor around the crystals. The 2 × 2 mm area enables the deposition of up to several microlitres of liquid. Imaging and visualization of the crystals are straightforward on the highly transparent membrane. Thanks to their affordable fabrication, these supports have the potential to become an attractive option for serial experiments at synchrotrons and free-electron lasers.

Modulation of Toll-like receptor 1 intracellular domain structure and activity by Zn2+ ions

Toll-like receptors (TLRs) play an important role in the innate immune response. While a lot is known about the structures of their extracellular parts, many questions are still left unanswered, when the structural basis of TLR activation is analyzed for the TLR intracellular domains. Here we report the structure and dynamics of TLR1 toll-interleukin like (TIR) cytoplasmic domain in crystal and in solution. We found that the TLR1-TIR domain is capable of specific binding of Zn with nanomolar affinity. Interactions with Zn are mediated by cysteine residues 667 and 686 and C667 is essential for the Zn binding. Potential structures of the TLR1-TIR/Zn complex were predicted in silico. Using the functional assays for the heterodimeric TLR1/2 receptor, we found that both Zn addition and Zn depletion affect the activity of TLR1, and C667A mutation disrupts the receptor activity. Analysis of C667 position in the TLR1 structure and possible effects of C667A mutation, suggests that zinc-binding ability of TLR1-TIR domain is critical for the receptor activation.

Lushpa et al report the structure and dynamics of the TLR1 toll-interleukin like (TIR) cytoplasmic domain in both crystal and solution. They demonstrate that the TLR1 TIR domain is capable of specific binding of Zn with nanomolar affinity, which appears to be critical for receptor activation, and provide potential structures TLR1-TIR/Zn complex based on in silico data.

The molecular basis of spectral tuning in blue- and red-shifted flavin-binding fluorescent proteins

Photoactive biological systems modify the optical properties of their chromophores, known as spectral tuning. Determining the molecular origin of spectral tuning is instrumental for understanding the function and developing applications of these biomolecules. Spectral tuning in flavin-binding fluorescent proteins (FbFPs), an emerging class of fluorescent reporters, is limited by their dependency on protein-bound flavins, whose structure and hence electronic properties cannot be altered by mutation. A blue-shifted variant of the plant-derived improved light, oxygen, voltage FbFP has been created by introducing a lysine within the flavin-binding pocket, but the molecular basis of this shift remains unconfirmed. We here structurally characterize the blue-shifted improved light, oxygen, voltage variant and construct a new blue-shifted CagFbFP protein by introducing an analogous mutation. X-ray structures of both proteins reveal displacement of the lysine away from the chromophore and opening up of the structure as instrumental for the blue shift. Site saturation mutagenesis and high-throughput screening yielded a red-shifted variant, and structural analysis revealed that the lysine side chain of the blue-shifted variant is stabilized close to the flavin by a secondary mutation, accounting for the red shift. Thus, a single additional mutation in a blue-shifted variant is sufficient to generate a red-shifted FbFP. Using spectroscopy, X-ray crystallography, and quantum mechanics molecular mechanics calculations, we provide a firm structural and functional understanding of spectral tuning in FbFPs. We also show that the identified blue- and red-shifted variants allow for two-color microscopy based on spectral separation. In summary, the generated blue- and red-shifted variants represent promising new tools for application in life sciences.

Deconstructing Noncovalent Kelch-like ECH-Associated Protein 1 (Keap1) Inhibitors into Fragments to Reconstruct New Potent Compounds

Targeting the protein-protein interaction (PPI) between nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keap1) is a potential therapeutic strategy to control diseases involving oxidative stress. Here, six classes of known small-molecule Keap1-Nrf2 PPI inhibitors were dissected into 77 fragments in a fragment-based deconstruction reconstruction (FBDR) study and tested in four orthogonal assays. This gave 17 fragment hits of which six were shown by X-ray crystallography to bind in the Keap1 Kelch binding pocket. Two hits were merged into compound 8 with a 220-380-fold stronger affinity (K_i = 16 μM) relative to the parent fragments. Systematic optimization resulted in several novel analogues with K_i values of 0.04-0.5 μM, binding modes determined by X-ray crystallography, and enhanced microsomal stability. This demonstrates how FBDR can be used to find new fragment hits, elucidate important ligand-protein interactions, and identify new potent inhibitors of the Keap1-Nrf2 PPI.

Structural determinants underlying the adduct lifetime in the LOV proteins of Pseudomonas putida

The primary photochemistry is similar among the flavin-bound sensory domains of light-oxygen-voltage (LOV) photoreceptors, where upon blue-light illumination a covalent adduct is formed on the microseconds time scale between the flavin chromophore and a strictly conserved cysteine residue. In contrast, the adduct-state decay kinetics vary from seconds to days or longer. The molecular basis for this variation among structurally conserved LOV domains is not fully understood. Here, we selected PpSB2-LOV, a fast-cycling (τ_rec 3.5 min, 20 °C) short LOV protein from Pseudomonas putida that shares 67% sequence identity with a slow-cycling (τ_rec 2467 min, 20 °C) homologous protein PpSB1-LOV. Based on the crystal structure of the PpSB2-LOV in the dark state reported here, we used a comparative approach, in which we combined structure and sequence information with molecular dynamic (MD) simulations to address the mechanistic basis for the vastly different adduct-state lifetimes in the two homologous proteins. MD simulations pointed toward dynamically distinct structural region, which were subsequently targeted by site-directed mutagenesis of PpSB2-LOV, where we introduced single- and multisite substitutions exchanging them with the corresponding residues from PpSB1-LOV. Collectively, the data presented identify key amino acids on the Aβ-Bβ, Eα-Fα loops, and the Fα helix, such as E27 and I66, that play a decisive role in determining the adduct lifetime. Our results additionally suggest a correlation between the solvent accessibility of the chromophore pocket and adduct-state lifetime. The presented results add to our understanding of LOV signaling and will have important implications in tuning the signaling behavior (on/off kinetics) of LOV-based optogenetic tools.

Protein Structure Analysis and Validation with X-Ray Crystallography

X-ray crystallography is the main technique for the determination of protein structures. About 85% of all protein structures known to date have been elucidated using X-ray crystallography. Knowledge of the three-dimensional structure of proteins can be used in various applications in biotechnology, biomedicine, drug design , and basic research and as a validation tool for protein modifications and ligand binding. Moreover, the requirement for pure, homogeneous, and stable protein solutions in crystallizations makes X-ray crystallography beneficial in other fields of protein research as well. Here, we describe the technique of X-ray protein crystallography and the steps involved for a successful three-dimensional crystal structure determination.

Novel approaches for the lipid sponge phase crystallization of the Rhodobacter sphaeroides photosynthetic reaction center

With the recent developments in the field of free-electron-laser-based serial femtosecond crystallography, the necessity to obtain a large number of high-quality crystals has emerged. In this work crystallization techniques were selected, tested and optimized for the lipid mesophase crystallization of the Rhodobacter sphaeroides membrane pigment-protein complex, known as the photosynthetic reaction center (RC). Novel approaches for lipid sponge phase crystallization in comparatively large volumes using Hamilton gas-tight glass syringes and plastic pipetting tips are described. An analysis of RC crystal structures obtained by lipid mesophase crystallization revealed non-native ligands that displaced the native electron-transfer cofactors (carotenoid sphero-idene and a ubi-quinone molecule) from their binding pockets. These ligands were identified and were found to be lipids that are major mesophase components. The selection of distinct co-crystallization conditions with the missing cofactors facilitated the restoration of sphero-idene in its binding site.

Multi-crystal data collection using synchrotron radiation as exemplified with low-symmetry crystals of Dps

Multi-crystal data collection using synchrotron radiation was successfully applied to determine the three-dimensional structure of a triclinic crystal form of Dps from Escherichia coli at 2.0 Å resolution. The final data set was obtained by combining 261 partial diffraction data sets measured from crystals with an average size of approximately 5 µm. The most important features of diffraction data measurement and processing for low-symmetry crystals are discussed.

Hydroxylation of Antitubercular Drug Candidate, SQ109, by Mycobacterial Cytochrome P450

Spreading of the multidrug-resistant (MDR) strains of the one of the most harmful pathogen Mycobacterium tuberculosis (Mtb) generates the need for new effective drugs. SQ109 showed activity against resistant Mtb and already advanced to Phase II/III clinical trials. Fast SQ109 degradation is attributed to the human liver Cytochrome P450s (CYPs). However, no information is available about interactions of the drug with Mtb CYPs. Here, we show that Mtb CYP124, previously assigned as a methyl-branched lipid monooxygenase, binds and hydroxylates SQ109 in vitro. A 1.25 Å-resolution crystal structure of the CYP124-SQ109 complex unambiguously shows two conformations of the drug, both positioned for hydroxylation of the ω-methyl group in the trans position. The hydroxylated SQ109 presumably forms stabilizing H-bonds with its target, Mycobacterial membrane protein Large 3 (MmpL3). We anticipate that Mtb CYPs could function as analogs of drug-metabolizing human CYPs affecting pharmacokinetics and pharmacodynamics of antitubercular (anti-TB) drugs.

Attaining atomic resolution from in situ data collection at room temperature using counter-diffusion-based low-cost microchips

Sample handling and manipulation for cryoprotection currently remain critical factors in X-ray structural determination. While several microchips for macromolecular crystallization have been proposed during the last two decades to partially overcome crystal-manipulation issues, increased background noise originating from the scattering of chip-fabrication materials has so far limited the attainable resolution of diffraction data. Here, the conception and use of low-cost, X-ray-transparent microchips for in situ crystallization and direct data collection, and structure determination at atomic resolution close to 1.0 Å, is presented. The chips are fabricated by a combination of either OSTEMER and Kapton or OSTEMER and Mylar materials for the implementation of counter-diffusion crystallization experiments. Both materials produce a sufficiently low scattering background to permit atomic resolution diffraction data collection at room temperature and the generation of 3D structural models of the tested model proteins lysozyme, thaumatin and glucose isomerase. Although the high symmetry of the three model protein crystals produced almost complete data sets at high resolution, the potential of in-line data merging and scaling of the multiple crystals grown along the microfluidic channels is also presented and discussed.

Resolution and dose dependence of radiation damage in biomolecular systems

The local Fourier-space relation between diffracted intensity I, diffraction wavevector q and dose D, , is key to probing and understanding radiation damage by X-rays and energetic particles in both diffraction and imaging experiments. The models used in protein crystallography for the last 50 years provide good fits to experimental I(q) versus nominal dose data, but have unclear physical significance. More recently, a fit to diffraction and imaging experiments suggested that the maximum tolerable dose varies as q -1 or linearly with resolution. Here, it is shown that crystallographic data have been strongly perturbed by the effects of spatially nonuniform crystal irradiation and diffraction during data collection. Reanalysis shows that these data are consistent with a purely exponential local dose dependence, = I 0(q)exp[-D/D e(q)], where D e(q) ∝ q α with α ≃ 1.7. A physics-based model for radiation damage, in which damage events occurring at random locations within a sample each cause energy deposition and blurring of the electron density within a small volume, predicts this exponential variation with dose for all q values and a decay exponent α ≃ 2 in two and three dimensions, roughly consistent with both diffraction and imaging experiments over more than two orders of magnitude in resolution. The B-factor model used to account for radiation damage in crystallographic scaling programs is consistent with α = 2, but may not accurately capture the dose dependencies of structure factors under typical nonuniform illumination conditions. The strong q dependence of radiation-induced diffraction decays implies that the previously proposed 20-30 MGy dose limit for protein crystallography should be replaced by a resolution-dependent dose limit that, for atomic resolution data sets, will be much smaller. The results suggest that the physics underlying basic experimental trends in radiation damage at T ≃ 100 K is straightforward and universal. Deviations of the local I(q, D) from strictly exponential behavior may provide mechanistic insights, especially into the radiation-damage processes responsible for the greatly increased radiation sensitivity observed at T ≃ 300 K.

Comparison of the Quality of Protein Crystals Grown by CLPC Seeds Method

We present a systematic quality comparison of protein crystals obtained with and without cross-linked protein crystal (CLPC) seeds. Four proteins were used to conduct the experiments, and the results showed that crystals obtained in the presence of CLPC seeds exhibited a better morphology. In addition, the X-ray diffraction data showed that the CLPC seeds method is a powerful tool to obtain high-quality protein crystals. Therefore, we recommend the use of CLPC seeds in preparing high-quality diffracting protein crystals.

A comparative anatomy of protein crystals: lessons from the automatic processing of 56 000 samples

The fully automatic processing of crystals of macromolecules has presented a unique opportunity to gather information on the samples that is not usually recorded. This has proved invaluable in improving sample-location, characterization and data-collection algorithms. After operating for four years, MASSIF-1 has now processed over 56 000 samples, gathering information at each stage, from the volume of the crystal to the unit-cell dimensions, the space group, the quality of the data collected and the reasoning behind the decisions made in data collection. This provides an unprecedented opportunity to analyse these data together, providing a detailed landscape of macromolecular crystals, intimate details of their contents and, importantly, how the two are related. The data show that mosaic spread is unrelated to the size or shape of crystals and demonstrate experimentally that diffraction intensities scale in proportion to crystal volume and molecular weight. It is also shown that crystal volume scales inversely with molecular weight. The results set the scene for the development of X-ray crystallography in a changing environment for structural biology.

The subatomic resolution study of laccase inhibition by chloride and fluoride anions using single-crystal serial crystallography: insights into the enzymatic reaction mechanism

Laccases are enzymes that catalyze the oxidation of a wide range of organic and inorganic substrates accompanied by the reduction of molecular oxygen to water. Here, a subatomic resolution X-ray crystallographic study of the mechanism of inhibition of the laccase from the basidiomycete fungus Steccherinum murashkinskyi by chloride and fluoride ions is presented. Three series of X-ray diffraction data sets were collected with increasing doses of absorbed X-ray radiation from a native S. murashkinskyi laccase crystal and from crystals of complexes of the laccase with chloride and fluoride ions. The data for the native laccase crystal confirmed the previously deduced enzymatic mechanism of molecular oxygen reduction. The structures of the complexes allowed the localization of chloride and fluoride ions in the channel near the T2 copper ion. These ions replace the oxygen ligand of the T2 copper ion in this channel and can play the role of this ligand in the enzymatic reaction. As follows from analysis of the structures from the increasing dose series, the inhibition of laccases by chloride and fluoride anions can be explained by the fact that the binding of these negatively charged ions at the position of the oxygen ligand of the T2 copper ion impedes the reduction of the T2 copper ion.

Structural analysis of a replication protein encoded by a plasmid isolated from a multiple sclerosis patient

Bovine meat and milk factors (BMMFs) are circular, single-stranded episomal DNAs that have been detected in bovine meat and milk products. BMMFs are thought to have roles in human malignant and degenerative diseases. BMMFs encode a replication initiator protein (Rep) that is actively transcribed and translated in human cells. In this study, a Rep WH1 domain encoded on a BMMF (MSBI1.176) isolated from a multiple sclerosis human brain sample was determined to 1.53 Å resolution using X-ray crystallography. The overall structure of the MSBI1.176 WH1 domain was remarkably similar to other Rep structures, despite having a low (28%) amino-acid sequence identity. The MSBI1.176 WH1 domain contained elements common to other Reps, including five α-helices, five β-strands and a hydrophobic pocket. These new findings suggest that the MSBI1.176 Rep might have comparable roles and functions to other known Reps of different origins.

Physical methods for studying flavoprotein photoreceptors

Molecular mechanisms of dark-to-light state transitions in flavoprotein photoreceptors have been the subject of intense investigation. Blue-light sensing flavoproteins fall into three general classes that share aspects of their activation processes: LOV domains, BLUF proteins, and cryptochromes. In all cases, light-induced changes in flavin redox, protonation, and bonding states result in hydrogen-bond and conformational rearrangements important for regulation of downstream targets. Physical characterization of these flavoprotein states can provide valuable insights into biological function, but clear conclusions are often challenging to draw owing to complexities of data collection and interpretation. In this chapter, we briefly review the three classes of flavoprotein photoreceptors and provide methods for their recombinant production, reconstitution with flavin cofactor, and characterization. We then relate best practices and special considerations for the application of several types of spectroscopies, redox potential measurements, and X-ray scattering experiments to photosensitive flavoproteins. The methods presented are generally accessible to most laboratories.

Igni18, a novel metallo-hydrolase from the hyperthermophilic archaeon Ignicoccus hospitalis KIN4/I: cloning, expression, purification and X-ray analysis

The hyperthermophilic crenarchaeon Ignicoccus hospitalis KIN4/I possesses at least 35 putative genes encoding enzymes that belong to the α/β-hydrolase superfamily. One of those genes, the metallo-hydrolase-encoding igni18, was cloned and heterologously expressed in Pichia pastoris. The enzyme produced was purified in its catalytically active form. The recombinant enzyme was successfully crystallized and the crystal diffracted to a resolution of 2.3 Å. The crystal belonged to space group R32, with unit-cell parameters a = b = 67.42, c = 253.77 Å, α = β = 90.0, γ = 120.0°. It is suggested that it contains one monomer of Igni18 within the asymmetric unit.

Pseudomonas aeruginosa esterase PA2949, a bacterial homolog of the human membrane esterase ABHD6: expression, purification and crystallization

The human membrane-bound α/β-hydrolase domain 6 (ABHD6) protein modulates endocannabinoid signaling, which controls appetite, pain and learning, as well as being linked to Alzheimer's and Parkinson's diseases, through the degradation of the key lipid messenger 2-arachidonylglycerol (2-AG). This makes ABHD6 an attractive therapeutic target that lacks structural information. In order to better understand the molecular mechanism of 2-AG-hydrolyzing enzymes, the PA2949 protein from Pseudomonas aeruginosa, which has 49% sequence similarity to the ABHD6 protein, was cloned, overexpressed, purified and crystallized. Overexpression of PA2949 in the homologous host yielded the membrane-bound enzyme, which was purified in milligram amounts. Besides their sequence similarity, the enzymes both show specificity for the hydrolysis of 2-AG and esters of medium-length fatty acids. PA2949 in the presence of n-octyl β-D-glucoside showed a higher activity and stability at room temperature than those previously reported for PA2949 overexpressed and purified from Escherichia coli. A suitable expression host and stabilizing detergent were crucial for obtaining crystals, which belonged to the tetragonal space group I4122 and diffracted to a resolution of 2.54 Å. This study provides hints on the functional similarity of ABHD6-like proteins in prokaryotes and eukaryotes, and might guide the structural study of these difficult-to-crystallize proteins.

Fab Fragment of VHH-Based Antibody Netakimab: Crystal Structure and Modeling Interaction with Cytokine IL-17A

Interleukin 17A (IL-17A) is a proinflammatory cytokine produced by Th17 cells. Antibody BCD-085 (netakimab) against human IL-17A is one of the new inhibitors of this cytokine. In netakimab, the VH domain is replaced by the VHH domain of Lama glama possessing a long complementarity determining region (CDR-H3) in its heavy chain. Here we demonstrate the high affinity of IL-17A to the Fab fragment of netakimab and to its integral part, the VHH domain. We have determined the crystal structure of the Fab fragment of netakimab at 1.9 Å resolution. High variability in the orientation of light and heavy chains of the Fab fragment of netakimab was shown, which is determined by the peculiarity of the structural organization of the CDR-H3. As the high conformational plasticity of the molecule hampers modeling the Fab fragment of netakimab complexed to IL-17A, we have carried out modeling the complex between the antigen and the integral part of the Fab fragment, the VHH domain. We explain the high netakimab Fab fragment affinity for IL-17A by a large number of protein–protein contacts due to additional interactions between CDR-H3 and the cytokine dimer.

MXCuBE2: the dawn of MXCuBE Collaboration

MXCuBE2 is the second-generation evolution of the MXCuBE beamline control software, initially developed and used at ESRF - the European Synchrotron. MXCuBE2 extends, in an intuitive graphical user interface (GUI), the functionalities and data collection methods available to users while keeping all previously available features and allowing for the straightforward incorporation of ongoing and future developments. MXCuBE2 introduces an extended abstraction layer that allows easy interfacing of any kind of macromolecular crystallography (MX) hardware component, whether this is a diffractometer, sample changer, detector or optical element. MXCuBE2 also works in strong synergy with the ISPyB Laboratory Information Management System, accessing the list of samples available for a particular experimental session and associating, either from instructions contained in ISPyB or from user input via the MXCuBE2 GUI, different data collection types to them. The development of MXCuBE2 forms the core of a fruitful collaboration which brings together several European synchrotrons and a software development factory and, as such, defines a new paradigm for the development of beamline control platforms for the European MX user community.

ID30B - a versatile beamline for macromolecular crystallography experiments at the ESRF

ID30B is an undulator-based high-intensity, energy-tuneable (6.0-20 keV) and variable-focus (20-200 µm in diameter) macromolecular crystallography (MX) beamline at the ESRF. It was the last of the ESRF Structural Biology Group's beamlines to be constructed and commissioned as part of the ESRF's Phase I Upgrade Program and has been in user operation since June 2015. Both a modified microdiffractometer (MD2S) incorporating an in situ plate screening capability and a new flexible sample changer (the FlexHCD) were specifically developed for ID30B. Here, the authors provide the current beamline characteristics and detail how different types of MX experiments can be performed on ID30B (http://www.esrf.eu/id30b).

Structure of the flavocytochrome c sulfide dehydrogenase associated with the copper-binding protein CopC from the haloalkaliphilic sulfur-oxidizing bacterium Thioalkalivibrio paradoxusARh 1

Flavocytochrome c sulfide dehydrogenase from Thioalkalivibrio paradoxus (TpFCC) is a heterodimeric protein consisting of flavin- and monohaem c-binding subunits. TpFCC was co-purified and co-crystallized with the dimeric copper-binding protein TpCopC. The structure of the TpFCC–(TpCopC)2 complex was determined by X-ray diffraction at 2.6 Å resolution. The flavin-binding subunit of TpFCC is structurally similar to those determined previously, and the structure of the haem-binding subunit is similar to that of the N-terminal domain of dihaem FCCs. According to classification based on amino-acid sequence, TpCopC belongs to a high-affinity CopC subfamily characterized by the presence of a conserved His1-Xxx-His3 motif at the N-terminus. Apparently, a unique α-helix which is present in each monomer of TpCopC at the interface with TpFCC plays a key role in complex formation. The structure of the copper-binding site in TpCopC is similar to those in other known CopC structures. His3 is not involved in binding to the copper ion and is 6–7 Å away from this ion. Therefore, the His1-Xxx-His3 motif cannot be considered to be a key factor in the high affinity of CopC for copper(II) ions. It is suggested that the TpFCC–(TpCopC)2 heterotetramer may be a component of a large periplasmic complex that is responsible for thiocyanate metabolism.

Multi-position data collection and dynamic beam sizing: recent improvements to the automatic data-collection algorithms on MASSIF-1

Macromolecular crystallography is now a mature and widely used technique that is essential in the understanding of biology and medicine. Increases in computing power combined with robotics have not only enabled large numbers of samples to be screened and characterized but have also enabled better decisions to be taken on data collection itself. This led to the development of MASSIF-1 at the ESRF, the first beamline in the world to run fully automatically while making intelligent decisions taking user requirements into account. Since opening in late 2014, the beamline has processed over 42 000 samples. Improvements have been made to the speed of the sample-handling robotics and error management within the software routines. The workflows initially put into place, while highly innovative at the time, have been expanded to include increased complexity and additional intelligence using the information gathered during characterization; this includes adapting the beam diameter dynamically to match the diffraction volume within the crystal. Complex multi-position and multi-crystal data collections have now also been integrated into the selection of experiments available. This has led to increased data quality and throughput, allowing even the most challenging samples to be treated automatically.

X-ray and UV radiation-damage-induced phasing using synchrotron serial crystallography

Multi-crystal serial crystallography data can be used for UV and X-ray radiation-damage-induced phasing.

Specific radiation damage can be used to determine phases de novo from macromolecular crystals. This method is known as radiation-damage-induced phasing (RIP). One limitation of the method is that the dose of individual data sets must be minimized, which in turn leads to data sets with low multiplicity. A solution to this problem is to use data from multiple crystals. However, the resulting signal can be degraded by a lack of isomorphism between crystals. Here, it is shown that serial synchrotron crystallography in combination with selective merging of data sets can be used to determine high-quality phases for insulin and thaumatin, and that the increased multiplicity can greatly enhance the success rate of the experiment.

The complex analysis of X-ray mesh scans for macromolecular crystallography

A method and software program, MeshBest, for the detection of individual crystals based on two-dimensional X-ray mesh scans are presented.

In macromolecular crystallography, mesh (raster) scans are carried out either as part of X-ray-based crystal-centring routines or to identify positions on the sample holder from which diffraction images can be collected. Here, the methods used in MeshBest, software which automatically analyses diffraction images collected during a mesh scan and produces a two-dimensional crystal map showing estimates of the dimensions, centre positions and diffraction qualities of each crystal contained in the mesh area, are presented. Sample regions producing diffraction images resulting from the superposition of more than one crystal are also distinguished from regions with single-crystal diffraction. The applicability of the method is demonstrated using several cases.

Radiation Damage in Macromolecular Crystallography—An Experimentalist’s View

Radiation damage still remains a major limitation and challenge in macromolecular X-ray crystallography. Some of the high-intensity radiation used for diffraction data collection experiments is absorbed by the crystals, generating free radicals. These give rise to radiation damage even at cryotemperatures (~100 K), which can lead to incorrect biological conclusions being drawn from the resulting structure, or even prevent structure solution entirely. Investigation of mitigation strategies and the effects caused by radiation damage has been extensive over the past fifteen years. Here, recent understanding of the physical and chemical phenomena of radiation damage is described, along with the global effects inflicted on the collected data and the specific effects observed in the solved structure. Furthermore, this review aims to summarise the progress made in radiation damage studies in macromolecular crystallography from the experimentalist’s point of view and to give an introduction to the current literature.

DA+ data acquisition and analysis software at the Swiss Light Source macromolecular crystallography beamlines

Data acquisition software is an essential component of modern macromolecular crystallography (MX) beamlines, enabling efficient use of beam time at synchrotron facilities. Developed at the Paul Scherrer Institute, the DA+ data acquisition software is implemented at all three Swiss Light Source (SLS) MX beamlines. DA+ consists of distributed services and components written in Python and Java, which communicate via messaging and streaming technologies. The major components of DA+ are the user interface, acquisition engine, online processing and database. Immediate data quality feedback is achieved with distributed automatic data analysis routines. The software architecture enables exploration of the full potential of the latest instrumentation at the SLS MX beamlines, such as the SmarGon goniometer and the EIGER X 16M detector, and development of new data collection methods.

A focused fragment library targeting the antibiotic resistance enzyme - Oxacillinase-48: Synthesis, structural evaluation and inhibitor design

β-Lactam antibiotics are of utmost importance when treating bacterial infections in the medical community. However, currently their utility is threatened by the emergence and spread of β-lactam resistance. The most prevalent resistance mechanism to β-lactam antibiotics is expression of β-lactamase enzymes. One way to overcome resistance caused by β-lactamases, is the development of β-lactamase inhibitors and today several β-lactamase inhibitors e.g. avibactam, are approved in the clinic. Our focus is the oxacillinase-48 (OXA-48), an enzyme reported to spread rapidly across the world and commonly identified in Escherichia coli and Klebsiella pneumoniae. To guide inhibitor design, we used diversely substituted 3-aryl and 3-heteroaryl benzoic acids to probe the active site of OXA-48 for useful enzyme-inhibitor interactions. In the presented study, a focused fragment library containing 49 3-substituted benzoic acid derivatives were synthesised and biochemically characterized. Based on crystallographic data from 33 fragment-enzyme complexes, the fragments could be classified into R¹ or R² binders by their overall binding conformation in relation to the binding of the R¹ and R² side groups of imipenem. Moreover, binding interactions attractive for future inhibitor design were found and their usefulness explored by the rational design and evaluation of merged inhibitors from orthogonally binding fragments. The best inhibitors among the resulting 3,5-disubstituted benzoic acids showed inhibitory potential in the low micromolar range (IC₅₀ = 2.9 μM). For these inhibitors, the complex X-ray structures revealed non-covalent binding to Arg250, Arg214 and Tyr211 in the active site and the interactions observed with the mono-substituted fragments were also identified in the merged structures.

Estimate your dose: RADDOSE-3D

We present the current status of RADDOSE-3D, a software tool allowing the estimation of the dose absorbed in a macromolecular crystallography diffraction experiment. The code allows a temporal and spatial dose contour map to be calculated for a crystal of any geometry and size as it is rotated in an X-ray beam, and gives several summary dose values: among them diffraction weighted dose. This allows experimenters to plan data collections which will minimize radiation damage effects by spreading the absorbed dose more homogeneously, and thus to optimize the use of their crystals. It also allows quantitative comparisons between different radiation damage studies, giving a universal "x-axis" against which to plot various metrics.

Fast iodide-SAD phasing for high-throughput membrane protein structure determination

We describe a fast, easy, and potentially universal method for the de novo solution of the crystal structures of membrane proteins via iodide-single-wavelength anomalous diffraction (I-SAD). The potential universality of the method is based on a common feature of membrane proteins-the availability at the hydrophobic-hydrophilic interface of positively charged amino acid residues with which iodide strongly interacts. We demonstrate the solution using I-SAD of four crystal structures representing different classes of membrane proteins, including a human G protein-coupled receptor (GPCR), and we show that I-SAD can be applied using data collection strategies based on either standard or serial x-ray crystallography techniques.

Structural study of the X-ray-induced enzymatic reduction of molecular oxygen to water bySteccherinum murashkinskyilaccase: insights into the reaction mechanism

The laccase fromSteccherinum murashkinskyiis a member of the large family of multicopper oxidases that catalyze the oxidation of a wide range of organic and inorganic substrates, accompanied by the reduction of dioxygen to water. The reducing properties of X-ray radiation and the high quality of the laccase crystals allow the study of the catalytic reduction of dioxygen to water directly in a crystal. A series of diffraction data sets with increasing absorbed radiation dose were collected from a single crystal ofSteccherinum murashkinskyilaccase at 1.35 Å resolution. Changes in the active-site structure associated with the reduction of molecular oxygen to water on increasing the absorbed dose of ionizing radiation were detected. The structures in the series are mixtures of different states of the enzyme–substrate complex. Nevertheless, it was possible to interpret these structures as complexes of various oxygen ligands with copper ions in different oxidation states. The results allowed the mechanism of oxygen reduction catalyzed by laccases to be refined.

Structure of a LOV protein in apo-state and implications for construction of LOV-based optical tools

Unique features of Light-Oxygen-Voltage (LOV) proteins like relatively small size (~12–19 kDa), inherent modularity, highly-tunable photocycle and oxygen-independent fluorescence have lately been exploited for the generation of optical tools. Structures of LOV domains reported so far contain a flavin chromophore per protein molecule. Here we report two new findings on the short LOV protein W619_1-LOV from Pseudomonas putida. First, the apo-state crystal structure of W619_1-LOV at 2.5 Å resolution reveals conformational rearrangements in the secondary structure elements lining the chromophore pocket including elongation of the Fα helix, shortening of the Eα-Fα loop and partial unfolding of the Eα helix. Second, the apo W619_1-LOV protein binds both natural and structurally modified flavin chromophores. Remarkably different photophysical and photochemical properties of W619_1-LOV bound to 7-methyl-8-chloro-riboflavin (8-Cl-RF) and lumichrome imply application of these variants as novel optical tools as they offer advantages such as no adduct state formation, and a broader choice of wavelengths for in vitro studies.

Identification of the point of diminishing returns in high-multiplicity data collection for sulfur SAD phasing

High-quality high-multiplicity X-ray diffraction data were collected on five different crystals of thaumatin using a homogeneous-profile X-ray beam at E = 8 keV to investigate the counteracting effects of increased multiplicity and increased radiation damage on the quality of anomalous diffraction data collected on macromolecular crystals. By comparing sulfur substructures obtained from subsets of the data selected as a function of absorbed X-ray dose with sulfur positions in the respective refined reference structures, the doses at which the highest quality of anomalous differences could be obtained were identified for the five crystals. A statistic σ{ΔF}D, calculated as the width σ of the normalized distribution of a set {ΔF} of anomalous differences collected at a dose D, is suggested as a measure of anomalous data quality as a function of dose. An empirical rule is proposed to identify the dose at which the gains in data quality due to increased multiplicity are outbalanced by the losses due to decreases in signal-to-noise as a consequence of radiation damage. Identifying this point of diminishing returns allows the optimization of the choice of data collection parameters and the selection of data to be used in subsequent crystal structure determination steps.

Comparison of helical scan and standard rotation methods in single-crystal X-ray data collection strategies

X-ray radiation in macromolecular crystallography can chemically alter the biological material and deteriorate the integrity of the crystal lattice with concomitant loss of resolution. Typical alterations include decarboxylation of glutamic and aspartic residues, breaking of disulfide bonds and the reduction of metal centres. Helical scans add a small translation to the crystal in the rotation method, so that for every image the crystal is shifted to expose a fresh part. On beamline PROXIMA 2A at Synchrotron SOLEIL, this procedure has been tested with various parameters in an attempt to understand how to mitigate the effects of radiation damage. Here, the strategies used and the crystallographic metrics for various scenarios are reported. Among these, the loss of bromine from bromophenyl moieties appears to be a useful monitor of radiation damage as the carbon-bromine bond is very sensitive to X-ray irradiation. Two cases are focused on where helical scans are shown to be superior in obtaining meaningful data compared with conventional methods. In one case the initial resolution of the crystal is extended over time, and in the second case the anomalous signal is preserved to provide greater effective multiplicity and easier phasing.

Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism

Ubiquitin receptors decode ubiquitin signals into many cellular responses. Ubiquitin receptors also undergo coupled monoubiquitylation, and rapid deubiquitylation has hampered the characterization of the ubiquitylated state. Using bacteria that express a ubiquitylation apparatus, we purified and determined the crystal structure of the proteasomal ubiquitin-receptor Rpn10 in its ubiquitylated state. The structure shows a novel ubiquitin-binding patch that directs K84 ubiquitylation. Superimposition of ubiquitylated-Rpn10 onto electron-microscopy models of proteasomes indicates that the Rpn10-conjugated ubiquitin clashes with Rpn9, suggesting that ubiquitylation might be involved in releasing Rpn10 from the proteasome. Indeed, ubiquitylation on immobilized proteasomes dissociates the modified Rpn10 from the complex, while unmodified Rpn10 mainly remains associated. In vivo experiments indicate that contrary to wild type, Rpn10-K84R is stably associated with the proteasomal subunit Rpn9. Similarly Rpn10, but not ubiquitylated-Rpn10, binds Rpn9 in vitro. Thus we suggest that ubiquitylation functions to dissociate modified ubiquitin receptors from their targets, a function that promotes cyclic activity of ubiquitin receptors.

Ubiquitin (Ub) receptors are responsible for the recognition of ubiquitylated proteins. Here the authors describe the crystal structure of the ubiquitylated form of the Ub-receptor Rpn10, which suggest that ubiquitylation of Rpn10 promotes its dissociation from the proteasome.

RNA protects a nucleoprotein complex against radiation damage

Radiation damage during macromolecular X-ray crystallographic data collection is still the main impediment for many macromolecular structure determinations. Even when an eventual model results from the crystallographic pipeline, the manifestations of radiation-induced structural and conformation changes, the so-called specific damage, within crystalline macromolecules can lead to false interpretations of biological mechanisms. Although this has been well characterized within protein crystals, far less is known about specific damage effects within the larger class of nucleoprotein complexes. Here, a methodology has been developed whereby per-atom density changes could be quantified with increasing dose over a wide (1.3-25.0 MGy) range and at higher resolution (1.98 Å) than the previous systematic specific damage study on a protein-DNA complex. Specific damage manifestations were determined within the large trp RNA-binding attenuation protein (TRAP) bound to a single-stranded RNA that forms a belt around the protein. Over a large dose range, the RNA was found to be far less susceptible to radiation-induced chemical changes than the protein. The availability of two TRAP molecules in the asymmetric unit, of which only one contained bound RNA, allowed a controlled investigation into the exact role of RNA binding in protein specific damage susceptibility. The 11-fold symmetry within each TRAP ring permitted statistically significant analysis of the Glu and Asp damage patterns, with RNA binding unexpectedly being observed to protect these otherwise highly sensitive residues within the 11 RNA-binding pockets distributed around the outside of the protein molecule. Additionally, the method enabled a quantification of the reduction in radiation-induced Lys and Phe disordering upon RNA binding directly from the electron density.