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We report the crystal structure of nuclear import receptor Importin-9 bound to its cargo, the histones H2A-H2B. Importin-9 wraps around the core, globular region of H2A-H2B to form an extensive interface. The nature of this interface coupled with quantitative analysis of deletion mutants of H2A-H2B suggests that the NLS-like sequences in the H2A-H2B tails play a minor role in import. Importin-9•H2A-H2B is reminiscent of interactions between histones and histone chaperones in that it precludes H2A-H2B interactions with DNA and H3-H4 as seen in the nucleosome. Like many histone chaperones, which prevent inappropriate non-nucleosomal interactions, Importin-9 also sequesters H2A-H2B from DNA. Importin-9 appears to act as a storage chaperone for H2A-H2B while escorting it to the nucleus. Surprisingly, RanGTP does not dissociate Importin-9•H2A-H2B but assembles into a RanGTP•Importin-9•H2A-H2B complex. The presence of Ran in the complex, however, modulates Imp9-H2A-H2B interactions to facilitate its dissociation by DNA and assembly into a nucleosome.

The modification of proteins by phosphorylation occurs in all life forms and is catalyzed by a large superfamily of enzymes known as protein kinases. We recently discovered a family of secretory pathway kinases that phosphorylate extracellular proteins. One member, family with sequence similarity 20C (Fam20C), is the physiological Golgi casein kinase. While examining distantly related protein sequences, we observed low levels of identity between the spore coat protein H (CotH), and the Fam20C-related secretory pathway kinases. CotH is a component of the spore in many bacterial and eukaryotic species, and is required for efficient germination of spores in Bacillus subtilis; however, the mechanism by which CotH affects germination is unclear. Here, we show that CotH is a protein kinase. The crystal structure of CotH reveals an atypical protein kinase-like fold with a unique mode of ATP binding. Examination of the genes neighboring cotH in B. subtilis led us to identify two spore coat proteins, CotB and CotG, as CotH substrates. Furthermore, we show that CotH-dependent phosphorylation of CotB and CotG is required for the efficient germination of B. subtilis spores. Collectively, our results define a family of atypical protein kinases and reveal an unexpected role for protein phosphorylation in spore biology.

The hypoxia-inducible factor complex (HIF-α·aryl hydrocarbon receptor nuclear translocator (ARNT)) requires association with several transcription coactivators for a successful cellular response to hypoxic stress. In addition to the conventional global transcription coactivator CREB-binding protein/p300 (CBP/p300) that binds to the HIF-α transactivation domain, a new group of transcription coactivators called the coiled-coil coactivators (CCCs) interact directly with the second PER-ARNT-SIM (PAS) domain of ARNT (ARNT PAS-B). These less studied transcription coactivators play essential roles in the HIF-dependent hypoxia response, and CCC misregulation is associated with several forms of cancer. To better understand CCC protein recruitment by the heterodimeric HIF transcription factor, we used x-ray crystallography, NMR spectroscopy, and biochemical methods to investigate the structure of the ARNT PAS-B domain in complex with the C-terminal fragment of a coiled-coil coactivator protein, transforming acidic coiled-coil coactivator 3 (TACC3). We found that the HIF-2α PAS-B domain also directly interacts with TACC3, motivating an NMR data-derived model suggesting a means by which TACC3 could form a ternary complex with HIF-2α PAS-B and ARNT PAS-B via β-sheet/coiled-coil interactions. These findings suggest that TACC3 could be recruited as a bridge to cooperatively mediate between the HIF-2α PAS-B·ARNT PAS-B complex, thereby participating more directly in HIF-dependent gene transcription than previously anticipated.

Arboviruses are a diverse group of insect-transmitted pathogens that pose global public health challenges. Identifying evolutionarily conserved host factors that combat arbovirus replication in disparate eukaryotic hosts is important as they may tip the balance between productive and abortive viral replication, and thus determine virus host range. Here, we exploit naturally abortive arbovirus infections that we identified in lepidopteran cells and use bacterial effector proteins to uncover host factors restricting arbovirus replication. Bacterial effectors are proteins secreted by pathogenic bacteria into eukaryotic hosts cells that can inhibit antimicrobial defenses. Since bacteria and viruses can encounter common host defenses, we hypothesized that some bacterial effectors may inhibit host factors that restrict arbovirus replication in lepidopteran cells. Thus, we used bacterial effectors as molecular tools to identify host factors that restrict four distinct arboviruses in lepidopteran cells. By screening 210 effectors encoded by seven different bacterial pathogens, we identify several effectors that individually rescue the replication of all four arboviruses. We show that these effectors encode diverse enzymatic activities that are required to break arbovirus restriction. We further characterize Shigella flexneri-encoded IpaH4 as an E3 ubiquitin ligase that directly ubiquitinates two evolutionarily conserved proteins, SHOC2 and PSMC1, promoting their degradation in insect and human cells. We show that depletion of either SHOC2 or PSMC1 in insect or human cells promotes arbovirus replication, indicating that these are ancient virus restriction factors conserved across invertebrate and vertebrate hosts. Collectively, our study reveals a novel pathogen-guided approach to identify conserved antimicrobial machinery, new effector functions, and conserved roles for SHOC2 and PSMC1 in virus restriction. Author summary: Microbial pathogens such as viruses and bacteria encounter diverse host cell responses during infection. While viruses possess antagonists to counter these responses in natural host species, their replication can be restricted in unnatural host cells where their antagonists are ineffective. Bacteria also employ a diverse repertoire of immune evasion proteins known as "effectors" that can inhibit antimicrobial responses found in invertebrate and vertebrate hosts. In this study, we hypothesized that some bacterial effectors may target host immunity proteins that restrict both bacteria and viruses. To test this hypothesis, we screened a bacterial effector library comprising 210 effectors from seven distinct bacterial pathogens for their ability to rescue the replication of four viruses in insect cells that are normally non-permissive to these viruses. Though numerous effectors were identified to rescue the replication of each virus, the uncharacterized IpaH4 protein encoded by the human pathogen Shigella flexneri was able to rescue all four viruses screened. We discovered that IpaH4 enhances arbovirus replication in both restrictive insect and permissive human cells by directly targeting two novel, evolutionarily conserved antiviral host proteins, SHOC2 and PSMC1, for degradation. Our study establishes bacterial effectors as valuable tools for identifying critical antimicrobial machinery employed by eukaryotic hosts. [ABSTRACT FROM AUTHOR]

Lipoxygenases are an important class of non-heme iron enzymes that catalyze the hydroperoxidation of unsaturated fatty acids. The details of the enzymatic mechanism of lipoxygenases are still not well understood. This study utilizes a combination of kinetic and structural probes to relate the lipoxygenase mechanism of action with structural modifications of the iron's second coordination sphere. The second coordination sphere consists of Gln(495) and Gln(697), which form a hydrogen bond network between the substrate cavity and the first coordination sphere (Asn(694)). In this investigation, we compared the kinetic and structural properties of four mutants (Q495E, Q495A, Q697N, and Q697E) with those of wild-type soybean lipoxygenase-1 and determined that changes in the second coordination sphere affected the enzymatic activity by hydrogen bond rearrangement and substrate positioning through interaction with Gln(495). The nature of the C-H bond cleavage event remained unchanged, which demonstrates that the mutations have not affected the mechanism of hydrogen atom tunneling. The unusual and dramatic inverse solvent isotope effect (SIE) observed for the Q697E mutant indicated that an Fe(III)-OH(-) is the active site base. A new transition state model for hydrogen atom abstraction is proposed.

The fidelity and specificity of information flow within a cell is controlled by scaffolding proteins that assemble and link enzymes into signalling circuits (1,2). These circuits can be inhibited by bacterial effector proteins that post-translationally modify individual pathway components (3-6). However, there is emerging evidence that pathogens directly organize higher-order signalling networks through enzyme scaffolding (7,8), and the identity of the effectors and their mechanisms of action are poorly understood. Here we identify the enterohaemorrhagic Escherichia coli O157:H7 type III effector EspG as a regulator of endomembrane trafficking using a functional screen, and report ADP-ribosylation factor (ARF) GTPases and p21-activated kinases (PAKs) as its relevant host substrates. The 2.5 A crystal structure of EspG in complex with ARF6 shows how EspG blocks GTPase-activating-protein-assisted GTP hydrolysis, revealing a potent mechanism of GTPase signalling inhibition at organelle membranes. In addition, the 2.8 Å crystal structure of EspG in complex with the autoinhibitory Iα3-helix of PAK2 defines a previously unknown catalytic site in EspG and provides an allosteric mechanism of kinase activation by a bacterial effector. Unexpectedly, ARF and PAKs are organized on adjacent surfaces of EspG, indicating its role as a 'catalytic scaffold' that effectively reprograms cellular events through the functional assembly of GTPase-kinase signalling complex., To identify new signalling pathways targetedby bacterial pathogens, we used a human growth hormone (hGH) secretion assay (9) to measure the ability of type III and type IV effector proteins [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.08.048 Byline: Maojun Yang (1), Bing Li (1), Diana R. Tomchick (2), Mischa Machius (2), Josep Rizo (1)(2), Hongtao Yu (1), Xuelian Luo (1) Keywords: CELLCYCLE Abstract: The status of spindle checkpoint signaling depends on the balance of two opposing dynamic processes that regulate the highly unusual two-state behavior of Mad2. In mitosis, a Mad1-Mad2 core complex recruits cytosolic Mad2 to kinetochores through Mad2 dimerization and converts Mad2 to a conformer amenable to Cdc20 binding, thereby facilitating checkpoint activation. p31.sup.comet inactivates the checkpoint through binding to Mad1- or Cdc20-bound Mad2, thereby preventing Mad2 activation and promoting the dissociation of the Mad2-Cdc20 complex. Here, we report the crystal structure of the Mad2-p31.sup.comet complex. The C-terminal region of Mad2 that undergoes rearrangement in different Mad2 conformers is a major structural determinant for p31.sup.comet binding, explaining the specificity of p31.sup.comet toward Mad1- or Cdc20-bound Mad2. p31.sup.comet adopts a fold strikingly similar to that of Mad2 and binds at the dimerization interface of Mad2. Thus, p31.sup.comet exploits the two-state behavior of Mad2 to block its activation by acting as an 'anti-Mad2.' Author Affiliation: (1) Department of Pharmacology, The University of Texas Southwestern Medical Center, 6001 Forest Park Road, Dallas, TX 75390, USA (2) Department of Biochemistry, The University of Texas Southwestern Medical Center, 6001 Forest Park Road, Dallas, TX 75390, USA Article History: Received 4 April 2007; Revised 28 June 2007; Accepted 30 August 2007 Article Note: (miscellaneous) Published: November 15, 2007

Research has been conducted on the cytochrome P450s catalyzing the oxidation of the structurally and chemically diverse hydrophobic substrates. The crystal structure of the complex between the bacterial P450BM-3 and substrate N-palmitoylglycine has been investigated and the results are presented.

The mechanism for synergistic inhibition of the Bacillus subtilis glutamine 5-phosphoribosyl-1-pyrophosphate (PRPP) amidotransfearse was analyzed through the utilization of nucleotide binding assay. It was indicated that ADP and GMP were the most synergistic pair and has demonstrated that synergistic inhibition resulted from substrate binding. It was indicated that ADP binds to the A site and GMP to the C site. Synergism accounted for the specific interaction between the nucleoside beta-diphosphate in the A site and a nucleoside phosphate in the C site.

Three enzymes and their capability for autocatalytic activation and engaging in nucleophilic attack are discussed. These three are glutamine PRPP amidotransferase, penicillin acylase and poteasome. They occupy a common fold that enables them to perform similar activities although they look divergent enough to mask their common evolutionary pattern and origin. A suggested name for this set of enzymes is Ntn hydrolases.