Structure of the human NK cell NKR-P1:LLT1 receptor:ligand complex reveals clustering in the immune synapse.

Signaling by the human C-type lectin-like receptor, natural killer (NK) cell inhibitory receptor NKR-P1, has a critical role in many immune-related diseases and cancer. C-type lectin-like receptors have weak affinities to their ligands; therefore, setting up a comprehensive model of NKR-P1-LLT1 interactions that considers the natural state of the receptor on the cell surface is necessary to understand its functions. Here we report the crystal structures of the NKR-P1 and NKR-P1:LLT1 complexes, which provides evidence that NKR-P1 forms homodimers in an unexpected arrangement to enable LLT1 binding in two modes, bridging two LLT1 molecules. These interaction clusters are suggestive of an inhibitory immune synapse. By observing the formation of these clusters in solution using SEC-SAXS analysis, by dSTORM super-resolution microscopy on the cell surface, and by following their role in receptor signaling with freshly isolated NK cells, we show that only the ligation of both LLT1 binding interfaces leads to effective NKR-P1 inhibitory signaling. In summary, our findings collectively support a model of NKR-P1:LLT1 clustering, which allows the interacting proteins to overcome weak ligand-receptor affinity and to trigger signal transduction upon cellular contact in the immune synapse.

Crystallographic fragment screening-based study of a novel FAD-dependent oxidoreductase from Chaetomium thermophilum. Corrigendum.

The synchrotron facilities used in collecting the data for the article by Svecová et al. [(2021), Acta Cryst. D77, 755-775] are acknowledged.

Crystallographic fragment screening-based study of a novel FAD-dependent oxidoreductase from Chaetomium thermophilum.

The FAD-dependent oxidoreductase from Chaetomium thermophilum (CtFDO) is a novel thermostable glycoprotein from the glucose-methanol-choline (GMC) oxidoreductase superfamily. However, CtFDO shows no activity toward the typical substrates of the family and high-throughput screening with around 1000 compounds did not yield any strongly reacting substrate. Therefore, protein crystallography, including crystallographic fragment screening, with 42 fragments and 37 other compounds was used to describe the ligand-binding sites of CtFDO and to characterize the nature of its substrate. The structure of CtFDO reveals an unusually wide-open solvent-accessible active-site pocket with a unique His-Ser amino-acid pair putatively involved in enzyme catalysis. A series of six crystal structures of CtFDO complexes revealed five different subsites for the binding of aryl moieties inside the active-site pocket and conformational flexibility of the interacting amino acids when adapting to a particular ligand. The protein is capable of binding complex polyaromatic substrates of molecular weight greater than 500 Da.

Trp-His covalent adduct in bilirubin oxidase is crucial for effective bilirubin binding but has a minor role in electron transfer.

Unlike any protein studied so far, the active site of bilirubin oxidase from Myrothecium verrucaria contains a unique type of covalent link between tryptophan and histidine side chains. The role of this post-translational modification in substrate binding and oxidation is not sufficiently understood. Our structural and mutational studies provide evidence that this Trp396-His398 adduct modifies T1 copper coordination and is an important part of the substrate binding and oxidation site. The presence of the adduct is crucial for oxidation of substituted phenols and it substantially influences the rate of oxidation of bilirubin. Additionally, we bring the first structure of bilirubin oxidase in complex with one of its products, ferricyanide ion, interacting with the modified tryptophan side chain, Arg356 and the active site-forming loop 393-398. The results imply that structurally and chemically distinct types of substrates, including bilirubin, utilize the Trp-His adduct mainly for binding and to a smaller extent for electron transfer.

Structural and Catalytic Properties of S1 Nuclease from Aspergillus oryzae Responsible for Substrate Recognition, Cleavage, Non-Specificity, and Inhibition.

The single-strand-specific S1 nuclease from Aspergillus oryzae is an archetypal enzyme of the S1-P1 family of nucleases with a widespread use for biochemical analyses of nucleic acids. We present the first X-ray structure of this nuclease along with a thorough analysis of the reaction and inhibition mechanisms and of its properties responsible for identification and binding of ligands. Seven structures of S1 nuclease, six of which are complexes with products and inhibitors, and characterization of catalytic properties of a wild type and mutants reveal unknown attributes of the S1-P1 family. The active site can bind phosphate, nucleosides, and nucleotides in several distinguished ways. The nucleoside binding site accepts bases in two binding modes-shallow and deep. It can also undergo remodeling and so adapt to different ligands. The amino acid residue Asp65 is critical for activity while Asn154 secures interaction with the sugar moiety, and Lys68 is involved in interactions with the phosphate and sugar moieties of ligands. An additional nucleobase binding site was identified on the surface, which explains the absence of the Tyr site known from P1 nuclease. For the first time ternary complexes with ligands enable modeling of ssDNA binding in the active site cleft. Interpretation of the results in the context of the whole S1-P1 nuclease family significantly broadens our knowledge regarding ligand interaction modes and the strategies of adjustment of the enzyme surface and binding sites to achieve particular specificity.

Crystallization of nepenthesin I using a low-pH crystallization screen.

Nepenthesins are aspartic proteases secreted by carnivorous pitcher plants of the genus Nepenthes. They significantly differ in sequence from other plant aspartic proteases. This difference, which provides more cysteine residues in the structure of nepenthesins, may contribute to their unique stability profile. Recombinantly produced nepenthesin 1 (rNep1) from N. gracilis in complex with pepstatin A was crystallized under two different crystallization conditions using a newly formulated low-pH crystallization screen. The diffraction data were processed to 2.9 and 2.8 Šresolution, respectively. The crystals belonged to space group P212121, with unit-cell parameters a = 86.63, b = 95.90, c = 105.40 Å, α = ß = γ = 90° and a = 86.28, b = 97.22, c = 103.78 Å, α = ß = γ = 90°, respectively. Matthews coefficient and solvent-content calculations suggest the presence of two molecules of rNep1 in the asymmetric unit. Here, the details of the crystallization experiment and analysis of the X-ray data are reported.

Phosphate binding in the active centre of tomato multifunctional nuclease TBN1 and analysis of superhelix formation by the enzyme.

Tomato multifunctional nuclease TBN1 belongs to the type I nuclease family, which plays an important role in apoptotic processes and cell senescence in plants. The newly solved structure of the N211D mutant is reported. Although the main crystal-packing motif (the formation of superhelices) is conserved, the details differ among the known structures. A phosphate ion was localized in the active site of the enzyme. The binding of the surface loop to the active centre is stabilized by the phosphate ion, which correlates with the observed aggregation of TBN1 in phosphate buffer. The conserved binding of the surface loop to the active centre suggests biological relevance of the contact in a regulatory function or in the formation of oligomers.

Four crystal structures of human LLT1, a ligand of human NKR-P1, in varied glycosylation and oligomerization states.

Human LLT1 is a C-type lectin-like ligand of NKR-P1 (CD161, gene KLRB1), a C-type lectin-like receptor of natural killer cells. Using X-ray diffraction, the first experimental structures of human LLT1 were determined. Four structures of LLT1 under various conditions were determined: monomeric, dimeric deglycosylated after the first N-acetylglucosamine unit in two forms and hexameric with homogeneous GlcNAc2Man5 glycosylation. The dimeric form follows the classical dimerization mode of human CD69. The monomeric form keeps the same fold with the exception of the position of an outer part of the long loop region. The hexamer of glycosylated LLT1 consists of three classical dimers. The hexameric packing may indicate a possible mode of interaction of C-type lectin-like proteins in the glycosylated form.

Rheological study of chitosan acetate solutions containing chitin nanofibrils.

Rheological properties of chitosan acetate solutions containing chitin nanofibrils (n-chitin) and biocompatible plasticizers intended for preparation of biodegradable films are reported in the steady, oscillatory and transient shear flow. The experiments were performed on slurries with an optimum proportion of 65/35 wt.% between chitosan and n-chitin in the films which was determined from our results of mechanical properties and absorption of water vapor. The time-dependent dynamic experiments revealed the chitin nanofibrils as an effective "gelling agent" of chitosan phase. The phenomenon is explained by a chitosan-like surface of n-chitin and by the interactions inducing orientational cooperativity of chitosan molecules dissolved in close neighborhood of the anisotropic chitin nanofibrils. Additions of glycerol or poly(ethylene glycol), improving mechanical properties of the films, delay significantly the onset of gelation of chitosan/n-chitin slurries. The effect is induced by an increase in viscosity of the slurries and by their enhanced chaotropic character.

Organophosphorus acid anhydrolase from Alteromonas macleodii: structural study and functional relationship to prolidases.

The bacterial enzyme organophosphorus acid anhydrolase (OPAA) is able to catalyze the hydrolysis of both proline dipeptides (Xaa-Pro) and several types of organophosphate (OP) compounds. The full three-dimensional structure of the manganese-dependent OPAA enzyme is presented for the first time. This enzyme, which was originally isolated from the marine bacterium Alteromonas macleodii, was prepared recombinantly in Escherichia coli. The crystal structure was determined at 1.8 Å resolution in space group C2, with unit-cell parameters a = 133.8, b = 49.2, c = 97.3 Å, ß = 125.0°. The enzyme forms dimers and their existence in solution was confirmed by dynamic light scattering and size-exclusion chromatography. The enzyme shares the pita-bread fold of its C-terminal domain with related prolidases. The binuclear manganese centre is located in the active site within the pita-bread domain. Moreover, an Ni(2+) ion from purification was localized according to anomalous signal. This study presents the full structure of this enzyme with complete surroundings of the active site and provides a critical analysis of its relationship to prolidases.

Plant multifunctional nuclease TBN1 with unexpected phospholipase activity: structural study and reaction-mechanism analysis.

Type I plant nucleases play an important role in apoptotic processes and cell senescence. Recently, they have also been indicated to be potent anticancer agents in in vivo studies. The first structure of tomato nuclease I (TBN1) has been determined, its oligomerization and activity profiles have been analyzed and its unexpected activity towards phospholipids has been discovered, and conclusions are drawn regarding its catalytic mechanism. The structure-solution process required X-ray diffraction data from two crystal forms. The first form was used for phase determination; the second form was used for model building and refinement. TBN1 is mainly α-helical and is stabilized by four disulfide bridges. Three observed oligosaccharides are crucial for its stability and solubility. The active site is localized at the bottom of the positively charged groove and contains a zinc cluster that is essential for enzymatic activity. An equilibrium between monomers, dimers and higher oligomers of TBN1 was observed in solution. Principles of the reaction mechanism of the phosphodiesterase activity are suggested, with central roles for the zinc cluster, the nucleobase-binding pocket (Phe-site) and Asp70, Arg73 and Asn167. Based on the distribution of surface residues, possible binding sites for dsDNA and other nucleic acids with secondary structure were identified. The phospholipase activity of TBN1, which is reported for the first time for a nuclease, significantly broadens the substrate promiscuity of the enzyme, and the resulting release of diacylglycerol, which is an important second messenger, can be related to the role of TBN1 in apoptosis.

Mouse Clr-g, a ligand for NK cell activation receptor NKR-P1F: crystal structure and biophysical properties.

Interactions between C-type lectin-like NK cell receptors and their protein ligands form one of the key recognition mechanisms of the innate immune system that is involved in the elimination of cells that have been malignantly transformed, virally infected, or stressed by chemotherapy or other factors. We determined an x-ray structure for the extracellular domain of mouse C-type lectin related (Clr) protein g, a ligand for the activation receptor NKR-P1F. Clr-g forms dimers in the crystal structure resembling those of human CD69. This newly reported structure, together with the previously determined structure of mouse receptor NKR-P1A, allowed the modeling and calculations of electrostatic profiles for other closely related receptors and ligands. Despite the high similarity among Clr-g, Clr-b, and human CD69, these molecules have fundamentally different electrostatics, with distinct polarization of Clr-g. The electrostatic profile of NKR-P1F is complementary to that of Clr-g, which suggests a plausible interaction mechanism based on contacts between surface sites of opposite potential.

New active HIV-1 protease inhibitors derived from 3-hexanol: conformation study of the free inhibitors in crystalline state and in complex with the enzyme.

Four novel linear non-peptidic HIV-1 protease inhibitors derived from 2,5-diamino-1,6-diphenyl-3-hexanol were synthesized and characterized. All of them exhibit tight binding to HIV-1 protease, with inhibition constants K(i) in the range 20 pm-5 nm. The investigated inhibitors were crystallized, and their crystal structures were determined by X-ray diffraction. In all cases, the conformations found in the crystalline state differ significantly from the conformations obtained by computational docking of the inhibitor in the binding cleft of native HIV-1 protease. Owing to the prevalence of hydrophobic substituents in all these inhibitors, the conformational mobility in water solution is restricted to their compact forms. The spectrum of low-energy conformations in solution dramatically changes during the formation of inhibitor crystals (phenyl ring stacking as a leading motif) or during the formation of a complex with HIV-1 protease (elongated conformation suitable to fit the enzyme pockets as a factor responsible for tight binding). High conformational flexibility and low conformational stress in the molecules of these inhibitors most likely increase their biological activity in comparison with more rigid compounds.

Structure of the H107R variant of the extracellular domain of mouse NKR-P1A at 2.3 Å resolution.

The structure of the H107R variant of the extracellular domain of the mouse natural killer cell receptor NKR-P1A has been determined by X-ray diffraction at 2.3 Å resolution from a merohedrally twinned crystal. Unlike the structure of the wild-type receptor in space group I4(1)22 with a single chain per asymmetric unit, the crystals of the variant belonged to space group I4(1) with a dimer in the asymmetric unit. Different degrees of merohedral twinning were detected in five data sets collected from different crystals. The mutation does not have a significant impact on the overall structure, but led to the binding of an additional phosphate ion at the interface of the molecules.

Chitinolytic enzymes from bacterium inhabiting human gastrointestinal tract -- critical parameters of protein isolation from anaerobic culture.

The object of this study are chitinolytic enzymes produced by bacterium Clostridium paraputrificum J4 isolated from the gastrointestinal tract of a healthy human. In particular, we focus on the development of purification protocols, determination of properties of the enzymes and their activity profiles. The process of bacteria cultivation and isolation of chitinolytic complex of enzymes showing specific activities of endo-, exo-chitinase and N-acetyl-ß-glucosaminidase was optimized. A range of various purification procedures were used such as ultrafiltration, precipitation, chromatographic separations (ion-exchange, size exclusion, chromatofocusing) in altered combinations. The optimal purification protocol comprises two or three steps. Individual samples were analyzed by SDS/PAGE electrophoresis and after renaturation their activity could be detected using zymograms. Mass spectroscopy peptide fragment analysis and MALDI analysis of the purest samples indicate presence of endochitinase B (molecular mass about 85 kDa) and of 60-kDa endo- and exochitinases.

Molecular architecture of mouse activating NKR-P1 receptors.

Receptors belonging to NKR-P1 family and their specific Clr ligands form an alternative missing self recognition system critical in immunity against tumors and viruses, elimination of tumor cells subjected to genotoxic stress, activation of T cell dependent immune response, and hypertension. The three-dimensional structure of the extracellular domain of the mouse natural killer (NK) cell receptor mNKR-P1Aex has been determined by X-ray diffraction. The core of the C-type lectin domain (CTLD) is homologous to the other CTLD receptors whereas one quarter of the domain forms an extended loop interacting tightly with a neighboring loop in the crystal. This domain swapping mechanism results in a compact interaction interface. A second dimerization interface resembles the known arrangement of other CTLD NK receptors. A functional dimeric form of the receptor is suggested, with the loop, evolutionarily conserved within this family, proposed to participate in interactions with ligands.

Structure of laccase from Streptomyces coelicolor after soaking with potassium hexacyanoferrate and at an improved resolution of 2.3†Å.

The paper reports the structure of the small laccase from Streptomyces coelicolor determined from a crystal soaked with potassium hexacyanoferrate [K4Fe(CN)6]. The decolorization of the natively blue crystal observed upon soaking indicates the reduction of the enzyme in the crystal. The ligand binds between laccase molecules and stabilizes the crystal. The increased diffraction limit of the diffraction data collected from this crystal enabled the refinement of the small laccase structure at 2.3 Šresolution, which is the highest resolution obtained to date.

Crystallization of recombinant bifunctional nuclease TBN1 from tomato.

The endonuclease TBN1 from Solanum lycopersicum (tomato) was expressed in Nicotiana benthamiana leaves and purified with suitable quality and in suitable quantities for crystallization experiments. Two crystal forms (orthorhombic and rhombohedral) were obtained and X-ray diffraction experiments were performed. The presence of natively bound Zn2+ ions was confirmed by X-ray fluorescence and by an absorption-edge scan. X-ray diffraction data were collected from the orthorhombic (resolution of 5.2 Å) and rhombohedral (best resolution of 3.2 Å) crystal forms. SAD, MAD and MR methods were applied for solution of the phase problem, with partial success. TBN1 contains three Zn2+ ions in a similar spatial arrangement to that observed in nuclease P1 from Penicillium citrinum.

Cystathionine gamma-lyase: Clinical, metabolic, genetic, and structural studies.

We report studies of six individuals with marked elevations of cystathionine in plasma and/or urine. Studies of CTH, the gene that encodes cystathionine gamma-lyase, revealed the presence among these individuals of either homozygous or compound heterozygous forms of a novel large deletion, p.Gly57_Gln196del, two novel missense mutations, c.589C>T (p.Arg197Cys) and c.932C>T (p.Thr311Ile), and one previously reported alteration, c.200C>T (p.Thr67Ile). Another novel missense mutation, c.185G>T (p.Arg62His), was found in heterozygous form in three mildly hypercystathioninemic members of a Taiwanese family. In one severely hypercystathioninemic individual no CTH mutation was found. Brief clinical histories of the cystathioninemic/cystathioninuric patients are presented. Most of the novel mutations were expressed and the CTH activities of the mutant proteins determined. The crystal structure of the human enzyme, hCTH, and the evidence available as to the effects of the mutations in question, as well as those of the previously reported p.Gln240Glu, on protein structure, enzymatic activity, and responsiveness to vitamin B(6) administration are discussed. Among healthy Czech controls, 9.3% were homozygous for CTH c.1208G>T (p.Ser403Ile), previously found homozygously in 7.5% of Canadians for whom plasma total homocysteine (tHcy) had been measured. Compared to wild-type homozygotes, among the 55 Czech c.1208G>T (p.Ser403Ile) homozygotes a greater level of plasma cystathionine was found only after methionine loading. Three of the four individuals homozygous or compound heterozygous for inactivating CTH mutations had mild plasma tHcy elevations, perhaps indicating a cause-and-effect relationship. The experience with the present patients provides no evidence that severe loss of CTH activity is accompanied by adverse clinical effects.

Crystallization of carbohydrate oxidase from Microdochium nivale.

Microdochium nivale carbohydrate oxidase was produced by heterologous recombinant expression in Aspergillus oryzae, purified and crystallized. The enzyme crystallizes with varying crystal morphologies depending on the crystallization conditions. Several different crystal forms were obtained using the hanging-drop vapour-diffusion method, two of which were used for diffraction measurements. Hexagon-shaped crystals (form I) diffracted to 2.66 A resolution, with unit-cell parameters a = b = 55.7, c = 610.4 A and apparent space group P6(2)22. Analysis of the data quality showed almost perfect twinning of the crystals. Attempts to solve the structure by molecular replacement did not give satisfactory results. Recently, clusters of rod-shaped crystals (form II) were grown in a solution containing PEG MME 550. These crystals belonged to the monoclinic system C2, with unit-cell parameters a = 132.9, b = 56.6, c = 86.5 A, beta = 95.7 degrees . Data sets were collected to a resolution of 2.4 A. The structure was solved by the molecular-replacement method. Model refinement is currently in progress.