Head-to-Tail Complex of Dishevelled and Axin-DIX Domains: Expression, Purification, Crystallographic Studies and Packing Analysis

Background: Head-to-tail polymerising domains generating heterogeneous aggregates are generally difficult to crystallise. DIX domains, exclusively found in the Wnt signalling pathway, are polymerising factors following this head-to-tail arrangement; moreover, they are considered to play a key role in the heterotypic interaction between Dishevelled (Dvl) and Axin, which are cytoplasmic proteins also positively and negatively regulating the canonical Wnt/β- catenin signalling pathway, but this interaction mechanism is still unknown. Objective: This study mainly aimed to clarify whether the Dvl2 and Axin-DIX domains (Dvl2-DIX and Axin-DIX, respectively) form a helical polymer in a head-to-tail way during complexation. Methods: Axin-DIX (DAX) and Dvl2-DIX (DIX), carrying polymerisation-blocking mutations, were expressed as a fusion protein by using a flexible peptide linker to fuse the C-terminal of the former to the N-terminal of the latter, enforcing a defined 1:1 stoichiometry between them. Results: The crystal of the DAX–DIX fusion protein diffracted to a resolution of about 0.3 nm and a data set was collected at a 0.309 nm resolution. The structure was solved via the molecular replacement method by using the DIX and DAX structures. A packing analysis of the crystal revealed the formation of a tandem heterodimer in a head-to-tail way, as predicted by the Wntsignalosome model. Conclusion: The results demonstrated that the combination of polymerisation-blocking mutations and a fusion protein of two head-to-tail polymerising domains is effective especially for crystallising complexes among heterologous polymerising proteins or domains.

Crystal structure of the aromatic-amino-acid aminotransferase fromStreptococcus mutans

Streptococcus mutans, a facultatively aerobic and Gram-positive bacterium, is the primary causative agent of dental caries and contributes to the multispecies biofilm known as dental plaque. In this study, the aromatic-amino-acid aminotransferase fromStreptococcus mutans(SmAroAT) was recombinantly expressed inEscherichia coli. An effective purification protocol was established. The recombinant protein was crystallized using the hanging-drop vapor-diffusion method with PEG 3350 as the primary precipitant. The crystal structure ofSmAroAT was solved at 2.2 Å resolution by the molecular-replacement method. Structural analysis indicated that the proteins of the aromatic-amino-acid aminotransferase family have conserved structural elements that might play a role in substrate binding. These results may help in obtaining a better understanding of the catabolism and biosynthesis of aromatic amino acids.

Crystal structure ofEscherichia colipurine nucleoside phosphorylase complexed with acyclovir

Escherichia colipurine nucleoside phosphorylase (PNP), which catalyzes the reversible phosphorolysis of purine ribonucleosides, belongs to the family I hexameric PNPs. Owing to their key role in the purine salvage pathway, PNPs are attractive targets for drug design against some pathogens. Acyclovir (ACV) is an acyclic derivative of the PNP substrate guanosine and is used as an antiviral drug for the treatment of some human viral infections. The crystalline complex ofE. coliPNP with acyclovir was prepared by co-crystallization in microgravity using counter-diffusion through a gel layer in a capillary. The structure of theE. coliPNP–ACV complex was solved at 2.32 Å resolution using the molecular-replacement method. The ACV molecule is observed in two conformations and sulfate ions were located in both the nucleoside-binding and phosphate-binding pockets of the enzyme. A comparison with the complexes of other hexameric and trimeric PNPs with ACV shows the similarity in acyclovir binding by these enzymes.

Periplasmic form of dipeptidyl aminopeptidase IV fromPseudoxanthomonas mexicanaWO24: purification, kinetic characterization, crystallization and X-ray crystallographic analysis

Dipeptidyl aminopeptidase IV (DAP IV or DPP IV) fromPseudoxanthomonas mexicanaWO24 (PmDAP IV) preferentially cleaves substrate peptides with Pro or Ala at the P1 position [NH2-P2-P1(Pro/Ala)-P1′-P2′…]. For crystallographic studies, the periplasmic form of PmDAP IV was overproduced inEscherichia coli, purified and crystallized in complex with the tripeptide Lys-Pro-Tyr using the hanging-drop vapour-diffusion method. Kinetic parameters of the purified enzyme against a synthetic substrate were also determined. X-ray diffraction data to 1.90 Å resolution were collected from a triclinic crystal form belonging to space groupP1, with unit-cell parametersa= 88.66,b= 104.49,c = 112.84 Å, α = 67.42, β = 68.83, γ = 65.46°. Initial phases were determined by the molecular-replacement method usingStenotrophomonas maltophiliaDPP IV (PDB entry 2ecf) as a template and refinement of the structure is in progress.

Crystal structure of the starch-binding domain of glucoamylase fromAspergillus niger

Glucoamylases are widely used commercially to produce glucose syrup from starch. The starch-binding domain (SBD) of glucoamylase fromAspergillus nigeris a small globular protein containing a disulfide bond. The structure ofA. nigerSBD has been determined by NMR, but the conformation surrounding the disulfide bond was unclear. Therefore, X-ray crystal structural analysis was used to attempt to clarify the conformation of this region. The SBD was purified from anEscherichia coli-based expression system and crystallized at 293 K. The initial phase was determined by the molecular-replacement method, and the asymmetric unit of the crystal contained four protomers, two of which were related by a noncrystallographic twofold axis. Finally, the structure was solved at 2.0 Å resolution. The SBD consisted of seven β-strands and eight loops, and the conformation surrounding the disulfide bond was determined from a clear electron-density map. Comparison of X-ray- and NMR-determined structures of the free SBD showed no significant difference in the conformation of each β-strand, but the conformations of the loops containing the disulfide bond and the L5 loop were different. In particular, the difference in the position of the Cαatom of Cys509 between the X-ray- and NMR-determined structures was 13.3 Å. In addition, theBfactors of the amino-acid residues surrounding the disulfide bond are higher than those of other residues. Therefore, the conformation surrounding the disulfide bond is suggested to be highly flexible.

Crystal structure of the YajQ-family protein XC_3703 fromXanthomonas campestrispv.campestris

As an important bacterial second messenger, bis-(3′,5′)-cyclic diguanylate (cyclic di-GMP or c-di-GMP) has been implicated in numerous biological activities, including biofilm formation, motility, survival and virulence. These processes are manipulated by the binding of c-di-GMP to its receptors. XC_3703 from the plant pathogenXanthomonas campestrispv.campestris, which belongs to the YajQ family of proteins, has recently been identified as a potential c-di-GMP receptor. XC_3703, together with XC_2801, functions as a transcription factor activating virulence-related genes, which can be reversed by the binding of c-di-GMP to XC_3703. However, the structural basis of how c-di-GMP regulates XC_3703 remains elusive. In this study, the structure of XC_3703 was determined to 2.1 Å resolution using the molecular-replacement method. The structure of XC_3703 consists of two domains adopting the same topology, which is similar to that of the RNA-recognition motif (RRM). Arg65, which is conserved among the c-di-GMP-binding subfamily of the YajQ family of proteins, together with Phe80 in domain II, forms a putative c-di-GMP binding site.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of AerF fromMicrocystis aeruginosa, a putative reductase participating in aeruginosin biosynthesis

The 2-carboxy-6-hydroxyoctahydroindole moiety is an essential residue for the antithrombotic activity of aeruginosins, which are a class of cyanobacteria-derived bioactive linear tetrapeptides. The biosynthetic pathway of the 2-carboxy-6-hydroxyoctahydroindole moiety has not yet been resolved. AerF was indicated to be involved in the biosynthesis of the 2-carboxy-6-hydroxyoctahydroindole moiety. This study reports the cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of AerF fromMicrocystis aeruginosawith a C-terminal His6tag. The crystal diffracted to a maximum resolution of 1.38 Å and belonged to the tetragonal space groupP4322, with unit-cell parametersa=b= 101.581,c= 116.094 Å. The calculated Matthews coefficient and solvent content of the crystal were 2.47 Å3 Da−1and 50.32%, respectively. The initial model of the structure was obtained by the molecular-replacement method and refinement of the structure is in progress.

Expression, purification, crystallization and preliminary X-ray crystallographic analysis of the histone-like HU protein fromSpiroplasma melliferumKC3

HU proteins belong to the nucleoid-associated proteins (NAPs) that are involved in vital processes such as DNA compaction and reparation, gene transcriptionetc.No data are available on the structures of HU proteins from mycoplasmas. To this end, the HU protein from the parasitic mycoplasmaSpiroplasma melliferumKC3 was cloned, overexpressed inEscherichia coliand purified to homogeneity. Prismatic crystals of the protein were obtained by the vapour-diffusion technique at 4°C. The crystals diffracted to 1.36 Å resolution (the best resolution ever obtained for a HU protein). The diffraction data were indexed in space groupC2 and the structure of the protein was solved by the molecular-replacement method with one monomer per asymmetric unit.

Cloning, expression, purification, crystallization and X-ray crystallographic analysis of PhaA fromRalstonia eutropha

Polyhydroxybutyrate (PHB) is a biopolymer that is in the spotlight because of its broad applications in bioplastics, fine chemicals, implant biomaterials and biofuels. PhaA fromRalstonia eutropha(RePhaA) is the first enzyme in the PHB biosynthetic pathway and catalyzes the condensation reaction of two acetyl-CoA molecules to give acetoacetyl-CoA.RePhaA was crystallized using the hanging-drop vapour-diffusion method in the presence of 20% polyethylene glycol monomethyl ether 2K, 0.1 MTris–HCl pH 8.5 and 0.2 MtrimethylamineN-oxide dihydrate at 295 K. X-ray diffraction data were collected to a maximum resolution of 1.96 Å on a synchrotron beamline. The crystal belonged to space groupP21, with unit-cell parametersa= 68.38,b= 105.47,c= 106.91 Å, α = γ = 90, β = 106.18°. With four subunits per asymmetric unit, the crystal volume per unit protein weight (VM) is 2.3 Å3 Da−1, which corresponds to a solvent content of approximately 46.2%. The structure was solved by the molecular-replacement method and refinement of the structure is in progress.

Purification, characterization and preliminary X-ray crystallographic studies of monodehydroascorbate reductase fromOryza sativaL.japonica

Monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) is a key enzyme in the reactive oxygen species (ROS) detoxification system of plants. The participation of MDHAR in ascorbate (AsA) recycling in the ascorbate–glutathione cycle is important in the acquired tolerance of crop plants to abiotic environmental stresses. Thus, MDHAR represents a strategic target protein for the improvement of crop yields. Although physiological studies have intensively characterized MDHAR, a structure-based functional analysis is not available. Here, a cytosolic MDHAR (OsMDHAR) derived fromOryza sativaL.japonicawas expressed usingEscherichia colistrain NiCo21 (DE3) and purified. The purified OsMDHAR showed specific enzyme activity (approximately 380 U per milligram of protein) and was crystallized using the hanging-drop vapour-diffusion method at pH 8.0 and 298 K. The crystal diffracted to 1.9 Å resolution and contained one molecule in the asymmetric unit (the Matthews coefficientVMis 1.98 Å3 Da−1, corresponding to a solvent content of 38.06%) in space groupP41212 with unit-cell parametersa=b= 81.89,c= 120.4 Å. The phase of the OsMDHAR structure was resolved by the molecular-replacement method using a ferredoxin reductase fromAcidovoraxsp. strain KKS102 (PDB entry 4h4q) as a model.