Crystal structure of Campylobacter jejuni lipoprotein Cj1090c.

In Gram-negative bacteria, lipopolysaccharide (LPS) is an essential component of the asymmetric outer membrane (OM). LptE is an OM lipoprotein that forms a complex with the ß-barrel OM protein, LptD. Incorporation of LPS into the OM outer leaflet is essential for bacterial viability, and mediated by the LptD/E complex. The genome of Campylobacter jejuni, a major foodborne pathogen, contains over 20 putative lipoproteins including Cj1090c. Here, we report the crystal structure of Cj1090c at 2.4 Å resolution, revealing structural evidence for LptE in C. jejuni. The analysis of this crystal structure, along with the genomic context, allows us to propose the C. jejuni LPS transport system for the first time, and permits for discussion of the features of the LptD/E complex of C. jejuni.

Conformation of Light-Harvesting Complex II Trimer Depends upon Its Binding Site.

The light-harvesting complex II (LHCII) trimer in plants functions as a major antenna complex and a quencher to protect it from photooxidative damage. Theoretical studies on the structure of an LHCII trimer have demonstrated that excitation energy transfer between chlorophylls (Chls) in LHCII can be modulated by its exquisite conformational fluctuation. However, conformational changes depending on its binding location have not yet been investigated, even though reorganization of protein complexes occurs by physiological regulations. In this study, we investigated conformational differences in LHCII by comparing published structures of an identical LHCII trimer in the three different photosystem supercomplexes from the green alga Chlamydomonas reinhardtii. Our results revealed distinct differences in Chl configurations as well as polypeptide conformations of the LHCII trimers depending on its binding location. We propose that these configurational differences readily modulate the function of LHCII and possibly lead to a change in excitation-energy flow over the photosynthetic supercomplex.

Crystal structure of Tam41 cytidine diphosphate diacylglycerol synthase from a Firmicutes bacterium.

Translocator assembly and maintenance 41 (Tam41) catalyses the synthesis of cytidine diphosphate diacylglycerol (CDP-DAG), which is a high-energy intermediate phospholipid critical for generating cardiolipin in mitochondria. Although Tam41 is present almost exclusively in eukaryotic cells, a Firmicutes bacterium contains the gene encoding Tam41-type CDP-DAG synthase (FbTam41). FbTam41 converted phosphatidic acid (PA) to CDP-DAG using a ternary complex mechanism in vitro. Additionally, FbTam41 functionally substituted yeast Tam41 in vivo. These results demonstrate that Tam41-type CDP-DAG synthase functions in some prokaryotic cells. We determined the crystal structure of FbTam41 lacking the C-terminal 18 residues in the cytidine triphosphate (CTP)-Mg2+ bound form at a resolution of 2.6 Å. The crystal structure showed that FbTam41 contained a positively charged pocket that specifically accommodated CTP-Mg2+ and PA in close proximity. By using this structure, we constructed a model for the full-length structure of FbTam41 containing the last a-helix, which was missing in the crystal structure. Based on this model, we propose a molecular mechanism for CDP-DAG synthesis in bacterial cells and mitochondria.

Single-molecule analysis reveals rotational substeps and chemo-mechanical coupling scheme of Enterococcus hirae V1-ATPase.

V1-ATPase (V1), the catalytic domain of an ion-pumping V-ATPase, is a molecular motor that converts ATP hydrolysis-derived chemical energy into rotation. Here, using a gold nanoparticle probe, we directly observed rotation of V1 from the pathogen Enterococcus hirae (EhV1). We found that 120° steps in each ATP hydrolysis event are divided into 40 and 80° substeps. In the main pause before the 40° substep and at low ATP concentration ([ATP]), the time constant was inversely proportional to [ATP], indicating that ATP binds during the main pause with a rate constant of 1.0 × 107 m-1 s-1 At high [ATP], we observed two [ATP]-independent time constants (0.5 and 0.7 ms). One of two time constants was prolonged (144 ms) in a rotation driven by slowly hydrolyzable ATPγS, indicating that ATP is cleaved during the main pause. In another subpause before the 80° substep, we noted an [ATP]-independent time constant (2.5 ms). Furthermore, in an ATP-driven rotation of an arginine-finger mutant in the presence of ADP, -80 and -40° backward steps were observed. The time constants of the pauses before -80° backward and +40° recovery steps were inversely proportional to [ADP] and [ATP], respectively, indicating that ADP- and ATP-binding events trigger these steps. Assuming that backward steps are reverse reactions, we conclude that 40 and 80° substeps are triggered by ATP binding and ADP release, respectively, and that the remaining time constant in the main pause represents phosphate release. We propose a chemo-mechanical coupling scheme of EhV1, including substeps largely different from those of F1-ATPases.

Plasmid-Based One-Pot Saturation Mutagenesis and Robot-Based Automated Screening for Protein Engineering.

We evaluated a method for protein engineering using plasmid-based one-pot saturation mutagenesis and robot-based automated screening. When the biases in nucleotides and amino acids were assessed for a loss-of-function point mutation in green fluorescent protein, the ratios of gain-of-function mutants were not significantly different from the expected values for the primers among the three different suppliers. However, deep sequencing analysis revealed that the ratios of nucleotides in the primers were highly biased among the suppliers. Biases for NNB were less severe than for NNN. We applied this method to screen a fusion protein of two chitinases, ChiA and ChiB (ChiAB). Three NNB codons as well as tyrosine and serine (X1YSX2X3) were inserted to modify the surface structure of ChiAB. We observed significant amino acid bias at the X3 position in water-soluble, active ChiAB-X1YSX2X3 mutants. Examination of the crystal structure of one active mutant, ChiAB-FYSFV, revealed that the X3 residue plays an important role in structure stabilization.

Chemical array system, a platform to identify novel hepatitis B virus entry inhibitors targeting sodium taurocholate cotransporting polypeptide.

Current anti-hepatitis B virus (HBV) agents including interferons and nucleos(t)ide analogs efficiently suppress HBV infection. However, as it is difficult to eliminate HBV from chronically infected liver, alternative anti-HBV agents targeting a new molecule are urgently needed. In this study, we applied a chemical array to high throughput screening of small molecules that interacted with sodium taurocholate cotransporting polypeptide (NTCP), an entry receptor for HBV. From approximately 30,000 compounds, we identified 74 candidates for NTCP interactants, and five out of these were shown to inhibit HBV infection in cell culture. One of such compound, NPD8716, a coumarin derivative, interacted with NTCP and inhibited HBV infection without causing cytotoxicity. Consistent with its NTCP interaction capacity, this compound was shown to block viral attachment to host hepatocytes. NPD8716 also prevented the infection with hepatitis D virus, but not hepatitis C virus, in agreement with NPD8716 specifically inhibiting NTCP-mediated infection. Analysis of derivative compounds showed that the anti-HBV activity of compounds was apparently correlated with the affinity to NTCP and the capacity to impair NTCP-mediated bile acid uptake. These results are the first to show that the chemical array technology represents a powerful platform to identify novel viral entry inhibitors.

Cyclosporin derivatives inhibit hepatitis B virus entry without interfering with NTCP transporter activity.

BACKGROUND & AIMS: The sodium taurocholate co-transporting polypeptide (NTCP) is the main target of most hepatitis B virus (HBV) specific entry inhibitors. Unfortunately, these agents also block NTCP transport of bile acids into hepatocytes, and thus have the potential to cause adverse effects. We aimed to identify small molecules that inhibit HBV entry while maintaining NTCP transporter function. METHODS: We characterized a series of cyclosporine (CsA) derivatives for their anti-HBV activity and NTCP binding specificity using HepG2 cells overexpressing NTCP and primary human hepatocytes. The four most potent derivatives were tested for their capacity to prevent HBV entry, but maintain NTCP transporter function. Their antiviral activity against different HBV genotypes was analysed. RESULTS: We identified several CsA derivatives that inhibited HBV infection with a sub-micromolar IC50. Among them, SCY446 and SCY450 showed low activity against calcineurin (CN) and cyclophilins (CyPs), two major CsA cellular targets. This suggested that instead, these compounds interacted directly with NTCP to inhibit viral attachment to host cells, and have no immunosuppressive function. Importantly, we found that SCY450 and SCY995 did not impair the NTCP-dependent uptake of bile acids, and inhibited multiple HBV genotypes including a clinically relevant nucleoside analog-resistant HBV isolate. CONCLUSIONS: This is the first example of small molecule selective inhibition of HBV entry with no decrease in NTCP transporter activity. It suggests that the anti-HBV activity can be functionally separated from bile acid transport. These broadly active anti-HBV molecules are potential candidates for developing new drugs with fewer adverse effects. LAY SUMMARY: In this study, we identified new compounds that selectively inhibited hepatitis B virus (HBV) entry, and did not impair bile acid uptake. Our evidence offers a new strategy for developing anti-HBV drugs with fewer side effects.

Crystal structure of MytiLec, a galactose-binding lectin from the mussel Mytilus galloprovincialis with cytotoxicity against certain cancer cell types.

MytiLec is a lectin, isolated from bivalves, with cytotoxic activity against cancer cell lines that express globotriaosyl ceramide, Galα(1,4)Galß(1,4)Glcα1-Cer, on the cell surface. Functional analysis shows that the protein binds to the disaccharide melibiose, Galα(1,6)Glc, and the trisaccharide globotriose, Galα(1,4)Galß(1,4)Glc. Recombinant MytiLec expressed in bacteria showed the same haemagglutinating and cytotoxic activity against Burkitt's lymphoma (Raji) cells as the native form. The crystal structure has been determined to atomic resolution, in the presence and absence of ligands, showing the protein to be a member of the ß-trefoil family, but with a mode of ligand binding unique to a small group of related trefoil lectins. Each of the three pseudo-equivalent binding sites within the monomer shows ligand binding, and the protein forms a tight dimer in solution. An engineered monomer mutant lost all cytotoxic activity against Raji cells, but retained some haemagglutination activity, showing that the quaternary structure of the protein is important for its cellular effects.

Structural insight into photoactivation of an adenylate cyclase from a photosynthetic cyanobacterium.

Cyclic-AMP is one of the most important second messengers, regulating many crucial cellular events in both prokaryotes and eukaryotes, and precise spatial and temporal control of cAMP levels by light shows great promise as a simple means of manipulating and studying numerous cell pathways and processes. The photoactivated adenylate cyclase (PAC) from the photosynthetic cyanobacterium Oscillatoria acuminata (OaPAC) is a small homodimer eminently suitable for this task, requiring only a simple flavin chromophore within a blue light using flavin (BLUF) domain. These domains, one of the most studied types of biological photoreceptor, respond to blue light and either regulate the activity of an attached enzyme domain or change its affinity for a repressor protein. BLUF domains were discovered through studies of photo-induced movements of Euglena gracilis, a unicellular flagellate, and gene expression in the purple bacterium Rhodobacter sphaeroides, but the precise details of light activation remain unknown. Here, we describe crystal structures and the light regulation mechanism of the previously undescribed OaPAC, showing a central coiled coil transmits changes from the light-sensing domains to the active sites with minimal structural rearrangement. Site-directed mutants show residues essential for signal transduction over 45 Å across the protein. The use of the protein in living human cells is demonstrated with cAMP-dependent luciferase, showing a rapid and stable response to light over many hours and activation cycles. The structures determined in this study will assist future efforts to create artificial light-regulated control modules as part of a general optogenetic toolkit.

A Novel Tricyclic Polyketide, Vanitaracin A, Specifically Inhibits the Entry of Hepatitis B and D Viruses by Targeting Sodium Taurocholate Cotransporting Polypeptide.

UNLABELLED: Anti-hepatitis B virus (HBV) drugs are currently limited to nucleos(t)ide analogs (NAs) and interferons. A challenge of drug development is the identification of small molecules that suppress HBV infection from new chemical sources. Here, from a fungus-derived secondary metabolite library, we identify a structurally novel tricyclic polyketide, named vanitaracin A, which specifically inhibits HBV infection. Vanitaracin A inhibited the viral entry process with a submicromolar 50% inhibitory concentration (IC50) (IC50 = 0.61 ± 0.23 µM), without evident cytotoxicity (50% cytotoxic concentration of >256 µM; selectivity index value of >419) in primary human hepatocytes. Vanitaracin A did not affect the HBV replication process. This compound was found to directly interact with the HBV entry receptor sodium taurocholate cotransporting polypeptide (NTCP) and impaired its bile acid transport activity. Consistent with this NTCP targeting, antiviral activity of vanitaracin A was observed with hepatitis D virus (HDV) but not hepatitis C virus. Importantly, vanitaracin A inhibited infection by all HBV genotypes tested (genotypes A to D) and clinically relevant NA-resistant HBV isolate. Thus, we identified a fungal metabolite, vanitaracin A, which was a potent, well-tolerated, and broadly active inhibitor of HBV and HDV entry. This compound, or its related analogs, could be part of an antiviral strategy for preventing reinfection with HBV, including clinically relevant nucleos(t)ide analog-resistant virus. IMPORTANCE: For achieving better treatment and prevention of hepatitis B virus (HBV) infection, anti-HBV agents targeting a new molecule are in great demand. Although sodium taurocholate cotransporting polypeptide (NTCP) has recently been reported to be an essential host factor for HBV entry, there is a limited number of reports that identify new compounds targeting NTCP and inhibiting HBV entry. Here, from an uncharacterized chemical library, we isolated a structurally new compound, named vanitaracin A, which inhibited the process of entry of HBV and hepatitis D virus (HDV). This compound was suggested to directly interact with NTCP and inhibit its transporter activity. Importantly, vanitaracin A inhibited the entry of all HBV genotypes examined and of a clinically relevant nucleos(t)ide analog-resistant HBV isolate.

Crystal structure of the Campylobacter jejuni Cj0090 protein reveals a novel variant of the immunoglobulin fold among bacterial lipoproteins.

Bacterial lipoproteins play an important role in bacterial pathogenesis and physiology. The genome of Campylobacter jejuni, a major foodborn pathogen, is predicted to contain over 20 lipoproteins. However, the functions of the majority of C. jejuni lipoproteins remain unknown. The Cj0090 protein is encoded by a lipoprotein operon composed of cj0089, cj0090, and cj0091. Here, we report the crystal structure of Cj0090 at 1.9 Å resolution, revealing a novel variant of the immunoglobulin fold with ß-sandwich architecture. The structure suggests that Cj0090 may be involved in protein-protein interactions, consistent with a possible role for bacterial lipoproteins.

Crystal structures of penicillin-binding protein 3 (PBP3) from methicillin-resistant Staphylococcus aureus in the apo and cefotaxime-bound forms.

Staphylococcus aureus is a widespread Gram-positive opportunistic pathogen, and a methicillin-resistant form (MRSA) is particularly difficult to treat clinically. We have solved two crystal structures of penicillin-binding protein (PBP) 3 (PBP3) from MRSA, the apo form and a complex with the ß-lactam antibiotic cefotaxime, and used electrospray mass spectrometry to measure its sensitivity to a variety of penicillin derivatives. PBP3 is a class B PBP, possessing an N-terminal non-penicillin-binding domain, sometimes called a dimerization domain, and a C-terminal transpeptidase domain. The model shows a different orientation of its two domains compared to earlier models of other class B PBPs and a novel, larger N-domain. Consistent with the nomenclature of "dimerization domain", the N-terminal region forms an apparently tight interaction with a neighboring molecule related by a 2-fold symmetry axis in the crystal structure. This dimer form is predicted to be highly stable in solution by the PISA server, but mass spectrometry and analytical ultracentrifugation provide unequivocal evidence that the protein is a monomer in solution.

Crystal structure of JlpA, a surface-exposed lipoprotein adhesin of Campylobacter jejuni.

The Campylobacter jejuni JlpA protein is a surface-exposed lipoprotein that was discovered as an adhesin promoting interaction with host epithelium cells, an early critical step in the pathogenesis of C. jejuni disease. Increasing evidence ascertained that JlpA is antigenic, indicating a role of JlpA in immune response during the infectious process. Here, we report the crystal structure of JlpA at 2.7Å resolution, revealing a catcher's mitt shaped unclosed half ß-barrel. Although the apparent architecture of JlpA is somewhat reminiscent of other bacterial lipoproteins such as LolB, the topology of JlpA is unique among the bacterial surface proteins reported to date and therefore JlpA represents a novel bacterial cell surface lipoprotein. The concave face of the structure results in an unusually large hydrophobic basin with a localized acidic pocket, suggesting a possibility that JlpA may accommodate multiple ligands. Therefore, the structure provides framework for determining the molecular function of JlpA and new strategies for the rational design of small molecule inhibitors efficiently targeting JlpA.

Structural insights into the glycosyltransferase activity of the Actinobacillus pleuropneumoniae HMW1C-like protein.

Glycosylation of proteins is a fundamental process that influences protein function. The Haemophilus influenzae HMW1 adhesin is an N-linked glycoprotein that mediates adherence to respiratory epithelium, an essential early step in the pathogenesis of H. influenzae disease. HMW1 is glycosylated by HMW1C, a novel glycosyltransferase in the GT41 family that creates N-glycosidic linkages with glucose and galactose at asparagine residues and di-glucose linkages at sites of glucose modification. Here we report the crystal structure of Actinobacillus pleuropneumoniae HMW1C (ApHMW1C), a functional homolog of HMW1C. The structure of ApHMW1C contains an N-terminal all α-domain (AAD) fold and a C-terminal GT-B fold with two Rossmann-like domains and lacks the tetratricopeptide repeat fold characteristic of the GT41 family. The GT-B fold harbors the binding site for UDP-hexose, and the interface of the AAD fold and the GT-B fold forms a unique groove with potential to accommodate the acceptor protein. Structure-based functional analyses demonstrated that the HMW1C protein shares the same structure as ApHMW1C and provided insights into the unique bi-functional activity of HMW1C and ApHMW1C, suggesting an explanation for the similarities and differences of the HMW1C-like proteins compared with other GT41 family members.

A novel intein-like autoproteolytic mechanism in autotransporter proteins.

Many virulence factors secreted by pathogenic Gram-negative bacteria are found to be members of the autotransporter protein family. These proteins share a common mechanism by which they exit the periplasm, involving the formation of a 12-stranded ß-barrel domain in the outer membrane. The role of this barrel in the secretion of the N-terminal passenger domain is controversial, and no model currently explains satisfactorily the entire body of experimental data. After secretion, some autotransporter barrels autoproteolytically cleave away the passenger, and one crystal structure is known for a barrel of this type in the postcleavage state. Hbp is an autotransporter of the self-cleaving type, which cuts the polypeptide between two absolutely conserved asparagine residues buried within the barrel lumen. Mutation of the first asparagine residue to isosteric aspartic acid prevents proteolysis. Here we present the crystal structure of a truncated Hbp mutant carrying the C-terminal residues of the passenger domain attached to the barrel. This model mimics the state of the protein immediately prior to separation of the passenger and barrel domains, and shows the role of residues in the so-called "linker" between the passenger and ß domains. This high-resolution membrane protein crystal structure also reveals the sites of many water molecules within the barrel. The cleavage mechanism shows similarities to those of inteins and some viral proteins, but with a novel means of promoting nucleophilic attack.

Crystallization and preliminary crystallographic studies of the butyrolactone autoregulator receptor protein (BarA) from Streptomyces virginiae.

The Streptomyces butyrolactone autoregulator receptor protein (BarA) is a DNA-binding protein that regulates the biosynthesis of the antibiotic virginiamycin. In this study, BarA from S. virginiae was overexpressed in Escherichia coli, purified and crystallized. Crystals of purified protein have been grown that diffracted to beyond 3.0 A resolution at 100 K using synchrotron radiation. The protein crystals belonged to the hexagonal space group P6(5)22, with unit-cell parameters a = b = 128.0, c = 286.2 A. With four molecules per asymmetric unit, the crystal volume per unit protein mass (V(M)) was 3.2 A(3) Da(-1) and the solvent content was 62%.

Expression, purification, physicochemical characterization and structural analysis of cytochrome c554 from Vibrio parahaemolyticus strain RIMD2210633.

The function of cytochrome c(554) of Vibrio parahaemolyticus has not yet been determined. We have determined the physicochemical properties and crystal structure of cytochrome c(554) at 1.8 A in order to help elucidate its function. The physicochemical properties and the tertiary structure of cytochrome c(554) resemble those of dimeric cytochrome c(552) from Pseudomonas nautica, but the Vibrio genus contains no gene for nitrite reductase, cytochrome cd(1), in its genome DNA. These results raise the possibility that both cytochromes denote an electron to an electron carrier and accept an electron from same electron carrier.

Crystal structures of penicillin-binding proteins 4 and 5 from Haemophilus influenzae.

We have determined high-resolution apo crystal structures of two low molecular weight penicillin-binding proteins (PBPs), PBP4 and PBP5, from Haemophilus influenzae, one of the most frequently found pathogens in the upper respiratory tract of children. Novel beta-lactams with notable antimicrobial activity have been designed, and crystal structures of PBP4 complexed with ampicillin and two of the novel molecules have also been determined. Comparing the apo form with those of the complexes, we find that the drugs disturb the PBP4 structure and weaken X-ray diffraction, to very different extents. PBP4 has recently been shown to act as a sensor of the presence of penicillins in Pseudomonas aeruginosa, and our models offer a clue to the structural basis for this effect. Covalently attached penicillins press against a phenylalanine residue near the active site and disturb the deacylation step. The ready inhibition of PBP4 by beta-lactams compared to PBP5 also appears to be related to the weaker interactions holding key residues in a catalytically competent position.

Visualizing breathing motion of internal cavities in concert with ligand migration in myoglobin.

Proteins harbor a number of cavities of relatively small volume. Although these packing defects are associated with the thermodynamic instability of the proteins, the cavities also play specific roles in controlling protein functions, e.g., ligand migration and binding. This issue has been extensively studied in a well-known protein, myoglobin (Mb). Mb reversibly binds gas ligands at the heme site buried in the protein matrix and possesses several internal cavities in which ligand molecules can reside. It is still an open question as to how a ligand finds its migration pathways between the internal cavities. Here, we report on the dynamic and sequential structural deformation of internal cavities during the ligand migration process in Mb. Our method, the continuous illumination of native carbonmonoxy Mb crystals with pulsed laser at cryogenic temperatures, has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino acid residues around the cavity, which results in the expansion of the cavity with a breathing motion. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand migration channel arising by induced fit, which is further supported by computational geometry analysis by the Delaunay tessellation method. This result suggests a crucial role of the breathing motion of internal cavities as a general mechanism of ligand migration in a protein matrix.

Mutational analysis of the substrate specificity of Escherichia coli penicillin binding protein 4.

Escherichia coli PBP4 is the archetypal class C, low molecular mass penicillin binding protein (LMM-PBP) and possesses both dd-carboxypeptidase and dd-endopeptidase activity. In contrast to other classes of PBP, class C LMM-PBPs show high dd-carboxypeptidase activity and rapidly hydrolyze synthetic fragments of peptidoglycan. The recently solved X-ray crystal structures of three class C LMM-PBPs (E. coli PBP4, Bacillus subtilis PBP4a, and Actinomadura R39 dd-peptidase) have identified several residues that form a pocket in the active site unique to this class of PBP. The X-ray cocrystal structure of the Actinomadura R39 DD-peptidase with a cephalosporin bearing a peptidoglycan-mimetic side chain showed that residues of this pocket interact with the third position meso-2,6-diaminopimelic acid residue of the peptidoglycan stem peptide. Equivalent residues of E. coli PBP4 (Asp155, Phe160, Arg361, and Gln422) were mutated, and the effect on both DD-carboxypeptidase and DD-endopeptidase activities was determined. Using N-acetylmuramyl-L-alanyl-gamma-D-glutamyl-meso-2,6-diaminopimelyl-D-alanyl-D-alanine as substrate, mutation of Asp155, Phe160, Arg361, and Gln422 to alanine reduced k(cat)/K(m) by 12.7-, 1.9-, 24.5-, and 13.8-fold, respectively. None of the k(cat) values deviated significantly from wild-type PBP4. PBP4 DD-endopeptidase activity was also affected, with substitution of Asp155, Arg361, and Gln422 reducing specific activity by 22%, 56%, and 40%, respectively. This provides the first direct demonstration of the importance of residues forming a subsite to accommodate meso-2,6-diaminopimelic acid in both the DD-carboxypeptidase and DD-endopeptidase activities of a class C LMM-PBP.