[Show abstract][Hide abstract] ABSTRACT: Cyclic dinucleotides (CDNs) and antitumor/antiviral agents (DMXAA and CMA) trigger STING-dependent innate immunity activation. Accumulative evidences have showed that DMXAA and CMA selectively activate mouse, but not human STING signaling. The mechanism underlying this species selectivity remains poorly understood. In this report, we have shown that human and rat STINGs display more similar signaling profiles toward DMXAA and CMA than that of human and mouse STINGs, suggesting that rat is more suitable for preclinical testing of STING-targeted drugs. We have also determined the crystal structures of both apo rat STING and its complex with cyclic GMP-AMP with 2'5' and 3'5' phosphodiester linkage (2'3'-cGAMP), a human endogenous CDN. Structure-guided biochemical analysis also revealed the functional importance of the connecting loop (A140-N152) between membrane and cytosolic domains in STING activation. Taken together, these findings reveal that rat STING is more closely related to human STING in terms of substrate preference, serving as a foundation for the development of STING-targeted drugs.
Full-text · Article · Dec 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: Importance:
Oncogenic herpesvirus KSHV is the etiological agent of Kaposi's sarcoma, primary effusion lymphoma and multicentric Castelman's disease. KSHV has developed a unique mechanism to subvert host antiviral immune responses by encoding four homologues of cellular interferon regulatory factors (vIRF1∼4). However, none of their DNA-binding profiles in human genome have been characterized till now, and the structural basis for their diverse DNA-binding properties remain poorly understood. In this study, we performed the first human genome and found it can bind to the promoter regions of 100 target cellular genes. X-ray structure analysis and functional studies provided unique insights into its DNA-binding potency and regulation of target genes expression. Our study suggested that vIRF2 could act as a transcription factor of its target genes and contribute to KSHV infection and pathogenesis through versatile functions.
No preview · Article · Nov 2015 · Journal of Virology
[Show abstract][Hide abstract] ABSTRACT: Early on, crystallography was a domain of mineralogy and mathematics and dealt mostly with symmetry properties and imaginary crystal lattices. This changed when Wilhelm Conrad Röntgen discovered X-rays in 1895, and in 1912, Max von Laue and his associates discovered that X-ray irradiated salt crystals would produce diffraction patterns that could reveal the internal atomic periodicity of the crystals. In the same year, the father-and-son team, Henry and Lawrence Bragg successfully solved the first crystal structure of sodium chloride and the era of modern crystallography began. Protein crystallography (PX) started some 20 years later with the pioneering work of British crystallographers. In the past 50–60 years, the achievements of modern crystallography and particularly those in PX have been due to breakthroughs in theoretical and technical advancements such as phasing and direct methods; to more powerful X-ray sources such as synchrotron radiation; to more sensitive and efficient X-ray detectors; to ever faster computers and to improvements in software. The exponential development of PX has been accelerated by the invention and applications of recombinant DNA technology that can yield nearly any protein of interest in large amounts and with relative ease. Novel methods, informatics platforms and technologies for automation and high-throughput have allowed the development of large-scale, high-efficiency macromolecular crystallography efforts in the field of structural genomics. Very recently, the X-ray free-electron laser sources and its applications in PX have shown great potential for revolutionizing the whole field again in the near future.
Full-text · Article · Jan 2015 · Crystallography Reviews
[Show abstract][Hide abstract] ABSTRACT: Iron(II) and 2-oxoglutarate (2OG)-dependent dioxygenases involved in histone and DNA/RNA demethylation convert the cosubstrate 2OG and oxygen to succinate and carbon dioxide, resulting in hydroxylation of the methyl group of the substrates and subsequent demethylation. Recent evidence has shown that these 2OG dioxygenases play vital roles in a variety of biological processes, including transcriptional regulation and gene expression. In this review, the structure and function of these dioxygenases in histone and nucleic acid demethylation will be discussed. Given the important roles of these 2OG dioxygenases, detailed analysis and comparison of the 2OG dioxygenases will guide the design of target-specific small-molecule chemical probes and inhibitors.
[Show abstract][Hide abstract] ABSTRACT: RsmI and RsmH are AdoMet-dependent methyltransferases that are responsible for the 2'-O-methylation and N(4)-methylation of C1402 of Escherichia coli 16S rRNA, respectively. Modification of this site has been found to play a role in fine-tuning the shape and function of the P-site to increase the decoding fidelity. It is interesting to study the mechanism by which C1402 can be methylated by both RsmI and RsmH. The crystal structure of RsmH in complex with AdoMet and cytidine has recently been determined and provided some implications for N(4)-methylation of this site. Here, the purification and crystallization of RsmI as well as its preliminary crystallographic analysis are reported. Co-crystallization of RsmI with AdoMet was carried out by the sitting-drop vapour-diffusion method and X-ray diffraction data were collected to 2.60 Å resolution on beamline 1W2B at BSRF. The crystal contained three molecules per asymmetric unit and belonged to space group C2, with unit-cell parameters a = 121.9, b = 152.5, c = 54.2 Å, β = 93.4°.
No preview · Article · Sep 2014 · Acta Crystallographica Section F Structural Biology and Crystallization Communications
[Show abstract][Hide abstract] ABSTRACT: A diverse superfamily of phospholipases consisting of the type VI lipase effectors Tle1–Tle5 secreted by the bacterial type VI secretion system (T6SS) have recently been identified as antibacterial effectors that hydrolyze membrane phospholipids. These effectors show no significant homology to known lipases, and their mechanism of membrane targeting and hydrolysis of phospholipids remains unknown. Here, the crystal structure of Tle1 (∼96.5 kDa) from
refined to 2.0 Å resolution is reported, representing the first structure of this superfamily. Its overall structure can be divided into two distinct parts, the phospholipase catalytic module and the putative membrane-anchoring module; this arrangement has not previously been observed in known lipase structures. The phospholipase catalytic module has a canonical α/β-hydrolase fold and mutation of any residue in the Ser-Asp-His catalytic triad abolishes its toxicity. The putative membrane-anchoring module adopts an open conformation composed of three amphipathic domains, and its partial folds are similar to those of several periplasmic or membrane proteins. A cell-toxicity assay revealed that the putative membrane-anchoring module is critical to Tle1 antibacterial activity. A molecular-dynamics (MD) simulation system in which the putative membrane-anchoring module embedded into a bilayer was stable over 50 ns. These structure–function studies provide insight into the hydrolysis and membrane-targeting process of the unique phospholipase Tle1.
No preview · Article · Aug 2014 · Acta Crystallographica Section D Biological Crystallography
[Show abstract][Hide abstract] ABSTRACT: The putative protein PA5089 from Pseudomonas aeruginosa has recently been identified as a Tle5 phospholipase effector from a type VI secretion system (T6SS), and its toxicity can be neutralized by the cognate immunity protein Tli5 (PA5088). Here, the expression, purification, crystallization and preliminary crystallographic analysis of PA5088 are reported. X-ray diffraction data were collected from selenomethionine-derivatized PA5088 crystals to a resolution of 2.55 Å. The crystals belonged to space group P21, with unit-cell parameters a = 64.002, b = 104.744, c = 90.168 Å.
No preview · Article · Jul 2014 · Acta Crystallographica Section F Structural Biology and Crystallization Communications
[Show abstract][Hide abstract] ABSTRACT: The bacterial type VI secretion system (T6SS) is used by donor cells to inject toxic effectors into receptor cells. The donor cells produce the corresponding immunity proteins to protect themselves against the effector proteins, thereby preventing their self-intoxication. Recently, the C-terminal domain of VgrG3 was identified as a T6SS effector. Information on the molecular mechanism of VgrG3 and its immunity protein TsaB has been lacking. Here, we determined the crystal structures of native TsaB and the VgrG3C-TsaB complex. VgrG3C adopts a canonical phage-T4-lysozyme-like fold. TsaB interacts with VgrG3C through molecular mimicry, and inserts into the VgrG3C pocket.
[Show abstract][Hide abstract] ABSTRACT: The bacterial type VI secretion system (T6SS), a dynamic organelle, participates in microbial competition by transporting toxic effector molecules to neighbouring cells to kill competitors. TsiV3, a recently defined T6SS immunity protein in Vibrio cholerae, possesses self-protection against killing by T6SS predatory cells by directly binding to and inhibiting their effector protein VgrG-3. Structural information about TsiV3 could help to illuminate its specific mechanism. In this study, TsiV3 from V. cholerae was cloned, expressed and crystallized and single-crystal X-ray diffraction data sets were collected to a resolution of 2.55 Å. Specifically, the crystal belonged to space group P212121, with unit-cell parameters a = 73.3, b = 78.12, c = 106.18 Å. Matthews coefficient calculations indicated that the crystal may contain six TsiV3 molecules in one asymmetric unit, with a VM value of 2.25 Å(3) Da(-1) and a solvent content of 45.42%.
No preview · Article · Mar 2014 · Acta Crystallographica Section F: Structural Biology Communications
[Show abstract][Hide abstract] ABSTRACT: Escherichia coli RnlA-RnlB is a newly identified toxin-antitoxin (TA) system that plays a role in bacteriophage resistance. RnlA functions as a toxin with mRNA endoribonuclease activity and the cognate antitoxin RnlB inhibits RnlA toxicity in E. coli cells. Interestingly, T4 phage encodes the antitoxin Dmd, which acts against RnlA to promote its own propagation, suggesting that RnlA-Dmd represents a novel TA system. Here, we have determined the crystal structure of RnlA refined to 2.10 Å. RnlA is composed of three independent domains: NTD (N-terminal domain), NRD (N repeated domain) and DBD (Dmd binding domain), which is an organization not previously observed among known toxin structures. Small-angle X-ray scattering (SAXS) analysis revealed that RnlA forms a dimer in solution via interactions between the DBDs from both monomers. The in vitro and in vivo functional studies showed that among the three domains, only the DBD is responsible for recognition and inhibition by Dmd and subcellular location of RnlA. In particular, the helix located at the C-terminus of DBD plays a vital role in binding Dmd. Our comprehensive studies reveal the key region responsible for RnlA toxicity and provide novel insights into its structure-function relationship.
No preview · Article · Sep 2013 · Molecular Microbiology
[Show abstract][Hide abstract] ABSTRACT: The Gram-negative bacteria type VI secretion system (T6SS) has been found to play an important role in interbacterial competition, biofilm formation and many other virulence-related processes. The bacteria harboring T6SS inject the effectors into their recipient's cytoplasm or periplasm to kill them and meanwhile, to avoid inhibiting itself, the cognate immunity proteins were produced to acts as the effector inhibitor. Tae4 (type VI amidase effector 4) and Tai4 (type VI amidase immunity 4) are newly identified T6SS effector-immunity (EI) pairs. We have recently solved the structures of StTae4-Tai4 and EcTae4-Tai4 complexes from the human pathogens Salmonella typhimurium and Enterobacter cloacae, respectively. It is very interesting and important to discover whether there is cross-neutralization between St- and EcTai4 and whether their effector inhibition mechanism is conserved. Here, we determined the crystal structure of StTae4 in complex with EcTai4. The solution conformation study revealed it is a compact heterotetramer that consists of an EcTai4 homodimer binding two StTae4 molecules in solution, different from that in crystal. A remarkable shift can be observed in both the flexible winding loop of StTae4 and protruding loop of EcTai4 and disulfide bonds are formed to stabilize their overall conformations. The in vitro and in vivo interactions studies showed EcTai4 can efficiently rescue the cells from the toxicity of its cognate effectors StTae4, but can not neutralize the toxic activities of the effectors from other families. These findings provide clear structural evidence to support the previous observation of cross-immunity within T6SS families and provide a basis for understanding their important roles in polymicrobial environments.
[Show abstract][Hide abstract] ABSTRACT: The recently described type VI secretion system (T6SS) acts as a needle that punctures the membrane of the target cells to deliver effector proteins. Type VI amidase effectors can be classified into four divergent families (Tae1-4). These effectors are secreted into the periplasmic space of neighboring cells via the T6SS and subsequently rupture peptidoglycan. However, the donor cells are protected from damage because of the presence of their cognate immunity proteins (Tai1-4). Here, we describe the structure of Tae3 in complex with Tai3. The Tae3-Tai3 complex exists as a stable heterohexamer, which is composed of two Tae3 molecules and two Tai3 homodimers (Tae3-Tai34-Tae3). Tae3 shares a common NlpC/P60 fold, which consists of an Nt-subdomain and a Ct-subdomain. Structural analysis indicates that two unique loops around the catalytic cleft adopt a closed conformation, resulting in a narrow and extended groove involved in the binding of the substrate. The inhibition of Tae3 is attributed to the insertion of the Ω-loop of Tai3 into the catalytic groove. Furthermore, a cell viability assay confirmed that a conserved motif (Q-D-X) in Tai3 members may play a key role in the inhibition process. Taken together, our studies have revealed a novel inhibition mechanism and provide insights into the role played by T6SS in interspecific competition.
No preview · Article · Jun 2013 · Biochemical Journal
[Show abstract][Hide abstract] ABSTRACT: RlmM is an AdoMet-dependent methyltransferase that is responsible for 2'-O-methylation of C2498 in the peptidyl-transferase loop of bacterial 23S rRNA. This modification occurs before assembly of the 50S ribosomal subunit, and lack of C2498 methylation can cause a slight reduction in bacterial fitness. Here, the purification and crystallization of RlmM from Escherichia coli as well as its preliminary crystallographic analysis are presented. Cocrystallization of RlmM with AdoMet was carried out and X-ray diffraction data were collected to a resolution of 2.30 Å on beamline BL17U at the SSRF. However, electron density for AdoMet cannot be observed by comprehensive crystallographic analysis, indicating that it is not bound by RlmM during the cocrystallization process. The structure was solved by molecular replacement and refinement is in progress. The crystal contained one molecule in the asymmetric unit and belonged to space group P21, with unit-cell parameters a = 56.07, b = 59.38, c = 54.35 Å, β = 94.84°, which differs from the P31 or P3121 space groups of previously reported RlmM structures (PDB entries 4auk, 4atn and 4b17).
No preview · Article · Jun 2013 · Acta Crystallographica Section F Structural Biology and Crystallization Communications
[Show abstract][Hide abstract] ABSTRACT: Type 2 diabetes is a chronic inflammatory metabolic disease, the key point being insulin resistance. Endoplasmic reticulum (ER) stress plays a critical role in the pathogenesis of type 2 diabetes. Previously, we found that hyperhomocysteinemia (HHcy) induced insulin resistance in adipose tissue. Here, we hypothesized that HHcy induces ER stress, which in turn promotes insulin resistance. In the present study, the direct effect of Hcy on adipose ER stress was investigated by use of primary rat adipocytes in vitro and mice with HHcy in vivo. The mechanism and the effect of G protein-coupled receptor 120 (GPR 120) were also investigated. We found that phosphorylation or expression of variant ER stress markers was elevated in adipose tissue of HHcy mice. HHcy activated c-Jun N-terminal kinase (JNK), the downstream signal of ER stress in adipose tissue, and activated JNK participated in insulin resistance by inhibiting Akt activation. Furthermore, JNK activated c-Jun and p65, which in turn triggered the transcription of pro-inflammatory cytokines. Both in vivo and in vitro assays revealed that Hcy-promoted macrophage infiltration aggravated ER stress in adipose tissue. Chemical chaperones PBA and TUDCA could reverse Hcy-induced inflammation and restore insulin-stimulated glucose uptake and Akt activation. Activation of GPR 120 reversed Hcy-induced JNK activation and prevented inflammation but not ER stress. Therefore, HHcy inhibited insulin sensitivity in adipose tissue by inducing ER stress, activating JNK to promote pro-inflammatory cytokine production and facilitating macrophage infiltration. These findings reveal a new mechanism of HHcy in the pathogenesis of insulin resistance.
Full-text · Article · Feb 2013 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The type VI secretion system (T6SS), a multisubunit needle-like apparatus, has recently been found to play a role in interspecies
interactions. The Gram-negative bacteria harboring T6SS (donor) deliver the effectors into their neighboring cells (recipient)
to kill them. Meanwhile, the cognate immunity proteins were employed to protect the donor cells against the toxic effectors.
Tae4 (type VI amidase effector 4) and Tai4 (type VI amidase immunity 4) are newly identified T6SS effector-immunity pairs. Here, we report the crystal structures of Tae4 from Enterobacter cloacae and Tae4-Tai4 complexes from both E. cloacae and Salmonella typhimurium. Tae4 acts as a dl-endopeptidase and displays a typical N1pC/P60 domain. Unlike Tsi1 (type VI secretion immunity 1), Tai4 is an all-helical protein and forms a dimer in solution. The small angle x-ray scattering study combined
with the analytical ultracentrifugation reveal that the Tae4-Tai4 complex is a compact heterotetramer that consists of a Tai4
dimer and two Tae4 molecules in solution. Structure-based mutational analysis of the Tae4-Tai4 interface shows that a helix
(α3) of one subunit in dimeric Tai4 plays a major role in binding of Tae4, whereas a protruding loop (L4) in the other subunit
is mainly responsible for inhibiting Tae4 activity. The inhibition process requires collaboration between the Tai4 dimer.
These results reveal a novel and unique inhibition mechanism in effector-immunity pairs and suggest a new strategy to develop
Preview · Article · Jan 2013 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: RsmE is the founding member of a new RNA methyltransferase (MTase) family responsible for methylation of U1498 in 16S ribosomal RNA in Escherichia coli. It is well conserved across bacteria and plants and may play an important role in ribosomal intersubunit communication. The crystal structure in monomer showed that it consists of two distinct but structurally related domains: the PUA (pseudouridine synthases and archaeosine-specific transglycosylases)-like RNA recognition and binding domain and the conserved MTase domain with a deep trefoil knot. Analysis of small-angle X-ray scattering data revealed that RsmE forms a flexible dimeric conformation that may be essential for substrate binding. The S-adenosyl-l-methionine (AdoMet)-binding characteristic determined by isothermal titration calorimetry suggested that there is only one AdoMet molecule bound in the subunit of the homodimer. In vitro methylation assay of the mutants based on the RsmE-AdoMet-uridylic acid complex model showed key residues involved in substrate binding and catalysis. Comprehensive comparisons of RsmE with closely related MTases, combined with the biochemical experiments, indicated that the MTase domain of one subunit in dimeric RsmE is responsible for binding of one AdoMet molecule and catalytic process while the PUA-like domain in the other subunit is mainly responsible for recognition of one substrate molecule (the ribosomal RNA fragment and ribosomal protein complex). The methylation process is required by collaboration of both subunits, and dimerization is functionally critical for catalysis. In general, our study provides new information on the structure-function relationship of RsmE and thereby suggests a novel catalytic mechanism.
No preview · Article · Aug 2012 · Journal of Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: RlmG is a specific AdoMet-dependent methyltransferase (MTase) responsible for N²-methylation of G1835 in 23S rRNA of Escherichia coli. Methylation of m²G1835 specifically enhances association of ribosomal subunits and provides a significant advantage for bacteria in osmotic and oxidative stress. Here, the crystal structure of RlmG in complex with AdoMet and its structure in solution were determined. The structure of RlmG is similar to that of the MTase RsmC, consisting of two homologous domains: the N-terminal domain (NTD) in the recognition and binding of the substrate, and the C-terminal domain (CTD) in AdoMet-binding and the catalytic process. However, there are distinct positively charged protuberances and a distribution of conserved residues contributing to the charged surface patch, especially in the NTD of RlmG for direct binding of protein-free rRNA. The RNA-binding properties of the NTD and CTD characterized by both gel electrophoresis mobility shift assays and isothermal titration calorimetry showed that NTD could bind RNA independently and RNA binding was achieved by the NTD, accomplished by a coordinating role of the CTD. The model of the RlmG-AdoMet-RNA complex suggested that RlmG may unfold its substrate RNA in the positively charged cleft between the NTD and CTD, and then G1835 disengages from its Watson-Crick pairing with C1905 and flips out to insert into the active site. Our structure and biochemical studies provide novel insights into the catalytic mechanism of G1835 methylation.
[Show abstract][Hide abstract] ABSTRACT: The type VI secretion systems (T6SS) have emerging roles in interspecies competition. In order to have an advantage in defense against other organisms, this system in Pseudomonas aeruginosa delivers a peptidoglycan amidase (Tse1) to the periplasmic space of a competitor. An immune protein (Tsi1) is also produced by the bacterium to protect itself from damage caused by Tse1. Tsi1 directly interacts with Tse1. We report that the crystal structure of Tse1 displays a common CHAP protein fold. Strikingly, our structures showed that the third residue in the catalytic triad may be novel as this residue type has not been observed previously.
[Show abstract][Hide abstract] ABSTRACT: RsmH is a specific AdoMet-dependent methyltransferase (MTase) responsible for N(4)-methylation of C1402 in 16S rRNA and conserved in almost all species of bacteria. The methylcytidine interacts with the P-site codon of the mRNA and increases ribosomal decoding fidelity. In this study, high resolution crystal structure (2.25Å) of Escherichia coli RsmH in complex with AdoMet and cytidine (the putative rRNA binding site) was determined. The structural analysis demonstrated that the complex consists of two distinct but structurally related domains: the typical MTase domain and the putative substrate recognition and binding domain. A deep pocket was found in the conserved AdoMet binding domain. It was also found that the cytidine bound far from AdoMet with the distance of 25.9Å. It indicates that the complex is not in a catalytically active state, and structural rearrangement of RsmH or the nucleotides neighboring C1402 may be necessary to trigger catalysis. Although there is only one molecule in the asymmetric unit of the crystals, RsmH can form a compact dimer across a crystallographic twofold axis. Further analysis of RsmH by small-angle X-ray scattering (SAXS) also revealed the dimer in solution, but with a more flexible conformation than that in crystal, likely resulting from the absence of the substrate. It implies that an active status of RsmH in vivo is achieved by a formation of the dimeric architecture. In general, crystal and solution structural analysis provides new information on the mechanism of the methylation of the fine-tuning ribosomal decoding center by the RsmH.
No preview · Article · Apr 2012 · Journal of Structural Biology