Unveiling success determinants for AMB-assisted phase expansion of fusion proteins in ARP/wARP.

Fusion proteins (FPs) are frequently utilized as a biotechnological tool in the determination of macromolecular structures using X-ray methods. Here, we explore the use of different protein tags in various FP, to obtain initial phases by using them in a partial molecular replacement (MR) and constructing the remaining FP structure with ARP/wARP. Usually, the tag is removed prior to crystallization, however leaving the tag on may facilitate crystal formation, and structural determination by expanding phases from known to unknown segments of the complex. In this study, the Protein Data Bank was mined for an up-to-date list of FPs with the most used protein tags, Maltose Binding Protein (MBP), Green Fluorescent Protein (GFP), Thioredoxin (TRX), Glutathione transferase (GST) and the Small Ubiquitin-like Modifier Protein (SUMO). Partial MR using the protein tag, followed by automatic model building, was tested on a subset of 116 FP. The efficiency of this method was analyzed and factors that influence the coordinate construction of a substantial portions of the fused protein were identified. Using MBP, GFP, and SUMO as phase generators it was possible to build at least 75 % of the protein of interest in 36 of the 116 cases tested. Our results reveal that tag selection has a significant impact; tags with greater structural stability, such as GFP, increase the success rate. Further statistical analysis identifies that resolution, Wilson B factor, solvent percentage, completeness, multiplicity, protein tag percentage in the FP (considering amino acids), and the linker length play pivotal roles using our approach. In cases where a structural homologous is absent, this method merits inclusion in the toolkit of protein crystallographers.

The structure of (E)-biformene synthase provides insights into the biosynthesis of bacterial bicyclic labdane-related diterpenoids.

The labdane-related diterpenoids (LRDs) are a large group of natural products with a broad range of biological activities. They are synthesized through two consecutive reactions catalyzed by class II and I diterpene synthases (DTSs). The structural complexity of LRDs mainly depends on the catalytic activity of class I DTSs, which catalyze the formation of bicyclic to pentacyclic LRDs, using as a substrate the catalytic product of class II DTSs. To date, the structural and mechanistic details for the biosynthesis of bicyclic LRDs skeletons catalyzed by class I DTSs remain unclear. This work presents the first X-ray crystal structure of an (E)-biformene synthase, LrdC, from the soil bacterium Streptomyces sp. strain K155. LrdC was identified as a part of an LRD cluster of five genes and was found to be a class I DTS that catalyzes the Mg2+-dependent synthesis of bicyclic LRD (E)-biformene by the dephosphorylation and rearrangement of normal copalyl pyrophosphate (CPP). Structural analysis of LrdC coupled with docking studies suggests that Phe189 prevents cyclization beyond the bicyclic LRD product through a strong stabilization of the allylic carbocation intermediate, while Tyr317 functions as a general base catalyst to deprotonate the CPP substrate. Structural comparisons of LrdC with homology models of bacterial bicyclic LRD-forming enzymes (CldD, RmnD and SclSS), as well as with the crystallographic structure of bacterial tetracyclic LRD ent-kaurene synthase (BjKS), provide further structural insights into the biosynthesis of bacterial LRD natural products.

X-ray driven reduction of Cpd I of Catalase-3 from N. crassa reveals differential sensitivity of active sites and formation of ferrous state.

Catalases are biotechnologically relevant enzymes because of their applications in food technology, bioremediation, and biomedicine. The dismutation of hydrogen peroxide occurs in two steps; in the first one, the enzyme forms an oxidized compound I (Cpd I) and in the second one, the enzyme is reduced to the ferric state. In this research work, we analyzed the reduction of Cpd I by X-ray radiation damage during diffraction experiments in crystals of CAT-3, a Large-Size Subunit Catalase (LSC) from Neurospora crassa. A Multi-Crystal Data collection Strategy was applied in order to obtain the Cpd I structure at a resolution of 2.2 Å; this intermediate was highly sensitive to X-ray and was easily reduced at very low deposited radiation dose, causing breakage of the Fe=O bond. The comparison of the structures showed reduced intermediates and also evidenced the differential sensitivity per monomer. The resting ferric state was reduced to the ferrous state, an intermediate without a previous report in LSC. The chemically obtained Cpd I and the X-ray reduced intermediates were identified by UV-visible microspectrometry coupled to data collection. The differential sensitivity and the formation of a ferrous state are discussed, emphasizing the importance of the correct interpretation in the oxidation state of the iron heme.

X-ray-induced catalytic active-site reduction of a multicopper oxidase: structural insights into the proton-relay mechanism and O2-reduction states.

During X-ray data collection from a multicopper oxidase (MCO) crystal, electrons and protons are mainly released into the system by the radiolysis of water molecules, leading to the X-ray-induced reduction of O2 to 2H2O at the trinuclear copper cluster (TNC) of the enzyme. In this work, 12 crystallographic structures of Thermus thermophilus HB27 multicopper oxidase (Tth-MCO) in holo, apo and Hg-bound forms and with different X-ray absorbed doses have been determined. In holo Tth-MCO structures with four Cu atoms, the proton-donor residue Glu451 involved in O2 reduction was found in a double conformation: Glu451a (∼7 Šfrom the TNC) and Glu451b (∼4.5 Šfrom the TNC). A positive peak of electron density above 3.5σ in an Fo - Fc map for Glu451a O(ℇ2) indicates the presence of a carboxyl functional group at the side chain, while its significant absence in Glu451b strongly suggests a carboxylate functional group. In contrast, for apo Tth-MCO and in Hg-bound structures neither the positive peak nor double conformations were observed. Together, these observations provide the first structural evidence for a proton-relay mechanism in the MCO family and also support previous studies indicating that Asp106 does not provide protons for this mechanism. In addition, eight composite structures (Tth-MCO-C1-8) with different X-ray-absorbed doses allowed the observation of different O2-reduction states, and a total depletion of T2Cu at doses higher than 0.2 MGy showed the high susceptibility of this Cu atom to radiation damage, highlighting the importance of taking radiation effects into account in biochemical interpretations of an MCO structure.

Structure of the complex between teicoplanin and a bacterial cell-wall peptide: use of a carrier-protein approach.

Multidrug-resistant bacterial infections are commonly treated with glycopeptide antibiotics such as teicoplanin. This drug inhibits bacterial cell-wall biosynthesis by binding and sequestering a cell-wall precursor: a D-alanine-containing peptide. A carrier-protein strategy was used to crystallize the complex of teicoplanin and its target peptide by fusing the cell-wall peptide to either MBP or ubiquitin via native chemical ligation and subsequently crystallizing the protein-peptide-antibiotic complex. The 2.05 Å resolution MBP-peptide-teicoplanin structure shows that teicoplanin recognizes its ligand through a combination of five hydrogen bonds and multiple van der Waals interactions. Comparison of this teicoplanin structure with that of unliganded teicoplanin reveals a flexibility in the antibiotic peptide backbone that has significant implications for ligand recognition. Diffraction experiments revealed an X-ray-induced dechlorination of the sixth amino acid of the antibiotic; it is shown that teicoplanin is significantly more radiation-sensitive than other similar antibiotics and that ligand binding increases radiosensitivity. Insights derived from this new teicoplanin structure may contribute to the development of next-generation antibacterials designed to overcome bacterial resistance.

Conformational stability and crystal packing: polymorphism in Neurospora crassa CAT-3.

Polymorphism is frequently observed from different crystallization conditions. In proteins, the effect on conformational variability is poorly documented, with only a few reported examples. Here, three polymorphic crystal structures determined for a large-subunit catalase, CAT-3 from Neurospora crassa, are reported. Two of them belonged to new space groups, P1 and P43212, and a third structure belonged to the same space group, P212121, as the previously deposited 2.3 Å resolution structure (PDB entry 3ej6), but had a higher resolution (1.95 Å). Comparisons between these polymorphic structures highlight the conformational stability of tetrameric CAT-3 and reveal a distortion in the tetrameric structure that has not previously been described.

Phase Heterogeneity in Cholesterol-Containing Ternary Phospholipid Lamellar Phases.

Pseudo-ternary mixtures of lamellar phase phospholipids (DPPC and brain sphingomyelin with cholesterol) were studied below T m while comparing the influence of cholesterol content, temperature, and the presence of small quantities of vitamin D binding protein (DBP) or vitamin D receptor (VDR). The measurements, conducted by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), cover a range of cholesterol concentrations (20% mol. wt to 40% mol. wt.) and physiologically relevant temperature range (294-314 K). In addition to rich intraphase behavior, data and modeling are used to approximate the lipids' headgroup location variations under the abovementioned experimental conditions.

Thermostable multicopper oxidase from Thermus thermophilus HB27: crystallization and preliminary X-ray diffraction analysis of apo and holo forms.

A thermostable multicopper oxidase from Thermus thermophilus HB27 (Tth-MCO) was successfully crystallized using the sitting-drop and hanging-drop vapour-diffusion methods. Crystallization conditions and preliminary X-ray diffraction data to 1.5 Å resolution obtained using synchrotron radiation at 100 K are reported. The crystals belonged to space group C222(1), with unit-cell parameters a = 93.6, b = 110.3, c = 96.3 Å. A monomer in the asymmetric unit yielded a Matthews coefficient (V(M)) of 2.60 Å(3) Da(-1) and a solvent content of 53%. An inactive enzyme form, apo-Tth-MCO, was also crystallized and diffraction data were collected to 1.7 Å resolution. In addition, a second inactive form of the enzyme, Hg-Tth-MCO, was obtained by soaking apo-Tth-MCO crystals with mercury(II) chloride and data were collected to a resolution of 1.7 Å.

From screen to structure with a harvestable microfluidic device.

Advances in automation have facilitated the widespread adoption of high-throughput vapour-diffusion methods for initial crystallization screening. However, for many proteins, screening thousands of crystallization conditions fails to yield crystals of sufficient quality for structural characterization. Here, the rates of crystal identification for thaumatin, catalase and myoglobin using microfluidic Crystal Former devices and sitting-drop vapour-diffusion plates are compared. It is shown that the Crystal Former results in a greater number of identified initial crystallization conditions compared with vapour diffusion. Furthermore, crystals of thaumatin and lysozyme obtained in the Crystal Former were used directly for structure determination both in situ and upon harvesting and cryocooling. On the basis of these results, a crystallization strategy is proposed that uses multiple methods with distinct kinetic trajectories through the protein phase diagram to increase the output of crystallization pipelines.

Picometer-scale conformational heterogeneity separates functional from nonfunctional states of a photoreceptor protein.

Protein structural fluctuations occur over a wide spatial scale, ranging from minute, picometer-scale displacements, to large, interdomain motions and partial unfolding. While large-scale protein structural changes and their effects on protein function have been the focus of much recent attention, small-scale fluctuations have been less well studied, and are generally assumed to have proportionally smaller effects. Here we use the bacterial photoreceptor photoactive yellow protein (PYP) to test if subtle structural changes do, indeed, imply equally subtle functional effects. We flash froze crystals of PYP to trap the protein's conformational ensemble, and probed the molecules in this ensemble for their ability to facilitate PYP's biological function (i.e., light-driven isomerization of its chromophore). Our results indicate that the apparently homogeneous structural state observed in a 0.82 A crystal structure in fact comprises an ensemble of conformational states, in which subpopulations with nearly identical structures display dramatically different functional properties.

Artificial covalent linkage of bacterial acyl carrier proteins for fatty acid production.

Acyl carrier proteins (ACPs) are essential to the production of fatty acids. In some species of marine bacteria, ACPs are arranged into tandem repeats joined by peptide linkers, an arrangement that results in high fatty acid yields. By contrast, Escherichia coli, a relatively low producer of fatty acids, uses a single-domain ACP. In this work, we have engineered the native E. coli ACP into tandem di- and tri-domain constructs joined by a naturally occurring peptide linker from the PUFA synthase of Photobacterium profundum. The size of these tandem fused ACPs was determined by size exclusion chromatography to be higher (21 kDa, 36 kDa and 141 kDa) than expected based on the amino acid sequence (12 kDa, 24 kDa and 37 kDa, respectively) suggesting the formation of a flexible extended conformation. Structural studies using small-angle X-ray scattering (SAXS), confirmed this conformational flexibility. The thermal stability for the di- and tri-domain constructs was similar to that of the unfused ACP, indicating a lack of interaction between domains. Lastly, E. coli cultures harboring tandem ACPs produced up to 1.6 times more fatty acids than wild-type ACP, demonstrating the viability of ACP fusion as a method to enhance fatty acid yield in bacteria.

High resolution X-ray crystallographic structure of bovine heart cytochrome c and its application to the design of an electron transfer biosensor.

Cytochrome c is one of the most studied proteins probably due to its electron-transfer properties in aerobic and anaerobic respiration. Particularly, cytochrome c from bovine heart is a small protein, M(r) 12,230 Da, globular (hydrodynamic diameter of 3.4 nm), soluble in different buffer solutions, and commercially available. Despite being a quite well-studied protein and relatively easy to manipulate from the biochemical and electrochemical viewpoint, its 3D structure has never been published. In this work, the purification, crystallization and 3D structure of one of the cytochrome c isoforms is presented to 1.5 A resolution. It is also shown how the presence of isoforms made both the purification and crystallization steps difficult. Finally, a new approach for protein electrocrystallization and design of biosensors is presented.

The crystal structure of the ring-hydroxylating dioxygenase from Sphingomonas CHY-1.

The ring-hydroxylating dioxygenase (RHD) from Sphingomonas CHY-1 is remarkable due to its ability to initiate the oxidation of a wide range of polycyclic aromatic hydrocarbons (PAHs), including PAHs containing four- and five-fused rings, known pollutants for their toxic nature. Although the terminal oxygenase from CHY-1 exhibits limited sequence similarity with well characterized RHDs from the naphthalene dioxygenase family, the crystal structure determined to 1.85 A by molecular replacement revealed the enzyme to share the same global alpha(3)beta(3) structural pattern. The catalytic domain distinguishes itself from other bacterial non-heme Rieske iron oxygenases by a substantially larger hydrophobic substrate binding pocket, the largest ever reported for this type of enzyme. While residues in the proximal region close to the mononuclear iron atom are conserved, the central region of the catalytic pocket is shaped mainly by the side chains of three amino acids, Phe350, Phe404 and Leu356, which contribute to the rather uniform trapezoidal shape of the pocket. Two flexible loops, LI and LII, exposed to the solvent seem to control the substrate access to the catalytic pocket and control the pocket length. Compared with other naphthalene dioxygenases residues Leu223 and Leu226, on loop LI, are moved towards the solvent, thus elongating the catalytic pocket by at least 2 A. An 11 A long water channel extends from the interface between the alpha and beta subunits to the catalytic site. The comparison of these structures with other known oxygenases suggests that the broad substrate specificity presented by the CHY-1 oxygenase is primarily due to the large size and particular topology of its catalytic pocket and provided the basis for the study of its reaction mechanism.

Purification, crystallization and preliminary X-ray analysis of struthiocalcin 1 from ostrich (Struthio camelus) eggshell.

The purification, crystallization and preliminary X-ray analysis of struthiocalcin 1 (SCA-1), a protein obtained from the intramineral part of ostrich (Struthio camelus) eggshell, is reported.

Biochemical and structural characterization of a novel arginine kinase from the spider Polybetes pythagoricus.

Energy buffering systems are key for homeostasis during variations in energy supply. Spiders are the most important predators for insects and therefore key in terrestrial ecosystems. From biomedical interest, spiders are important for their venoms and as a source of potent allergens, such as arginine kinase (AK, EC 2.7.3.3). AK is an enzyme crucial for energy metabolism, keeping the pool of phosphagens in invertebrates, and also an allergen for humans. In this work, we studied AK from the Argentininan spider Polybetes pythagoricus (PpAK), from its complementary DNA to the crystal structure. The PpAK cDNA from muscle was cloned, and it is comprised of 1068 nucleotides that encode a 384-amino acids protein, similar to other invertebrate AKs. The apparent Michaelis-Menten kinetic constant (Km ) was 1.7 mM with a kcat of 75 s-1. Two crystal structures are presented, the apoPvAK and PpAK bound to arginine, both in the open conformation with the active site lid (residues 310-320) completely disordered. The guanidino group binding site in the apo structure appears to be organized to accept the arginine substrate. Finally, these results contribute to knowledge of mechanistic details of the function of arginine kinase.

Self-Assembly of 3D DNA Crystals Containing a Torsionally Stressed Component.

There is an increasing appreciation for structural diversity of DNA that is of interest to both DNA nanotechnology and basic biology. Here, we have explored how DNA responds to torsional stress by building on a previously reported two-turn DNA tensegrity triangle and demonstrating that we could introduce an extra nucleotide pair (np) into the original sequence without affecting assembly and crystallization. The extra np imposes a significant torsional stress, which is accommodated by global changes throughout the B-DNA duplex and the DNA lattice. The work reveals a near-atomic structure of naked DNA under a torsional stress of approximately 14%, and thus provides an example of DNA distortions that occur without a requirement for either an external energy source or the free energy available from protein or drug binding.

Insights into ligand binding to a glutathione S-transferase from mango: Structure, thermodynamics and kinetics.

We studied a mango glutathione S-transferase (GST) (Mangifera indica) bound to glutathione (GSH) and S-hexyl glutathione (GSX). This GST Tau class (MiGSTU) had a molecular mass of 25.5 kDa. MiGSTU Michaelis-Menten kinetic constants were determined for their substrates obtaining a Km, Vmax and kcat for CDNB of 0.792 mM, 80.58 mM min-1 and 68.49 s-1 respectively and 0.693 mM, 105.32 mM min-1 and 89.57 s-1, for reduced GSH respectively. MiGSTU had a micromolar affinity towards GSH (5.2 µM) or GSX (7.8 µM). The crystal structure of the MiGSTU in apo or bound to GSH or GSX generated a model that explains the thermodynamic signatures of binding and showed the importance of enthalpic-entropic compensation in ligand binding to Tau-class GST enzymes.

Small-molecule modulators of methyl-lysine binding for the CBX7 chromodomain.

Chromobox homolog 7 (CBX7) plays an important role in gene transcription in a wide array of cellular processes, ranging from stem cell self-renewal and differentiation to tumor progression. CBX7 functions through its N-terminal chromodomain (ChD), which recognizes trimethylated lysine 27 of histone 3 (H3K27me3), a conserved epigenetic mark that signifies gene transcriptional repression. In this study, we report the discovery of small molecules that inhibit CBX7ChD binding to H3K27me3. Our crystal structures reveal the binding modes of these molecules that compete against H3K27me3 binding through interactions with key residues in the methyl-lysine binding pocket of CBX7ChD. We further show that a lead compound, MS37452, derepresses transcription of Polycomb repressive complex target gene p16/CDKN2A by displacing CBX7 binding to the INK4A/ARF locus in prostate cancer cells. These small molecules have the potential to be developed into high-potency chemical modulators that target CBX7 functions in gene transcription in different disease pathways.

Crystallization and X-ray diffraction analysis of a putative bacterial class I labdane-related diterpene synthase.

Labdane-related diterpenoids are natural products with potential pharmaceutical applications that are rarely found in bacteria. Here, a putative class I labdane-related diterpene synthase (LrdC) identified by genome mining in a streptomycete was successfully crystallized using the microbatch method. Crystals of the LrdC enzyme were obtained in a holo form with its natural cofactor Mg(2+) (LrdC-Mg(2+)) and in complex with inorganic pyrophosphate (PPi) (LrdC-Mg(2+)-PPi). Crystals of native LrdC-Mg(2+) diffracted to 2.50 Šresolution and belonged to the trigonal space group P3221, with unit-cell parameters a = b = 107.1, c = 89.2 Å. Crystals of the LrdC-Mg(2+)-PPi complex grown in the same conditions as the native enzyme with PEG 8000 diffracted to 2.36 Šresolution and also belonged to the trigonal space group P3221. Crystals of the LrdC-Mg(2+)-PPi complex grown in a second crystallization condition with PEG 3350 diffracted to 2.57 Šresolution and belonged to the monoclinic space group P21, with unit-cell parameters a = 49.9, b = 104.1, c = 66.5 Å, ß = 111.4°. The structure was determined by the single-wavelength anomalous dispersion (SAD) technique using the osmium signal from a potassium hexachloroosmate (IV) derivative.

High resolution imaging as a characterization tool for biological crystals.

Biomolecular crystals consist of large unit cells that form a rather flexible medium that is able to accommodate a certain degree of lattice distortion, leading to several interesting issues ranging from structural to physical properties. Several techniques, from X-ray diffraction to microscopy, have been adapted to study the structural and physical properties of biomolecular crystals systematically. The use of synchrotron-based monochromatic X-ray diffraction topography, with triple axis diffractometry and rocking curve measurements, to characterize biomolecular crystals is reviewed. Recent X-ray diffraction images from gel and solution grown lysozyme crystals are presented. Defect structures in these crystals are discussed, together with reciprocal space mapping, and compared with results obtained from crystals grown in a low gravity environment.