Crystal structure of the Melampsora lini effector AvrP reveals insights into a possible nuclear function and recognition by the flax disease resistance protein P

The effector protein AvrP is secreted by the flax rust fungal pathogen (Melampsora lini) and recognized specifically by the flax (Linum usitatissimum) P disease resistance protein, leading to effector‐triggered immunity. To investigate the biological function of this effector and the mechanisms of specific recognition by the P resistance protein, we determined the crystal structure of AvrP. The structure reveals an elongated zinc‐finger‐like structure with a novel interleaved zinc‐binding topology. The residues responsible for zinc binding are conserved in AvrP effector variants and mutations of these motifs result in a loss of P‐mediated recognition. The first zinc‐coordinating region of the structure displays a positively charged surface and shows some limited similarities to nucleic acid‐binding and chromatin‐associated proteins. We show that the majority of the AvrP protein accumulates in the plant nucleus when transiently expressed in Nicotiana benthamiana cells, suggesting a nuclear pathogenic function. Polymorphic residues in AvrP and its allelic variants map to the protein surface and could be associated with differences in recognition specificity. Several point mutations of residues on the non‐conserved surface patch result in a loss of recognition by P, suggesting that these residues are required for recognition.     

Isolation and characterization of microsatellite loci from the rust pathogen,Melampsora lini

We developed and characterized primers for 11 variable microsatellite loci present in the genome of the flax rust, Melampsora lini. The microsatellite loci were identified by sequencing clones from a library of EcoRI DNA fragments enriched for four simple sequence repeat motifs (AAG, AAT, TC and TG). All 11 primer pairs successfully amplified DNA fragments from a sample of 102 M. lini isolates (98 isolated from Linum marginale and four from Linum usitatissimum), revealing a total of 32 alleles. Allelic diversity at the 11 loci ranged from 0.030 to 0.449.     

Structural and functional study of Legionella pneumophila effector RavA

Legionella pneumophila is an intracellular pathogen that causes Legionnaire''s disease in humans. This bacterium can be found in freshwater environments as a free‐living organism, but it is also an intracellular parasite of protozoa. Human infection occurs when inhaled aerosolized pathogen comes into contact with the alveolar mucosa and replicates in alveolar macrophages. Legionella enters the host cell by phagocytosis and redirects the Legionella‐containing phagosomes from the phagocytic maturation pathway. These nascent phagosomes fuse with ER‐derived secretory vesicles and membranes forming the Legionella‐containing vacuole. Legionella subverts many host cellular processes by secreting over 300 effector proteins into the host cell via the Dot/Icm type IV secretion system. The cellular function for many Dot/Icm effectors is still unknown. Here, we present a structural and functional study of L. pneumophila effector RavA (Lpg0008). Structural analysis revealed that the RavA consists of four ~85 residue long α‐helical domains with similar folds, which show only a low level of structural similarity to other protein domains. The ~90 residues long C‐terminal segment is predicted to be natively unfolded. We show that during L. pneumophila infection of human cells, RavA localizes to the Golgi apparatus and to the plasma membrane. The same localization is observed when RavA is expressed in human cells. The localization signal resides within the C‐terminal sequence C⁴⁰⁹WTSFCGLF⁴¹⁷. Yeast‐two‐hybrid screen using RavA as bait identified RAB11A as a potential binding partner. RavA is present in L. pneumophila strains but only distant homologs are found in other Legionella species, where the number of repeats varies.     

Structural insights into caspase ADPR deacylization catalyzed by a bacterial effector and host calmodulin

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Structural insights into the catalytic mechanism of aldehyde-deformylating oxygenases

The fatty alk(a/e)ne biosynthesis pathway found in cyanobacteria gained tremendous attention in recent years as a promising alternative approach for biofuel production. Cyanobacterial aldehyde-deformylating oxygenase (cADO), which catalyzes the conversion of Cn fatty aldehyde to its corresponding Cn-1 alk(a/e)ne, is a key enzyme in that pathway. Due to its low activity, alk(a/e)ne production by cADO is an inefficient process. Previous biochemical and structural investigations of cADO have provided some information on its catalytic reaction. However, the details of its catalytic processes remain unclear. Here we report five crystal structures of cADO from the Synechococcus elongates strain PCC7942 in both its iron-free and iron-bound forms, representing different states during its catalytic process. Structural comparisons and functional enzyme assays indicate that Glu144, one of the iron-coordinating residues, plays a vital role in the catalytic reaction of cADO. Moreover, the helix where Glu144 resides exhibits two distinct conformations that correlates with the different binding states of the di-iron center in cADO structures. Therefore, our results provide a structural explanation for the highly labile feature of cADO di-iron center, which we proposed to be related to its low enzymatic activity. On the basis of our structural and biochemical data, a possible catalytic process of cADO was proposed, which could aid the design of cADO with improved activity.     

Structural and mechanistic insights into the precise product synthesis by a bifunctional miltiradiene synthase

Selaginella moellendorffii miltiradiene synthase (SmMDS) is a unique bifunctional diterpene synthase (diTPS) that catalyses the successive cyclization of (E,E,E)-geranylgeranyl diphosphate (GGPP) via (+)-copalyl diphosphate (CPP) to miltiradiene, which is a crucial precursor of important medicinal compounds, such as triptolide, ecabet sodium and carnosol. Miltiradiene synthetic processes have been studied in monofunctional diTPSs, while the precise mechanism by which active site amino acids determine product simplicity and the experimental evidence for reaction intermediates remain elusive. In addition, how bifunctional diTPSs work compared to monofunctional enzymes is attractive for detailed research. Here, by mutagenesis studies of SmMDS, we confirmed that pimar-15-en-8-yl⁺ is an intermediate in miltiradiene synthesis. Moreover, we determined the apo-state and the GGPP-bound state crystal structures of SmMDS. By structure analysis and mutagenesis experiments, possible contributions of key residues both in class I and II active sites were suggested. Based on the structural and functional analyses, we confirmed the copal-15-yl⁺ intermediate and unveiled more details of the catalysis process in the SmMDS class I active site. Moreover, the structural and experimental results suggest an internal channel for (+)-CPP produced in the class II active site moving towards the class I active site. Our research is a good example for intermediate identification of diTPSs and provides new insights into the product specificity determinants and intermediate transport, which should greatly facilitate the precise controlled synthesis of various diterpenes.     

Structural insights into the regulation of peptidoglycan DL-endopeptidases by inhibitory protein IseA

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Structural and functional analysis of the Lmo2642 cyclic nucleotide phosphodiesterase from Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular pathogen invading humans and animals with the highest fatality rate among the food-borne pathogens. The Listeria pathogenic processes, such as cell entry and escape from phagosomes, depend on the actions of diverse bacterial factors, including lipoproteins. Here, we report the crystal structure of Lmo2642, a conserved putative lipoprotein containing a Ser/Thr phosphatase domain. The protein consists of two distinct domains: a catalytic domain that belongs to the metallophosphoesterase superfamily and an auxiliary α-helical bundle domain. The active site in the catalytic domain of Lmo2642 contains a dinuclear metal center in which Mn2(+) and Fe3(+) are preferentially positioned at the site1 and site2, respectively. On the basis of the structural analysis and enzymatic assays, we identified the biochemical activity of the protein as a cyclic nucleotide phosphodiesterase toward 2',3'- and 3',5'-cyclic nucleotides. Considering the cNMP phosphodiesterase activity and the putative surface localization of Lmo2642, we speculate that Lmo2642 has some potential roles in the host-pathogen interactions by changing the cAMP concentration of host cells during L. monocytogenes infection.     

\Cytokinin oxidase/dehydrogenase (CKX) activity in wild and ethylene-insensitive mutant eti5 type of Arabidopsis thaliana (L.) Heynh plants and the effect of cytokinin N1-(2-chloro-4-pyridyl)-N2-phenylurea on enzymatic activity and leaf morphology

The specific activity of cytokinin oxidase/dehydrogenase (EC 1.5.99.12) (CKX) was determined in leaves of wild type (wt) and ethylene-insensitive mutant (eti5) of Arabidopsis thaliana (L.) Heynh plants. Comparative studies showed that this mutation has lower basal CKX activity than wt. Application of 4PU-30 (N¹-(2-chloro-4-pyridyl)-N²-phenylurea) resulted in decreased CKX activity in both wt and mutant plants. The treatment increased leaf blade thickness and the volume of chlorophyll-containing cells per unit leaf area in wt but these changes were not observed in the eti5 mutant. The reduction in chlorophyll “a” and “b”, as well as in carotenoids content in the treated wt tissues resulting from altered leaf morphology was not detected in eti5 plants.     

Structural Insights into the Induced-fit Inhibition of Fascin by a Small-Molecule Inhibitor

Tumor metastasis is responsible for ~ 90% of all cancer deaths. One of the key steps of tumor metastasis is tumor cell migration and invasion. Filopodia are cell surface extensions that are critical for tumor cell migration. Fascin protein is the main actin-bundling protein in filopodia. Small-molecule fascin inhibitors block tumor cell migration, invasion, and metastasis. Here we present the structural basis for the mechanism of action of these small-molecule fascin inhibitors. X-ray crystal structural analysis of a complex of fascin and a fascin inhibitor shows that binding of the fascin inhibitor to the hydrophobic cleft between the domains 1 and 2 of fascin induces a ~ 35^o rotation of domain 1, leading to the distortion of both the actin-binding sites 1 and 2 on fascin. Furthermore, the crystal structures of an inhibitor alone indicate that the conformations of the small-molecule inhibitors are dynamic. Mutations of the inhibitor-interacting residues decrease the sensitivity of fascin to the inhibitors. Our studies provide structural insights into the molecular mechanism of fascin protein function as well as the action of small-molecule fascin inhibitors.     

Structural insights into the stabilization of active,tetrameric DszC by its C‐terminus

Dibenzothiophene (DBT) is a typical sulfur‐containing compound found in fossil fuels. This compound and its derivatives are resistant to the hydrodesulfurization method often used in industry, but they are susceptible to enzymatic desulfurization via the 4S pathway, which is a well‐studied biochemical pathway consisting of four enzymes. DBT monooxygenase (DszC) from Rhodococcus erythropolis is involved in the first step of the 4S pathway. We determined the crystal structure of DszC, which reveals that, in contrast to several homologous proteins, the C‐terminus (410–417) of DszC participates in the stabilization of the substrate‐binding pocket. Analytical ultracentrifugation analysis and enzymatic assays confirmed that the C‐terminus is important for the stabilization of the active conformation of the substrate‐binding pocket and the tetrameric state. Therefore, the C‐terminus of DszC plays a significant role in the catalytic activity of this enzyme. Proteins 2014; 82:2733–2743. © 2014 Wiley Periodicals, Inc.     

Structural and functional insights into the mechanism by which MutS2 recognizes a DNA junction

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Structural and mechanistic insights into the inhibition of type I-F CRISPR-Cas system by anti-CRISPR protein AcrIF23

Prokaryotes evolved clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins as a kind of adaptive immune defense against mobile genetic elements including harmful phages. To counteract this defense, many mobile genetic elements in turn encode anti-CRISPR proteins (Acrs) to inactivate the CRISPR-Cas system. While multiple mechanisms of Acrs have been uncovered, it remains unknown whether other mechanisms are utilized by uncharacterized Acrs. Here, we report a novel mechanism adopted by recently identified AcrIF23. We show that AcrIF23 interacts with the Cas2/3 helicase-nuclease in the type I-F CRISPR-Cas system, similar to AcrIF3. The structure of AcrIF23 demonstrated a novel fold and structure-based mutagenesis identified a surface region of AcrIF23 involved in both Cas2/3-binding and its inhibition capacity. Unlike AcrIF3, however, we found AcrIF23 only potently inhibits the DNA cleavage activity of Cas2/3 but does not hinder the recruitment of Cas2/3 to the CRISPR RNA-guided surveillance complex (the Csy complex). Also, in contrast to AcrIF3 which hinders substrate DNA recognition by Cas2/3, we show AcrIF23 promotes DNA binding to Cas2/3. Taken together, our study identifies a novel anti-CRISPR mechanism used by AcrIF23 and highlights the diverse mechanisms adopted by Acrs.     

Structural insights into the modulation of the redox properties of two Geobacter sulfurreducens homologous triheme cytochromes

The redox properties of a periplasmic triheme cytochrome, PpcB from Geobacter sulfurreducens, were studied by NMR and visible spectroscopy. The structure of PpcB was determined by X-ray diffraction. PpcB is homologous to PpcA (77% sequence identity), which mediates cytoplasmic electron transfer to extracellular acceptors and is crucial in the bioenergetic metabolism of Geobacter spp. The heme core structure of PpcB in solution, probed by 2D-NMR, was compared to that of PpcA. The results showed that the heme core structures of PpcB and PpcA in solution are similar, in contrast to their crystal structures where the heme cores of the two proteins differ from each other. NMR redox titrations were carried out for both proteins and the order of oxidation of the heme groups was determined. The microscopic properties of PpcB and PpcA redox centers showed important differences: (i) the order in which hemes become oxidized is III-I-IV for PpcB, as opposed to I-IV-III for PpcA; (ii) the redox-Bohr effect is also different in the two proteins. The different redox features observed between PpcB and PpcA suggest that each protein uniquely modulates the properties of their co-factors to assure effectiveness in their respective metabolic pathways. The origins of the observed differences are discussed.     

Structural insights into initial and intermediate steps of the ribosome-recycling process

The ribosome-recycling factor (RRF) and elongation factor-G (EF-G) disassemble the 70S post-termination complex (PoTC) into mRNA, tRNA, and two ribosomal subunits. We have determined cryo-electron microscopic structures of the PoTC·RRF complex, with and without EF-G. We find that domain II of RRF initially interacts with universally conserved residues of the 23S rRNA helices 43 and 95, and protein L11 within the 50S ribosomal subunit. Upon EF-G binding, both RRF and tRNA are driven towards the tRNA-exit (E) site, with a large rotational movement of domain II of RRF towards the 30S ribosomal subunit. During this intermediate step of the recycling process, domain II of RRF and domain IV of EF-G adopt hitherto unknown conformations. Furthermore, binding of EF-G to the PoTC·RRF complex reverts the ribosome from ratcheted to unratcheted state. These results suggest that (i) the ribosomal intersubunit reorganizations upon RRF binding and subsequent EF-G binding could be instrumental in destabilizing the PoTC and (ii) the modes of action of EF-G during tRNA translocation and ribosome-recycling steps are markedly different.     

Crystal structure of the effector AvrLm4–7 of Leptosphaeria maculans reveals insights into its translocation into plant cells and recognition by resistance proteins

The avirulence gene AvrLm4–7 of Leptosphaeria maculans, the causal agent of stem canker in Brassica napus (oilseed rape), confers a dual specificity of recognition by two resistance genes (Rlm4 and Rlm7) and is strongly involved in fungal fitness. In order to elucidate the biological function of AvrLm4–7 and understand the specificity of recognition by Rlm4 and Rlm7, the AvrLm4–7 protein was produced in Pichia pastoris and its crystal structure was determined. It revealed the presence of four disulfide bridges, but no close structural analogs could be identified. A short stretch of amino acids in the C terminus of the protein, (R/N)(Y/F)(R/S)E(F/W), was well‐conserved among AvrLm4–7 homologs. Loss of recognition of AvrLm4–7 by Rlm4 is caused by the mutation of a single glycine to an arginine residue located in a loop of the protein. Loss of recognition by Rlm7 is governed by more complex mutational patterns, including gene loss or drastic modifications of the protein structure. Three point mutations altered residues in the well‐conserved C–terminal motif or close to the glycine involved in Rlm4‐mediated recognition, resulting in the loss of Rlm7‐mediated recognition. Transient expression in Nicotiana benthamiana (tobacco) and particle bombardment experiments on leaves from oilseed rape suggested that AvrLm4–7 interacts with its cognate R proteins inside the plant cell, and can be translocated into plant cells in the absence of the pathogen. Translocation of AvrLm4–7 into oilseed rape leaves is likely to require the (R/N)(Y/F)(R/S)E(F/W) motif as well as an RAWG motif located in a nearby loop that together form a positively charged region.     

Functional and Structural Characterization of the Antiphagocytic Properties of a Novel Transglutaminase from Streptococcus suis

Streptococcus suis serotype 2 (Ss2) is an important swine and human zoonotic pathogen. In the present study, we identified a novel secreted immunogenic protein, SsTGase, containing a highly conserved eukaryotic-like transglutaminase (TGase) domain at the N terminus. We found that inactivation of SsTGase significantly reduced the virulence of Ss2 in a pig infection model and impaired its antiphagocytosis in human blood. We further solved the crystal structure of the N-terminal portion of the protein in homodimer form at 2.1 Å. Structure-based mutagenesis and biochemical studies suggested that disruption of the homodimer directly resulted in the loss of its TGase activity and antiphagocytic ability. Characterization of SsTGase as a novel virulence factor of Ss2 by acting as a TGase would be beneficial for developing new therapeutic agents against Ss2 infections.