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Preview: Acta Crystallographica Section F

Acta Crystallographica Section F

Acta Crystallographica Section F: Structural Biology Communications is a rapid all-electronic journal, which provides a home for short communications on the crystallization and structure of biological macromolecules. Structures determined through structur

Published: 2016-10-01


Crystal structure of yeast monothiol glutaredoxin Grx6 in complex with a glutathione-coordinated [2Fe–2S] cluster


Glutaredoxins (Grxs) constitute a superfamily of proteins that perform diverse biological functions. The Saccharomyces cerevisiae glutaredoxin Grx6 not only serves as a glutathione (GSH)-dependent oxidoreductase and as a GSH transferase, but also as an essential [2Fe–2S]-binding protein. Here, the dimeric structure of the C-terminal domain of Grx6 (holo Grx6C), bridged by one [2Fe–2S] cluster coordinated by the active-site Cys136 and two external GSH molecules, is reported. Structural comparison combined with multiple-sequence alignment demonstrated that holo Grx6C is similar to the [2Fe–2S] cluster-incorporated dithiol Grxs, which share a highly conserved [2Fe–2S] cluster-binding pattern and dimeric conformation that is distinct from the previously identified [2Fe–2S] cluster-ligated monothiol Grxs.

Arabidopsis receptor-like cytoplasmic kinase BIK1: purification, crystallization and X-ray diffraction analysis


Receptor-like cytoplasmic kinases (RLCKs) in Arabidopsis play a central role in the integration of signaling input from various growth and immune signaling pathways. BOTRYTIS-INDUCED KINASE 1 (BIK1), belonging to the RLCK family, is an important player in defense against bacterial and fungal pathogens and in ethylene and brassinosteroid hormone signaling. In this study, the purification and crystallization of a first member of the class VI family of RLCK proteins, BIK1, are reported. BIK1 was crystallized using the microbatch-under-oil method. X-ray diffraction data were collected to 2.35 Å resolution. The crystals belonged to the monoclinic space group C2, with two monomers per asymmetric unit.

Crystal and solution structural studies of mouse phospholipid hydroperoxide glutathione peroxidase 4


The mammalian glutathione peroxidase (GPx) family is a key component of the cellular antioxidative defence system. Within this family, GPx4 has unique features as it accepts a large class of hydroperoxy lipid substrates and has a plethora of biological functions, including sperm maturation, regulation of apoptosis and cerebral embryogenesis. In this paper, the structure of the cytoplasmic isoform of mouse phospholipid hydroperoxide glutathione peroxidase (O70325-2 GPx4) with selenocysteine 46 mutated to cysteine is reported solved at 1.8 Å resolution using X-ray crystallography. Furthermore, solution data of an isotope-labelled GPx protein are presented.

The structure of a glycoside hydrolase 29 family member from a rumen bacterium reveals unique, dual carbohydrate-binding domains


Glycoside hydrolase (GH) family 29 consists solely of α-l-fucosidases. These enzymes catalyse the hydrolysis of glycosidic bonds. Here, the structure of GH29_0940, a protein cloned from metagenomic DNA from the rumen of a cow, has been solved, which reveals a multi-domain arrangement that has only recently been identified in bacterial GH29 enzymes. The microbial species that provided the source of this enzyme is unknown. This enzyme contains a second carbohydrate-binding domain at its C-terminal end in addition to the typical N-terminal catalytic domain and carbohydrate-binding domain arrangement of GH29-family proteins. GH29_0940 is a monomer and its overall structure consists of an N-terminal TIM-barrel-like domain, a central β-sandwich domain and a C-terminal β-sandwich domain. The TIM-barrel-like catalytic domain exhibits a (β/α)8/7 arrangement in the core instead of the typical (β/α)8 topology, with the `missing' α-helix replaced by a long meandering loop that `closes' the barrel structure and suggests a high degree of structural flexibility in the catalytic core. This feature was also noted in all six other structures of GH29 enzymes that have been deposited in the PDB. Based on sequence and structural similarity, the residues Asp162 and Glu220 are proposed to serve as the catalytic nucleophile and the proton donor, respectively. Like other GH29 enzymes, the GH29_0940 structure shows five strictly conserved residues in the catalytic pocket. The structure shows two glycerol molecules in the active site, which have also been observed in other GH29 structures, suggesting that the enzyme catalyses the hydrolysis of small carbohydrates. The two binding domains are classed as family 32 carbohydrate-binding modules (CBM32). These domains have residues involved in ligand binding in the loop regions at the edge of the β-sandwich. The predicted substrate-binding residues differ between the modules, suggesting that different modules bind to different groups on the substrate(s). Enzymes that possess multiple copies of CBMs are thought to have a complex mechanism of ligand recognition. Defined electron density identifying a long 20-amino-acid hydrophilic loop separating the two CBMs was observed. This suggests that the additional C-terminal domain may have a dynamic range of movement enabled by the loop, allowing a unique mode of action for a GH29 enzyme that has not been identified previously.

Comparative analysis of anti-polyglutamine Fab crystals grown on Earth and in microgravity


Huntington's disease is one of nine neurodegenerative diseases caused by a polyglutamine (polyQ)-repeat expansion. An anti-polyQ antigen-binding fragment, MW1 Fab, was crystallized both on Earth and on the International Space Station, a microgravity environment where convection is limited. Once the crystals returned to Earth, the number, size and morphology of all crystals were recorded, and X-ray data were collected from representative crystals. The results generally agreed with previous microgravity crystallization studies. On average, microgravity-grown crystals were 20% larger than control crystals grown on Earth, and microgravity-grown crystals had a slightly improved mosaicity (decreased by 0.03°) and diffraction resolution (decreased by 0.2 Å) compared with control crystals grown on Earth. However, the highest resolution and lowest mosaicity crystals were formed on Earth, and the highest-quality crystal overall was formed on Earth after return from microgravity.

Crystal structure of rofecoxib bound to human cyclooxygenase-2


Rofecoxib (Vioxx) was one of the first selective cyclooxygenase-2 (COX-2) inhibitors (coxibs) to be approved for use in humans. Within five years after its release to the public, Vioxx was withdrawn from the market owing to the adverse cardiovascular effects of the drug. Despite the widespread knowledge of the development and withdrawal of Vioxx, relatively little is known at the molecular level about how the inhibitor binds to COX-2. Vioxx is unique in that the inhibitor contains a methyl sulfone moiety in place of the sulfonamide moiety found in other coxibs such as celecoxib and valdecoxib. Here, new crystallization conditions were identified that allowed the structural determination of human COX-2 in complex with Vioxx and the structure was subsequently determined to 2.7 Å resolution. The crystal structure provides the first atomic level details of the binding of Vioxx to COX-2. As anticipated, Vioxx binds with its methyl sulfone moiety located in the side pocket of the cyclooxygenase channel, providing support for the isoform selectivity of this drug.

Crystallographic study of the 2-thioribothymidine-synthetic complex TtuA–TtuB from Thermus thermophilus


The ubiquitin-like protein TtuB is a sulfur carrier for the biosynthesis of 2-thioribothymidine (s2T) at position 54 in some thermophilic bacterial tRNAs. TtuB captures a S atom at its C-terminus as a thiocarboxylate and transfers it to tRNA by the transferase activity of TtuA. TtuB also functions to suppress s2T formation by forming a covalent bond with TtuA. To explore how TtuB interacts with TtuA and switches between these two different functions, high-resolution structure analysis of the TtuA–TtuB complex is required. In this study, the TtuA–TtuB complex from Thermus thermophilus was expressed, purified and crystallized. To mimic the thiocarboxylated TtuB, the C-terminal Gly residue was replaced with Cys (G65C) to obtain crystals of the TtuA–TtuB complex. A Zn-MAD data set was collected to a resolution of 2.5 Å. MAD analysis successfully determined eight Zn sites, and a partial structure model composed of four TtuA–TtuB complexes in the asymmetric unit was constructed.

X-ray crystallographic studies of the extracellular domain of the first plant ATP receptor, DORN1, and the orthologous protein from Camelina sativa


Does not respond to nucleotides 1 (DORN1) has recently been identified as the first membrane-integral plant ATP receptor, which is required for ATP-induced calcium response, mitogen-activated protein kinase activation and defense responses in Arabidopsis thaliana. In order to understand DORN1-mediated ATP sensing and signal transduction, crystallization and preliminary X-ray studies were conducted on the extracellular domain of DORN1 (atDORN1-ECD) and that of an orthologous protein, Camelina sativa lectin receptor kinase I.9 (csLecRK-I.9-ECD or csI.9-ECD). A variety of deglycosylation strategies were employed to optimize the glycosylated recombinant atDORN1-ECD for crystallization. In addition, the glycosylated csI.9-ECD protein was crystallized at 291 K. X-ray diffraction data were collected at 4.6 Å resolution from a single crystal. The crystal belonged to space group C222 or C2221, with unit-cell parameters a = 94.7, b = 191.5, c = 302.8 Å. These preliminary studies have laid the foundation for structural determination of the DORN1 and I.9 receptor proteins, which will lead to a better understanding of the perception and function of extracellular ATP in plants.

Biochemical, spectroscopic and X-ray structural analysis of deuterated multicopper oxidase CueO prepared from a new expression construct for neutron crystallography


Multicopper oxidases oxidize various phenolic and nonphenolic compounds by using molecular oxygen as an electron acceptor to produce water. A multicopper oxidase protein, CueO, from Escherichia coli is involved in copper homeostasis in the bacterial cell. Although X-ray crystallographic studies have been conducted, the reduction mechanism of oxygen and the proton-transfer pathway remain unclear owing to the difficulty in identifying H atoms from X-ray diffraction data alone. To elucidate the reaction mechanism using neutron crystallography, a preparation system for obtaining large, high-quality single crystals of deuterated CueO was developed. Tiny crystals were obtained from the deuterated CueO initially prepared from the original construct. The X-ray crystal structure of the deuterated CueO showed that the protein contained an incompletely truncated signal sequence at the N-terminus, which resulted in the heterogeneity of the protein sample for crystallization. Here, a new CueO expression system that had an HRV3C cleavage site just after the signal sequence was constructed. Deuterated CueO from the new construct was expressed in cells cultured in deuterated algae-extract medium and the signal sequence was completely eliminated by HRV3C protease. The deuteration level of the purified protein was estimated by MALDI-TOF mass spectrometry to be at least 83.2% compared with nondeuterated protein. Nondeuterated CueO crystallized in space group P21, with unit-cell parameters a = 49.51, b = 88.79, c = 53.95 Å, β = 94.24°, and deuterated CueO crystallized in space group P212121, with unit-cell parameters a = 49.91, b = 106.92, c = 262.89 Å. The crystallographic parameters for the crystals of the new construct were different from those previously reported for nondeuterated crystals. The nondeuterated and deuterated CueO from the new construct had similar UV–Vis spectra, enzymatic activities and overall structure and geometry of the ligands of the Cu atoms in the active site to those of previously reported CueO structures. These results indicate that the CueO protein prepared using the new construct is suitable for further neutron diffraction studies.

Expression and crystallographic studies of the Arabidopsis thaliana GDP-d-mannose pyrophosphorylase VTC1


GDP-d-mannose pyrophosphorylase catalyzes the production of GDP-d-mannose, an intermediate product in the plant ascorbic acid (AsA) biosynthetic pathway. This enzyme is a key regulatory target in AsA biosynthesis and is encoded by VITAMIN C DEFECTIVE 1 (VTC1) in the Arabidopsis thaliana genome. Here, recombinant VTC1 was expressed, purified and crystallized. Diffraction data were obtained from VTC1 crystals grown in the absence and presence of substrate using X-rays. The ligand-free VTC1 crystal diffracted X-rays to 3.3 Å resolution and belonged to space group R32, with unit-cell parameters a = b = 183.6, c = 368.5 Å, α = β = 90, γ = 120°; the crystal of VTC1 in the presence of substrate diffracted X-rays to 1.75 Å resolution and belonged to space group P21, with unit-cell parameters a = 70.8, b = 83.9, c = 74.5 Å, α = γ = 90.0, β = 114.9°.