Type III effectors are virulence factors of Gram-negative bacterial pathogens delivered directly into host cells by the type III secretion nanomachine where they manipulate host cell processes such as the innate immunity and gene expression. Here, we show that the novel type III effector XopL from the model plant pathogen Xanthomonas campestris pv. vesicatoria exhibits E3 ubiquitin ligase activity in vitro and in planta, induces plant cell death and subverts plant immunity. E3 ligase activity is associated with the C-terminal region of XopL, which specifically interacts with plant E2 ubiquitin conjugating enzymes and mediates formation of predominantly K11-linked polyubiquitin chains. The crystal structure of the XopL C-terminal domain revealed a single domain with a novel fold, termed XL-box, not present in any previously characterized E3 ligase. Mutation of amino acids in the central cavity of the XL-box disrupts E3 ligase activity and prevents XopL-induced plant cell death. The lack of cysteine residues in the XL-box suggests the absence of thioester-linked ubiquitin-E3 ligase intermediates and a non-catalytic mechanism for XopL-mediated ubiquitination. The crystal structure of the N-terminal region of XopL confirmed the presence of a leucine-rich repeat (LRR) domain, which may serve as a protein-protein interaction module for ubiquitination target recognition. While the E3 ligase activity is required to provoke plant cell death, suppression of PAMP responses solely depends on the N-terminal LRR domain. Taken together, the unique structural fold of the E3 ubiquitin ligase domain within the Xanthomonas XopL is unprecedented and highlights the variation in bacterial pathogen effectors mimicking this eukaryote-specific activity.
Numerous bacterial pathogens infecting plants, animals and humans use a common strategy of host colonization, which involves injection of specific proteins termed effectors into the host cell. Identification of effector proteins and elucidation of their individual functions is essential for our understanding of the pathogenesis process. Here, we identify a novel effector, XopL, from Xanthomonas campestris pv. vesicatoria, which causes disease in tomato and pepper plants. We show that XopL suppresses PAMP-related defense gene expression and further characterize XopL as an E3 ubiquitin ligase. This eukaryote-specific function involves attachment of ubiquitin molecule(s) to a particular protein targeted for degradation or localisation to specific cell compartments. Ubiquitination processes play a central role in cell-cycle regulation, DNA repair, cell growth and immune responses. In the case of XopL this activity triggers plant cell death. Through structural and functional analysis we demonstrate that XopL contains two distinct domains, one of which demonstrates a novel fold never previously observed in E3 ubiquitin ligases. This novel domain specifically interacts with plant ubiquitination system components. Our findings provide the first insights into the function of a previously unknown XopL effector and identify a new member of the growing family of bacterial pathogenic factors hijacking the host ubiquitination system.
The structural characterization of acyl-carrier-protein synthase (AcpS) from three different pathogenic microorganisms is reported. One interesting finding of the present work is a crystal artifact related to the activity of the enzyme, which fortuitously represents an opportunity for a strategy to design a potential inhibitor of a pathogenic AcpS.
Some bacterial type II fatty-acid synthesis (FAS II) enzymes have been shown to be important candidates for drug discovery. The scientific and medical quest for new FAS II protein targets continues to stimulate research in this field. One of the possible additional candidates is the acyl-carrier-protein synthase (AcpS) enzyme. Its holo form post-translationally modifies the apo form of an acyl carrier protein (ACP), which assures the constant delivery of thioester intermediates to the discrete enzymes of FAS II. At the Center for Structural Genomics of Infectious Diseases (CSGID), AcpSs from Staphylococcus aureus (AcpSSA), Vibrio cholerae (AcpSVC) and Bacillus anthracis (AcpSBA) have been structurally characterized in their apo, holo and product-bound forms, respectively. The structure of AcpSBA is emphasized because of the two 3′,5′-adenosine diphosphate (3′,5′-ADP) product molecules that are found in each of the three coenzyme A (CoA) binding sites of the trimeric protein. One 3′,5′-ADP is bound as the 3′,5′-ADP part of CoA in the known structures of the CoA–AcpS and 3′,5′-ADP–AcpS binary complexes. The position of the second 3′,5′-ADP has never been described before. It is in close proximity to the first 3′,5′-ADP and the ACP-binding site. The coordination of two ADPs in AcpSBA may possibly be exploited for the design of AcpS inhibitors that can block binding of both CoA and ACP.
acyl-carrier-protein synthase; acyl carrier protein; type II fatty-acid synthesis; inhibition; 3′,5′-adenosine diphosphate; coenzyme A
For the last decade, worldwide efforts for the treatment of anthrax infection have focused on developing effective vaccines. Patients that are already infected are still treated traditionally, using different types of standard antimicrobial agents. The most popular are antibiotics such as tetracyclines and fluoroquinolones. While aminoglycosides appear to be less effective antimicrobial agents than other antibiotics, synthetic aminoglycosides have been shown to act as potent inhibitors of anthrax lethal factor and may have potential application as antitoxins. Here, we present a structural analysis of the BA2930 protein, a putative aminoglycoside acetyltransferase, which may be a component of the bacterium’s aminoglycoside resistance mechanism. The determined structures revealed details of a fold characteristic only for one other protein structure in the PDB, namely YokD from Bacillus subtilis. Both BA2930 and YokD are members of the Antibiotic_NAT superfamily (PF02522). Sequential and structural analysis showed that residues conserved throughout the Antibiotic_NAT superfamily are responsible for the binding of the cofactor acetyl coenzyme A. The interaction of BA2930 with cofactors was characterized by both crystallographic and binding studies.
Antibiotics resistance; Aminoglycoside N-acetyltransferase; Bacillus anthracis; AcCoA; crystal structure
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) and the associated proteins (Cas) comprise a system of adaptive immunity against viruses and plasmids in prokaryotes. Cas1 is a CRISPR-associated protein that is common to all CRISPR-containing prokaryotes but its function remains obscure. Here we show that the purified Cas1 protein of Escherichia coli (YgbT) exhibits nuclease activity against single-stranded and branched DNAs including Holliday junctions, replication forks, and 5′-flaps. The crystal structure of YgbT and site-directed mutagenesis have revealed the potential active site. Genome-wide screens show that YgbT physically and genetically interacts with key components of DNA repair systems, including recB, recC and ruvB. Consistent with these findings, the ygbT deletion strain showed increased sensitivity to DNA damage and impaired chromosomal segregation. Similar phenotypes were observed in strains with deletion of CRISPR clusters, suggesting that the function of YgbT in repair involves interaction with the CRISPRs. These results show that YgbT belongs to a novel, structurally distinct family of nucleases acting on branched DNAs and suggest that, in addition to antiviral immunity, at least some components of the CRISPR-Cas system have a function in DNA repair.
Cas1; CRISPR; DNA recombination; DNA repair; nuclease; YgbT
A crystal structure of the putative N-carbamoylsarcosine amidase (CSHase) Ta0454 from Thermoplasma acidophilum was solved by single-wavelength anomalous diffraction and refined at a resolution of 2.35 Å. CSHases are involved in the degradation of creatinine. Ta0454 shares a similar fold and a highly conserved C-D-K catalytic triad (Cys123, Asp9, and Lys90) with the structures of three cysteine hydrolases (PDB codes 1NBA, 1IM5, and 2H0R). Molecular dynamics (MD) simulations of Ta0454/N-carbamoylsarcosine and Ta0454/pyrazinamide complexes were performed to determine the structural basis of the substrate binding pattern for each ligand. Based on the MD simulated-trajectories, the MM/PBSA method predicts binding free energies of −24.5 and −17.1 kcal/mol for the two systems, respectively. The predicted binding free energies suggest that Ta0454 is selective for N-carbamoylsarcosine over pyrazinamide, and zinc ions play an important role in the favorable substrate bound states.
N-carbamoylsarcosine amidase; C-D-K catalytic triad; creatinine degradation; crystal structure; MM/PBSA; molecular dynamics simulations
Comparative genomic studies have identified many proteins that are found only in various Chlamydiae species and exhibit no significant sequence similarity to any protein in organisms that do not belong to this group. The CT670 protein of Chlamydia trachomatis is one of the proteins whose genes are in one of the type III secretion gene clusters but whose cellular functions are not known. CT670 shares several characteristics with the YscO protein of Yersinia pestis, including the neighboring genes, size, charge, and secondary structure, but the structures and/or functions of these proteins remain to be determined. Although a BLAST search with CT670 did not identify YscO as a related protein, our analysis indicated that these two proteins exhibit significant sequence similarity. In this paper, we report that the CT670 crystal, solved at a resolution of 2 Å, consists of a single coiled coil containing just two long helices. Gel filtration and analytical ultracentrifugation studies showed that in solution CT670 exists in both monomeric and dimeric forms and that the monomer predominates at lower protein concentrations. We examined the interaction of CT670 with many type III secretion system-related proteins (viz., CT091, CT665, CT666, CT667, CT668, CT669, CT671, CT672, and CT673) by performing bacterial two-hybrid assays. In these experiments, CT670 was found to interact only with the CT671 protein (YscP homolog), whose gene is immediately downstream of ct670. A specific interaction between CT670 and CT671 was also observed when affinity chromatography pull-down experiments were performed. These results suggest that CT670 and CT671 are putative homologs of the YcoO and YscP proteins, respectively, and that they likely form a chaperone-effector pair.
The Pseudomonas aeruginosa PsrA autorepressor has dual roles as a repressor of the fadBA5 β-oxidation-operon and an activator of the stationary-phase sigma factor rpoS and exsCEBA-operon of the type III secretion system (TTSS). Previously, we demonstrated that the repression of the fadBA5-operon by PsrA is relieved by long-chain fatty acids (LCFA). However, the signal affecting the activation of rpoS and exsC via PsrA is unknown. In this study, microarray and gene-fusion data suggested that LCFA (e.g. oleate) affected the expression of rpoS and exsC. DNA binding studies confirmed that PsrA binds to the rpoS and exsC promoter regions. This binding was inhibited by LCFA, indicating that LCFA directly affects the activation of these two genes through PsrA. LCFA decreased rpoS and exsC expression, resulting in increased N-(butyryl)-l-homoserine-lactone quorum-sensing signal and decreased ExoS/T production, respectively. Based on the crystal structure of PsrA, site-directed mutagenesis of amino acid residues, within the hydrophobic channel thought to accommodate LCFA, created two LCFA-nonresponsive PsrA mutants. The binding and activation of rpoS and exsC by these PsrA mutants was no longer inhibited by LCFA. These data support a mechanistic model where LCFA influence PsrA regulation to control LCFA metabolism and some virulence genes in P. aeruginosa.
NleG homologues constitute the largest family of type 3 effectors delivered by pathogenic E. coli, with fourteen members in the enterohaemorrhagic (EHEC) O157:H7 strain alone. Identified recently as part of the non-LEE-encoded (Nle) effector set, this family remained uncharacterised and shared no sequence homology to other proteins including those of known function. The C-terminal domain of NleG2-3 (residues 90 to 191) is the most conserved region in NleG proteins and was solved by NMR. Structural analysis of this structure revealed the presence of a RING finger/U-box motif. Functional assays demonstrated that NleG2-3 as well as NleG5-1, NleG6-2 and NleG9′ family members exhibited a strong autoubiquitination activity in vitro; a characteristic usually expressed by eukaryotic ubiquitin E3 ligases. When screened for activity against a panel of 30 human E2 enzymes, the NleG2-3 and NleG5-1 homologues showed an identical profile with only UBE2E2, UBE2E3 and UBE2D2 enzymes supporting NleG activity. Fluorescence polarization analysis yielded a binding affinity constant of 56±2 µM for the UBE2D2/NleG5-1 interaction, a value comparable with previous studies on E2/E3 affinities. The UBE2D2 interaction interface on NleG2-3 defined by NMR chemical shift perturbation and mutagenesis was shown to be generally similar to that characterised for human RING finger ubiquitin ligases. The alanine substitutions of UBE2D2 residues Arg5 and Lys63, critical for activation of eukaryotic E3 ligases, also significantly decreased both NleG binding and autoubiquitination activity. These results demonstrate that bacteria-encoded NleG effectors are E3 ubiquitin ligases analogous to RING finger and U-box enzymes in eukaryotes.
Many bacterial pathogens utilize a multiprotein ‘‘injection needle’’ termed the type III secretion system to deliver a set of proteins called effectors into the host cell. These effectors then manipulate host signalling pathways to the advantage of the pathogen, often mimicking eukaryote-specific activities. We present a study of an uncharacterised family of effectors called NleG, secreted primarily by enterohaemorrhagic E. coli (EHEC) O157:H7, a causative agent of human gastroenteritis. We solved the solution structure of a conserved C-terminal region of an NleG family member by NMR. Structural analysis demonstrated that the NleG structure is similar to the RING finger/U-box domain found primarily in eukaryotic ubiquitin ligases. The activity of these domains in eukaryotes is an essential part of the ubiquitination signalling system. Due to its central role in cell metabolism and the host immune response, the ubiquitination system has emerged as a primary target for bacterial effectors. Our biochemical analysis demonstrated that NleG proteins selectively interact with human E2 ubiquitin conjugating enzymes and exhibit in vitro activity typical of eukaryotic E3 ligases. Our data reveal that NleG effectors structurally and functionally mimic host U-box/RING E3 ubiquitin ligases. Future research will focus on determining targets of NleG ubiquitin ligase activity and the role in E. coli pathogenesis.
N2-Methylguanine 966 is located in the loop of Escherichia coli 16 S rRNA helix 31, forming a part of the P-site tRNA-binding pocket. We found yhhF to be a gene encoding for m2G966 specific 16 S rRNA methyltransferase. Disruption of the yhhF gene by kanamycin resistance marker leads to a loss of modification at G966. The modification could be rescued by expression of recombinant protein from the plasmid carrying the yhhF gene. Moreover, purified m2G966 methyltransferase, in the presence of S-adenosylomethionine (AdoMet), is able to methylate 30 S ribosomal subunits that were purified from yhhF knock-out strain in vitro. The methylation is specific for G966 base of the 16 S rRNA. The m2G966 methyltransferase was crystallized, and its structure has been determined and refined to 2.05 Å. The structure closely resembles RsmC rRNA methyltransferase, specific for m2G1207 of the 16 S rRNA. Structural comparisons and analysis of the enzyme active site suggest modes for binding AdoMet and rRNA to m2G966 methyltransferase. Based on the experimental data and current nomenclature the protein expressed from the yhhF gene was renamed to RsmD. A model for interaction of RsmD with ribosome has been proposed.
Ribose-5-phosphate isomerase A (RpiA; EC 18.104.22.168) interconverts ribose-5-phosphate and ribulose-5-phosphate. This enzyme plays essential roles in carbohydrate anabolism and catabolism; it is ubiquitous and highly conserved. The structure of RpiA from Escherichia coli was solved by multiwavelength anomalous diffraction (MAD) phasing, and refined to 1.5 Å resolution (R factor 22.4%, Rfree 23.7%). RpiA exhibits an α/β/(α/β)/β/α fold, some portions of which are similar to proteins of the alcohol dehydrogenase family. The two subunits of the dimer in the asymmetric unit have different conformations, representing the opening/closing of a cleft. Active site residues were identified in the cleft using sequence conservation, as well as the structure of a complex with the inhibitor arabinose-5-phosphate at 1.25 Å resolution. A mechanism for acid-base catalysis is proposed.
ribose-5-phosphate isomerase; MAD; X-ray crystallography; pentose phosphate pathway; Calvin cycle; arabinose-5-phosphate
Ribose-5-phosphate isomerases (EC 22.214.171.124) interconvert ribose 5-phosphate and ribulose 5-phosphate. This reaction permits the synthesis of ribose from other sugars, as well as the recycling of sugars from nucleotide breakdown. Two unrelated types of enzyme can catalyze the reaction. The most common, RpiA, is present in almost all organisms (including Escherichia coli), and is highly conserved. The second type, RpiB, is present in some bacterial and eukaryotic species and is well conserved. In E. coli, RpiB is sometimes referred to as AlsB, because it can take part in the metabolism of the rare sugar, allose, as well as the much more common ribose sugars. We report here the structure of RpiB/AlsB from E. coli, solved by multi-wavelength anomalous diffraction (MAD) phasing, and refined to 2.2 Å resolution. RpiB is the first structure to be solved from pfam02502 (the RpiB/LacAB family). It exhibits a Rossmann-type αβα-sandwich fold that is common to many nucleotide-binding proteins, as well as other proteins with different functions. This structure is quite distinct from that of the previously solved RpiA; although both are, to some extent, based on the Rossmann fold, their tertiary and quaternary structures are very different. The four molecules in the RpiB asymmetric unit represent a dimer of dimers. Active-site residues were identified at the interface between the subunits, such that each active site has contributions from both subunits. Kinetic studies indicate that RpiB is nearly as efficient as RpiA, despite its completely different catalytic machinery. The sequence and structural results further suggest that the two homologous components of LacAB (galactose-6-phosphate isomerase) will compose a bi-functional enzyme; the second activity is unknown.
ribose-5-phosphate isomerase; pentose phosphate pathway; galactose-6-phosphate isomerase; MAD; X-ray crystallography
Structural proteomics projects are generating three-dimensional structures of novel, uncharacterized proteins at an increasing rate. However, structure alone is often insufficient to deduce the specific biochemical function of a protein. Here we determined the function for a protein using a strategy that integrates structural and bioinformatics data with parallel experimental screening for enzymatic activity. BioH is involved in biotin biosynthesis in Escherichia coli and had no previously known biochemical function. The crystal structure of BioH was determined at 1.7 Å resolution. An automated procedure was used to compare the structure of BioH with structural templates from a variety of different enzyme active sites. This screen identified a catalytic triad (Ser82, His235, and Asp207) with a configuration similar to that of the catalytic triad of hydrolases. Analysis of BioH with a panel of hydrolase assays revealed a carboxylesterase activity with a preference for short acyl chain substrates. The combined use of structural bioinformatics with experimental screens for detecting enzyme activity could greatly enhance the rate at which function is determined from structure.
Polyamines are essential in all branches of life. Spermidine synthase (putrescine aminopropyltransferase, PAPT) catalyzes the biosynthesis of spermidine, a ubiquitous polyamine. The crystal structure of the PAPT from Thermotoga maritima (TmPAPT) has been solved to 1.5 Å resolution in the presence and absence of AdoDATO (S-adenosyl-1,8-diamino-3-thiooctane), a compound containing both substrate and product moieties. This, the first structure of an aminopropyltransferase, reveals deep cavities for binding substrate and cofactor, and a loop that envelops the active site. The AdoDATO binding site is lined with residues conserved in PAPT enzymes from bacteria to humans, suggesting a universal catalytic mechanism. Other conserved residues act sterically to provide a structural basis for polyamine specificity. The enzyme is tetrameric; each monomer consists of a C-terminal domain with a Rossmann-like fold and an N-terminal β-stranded domain. The tetramer is assembled using a novel barrel-type oligomerization motif.
Members of the IclR family of transcription regulators modulate signal-dependent expression of genes involved in carbon metabolism in bacteria and archaea. The Thermotoga maritima TM0065 gene codes for a protein (TM-IclR) that is homologous to the IclR family. We have determined the crystal structure of TM-IclR at 2.2 Å resolution using MAD phasing and synchrotron radiation. The protein is composed of two domains: the N-terminal DNA-binding domain contains the winged helix-turn-helix motif, and the C-terminal presumed regulatory domain is involved in binding signal molecule. In a proposed signal-binding site, a bound Zn2+ ion was found. In the crystal, TM-IclR forms a dimer through interactions between DNA-binding domains. In the dimer, the DNA-binding domains are 2-fold related, but the dimer is asymmetric with respect to the orientation of signal-binding domains. Crystal packing analysis showed that TM-IclR dimers form a tetramer through interactions exclusively by signal-binding domains. A model is proposed for binding of IclR-like factors to DNA, and it suggests that signal-dependent transcription regulation is accomplished by affecting an oligomerization state of IclR and therefore its affinity for DNA target.
IpaH proteins are E3 ubiquitin ligases delivered by the type III secretion apparatus into host cells upon infection of humans by the Gram-negative pathogen Shigella flexneri. These proteins comprise a variable leucine-rich repeat—containing N-terminal domain and a conserved C-terminal domain harboring an invariant cysteine residue that is crucial for activity. IpaH homologs are encoded by diverse animal and plant pathogens. Here we demonstrate that the IpaH C-terminal domain carries the catalytic activity for ubiquitin transfer and that the N-terminal domain carries the substrate specificity. The structure of the IpaH C-terminal domain, determined to 2.65-Å resolution, represents an all-helical fold bearing no resemblance to previously defined E3 ubiquitin ligases. The conserved and essential cysteine residue lies on a flexible, surface-exposed loop surrounded by conserved acidic residues, two of which are crucial for IpaH activity.
Glutaminases belong to the large superfamily of serine-dependent β-lactamases and penicillin-binding proteins, and they catalyze the hydrolytic deamidation of l-glutamine to l-glutamate. In this work, we purified and biochemically characterized four predicted glutaminases from Escherichia coli (YbaS and YneH) and Bacillus subtilis (YlaM and YbgJ). The proteins demonstrated strict specificity to l-glutamine and did not hydrolyze d-glutamine or l-asparagine. In each organism, one glutaminase showed higher affinity to glutamine (E. coli YbaS and B. subtilis YlaM; Km 7.3 and 7.6 mM, respectively) than the second glutaminase (E. coli YneH and B. subtilis YbgJ; Km 27.6 and 30.6 mM, respectively). The crystal structures of the E. coli YbaS and the B. subtilis YbgJ revealed the presence of a classical β-lactamase-like fold and conservation of several key catalytic residues of β-lactamases (Ser74, Lys77, Asn126, Lys268, and Ser269 in YbgJ). Alanine replacement mutagenesis demonstrated that most of the conserved residues located in the putative glutaminase catalytic site are essential for activity. The crystal structure of the YbgJ complex with the glutaminase inhibitor 6-diazo-5-oxo-l-norleucine revealed the presence of a covalent bond between the inhibitor and the hydroxyl oxygen of Ser74, providing evidence that Ser74 is the primary catalytic nucleophile and that the glutaminase reaction proceeds through formation of an enzyme–glutamyl intermediate. Growth experiments with the E. coli glutaminase deletion strains revealed that YneH is involved in the assimilation of l-glutamine as a sole source of carbon and nitrogen and suggested that both glutaminases (YbaS and YneH) also contribute to acid resistance in E. coli.
The 3′ processing of most bacterial precursor tRNAs involves exonucleolytic trimming to yield a mature CCA end. This step is carried out by RNase T, a member of the large DEDD family of exonucleases. We report the crystal structures of RNase T from Escherichia coli and Pseudomonas aeruginosa, which show that this enzyme adopts an opposing dimeric arrangement, with the catalytic DEDD residues from one monomer closely juxtaposed with a large basic patch on the other monomer. This arrangement suggests that RNase T has to be dimeric for substrate specificity, and agrees very well with prior site-directed mutagenesis studies. The dimeric architecture of RNase T is very similar to the arrangement seen in oligoribonuclease, another bacterial DEDD family exoribonuclease. The catalytic residues in these two enzymes are organized very similarly to the catalytic domain of the third DEDD family exoribonuclease in E. coli, RNase D, which is monomeric.
The crystal structure of phophoribosylaminoimidazole-succinocarboxamide or SAICAR synthase from T. maritima at 2.2 Å revealed an unusual covalent dimer.
As a part of a structural genomics program, the 2.2 Å resolution crystal structure of the PurC gene product from Thermotoga maritima has been solved. This 26.2 kDa protein belongs to the phophoribosylaminoimidazole-succinocarboxamide or SAICAR synthase family of enzymes, the members of which are involved in de novo purine biosynthesis. SAICAR synthase can be divided into three subdomains: two α+β regions exhibiting structural homology with ATP-binding proteins and a carboxy-terminal subdomain of two α-helices. The asymmetric unit contains two copies of the protein which are covalently linked by a disulfide bond between Cys126(A) and Cys126(B). This 230-amino-acid protein exhibits high structural homology with SAICAR synthase from baker’s yeast. The protein structure is described and compared with that of the ATP–SAICAR synthase complex from yeast.
SAICAR synthase; purine biosynthesis; Thermotoga maritima