Crystal structure of the parasporin-2 Bacillus thuringiensis toxin that recognizes cancer cells.

Parasporin-2 is a protein toxin that is isolated from parasporal inclusions of the Gram-positive bacterium Bacillus thuringiensis. Although B. thuringiensis is generally known as a valuable source of insecticidal toxins, parasporin-2 is not insecticidal, but has a strong cytocidal activity in liver and colon cancer cells. The 37-kDa inactive nascent protein is proteolytically cleaved to the 30-kDa active form that loses both the N-terminal and the C-terminal segments. Accumulated cytological and biochemical observations on parasporin-2 imply that the protein is a pore-forming toxin. To confirm the hypothesis, we have determined the crystal structure of its active form at a resolution of 2.38 A. The protein is unusually elongated and mainly comprises long beta-strands aligned with its long axis. It is similar to aerolysin-type beta-pore-forming toxins, which strongly reinforce the pore-forming hypothesis. The molecule can be divided into three domains. Domain 1, comprising a small beta-sheet sandwiched by short alpha-helices, is probably the target-binding module. Two other domains are both beta-sandwiches and thought to be involved in oligomerization and pore formation. Domain 2 has a putative channel-forming beta-hairpin characteristic of aerolysin-type toxins. The surface of the protein has an extensive track of exposed side chains of serine and threonine residues. The track might orient the molecule on the cell membrane when domain 1 binds to the target until oligomerization and pore formation are initiated. The beta-hairpin has such a tight structure that it seems unlikely to reform as postulated in a recent model of pore formation developed for aerolysin-type toxins. A safety lock model is proposed as an inactivation mechanism by the N-terminal inhibitory segment.

Crystal structure of a ten-amino acid protein.

What is the smallest protein? This is actually not such a simple question to answer, because there is no established consensus among scientists as to the definition of a protein. We describe here a designed molecule consisting of only 10 amino acids. Despite its small size, its essential characteristics, revealed by its crystal structure, solution structure, thermal stability, free energy surface, and folding pathway network, are consistent with the properties of natural proteins. The existence of this kind of molecule deepens our understanding of proteins and impels us to define an "ideal protein" without inquiring whether the molecule actually occurs in nature.

Implication for buried polar contacts and ion pairs in hyperthermostable enzymes.

Understanding the structural basis of thermostability and thermoactivity, and their interdependence, is central to the successful future exploitation of extremophilic enzymes in biotechnology. However, the structural basis of thermostability is still not fully characterized. Ionizable residues play essential roles in proteins, modulating protein stability, folding and function. The dominant roles of the buried polar contacts and ion pairs have been reviewed by distinguishing between the inside polar contacts and the total intramolecular polar contacts, and by evaluating their contribution as molecular determinants for protein stability using various protein structures from hyperthermophiles, thermophiles and mesophilic organisms. The analysis revealed that the remarkably increased number of internal polar contacts in a monomeric structure probably play a central role in enhancing the melting temperature value up to 120 degrees C for hyperthermophilic enzymes from the genus Pyrococcus. These results provide a promising contribution for improving the thermostability of enzymes by modulating buried polar contacts and ion pairs.

Effect of a sodium ion on the dehydration-induced phase transition of monoclinic lysozyme crystals.

A monoclinic lysozyme crystal grown in NaCl solution was transformed into a new monoclinic crystal form by controlled dehydration. This crystal-to-crystal phase transition was accompanied by 20-40% solvent loss and the transformed crystal diffracted to prominently high resolution. The structures of the native and transformed crystals were determined at 1.4 and 1.15 A resolution, respectively. In the native crystal a sodium ion was bound to the loop region Ser60-Asn74; however, it was released in the transformed crystal and a water molecule occupied this position. In the transformed crystal a sodium ion was bound to the carboxyl group of Asp52, a catalytic residue. The same structural change was observed in the phase transition of a crystal soaked in a saturated NaCl solution. In contrast, a crystal soaked in 10% NaCl solution was transformed in a shorter time with a smaller loss of solvent and the structure of the sodium-binding site was conserved in the transformed crystal. The high concentration of NaCl is likely to stabilize the crystal structure against dehydration by forming salt linkages between protein molecules. This suggests that the sodium ion in the crystal regulates not only the structural change of the loop region Ser60-Asn74 but also the molecular rearrangement caused by dehydration.

Structural basis for recognition of the matrix attachment region of DNA by transcription factor SATB1.

Special AT-rich sequence binding protein 1 (SATB1) regulates gene expression essential in immune T-cell maturation and switching of fetal globin species, by binding to matrix attachment regions (MARs) of DNA and inducing a local chromatin remodeling. Previously we have revealed a five-helix structure of the N-terminal CUT domain, which is essentially the folded region in the MAR-binding domain, of human SATB1 by NMR. Here we determined crystal structure of the complex of the CUT domain and a MAR DNA, in which the third helix of the CUT domain deeply enters the major groove of DNA in the B-form. Bases of 5'-CTAATA-3' sequence are contacted by this helix, through direct and water-mediated hydrogen bonds and apolar and van der Waals contacts. Mutations at conserved base-contacting residues, Gln402 and Gly403, reduced the DNA-binding activity, which confirmed the importance of the observed interactions involving these residues. A significant number of equivalent contacts are observed also for typically four-helix POU-specific domains of POU-homologous proteins, indicating that these domains share a common framework of the DNA-binding mode, recognizing partially similar DNA sequences.

Structure of an orthorhombic form of xylanase II from Trichoderma reesei and analysis of thermal displacement.

An orthorhombic crystal of xylanase II from Trichoderma reesei was grown in the presence of sodium iodide. Crystal structures at atomic resolution were determined at 100 and 293 K. Protein molecules were aligned along a crystallographic twofold screw axis, forming a helically extended polymer-like chain mediated by an iodide ion. The iodide ion connected main-chain peptide groups between two adjacent molecules by an N-H...I-...H-N hydrogen-bond bridge, thus contributing to regulation of the molecular arrangement and suppression of the rigid-body motion in the crystal with high diffraction quality. The structure at 293 K showed considerable thermal motion in the loop regions connecting the beta-strands that form the active-site cleft. TLS model analysis of the thermal motion and a comparison between this structure and that at 100 K suggest that the fluctuation of these loop regions is attributable to the hinge-like movement of the beta-strands.

Molecular structure of a novel membrane protease specific for a stomatin homolog from the hyperthermophilic archaeon Pyrococcus horikoshii.

Membrane-bound proteases are involved in various regulatory functions. Our previous study indicated that the N-terminal region of an open reading frame, PH1510 (residues 16-236, designated as 1510-N) from the hyperthermophilic archaeon Pyrococcus horikoshii, is a serine protease with a catalytic Ser-Lys dyad that specifically cleaves the C-terminal hydrophobic residues of a membrane protein, the stomatin-homolog PH1511. In humans, an absence of stomatin is associated with a form of hemolytic anemia known as hereditary stomatocytosis, but the function of stomatin is not fully understood. Here, we report the crystal structure of 1510-N in dimeric form. Each active site of 1510-N is rich in hydrophobic residues, which accounts for the substrate-specificity. The monomer of 1510-N shows structural similarity to one monomer of Escherichia coli ClpP, an ATP-dependent tetradecameric protease. But, their oligomeric forms are different. Major contributors to dimeric interaction in 1510-N are the alpha7 helix and beta9 strand, both of which are missing from ClpP. While the long handle region of ClpP contributes to the stacking of two heptameric rings, the corresponding L2 loop of 1510-N is disordered because the region has little interaction with other residues of the same molecule. The catalytic Ser97 of 1510-N is in almost the same location as the catalytic Ser97 of E.coli ClpP, whereas another residue, Lys138, presumably forming the catalytic dyad, is located in the disordered L2 region of 1510-N. These findings suggest that the binding of the substrate to the catalytic site of 1510-N induces conformational changes in a region that includes loop L2 so that Lys138 approaches the catalytic Ser97.

Structural phase transition of monoclinic crystals of hen egg-white lysozyme.

Two monoclinic crystals (space group P2(1)) of hen egg-white lysozyme, a type I crystal grown at room temperature in a D2O solution with pD 4.5 containing 2%(w/v) sodium nitrate and a type II crystal grown at 313 K in a 10%(w/v) sodium chloride solution with pH 7.6, were each transformed into another monoclinic crystal with the same space group by dehydration-induced phase transition. Changes in X-ray diffraction were recorded to monitor the progress of the crystal transformation, which started with the appearance of diffuse streaks. In both crystals, the intensity of h + l odd reflections gradually weakened and finally disappeared on completion of the transformation. X-ray diffraction in the intermediate state indicated the presence of lattices of both the native and transformed crystals. In the native type I crystal, two alternate conformations were observed in the main chain of the region Gly71-Asn74. One conformer bound a sodium ion which was replaced with a water molecule in the other conformer. In the transformed crystal, the sodium ion was removed and the main-chain conformation of this region was converted to that of the water-bound form. The transformed crystal diffracted to a higher resolution than the native crystal, while the peak width of the diffraction spots increased. Analysis of the thermal motion of protein molecules using the TLS model has shown that the enhancement of the diffraction power in the transformed crystal is mainly ascribable to the suppression of rigid-body motion owing to an increase in intermolecular contacts as a result of the loss of bulk solvent.

Role of Trp140 at subsite -6 on the maltohexaose production of maltohexaose-producing amylase from alkalophilic Bacillus sp.707.

Maltohexaose-producing amylase (G6-amylase) from alkalophilic Bacillus sp.707 predominantly produces maltohexaose (G6) in the yield of >30% of the total products from short-chain amylose (DP=17). Our previous crystallographic study showed that G6-amylase has nine subsites, from -6 to +3, and pointed out the importance of the indole moiety of Trp140 in G6 production. G6-amylase has very low levels of hydrolytic activities for oligosaccharides shorter than maltoheptaose. To elucidate the mechanism underlying G6 production, we determined the crystal structures of the G6-amylase complexes with G6 and maltopentaose (G5). In the active site of the G6-amylase/G5 complex, G5 is bound to subsites -6 to -2, while G1 and G6 are found at subsites +2 and -7 to -2, respectively, in the G6-amylase/G6 complex. In both structures, the glucosyl residue located at subsite -6 is stacked to the indole moiety of Trp140 within a distance of 4A. The measurement of the activities of the mutant enzymes when Trp140 was replaced by leucine (W140L) or by tyrosine (W140Y) showed that the G6 production from short-chain amylose by W140L is lower than that by W140Y or wild-type enzyme. The face-to-face short contact between Trp140 and substrate sugars is suggested to regulate the disposition of the glucosyl residue at subsite -6 and to govern product specificity for G6 production.

Structure of RadB recombinase from a hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1: an implication for the formation of a near-7-fold helical assembly.

The X-ray crystal structure of RadB from Thermococcus kodakaraensis KOD1, an archaeal homologue of the RecA/Rad51 family proteins, have been determined in two crystal forms. The structure represents the core ATPase domain of the RecA/Rad51 proteins. Two independent molecules in the type 1 crystal were roughly related by 7-fold screw symmetry whereas non-crystallographic 2-fold symmetry was observed in the type 2 crystal. The dimer structure in the type 1 crystal is extended to construct a helical assembly, which resembles the filamentous structures reported for other RecA/Rad51 proteins. The molecular interface in the type 1 dimer is formed by facing a basic surface patch of one monomer to an acidic one of the other. The empty ATP binding pocket is located at the interface and barely concealed from the outside similarly to that in the active form of the RecA filament. The model assembly has a positively charged belt on one surface bordering the helical groove suitable for facile binding of DNA. Electron microscopy has revealed that, in the absence of ATP and DNA, RadB forms a filament with a similar diameter to that of the hypothetical assembly, although its helical properties were not confirmed.

Amyloid fibril formation by the CAD domain of caspase-activated DNase.

Caspase-activated DNase (CAD) is a key protein in the process of apoptosis that degrades DNA through the action of caspases. Its N-terminal region, the CAD domain (CAD-CD), is highly conserved among CAD family proteins and is responsible for the interaction with its inhibitor. We report here that CAD-CD spontaneously aggregates to form amyloid fibrils, without a lag time, under the conditions of low pH (below 4) and the presence of anions. Interestingly, the secondary structure of CAD-CD in the fibril state comprised not only beta-sheet but also alpha-helix, as found in CD, FTIR, and x-ray fiber diffraction experiments. Aromatic side chains have a defined orientation and are in the hydrophobic environment occurring with the CAD-CD fibrillogenesis. These findings provide new insights into the architecture of amyloid fibrils.

Crystallization and preliminary X-ray studies on the reaction center-light-harvesting 1 core complex from Rhodopseudomonas viridis.

The reaction center-light-harvesting 1 (RC-LH1) core complex is the photosynthetic apparatus in the membrane of the purple photosynthetic bacterium Rhodopseudomonas viridis. The RC is surrounded by an LH1 complex that is constituted of oligomers of three types of apoproteins (alpha, beta and gamma chains) with associated bacteriochlorophyll bs and carotenoid. It has been crystallized by the sitting-drop vapour-diffusion method. A promising crystal diffracted to beyond 8.0 A resolution. It belonged to space group P1, with unit-cell parameters a = 141.4, b = 136.9, c = 185.3 A, alpha = 104.6, beta = 94.0, gamma = 110.7 degrees. A Patterson function calculated using data between 15.0 and 8.0 A resolution suggested that the LH1 complex is distributed with quasi-16-fold rotational symmetry around the RC.

Crystallization of parasporin-2, a Bacillus thuringiensis crystal protein with selective cytocidal activity against human cells.

Bacillus thuringiensis is a valuable source of protein toxins that are specifically effective against certain insects and worms but harmless to mammals. In contrast, a protein toxin obtained from B. thuringiensis strain A1547, designated parasporin-2, is not insecticidal but has a strong cytocidal activity against human cells with markedly divergent target specificity. The 37 kDa inactive protein is proteolytically activated to a 30 kDa active form. The active form of the recombinant protein toxin was crystallized in the presence of ethylene glycol and polyethylene glycol 8000 at neutral pH. The crystals belong to the hexagonal space group P6(1) or P6(5), with unit-cell parameters a = b = 134.37, c = 121.24 A. Diffraction data from a native crystal were collected to 2.75 A resolution using a synchrotron-radiation source.

Biochemical and crystallographic analyses of maltohexaose-producing amylase from alkalophilic Bacillus sp. 707.

Maltohexaose-producing amylase, called G6-amylase (EC 3.2.1.98), from alkalophilic Bacillus sp.707 predominantly produces maltohexaose (G6) from starch and related alpha-1,4-glucans. To elucidate the reaction mechanism of G6-amylase, the enzyme activities were evaluated and crystal structures were determined for the native enzyme and its complex with pseudo-maltononaose at 2.1 and 1.9 A resolutions, respectively. The optimal condition for starch-degrading reaction activity was found at 45 degrees C and pH 8.8, and the enzyme produced G6 in a yield of more than 30% of the total products from short-chain amylose (DP = 17). The crystal structures revealed that Asp236 is a nucleophilic catalyst and Glu266 is a proton donor/acceptor. Pseudo-maltononaose occupies subsites -6 to +3 and induces the conformational change of Glu266 and Asp333 to form a salt linkage with the N-glycosidic amino group and a hydrogen bond with secondary hydroxyl groups of the cyclitol residue bound to subsite -1, respectively. The indole moiety of Trp140 is stacked on the cyclitol and 4-amino-6-deoxyglucose residues located at subsites -6 and -5 within a 4 A distance. Such a face-to-face short contact may regulate the disposition of the glucosyl residue at subsite -6 and would govern the product specificity for G6 production.

X-ray structure of a membrane-bound beta-glycosidase from the hyperthermophilic archaeon Pyrococcus horikoshii.

The beta-glycosidase of the hyperthermophilic Archaeon Pyrococcus horikoshii is a membrane-bound enzyme with the preferred substrate of alkyl-beta-glycosides. In this study, the unusual structural features that confer the extreme thermostability and substrate preferences of this enzyme were investigated by X-ray crystallography and docking simulation. The enzyme was crystallized in the presence of a neutral surfactant, and the crystal structure was solved by the molecular replacement method and refined at 2.5 A. The main-chain fold of the enzyme belongs to the (betaalpha)8 barrel structure common to the Family 1 glycosyl hydrolases. The active site is located at the center of the C-termini of the barrel beta-strands. The deep pocket of the active site accepts one sugar unit, and a hydrophobic channel extending radially from there binds the nonsugar moiety of the substrate. The docking simulation for oligosaccharides and alkylglucosides indicated that alkylglucosides with a long aliphatic chain are easily accommodated in the hydrophobic channel. This sparingly soluble enzyme has a cluster of hydrophobic residues on its surface, situated at the distal end of the active site channel and surrounded by a large patch of positively charged residues. We propose that this hydrophobic region can be inserted into the membrane while the surrounding positively charged residues make favorable contacts with phosphate groups on the inner surface of the membrane. The enzyme could thus adhere to the membrane in the proximity of its glycolipid substrate.

Phase transition of triclinic hen egg-white lysozyme crystal associated with sodium binding.

A triclinic crystal of hen egg-white lysozyme obtained from a D2O solution at 313 K was transformed into a new triclinic crystal by slow release of solvent under a temperature-regulated nitrogen-gas stream. The progress of the transition was monitored by X-ray diffraction. The transition started with the appearance of strong diffuse streaks. The diffraction spots gradually fused and faded with the emergence of diffraction from the new lattice; the scattering power of the crystal fell to a resolution of 1.5 A from the initial 0.9 A resolution. At the end of the transition, the diffuse streaks disappeared and the scattering power recovered to 1.1 A resolution. The transformed crystal contained two independent molecules and the solvent content had decreased to 18% from the 32% solvent content of the native crystal. The structure was determined at 1.1 A resolution and compared with the native structure refined at the same resolution. The backbone structures of the two molecules in the transformed crystal were superimposed on the native structure with root-mean-square deviations of 0.71 and 0.96 A. A prominent structural difference was observed in the loop region of residues Ser60-Leu75. In the native crystal, a water molecule located at the centre of this helical loop forms hydrogen bonds to main-chain peptide groups. In the transformed crystal, this water molecule is replaced by a sodium ion with octahedral coordination that involves water molecules and a nitrate ion. The peptide group connecting Arg73 and Asn74 is rotated by 180 degrees so that the CO group of Arg73 can coordinate to the sodium ion. The change in the X-ray diffraction pattern during the phase transition suggests that the transition proceeds at the microcrystal level. A mechanism is proposed for the crystal transformation.

Role of Phe283 in enzymatic reaction of cyclodextrin glycosyltransferase from alkalophilic Bacillus sp.1011: Substrate binding and arrangement of the catalytic site.

Cyclodextrin glycosyltransferase (CGTase) belonging to the alpha-amylase family mainly catalyzes transglycosylation and produces cyclodextrins from starch and related alpha-1,4-glucans. The catalytic site of CGTase specifically conserves four aromatic residues, Phe183, Tyr195, Phe259, and Phe283, which are not found in alpha-amylase. To elucidate the structural role of Phe283, we determined the crystal structures of native and acarbose-complexed mutant CGTases in which Phe283 was replaced with leucine (F283L) or tyrosine (F283Y). The temperature factors of the region 259-269 in native F283L increased >10 A(2) compared with the wild type. The complex formation with acarbose not only increased the temperature factors (>10 A(2)) but also changed the structure of the region 257-267. This region is stabilized by interactions of Phe283 with Phe259 and Leu260 and plays an important role in the cyclodextrin binding. The conformation of the side-chains of Glu257, Phe259, His327, and Asp328 in the catalytic site was altered by the mutation of Phe283 with leucine, and this indicates that Phe283 partly arranges the structure of the catalytic site through contacts with Glu257 and Phe259. The replacement of Phe283 with tyrosine decreased the enzymatic activity in the basic pH range. The hydroxyl group of Tyr283 forms hydrogen bonds with the carboxyl group of Glu257, and the pK(a) of Glu257 in F283Y may be lower than that in the wild type.

Distinct domain functions regulating de novo DNA synthesis of thermostable DNA primase from hyperthermophile Pyrococcus horikoshii.

DNA primases are essential components of the DNA replication apparatus in every organism. Reported here are the biochemical characteristics of a thermostable DNA primase from the thermophilic archaeon Pyrococcus horikoshii, which formed the oligomeric unit L(1)S(1) and synthesized long DNA primers 10 times more effectively than RNA primers. The N-terminal (25KL) and C-terminal halves (20KL) of the large subunit (L) play distinct roles in regulating de novo DNA synthesis of the small catalytic subunit (S). The 25KL domain has a dual function. One function is to depress the large affinity of the intrasubunit domain 20KL for the template DNA until complex (L(1)S(1) unit) formation. The other function is to tether the L subunit tightly to the S subunit, probably to promote effective interaction between the intrasubunit domain 20KL and the active center of the S subunit. The 20KL domain is a central factor to enhance the de novo DNA synthesis activity of the catalytic S subunit since the total affinity of the L(1)S(1) unit is mainly derived from the affinity of 20KL, which is elevated more than 10 times by the heterodimer formation, presumably due to the cancellation of the inhibitory activity of 25KL through tight binding to the S subunit.

X-ray structural analysis of the ligand-recognition mechanism in the dual-affinity labeling of c-type lysozyme with 2',3'-epoxypropyl beta-glycoside of N-acetyllactosamine.

In spite of the belonging to the same c-type lysozyme family, hen egg-white lysozyme (HEWL) was much less susceptible to the dual-affinity labeling with 2',3'-epoxypropyl beta-glycoside of N-acetyllactosamine (Galbeta1,4GlcNAc-Epo) than human lysozyme (HL). The three-dimensional structures of the HEWL labeled with single Galbeta1,4GlcNAc-Epo and the Glu102-mutant HL labeled with double Galbeta1,4GlcNAc-Epo were determined by X-ray crystallography at resolutions of 1.85 and 2.0 A, respectively. The overall conformation and the interaction mode of the carbohydrate ligand part in the singly labeled HEWL and the doubly labeled Glu102-mutant HL were basically identical to those of the correspondingly labeled wild-type HL with minor alterations in some stereochemical parameters. A detailed comparison of the structures revealed the key protein-carbohydrate and carbohydrate-carbohydrate interactions essential for the dual labeling. It was suggested that the difference in the efficiency of the dual labeling was caused by the structural difference between Gln104 in HL and Asn103 in HEWL. The relevance to our previous study and the carbohydrate-carbohydrate interaction on cell-surface membranes were discussed.