The structure of an uncomplexed form of α-amylase from B. halmapalus is compared with a form in which maltose, glucose and a nonasaccharide derived from acarbose and maltose are bound.
Recombinant Bacillus halmapalus α-amylase (BHA) was studied in two different crystal forms. The first crystal form was obtained by crystallization of BHA at room temperature in the presence of acarbose and maltose; data were collected at cryogenic temperature to a resolution of 1.9 Å. It was found that the crystal belonged to space group P212121, with unit-cell parameters a = 47.0, b = 73.5, c = 151.1 Å. A maltose molecule was observed and found to bind to BHA and previous reports of the binding of a nonasaccharide were confirmed. The second crystal form was obtained by pH-induced crystallization of BHA in a MES–HEPES–boric acid buffer (MHB buffer) at 303 K; the solubility of BHA in MHB has a retrograde temperature dependency and crystallization of BHA was only possible by raising the temperature to at least 298 K. Data were collected at cryogenic temperature to a resolution of 2.0 Å. The crystal belonged to space group P212121, with unit-cell parameters a = 38.6, b = 59.0, c = 209.8 Å. The structure was solved using molecular replacement. The maltose-binding site is described and the two structures are compared. No significant changes were seen in the structure upon binding of the substrates.
α-amylase; maltose-binding site; Bacillus halmapalus
When properly applied, pseudosymmetry can be used to improve crystallographic phases through averaging and to facilitate crystal structure determination.
Here, a case is presented of an unusual structure determination which was facilitated by the use of pseudosymmetry. Group A streptococcus uses cysteine protease Mac-1 (also known as IdeS) to evade the host immune system. Native Mac-1 was crystallized in the orthorhombic space group P21212. Surprisingly, crystals of the inactive C94A mutant of Mac-1 displayed monoclinic symmetry with space group P21, despite the use of native orthorhombic Mac-1 microcrystals for seeding. Attempts to solve the structure of the C94A mutant by MAD phasing in the monoclinic space group did not produce an interpretable map. The native Patterson map of the C94A mutant showed two strong peaks along the (1 0 1) diagonal, indicating possible translational pseudosymmetry in space group P21. Interestingly, one-third of the monoclinic reflections obeyed pseudo-orthorhombic P21212 symmetry similar to that of the wild-type crystals and could be indexed and processed in this space group. The pseudo-orthorhombic and monoclinic unit cells were related by the following vector operations: a
m = b
o − c
m = a
o and c
m = −2c
o − b
o. The pseudo-orthorhombic subset of data produced good SAD phases, leading to structure determination with one monomer in the asymmetric unit. Subsequently, the structure of the Mac-1 mutant in the monoclinic form was determined by molecular replacement, which showed six molecules forming three translationally related dimers aligned along the (1 0 1) diagonal. Knowing the geometric relationship between the pseudo-orthorhombic and the monoclinic unit cells, all six molecules can be generated in the monoclinic unit cell directly without the use of molecular replacement. The current case provides a successful example of the use of pseudosymmetry as a powerful phase-averaging method for structure determination by anomalous diffraction techniques. In particular, a structure can be solved in a higher pseudosymmetry subcell in which an NCS operator becomes a crystallographic operator. The geometrical relationships between the subcell and parental cell can be used to generate a complete molecular representation of the parental asymmetric unit for refinement.
pseudosymmetry; structure determination; cysteine proteases; Mac-1
The flavin-dependent enzyme FerB from P. denitrificans has been purified and both native and SeMet-substituted FerB have been crystallized. The two variants crystallized in two different crystallographic forms belonging to the monoclinic space group P21 and the orthorhombic space group P21212, respectively. X-ray diffraction data were collected to 1.75 Å resolution for both forms.
The flavin-dependent enzyme FerB from Paracoccus denitrificans reduces a broad range of compounds, including ferric complexes, chromate and most notably quinones, at the expense of the reduced nicotinamide adenine dinucleotide cofactors NADH or NADPH. Recombinant unmodified and SeMet-substituted FerB were crystallized under similar conditions by the hanging-drop vapour-diffusion method with microseeding using PEG 4000 as the precipitant. FerB crystallized in several different crystal forms, some of which diffracted to approximately 1.8 Å resolution. The crystals of native FerB belonged to space group P21, with unit-cell parameters a = 61.6, b = 110.1, c = 65.2 Å, β = 118.2° and four protein molecules in the asymmetric unit, whilst the SeMet-substituted form crystallized in space group P21212, with unit-cell parameters a = 61.2, b = 89.2, c = 71.5 Å and two protein molecules in the asymmetric unit. Structure determination by the three-wavelength MAD/MRSAD method is now in progress.
flavoenzymes; quinone reductases; Paracoccus denitrificans
Insulin is a therapeutic protein that is widely used for the treatment of diabetes. Its biological function was discovered more than 80 years ago and it has since then been characterized extensively. Crystallization of the insulin molecule has always been a key activity since the protein is often administered by subcutaneous injections of crystalline insulin formulations. Over the years, insulin has been crystallized and characterized in a number of crystal systems.
Interestingly, we have now discovered two new crystal forms of human insulin. The crystals were obtained when the two chaotropic agents, urea and thiocyanate were present in the crystallization experiments, and their structures were determined by X-ray crystallography. The crystals belong to the orthorhombic and monoclinic crystal systems, with space groups C2221 and C2 respectively. The orthorhombic crystals were obtained at pH 6.5 and contained three insulin hexamers in R6 conformation in the asymmetric unit whilst the monoclinic C2 crystals were obtained at pH 7.0 and contained one R6 hexamer in the asymmetric unit. Common for the two new crystals is a hexamer-hexamer interaction that has not been found in any of the previous crystal forms of insulin. The contacts involve a tight glutamate-glutamate interaction with a distance of 2.3 Å between groups. The short distance suggests a low barrier hydrogen bond. In addition, two tyrosine-tyrosine interactions occupying a known phenol binding pocket contribute to the stabilization of the contacts. Within the crystals, distinct binding sites for urea were found, adding further to the discussion on the role of urea in protein denaturation.
The change in space group from C2221 to C2 was primarily caused by an increase in pH. The fewer number of hexamer-hexamer interactions comprising the short hydrogen bond in the C2 space group suggest that pH is the driving force. In addition, the distance between the two glutamates increases from 2.32 Å in the C2221 crystals to 2.4 Å in the C2 crystals. However, in both cases the low barrier hydrogen bond and the tyrosine-tyrosine interaction should contribute to the stability of the crystals which is crucial when used in pharmaceutical formulations.
Crystals of the human Plk1 Polo-box domain in complex with a Cdc25C target peptide in an unphosphorylated and a phosphorylated state have been obtained in orthorhombic and monoclinic forms that diffract to 2.1 and 2.85 Å, respectively, using synchrotron radiation.
Polo-like kinase (Plk1) is crucial for cell-cycle progression via mitosis. Members of the Polo-like kinase family are characterized by the presence of a C-terminal domain termed the Polo-box domain (PBD) in addition to the N-terminal kinase domain. The PBD of Plk1 was cloned and overexpressed in Escherichia coli. Crystallization experiments of the protein in complex with an unphosphorylated and a phosphorylated target peptide from Cdc25C yield crystals suitable for X-ray diffraction analysis. Crystals of the PBD in complex with the phosphorylated peptide belong to the orthorhombic space group P212121, with unit-cell parameters a = 38.23, b = 67.35, c = 88.25 Å, α = γ = β = 90°, and contain one molecule per asymmetric unit. Crystals of the PBD in complex with the unphosphorylated peptide belong to the monoclinic space group P21, with unit-cell parameters a = 40.18, b = 49.17, c = 56.23 Å, α = γ = 90, β = 109.48°, and contain one molecule per asymmetric unit. The crystals diffracted to resolution limits of 2.1 and 2.85 Å using synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) and the Swiss Light Source (SLS), respectively.
Polo-like kinase; Polo-box domain; Cdc25C
The PH domain and ORD of the oxysterol-binding protein Osh3 from S. cerevisae were crystallized and X-ray diffraction data were collected.
Oxysterol-binding protein (OSBP) related proteins (ORPs) are conserved from yeast to humans and are implicated in regulation of sterol homeostasis and in signal transduction pathways. Osh3 of Saccharomyces cerevisiae is a pleckstrin-homology (PH) domain-containing ORP member that regulates phosphoinositide metabolism at endoplasmic reticulum–plasma membrane contact sites. The N-terminal PH domain of Osh3 was purified and crystallized as a lysozyme fusion and the resulting crystal diffracted to 2.3 Å resolution. The crystal belonged to the monoclinic space group C2, with unit-cell parameters a = 98.03, b = 91.31, c = 84.13 Å, β = 81.41°. With two molecules in the asymmetric unit, the Matthews coefficient was 3.13 Å3 Da−1. Initial attempts to solve the structure by molecular-replacement techniques using T4 lysozyme as a search model were successful. The C-terminal OSBP-related domain (OBD) of Osh3 was crystallized by the vapour-diffusion method and the resulting crystal diffracted to 1.5 Å resolution. The crystal was orthorhombic, belonging to space group P212121, with unit-cell parameters a = 41.57, b = 87.52, c = 100.58 Å. With one molecule in the asymmetric unit, the Matthews coefficient was 2.01 Å3 Da−1. Initial attempts to solve the structure by the single-wavelength anomalous dispersion technique using bromine were successful.
oxysterol-binding protein; Osh3; Saccharomyces cerevisiae
Aminoglycoside-2′′-phosphotransferase-IVa [APH(2′′)-IVa] is an enzyme that is responsible for high-level gentamicin resistance in E. casseliflavus isolates. Three different crystals of wild-type substrate-free APH(2′′)-IVa have been prepared and preliminary X-ray diffraction experiments have been undertaken on all three crystal forms.
The deactivation of aminoglycoside antibiotics by chemical modification is one of the major sources of bacterial resistance to this family of therapeutic compounds, which includes the clinically relevant drugs streptomycin, kanamycin and gentamicin. The aminoglycoside phosphotransferases (APHs) form one such family of enzymes responsible for this resistance. The gene encoding one of these enzymes, aminoglycoside-2′′-phosphotransferase-IVa [APH(2′′)-IVa] from Enterococcus casseliflavus, has been cloned and the protein (comprising 306 amino-acid residues) has been expressed in Escherichia coli and purified. The enzyme was crystallized in three substrate-free forms. Two of the crystal forms belonged to the orthorhombic space group P212121 with similar unit-cell parameters, although one of the crystal forms had a unit-cell volume that was approximately 13% smaller than the other and a very low solvent content of around 38%. The third crystal form belonged to the monoclinic space group P21 and preliminary X-ray diffraction analysis was consistent with the presence of two molecules in the asymmetric unit. The orthorhombic crystal forms of apo APH(2′′)-IVa both diffracted to 2.2 Å resolution and the monoclinic crystal form diffracted to 2.4 Å resolution; synchrotron diffraction data were collected from these crystals at SSRL (Stanford, California, USA). Structure determination by molecular replacement using the structure of the related enzyme APH(2′′)-IIa is proceeding.
aminoglycoside-2′′-phosphotransferase-IVa; Enterococcus casseliflavus; antibiotic resistance
A single-point mutant (T109S) of E. coli dihydroorotase initially crystallizes so that the two monomers of the dimer are related by a crystallographic twofold axis. In the presence of substrate, conversion to the previously observed asymmetric dimer with substrate bound in one subunit and product in the other is observed.
Crystals of a single-point mutant (T109S) of Escherichia coli dihydroorotase (DHOase) with diminished activity grown in the presence of l-dihydroorotate (l-DHO) are tetragonal, with a monomer in the asymmetric unit. These crystals are extremely unstable and disintegrate shortly after formation, which is followed by the growth of orthorhombic crystals from the remnants of the tetragonal crystals or at new nucleation sites. Orthorhombic crystals, for which a structure has previously been reported [Thoden et al. (2001 ▶), Biochemistry, 40, 6989–6997; Lee et al. (2005 ▶), J. Mol. Biol.
348, 523–533], contain a dimer of DHOase in the asymmetric unit; the active site of one monomer contains the substrate N-carbamyl-l-aspartate (l-CA-asp) and the active site of the other monomer contains the product of the reaction, l-DHO. In the subunit with l-DHO in the active site, a surface loop (residues 105–115) is ‘open’. In the other subunit, with l-CA-asp in the active site, the loop folds inwards, forming specific hydrogen bonds from the loop to the l-CA-asp. The tetragonal crystal form can be stabilized by crystallization in the presence of the inhibitor 5-fluoroorotate (FOA), a product (l-DHO) mimic. Crystals of the complex of T109S DHOase with FOA are tetragonal, space group P41212, with unit-cell parameters a = b = 72.6, c = 176.1 Å. The structure has been refined to R and R
free values of 0.218 and 0.257, despite severe anisotropy of the diffraction. In this structure, the flexible loops are both in the ‘open’ conformation, which is consistent with FOA, like l-DHO, binding at both sites. The behaviour of the T109S mutant crystals of DHOase in the presence of l-DHO is explained by initial binding of l-DHO to both subunits, followed by slow conversion to l-CA-asp, with consequent movement of the flexible loop and dissolution of the crystals. Orthorhombic crystals are then able to grow in the presence of l-DHO and l-CA-asp.
dihydroorotase; conformational change; loop movement; catalytic state; crystal contacts; crystal instability
Crystals of DDX3 RNA helicase domain have been obtained in a monoclinic form that diffract to 2.2 Å resolution using synchrotron radiation at the ID14-1 ESRF beamline.
DDX3 is a human RNA helicase that is involved in RNA processing and important human diseases. This enzyme belongs to the DEAD-box protein family, the members of which are characterized by the presence of nine conserved motifs including the Asp-Glu-Ala-Asp motif that defines the family. DDX3 has two distinct domains: an ATP-binding domain in the central region of the protein and a helicase domain in the carboxy-terminal region. The helicase domain of DDX3 was cloned and overexpressed in Escherichia coli. Crystallization experiments yielded crystals that were suitable for X-ray diffraction analysis. The final crystallization conditions were a reservoir solution consisting of 2 M ammonium sulfate, 0.1 M imidazole pH 6.4 plus 5 mM spermine tetrahydrochloride and a protein solution containing 10 mM HEPES, 500 mM ammonium sulfate pH 8.0. The crystals of the helicase domain belong to the monoclinic space group P21, with unit-cell parameters a = 43.85, b = 60.72, c = 88.39 Å, α = γ = 90, β = 101.02°, and contained three molecules per asymmetric unit. These crystals diffracted to a resolution limit of 2.2 Å using synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) and the Swiss Light Source (SLS).
DDX3; RNA helicases
The crystallogenesis of bacteriophage P22 tail-fiber gp26 is described. To study possible pH-induced conformational changes in gp26 structure, native trimeric gp26 has been crystallized at acidic pH (4.6) and a chimera of gp26 fused to maltose-binding protein (MBP-gp26) has been crystallized at neutral and alkaline pH (7-10).
Gp26 is one of three phage P22-encoded tail accessory factors essential for stabilization of viral DNA within the mature capsid. In solution, gp26 exists as an extended triple-stranded coiled-coil protein which shares profound structural similarities with class I viral membrane-fusion protein. In the cryo-EM reconstruction of P22 tail extracted from mature virions, gp26 forms an ∼220 Å extended needle structure emanating from the neck of the tail, which is likely to be brought into contact with the cell’s outer membrane when the viral DNA-injection process is initiated. To shed light on the potential role of gp26 in cell-wall penetration and DNA injection, gp26 has been crystallized at acidic, neutral and alkaline pH. Crystals of native gp26 grown at pH 4.6 diffract X-rays to 2.0 Å resolution and belong to space group P21, with a dimer of trimeric gp26 molecules in the asymmetric unit. To study potential pH-induced conformational changes in the gp26 structure, a chimera of gp26 fused to maltose-binding protein (MBP-gp26) was generated. Hexagonal crystals of MBP-gp26 were obtained at neutral and alkaline pH using the high-throughput crystallization robot at the Hauptman–Woodward Medical Research Institute, Buffalo, NY, USA. These crystals diffract X-rays to beyond 2.0 Å resolution. Structural analysis of gp26 crystallized at acidic, neutral and alkaline pH is in progress.
bacteriophage P22; membrane-fusion proteins; coiled coil; cryo-annealing
Recombinant nucleoside triphosphate hydrolases from N. caninum and T. gondii have been purified and crystallized for X-ray structure analysis.
The nucleoside triphosphate hydrolases that are produced by Neospora caninum (NcNTPase) and Toxoplasma gondii (TgNTPase-I) have a different physiological function from the ubiquitous ecto-ATPases. The recombinant enzymes were crystallized at 293 K using polyethylene glycol 3350 as a precipitant and X-ray diffraction data sets were collected for NcNTPase (to 2.8 Å resolution) and TgNTPase-I (to 3.1 Å resolution) at 100 K using synchrotron radiation. The crystals of NcNTPase and TgNTPase-I belonged to the orthorhombic space group I222 (unit-cell parameters a = 93.6, b = 140.8, c = 301.1 Å) and the monoclinic space group P21 (unit-cell parameters a = 87.1, b = 123.5, c = 120.2 Å, β = 96.6°), respectively, with two NcNTPase (V
M = 3.7 Å3 Da−1) and four TgNTPase-I (V
M = 2.7 Å3 Da−1) molecules per asymmetric unit. SAD phasing trials using a data set (λ = 0.97904 Å) collected from a crystal of selenomethionylated NcNTPase gave an initial electron-density map of sufficient quality to build a molecular model of NcNTPase.
nucleoside triphosphate hydrolases; Neospora caninum; Toxoplasma gondii
A mutated version of InsP5 2-K allows the production of crystals of the apo form and structure determination using X-ray crystallography.
Inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IP5 2-K) is a key enzyme that catalyzes the synthesis of phytic acid (IP6) from inositol 1,3,4,5,6-pentakisphosphate (IP5) and ATP. The first structure of IP5 2-K, that from Arabidopsis thaliana, has been solved previously; it only crystallized in the presence of inositol, either the substrate IP5 or the product IP6, and failed to crystallize in its free state (without inositol). Based on structural analysis, a point mutation of IP5 2-K (W129A) has been produced in order to overcome this limitation and obtain information about protein conformational changes upon substrate binding. Here, the production and crystallization of W129A IP5 2-K in its free state and with bound nucleotide is described. These crystals differed from the native crystals and belonged to the orthorhombic space group P21212, with unit-cell parameters a = 66.00, b = 68.23, c = 105.80 Å and a = 63.06, b = 71.80, c = 100.23 Å, respectively. The crystals diffracted to resolutions of 2.22 Å (apo) and 2.05 Å (nucleotide bound) using synchrotron radiation and contained one molecule per asymmetric unit. The structures have been determined using the molecular-replacement method and refinement is being undertaken.
inositol kinases; inositol phosphate; phytic acid; IP6; IP5 2-K
The crystallization of rice α-amylase/subtilisin bifunctional inhibitor is reported.
Rice bifunctional α-amylase/subtilisin inhibitor (RASI) can inhibit both α-amylase from larvae of the red flour beetle (Tribolium castaneum) and subtilisin from Bacillus subtilis. The synthesis of RASI is up-regulated during the late milky stage in developing seeds. The 8.9 kDa molecular-weight RASI from rice has been crystallized using the hanging-drop vapour-diffusion method. According to 1.81 Å resolution X-ray diffraction data from rice RASI crystals, the crystal belongs to space group P21212, with unit-cell parameters a = 79.99, b = 62.95, c = 66.70 Å. Preliminary analysis indicates two RASI molecules in an asymmetric unit with a solvent content of 44%.
α-amylase/subtilisin inhibitor; rice
A truncated variant of the human RuvBL1–RuvBL2 complex was cloned, expressed, purified and crystallised. Synchrotron diffraction data to 4 Å resolution were used to carry out a preliminary crystallographic analysis of the complex.
The complex of RuvBL1 and its homologue RuvBL2, two evolutionarily highly conserved eukaryotic proteins belonging to the AAA+ (ATPase associated with diverse cellular activities) family of ATPases, was co-expressed in Escherichia coli. For crystallization purposes, the flexible domains II of RuvBL1 and RuvBL2 were truncated. The truncated RuvBL1–RuvBL2 complex was crystallized using the hanging-drop vapour-diffusion method at 293 K. The crystals were hexagonal-shaped plates and belonged to either the orthorhombic space group C2221, with unit-cell parameters a = 111.4, b = 188.0, c = 243.4 Å and six monomers in the asymmetric unit, or the monoclinic space group P21, with unit-cell parameters a = 109.2, b = 243.4, c = 109.3 Å, β = 118.7° and 12 monomers in the asymmetric unit. The crystal structure could be solved by molecular replacement in both possible space groups and the solutions obtained showed that the complex forms a dodecamer.
RuvBL1; RuvBL2; ATPases
Isomaltooligosaccharides (IMO) have been suggested as promising prebiotics that stimulate the growth of probiotic bacteria. Genomes of probiotic lactobacilli from the acidophilus group, as represented by Lactobacillus acidophilus NCFM, encode α-1,6 glucosidases of the family GH13_31 (glycoside hydrolase family 13 subfamily 31) that confer degradation of IMO. These genes reside frequently within maltooligosaccharide utilization operons, which include an ATP-binding cassette transporter and α-glucan active enzymes, e.g., maltogenic amylases and maltose phosphorylases, and they also occur separated from any carbohydrate transport or catabolism genes on the genomes of some acidophilus complex members, as in L. acidophilus NCFM. Besides the isolated locus encoding a GH13_31 enzyme, the ABC transporter and another GH13 in the maltooligosaccharide operon were induced in response to IMO or maltotetraose, as determined by reverse transcription-PCR (RT-PCR) transcriptional analysis, suggesting coregulation of α-1,6- and α-1,4-glucooligosaccharide utilization loci in L. acidophilus NCFM. The L. acidophilus NCFM GH13_31 (LaGH13_31) was produced recombinantly and shown to be a glucan 1,6-α-glucosidase active on IMO and dextran and product-inhibited by glucose. The catalytic efficiency of LaGH13_31 on dextran and the dextran/panose (trisaccharide) efficiency ratio were the highest reported for this class of enzymes, suggesting higher affinity at distal substrate binding sites. The crystal structure of LaGH13_31 was determined to a resolution of 2.05 Å and revealed additional substrate contacts at the +2 subsite in LaGH13_31 compared to the GH13_31 from Streptococcus mutans (SmGH13_31), providing a possible structural rationale to the relatively high affinity for dextran. A comprehensive phylogenetic and activity motif analysis mapped IMO utilization enzymes from gut microbiota to rationalize preferential utilization of IMO by gut residents.
Crystals of the N-terminal domain of Gram-negative bacteria-binding protein 3 of D. melanogaster grown from PEG solutions are monoclinic (space group C2) and diffract to 1.7 Å resolution.
Gram-negative bacteria-binding protein 3 (GNBP3) is a pattern-recognition receptor which contributes to the defensive response against fungal infection in Drosophila. The protein consists of an N-terminal domain, which is considered to recognize β-glucans from the fungal cell wall, and a C-terminal domain, which is homologous to bacterial glucanases but devoid of activity. The N-terminal domain of GNBP3 (GNBP3-Nter) was successfully purified after expression in Drosophila S2 cells. Diffraction-quality crystals were produced by the hanging-drop vapour-diffusion method using PEG 2000 and PEG 8000 as precipitants. Preliminary X-ray diffraction analysis revealed that the GNBP3-Nter crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 134.79, b = 30.55, c = 51.73 Å, β = 107.4°, and diffracted to 1.7 Å using synchrotron radiation. The asymmetric unit is expected to contain two copies of GNBP3-Nter. Heavy-atom derivative data were collected and a samarium derivative showed one high-occupancy site per molecule.
GNBP3; N-terminal domain; Drosophila melanogaster; pattern-recognition receptors
The structure of a triclinic crystal form of 4-diphosphocytidyl-2C-methyl-d-erythritol kinase has been determined. Comparisons with a previously reported monoclinic crystal form raise questions about our knowledge of the quaternary structure of this enzyme.
4-Diphosphocytidyl-2C-methyl-d-erythritol kinase (IspE; EC 18.104.22.168) contributes to the 1-deoxy-d-xylulose 5-phosphate or mevalonate-independent biosynthetic pathway that produces the isomers isopentenyl diphosphate and dimethylallyl diphosphate. These five-carbon compounds are the fundamental building blocks for the biosynthesis of isoprenoids. The mevalonate-independent pathway does not occur in humans, but is present and has been shown to be essential in many dangerous pathogens, i.e. Plasmodium species, which cause malaria, and Gram-negative bacteria. Thus, the enzymes involved in this pathway have attracted attention as potential drug targets. IspE produces 4-diphosphosphocytidyl-2C-methyl-d-erythritol 2-phosphate by ATP-dependent phosphorylation of 4-diphosphocytidyl-2C-methyl-d-erythritol. A triclinic crystal structure of the Escherichia coli IspE–ADP complex with two molecules in the asymmetric unit was determined at 2 Å resolution and compared with a monoclinic crystal form of a ternary complex of E. coli IspE also with two molecules in the asymmetric unit. The molecular packing is different in the two forms. In the asymmetric unit of the triclinic crystal form the substrate-binding sites of IspE are occluded by structural elements of the partner, suggesting that the ‘triclinic dimer’ is an artefact of the crystal lattice. The surface area of interaction in the triclinic form is almost double that observed in the monoclinic form, implying that the dimeric assembly in the monoclinic form may also be an artifact of crystallization.
mevalonate-independent pathway; isoprenoid biosynthesis; kinases
A trypsin-resistant catalytic domain of human calcineurin α (A subunit, residues 20–347) was crystallized in space group P21212. An X-ray diffraction data set was collected to 2.87 Å resolution and the structure was solved by molecular replacement.
Calcineurin, a Ca2+/calmodulin-dependent serine/threonine protein phosphatase, plays a key role in a number of cellular pathways, including T-cell activation, and is an important molecular target of the immunosuppressive drugs cyclosporin A and FK506. To understand the structural basis underlying the activation of calcineurin by calmodulin, X-ray crystallography was employed to solve the three-dimensional structure of the free calcineurin catalytic domain (residues 20–347 of the A subunit). To accomplish this, a bacterially expressed glutathione S-transferase (GST) fusion protein of the human calcineurin catalytic domain was first purified by GST-affinity chromatography. After limited digestion by trypsin, the catalytic domain (Cncat) was purified using anion-exchange and size-exclusion chromatography. Crystallization of Cncat was achieved by the hanging-drop vapour-diffusion method at pH 6.5 using PEG 6000 as precipitant. The diffraction results showed that the Cncat crystal belonged to the orthorhombic space group P21212, with unit-cell parameters a = 161.6, b = 87.4, c = 112.0 Å. There are four Cncat molecules in the asymmetric unit, with 49.5% solvent content. An X-ray diffraction data set was collected to 2.87 Å resolution and a clear molecular-replacement solution was obtained. The active site of Cncat is open to the solvent channels in the crystal packing.
human calcineurin; catalytic domain
Crystallization of the prefoldin β subunit Yke2 is reported. This protein is a novel and potentially important target for anti-cancer therapeutics.
The Gim complex (GimC) from Saccharomyces cerevisiae is a heterohexameric protein complex, also known as prefoldin (PFD), which binds and stabilizes unfolded target polypeptides and subsequently delivers them to chaperonins for completion of folding. In this study, the crystallization and preliminary X-ray analysis of one of the β subunits of the Gim complex (Yke2) from S. cerevisiae are described. The purified protein was crystallized by the vapour-diffusion method, producing two types of crystals that belonged to the orthorhombic space group C222 or the primitive monoclinic space group P21. The unit-cell parameters for the C-centred orthorhombic crystal were a = 48.2, b = 168.86, c = 131.81 Å and the unit-cell parameters for the primitive monoclinic crystal were a = 47.83, b = 134.90, c = 81.50 Å, β = 100.71°. The Yke2 crystals diffracted to 4.2 and 3.1 Å resolution, respectively, on a rotating-anode generator under cryoconditions. This is the first report concerning the crystallization of a β subunit of a eukaryotic prefoldin.
prefoldin; β subunit; Gim complex; Yke2
The regulatory domain of M. tuberculosis aspartokinase, the enzyme which catalyses the first reaction step in the biosynthesis of the amino acids lysine, methionine and threonine, has been cloned, expressed, purified and crystallized. Preliminary X-ray diffraction analysis of several crystals revealed the presence of five distinct crystal forms.
The regulatory domain of Mycobacterium tuberculosis aspartokinase (Mtb-AK, Mtb-Ask, Rv3709c) has been cloned, heterologously expressed in Escherichia coli and purified using standard chromatographic techniques. Screening for initial crystallization conditions using the regulatory domain (AK-β) in the presence of the potential feedback inhibitor threonine identified four conditions which yielded crystals suitable for X-ray diffraction analysis. From these four conditions five different crystal forms of Mtb-AK-β resulted, three of which belonged to the orthorhombic system, one to the tetragonal system and one to the monoclinic system. The highest resolution (1.6 Å) was observed for a crystal form belonging to space group P212121, with unit-cell parameters a = 53.70, b = 63.43, c = 108.85 Å and two molecules per asymmetric unit.
aspartokinase; Rv3709c; Mycobacterium tuberculosis; tuberculosis
In order to clarify the structural basis of the pathogenesis-related-1 domain, Na-ASP-1, the first multi-domain ASP from the human hookworm parasite N. americanus, has been crystallized. 2.2 Å resolution data have been collected from a crystal belonging to the monoclinic space group P21.
Human hookworm infection is a major cause of anemia and malnutrition in the developing world. In an effort to control hookworm infection, the Human Hookworm Vaccine Initiative has identified candidate vaccine antigens from the infective larval stage (L3) of the parasite, including a family of pathogenesis-related-1 (PR-1) proteins known as the ancylostoma-secreted proteins (ASPs). The functions of the ASPs are unknown. In addition, it is unclear why some ASPs have one while others have multiple PR-1 domains. There are no known structures of a multi-domain ASP and in an effort to remedy this situation, recombinant Na-ASP-1 has been expressed, purified and crystallized. Na-ASP-1 is a 406-amino-acid multi-domain ASP from the prevalent human hookworm parasite Necator americanus. Useful X-ray data to 2.2 Å have been collected from a crystal that belongs to the monoclinic space group P21 with unit-cell parameters a = 67.7, b = 74.27, c = 84.60 Å, β = 112.12°. An initial molecular-replacement solution has been obtained with one monomer in the asymmetric unit.
pathogenesis-related proteins; hookworm; ASP; Necator americanus; ancylostoma; vaccines
Crystallization of the α-glucosidase MalA from S. solfataricus belonging to glycoside hydrolase family 31.
MalA is an α-glucosidase from the hyperthermophilic archaeon Sulfolobus solfataricus. It belongs to glycoside hydrolase family 31, which includes several medically interesting α-glucosidases. MalA and its selenomethionine derivative have been overproduced in Escherichia coli and crystallized in four different crystal forms. Microseeding was essential for the formation of good-quality crystals of forms 2 and 4. For three of the crystal forms (2, 3 and 4) full data sets could be collected. The most suitable crystals for structure determination are the monoclinic form 4 crystals, belonging to space group P21, from which data sets extending to 2.5 Å resolution have been collected. Self-rotation functions calculated for this form and for the orthorhombic (P212121) form 2 indicate the presence of six molecules in the asymmetric unit related by 32 symmetry.
α-glucosidases; glycoside hydrolase family 31; carbohydrate metabolism; protein expression
The crystallization of E. coli maltoporin in a new crystal form that diffracts to high resolution is reported.
Maltoporin is an outer-membrane protein that forms a β-barrel composed of three monomers and ensures the transport of maltose and maltodextrin in Gram-negative bacteria. Previously, the crystallization of Escherichia coli or Salmonella typhimurium maltoporin has been achieved in the presence of a mixture of the detergents β-decylmaltoside and dodecyl nonaoxyethylene. These crystals all belonged to the orthorhombic space group C2221 and gave rise to several structures of maltoporin in complex with different carbohydrates determined at resolutions between 3.2 and 2.4 Å. Here, the crystallization of E. coli maltoporin in a new crystal form is reported; the crystals belonged to the trigonal R3 space group and diffracted to 1.9 Å resolution. These crystals were obtained using n-dodecyl-β-d-maltoside as a detergent. Crystals with a lens or pyramidal morphology could be obtained using sitting or hanging drops, respectively, and despite their very different shapes they presented the same space group and very similar unit-cell parameters.
outer-membrane proteins; maltoporin; carbohydrates
The bicupin YwfC from B. subtilis was crystallized in two crystal forms and diffraction data were collected to 2.2 Å resolution.
A central tenet of evolutionary biology is that proteins with diverse biochemical functions evolved from a single ancestral protein. A variation on this theme is that the functional repertoire of proteins in a living organism is enhanced by the evolution of single-chain multidomain polypeptides by gene-fusion or gene-duplication events. Proteins with a double-stranded β-helix (cupin) scaffold perform a diverse range of functions. Bicupins are proteins with two cupin domains. There are four bicupins in Bacillus subtilis, encoded by the genes yvrK, yoaN, yxaG and ywfC. The extensive phylogenetic information on these four proteins makes them a good model system to study the evolution of function. The proteins YvrK and YoaN are oxalate decarboxylases, whereas YxaG is a quercetin dioxygenase. In an effort to aid the functional annotation of YwfC as well as to obtain a complete structure–function data set of bicupins, it was proposed to determine the crystal structure of YwfC. The bicupin YwfC was crystallized in two crystal forms. Preliminary crystallographic studies were performed on the diamond-shaped crystals, which belonged to the tetragonal space group P422. These crystals were grown using the microbatch method at 298 K. Native X-ray diffraction data from these crystals were collected to 2.2 Å resolution on a home source. These crystals have unit-cell parameters a = b = 68.7, c = 211.5 Å. Assuming the presence of two molecules per asymmetric unit, the V
M value was 2.3 Å3 Da−1 and the solvent content was approximately 45%. Although the crystals appeared less frequently than the tetragonal form, YwfC also crystallizes in the monoclinic space group P21, with unit-cell parameters a = 46.7, b = 106.3, c = 48.7 Å, β = 92.7°.
cupin fold; gene duplication; functional diversity
The crystal structure of the Z-isomer of 2,4-diamino-5-[2-(2′-methoxyphenyl)-propenyl]-furo[2,3-d]pyrimidine shows an unusual packing arrangement in which the conserved active site Arg70 forms a salt bridge to the side chain of Glu44 from a symmetry-related molecule.
The crystal structure of the ternary complex of human dihydrofolate reductase (hDHFR) with NADPH and the Z isomer of 2,4-diamino-5-[2-(2′-methoxyphenyl)propenyl]-furo[2,3-d]pyrimidine (Z1) shows that the Z isomer binds in the normal antifolate orientation in which the furo oxygen occupies the 8-amino position observed in the binding of 2,4-diaminopteridine antifolates such as methotrexate and with the methoxyphenyl moiety cis to and coplanar with the furo[2,3-d]pyrimidine ring. The hDHFR ternary complex crystallized in the orthorhombic space group P212121 and its structure was refined to 1.7 Å resolution. Although other hDHFR complexes crystallize in this space group, these data provide only the second example of an unusual packing arrangement in which the conserved active-site Arg70 forms a salt bridge to the side chain of Glu44 from a symmetry-related molecule. As a result, the conformations of Phe31 and Gln35 shift with respect to those observed in the structure of mouse DHFR bound to Z1, which crystallizes in the monoclinic space group P21 and shows that Gln35 interacts with Arg70.
human dihydrofolate reductase; 2,4-diaminofuro[2,3-d]pyrimidine antifolate