Enhanced Azidolysis by the Formation of Stable Ser-His Catalytic Dyad in a Glycoside Hydrolase Family 10 Xylanase Mutant.

Glycoside hydrolases require carboxyl groups as catalysts for their activity. A retaining xylanase from Streptomyces olivaceoviridis E-86 belonging to glycoside hydrolase family 10 possesses Glu128 and Glu236 that respectively function as acid/base and nucleophile. We previously developed a unique mutant of the retaining xylanase, N127S/E128H, whose deglycosylation is triggered by azide. A crystallographic study reported that the transient formation of a Ser-His catalytic dyad in the reaction cycle possibly reduced the azidolysis reaction. In the present study, we engineered a catalytic dyad with enhanced stability by site-directed mutagenesis and crystallographic study of N127S/E128H. Comparison of the Michaelis complexes of N127S/E128H with pNP-X2 and with xylopentaose showed that Ser127 could form an alternative hydrogen bond with Thr82, which disrupts the formation of the Ser-His catalytic dyad. The introduction of T82A mutation in N127S/E128H produces an enhanced first-order rate constant (6 times that of N127S/E128H). We confirmed the presence of a stable Ser-His hydrogen bond in the Michaelis complex of the triple mutant, which forms the productive tautomer of His128 that acts as an acid catalyst. Because the glycosyl azide is applicable in the bioconjugation of glycans by using click chemistry, the enzyme-assisted production of the glycosyl azide may contribute to the field of glycobiology.

Studies on crenarchaeal tyrosylation accuracy with mutational analyses of tyrosyl-tRNA synthetase and tyrosine tRNA from Aeropyrum pernix.

Aminoacyl-tRNA synthetases play a key role in the translation of genetic code into correct protein sequences. These enzymes recognize cognate amino acids and tRNAs from noncognate counterparts, and catalyze the formation of aminoacyl-tRNAs. While Although several tyrosyl-tRNA synthetases (TyrRSs) from various species have been structurally and functionally well characterized, the crenarchaeal TyrRS remains poorly understood. In this study, we performed mutational analyses on tyrosine tRNA (tRNA(Tyr)) and TyrRS from the crenarchaeon, Aeropyrum pernix, to investigate the molecular recognition mechanism. Kinetics for tyrosylation using in vitro transcript indicated that the discriminator base A73 and adjacent G72 in the acceptor stem are identity elements of tRNA(Tyr), whereas the C1 base and anticodon had modest roles as identity determinants. Intriguingly, in contrast to the identity element of eukaryotic/euryarchaeal TyrRSs, the first base-pair (C1-G72) of the acceptor stem was not essential in crenarchaeal TyrRS as a pair. Furthermore, A. pernix TyrRS mutants were constructed at positions Tyr39 and Asp172, which could form hydrogen bonds with the 4-hydroxyl group of l-tyrosine. The tyrosylation activities with the mutants resulted that Asp172 mutants completely abolished tyrosylation activity, whereas Tyr39 mutants had no effect on activity. Thus, crenarchaeal TyrRS appears to adopt different molecular recognition mechanism from other TyrRSs.

An RNA aptamer containing two binding sites against the HCV minus-IRES domain I.

The higher order structure of HCV (-)IRES containing five stem-loop structures (domain I) is essential for HCV replication because the viral RNA-dependent RNA polymerase, NS5B, recognizes it as the initiation site for plus-strand synthesis. To inhibit a de novo synthesis of plus-strand RNA molecules, in vitro selection against (-)IRES domain I was performed. One of the obtained aptamers, AP30, contained two consensus sequences within a random sequence region. Two consensus sequences form two apical loops and mutational analysis showed that both sequences were essential for binding to the target and for inhibiting NS5B-mediated RNA synthesis in vitro.

Increased inhibitory ability of conjugated RNA aptamers against the HCV IRES.

Hepatitis C virus (HCV) translation begins within the internal ribosome entry site (IRES). We have previously isolated two RNA aptamers, 2-02 and 3-07, which specifically bind to domain II and domain III-IV of the HCV IRES, respectively, and inhibit IRES-dependent translation. To improve the function of these aptamers, we constructed two conjugated molecules of 2-02 and 3-07. These bound to the target RNA more efficiently than the two parental aptamers. Furthermore, they inhibited IRES-dependent translation about 10 times as efficiently as the 3-07 aptamer. This result indicates that combining aptamers for different target recognition sites potentiates the inhibition activity by enhancing the domain-binding efficiency.

NMR studies on the interaction of sugars with the C-terminal domain of an R-type lectin from the earthworm Lumbricus terrestris.

The R-type lectin EW29, isolated from the earthworm Lumbricus terrestris, consists of two homologous domains (14,500 Da) showing 27% identity with each other. The C-terminal domain (Ch; C-half) of EW29 (EW29Ch) has two sugar-binding sites in subdomains alpha and gamma, and the protein uses these sugar-binding sites for its function as a single-domain-type hemagglutinin. In order to determine the sugar-binding ability and specificity for each of the two sugar-binding sites in EW29Ch, ligand-induced chemical-shift changes in EW29Ch were monitored using (1)H-(15)N HSQC spectra as a function of increasing concentrations of lactose, melibiose, D-galactose, methyl alpha-D-galactopyranoside and methyl beta-D-galactopyranoside. Shift perturbation patterns for well-resolved resonances confirmed that all of these sugars associated independently with the two sugar-binding sites of EW29Ch. NMR titration experiments showed that the sugar-binding site in subdomain alpha had a slow or intermediate exchange regime on the chemical-shift timescale (K(d) = 10(-2) to 10(-1) mM), whereas that in subdomain gamma had a fast exchange regime for these sugars (K(d) = 2-6 mM). Thus, our results suggest that the two sugar-binding sites of EW29Ch in the same molecule retain its hemagglutinating activity, but this activity is 10-fold lower than that of the whole protein because EW29Ch has two sugar-binding sites in the same molecule, one of which has a weak binding mode.

Crystallographic snapshots of an entire reaction cycle for a retaining xylanase from Streptomyces olivaceoviridis E-86.

Retaining glycosyl hydrolases, which catalyse both glycosylation and deglycosylation in a concerted manner, are the most abundant hydrolases. To date, their visualization has tended to be focused on glycosylation because glycosylation reactions can be visualized by inactivating deglycosylation step and/or using substrate analogues to isolate covalent intermediates. Furthermore, during structural analyses of glycosyl hydrolases with hydrolytic reaction products by the conventional soaking method, mutarotation of an anomeric carbon in the reaction products promptly and certainly occurs. This undesirable structural alteration hinders visualization of the second step in the reaction. Here, we investigated X-ray crystallographic visualization as a possible method for visualizing the conformational itinerary of a retaining xylanase from Streptomyces olivaceoviridis E-86. To clearly define the stereochemistry at the anomeric carbon during the deglycosylation step, extraneous nucleophiles, such as azide, were adopted to substitute for the missing base catalyst in an appropriate mutant. The X-ray crystallographic visualization provided snapshots of the components of the entire reaction, including the E*S complex, the covalent intermediate, breakdown of the intermediate and the enzyme-product (E*P)complex.

Sugar-complex structures of the C-half domain of the galactose-binding lectin EW29 from the earthworm Lumbricus terrestris.

R-type lectins are one of the most prominent types of lectin; they exist ubiquitously in nature and mainly bind to the galactose unit of sugar chains. The galactose-binding lectin EW29 from the earthworm Lumbricus terrestris belongs to the R-type lectin family as represented by the plant lectin ricin. It shows haemagglutination activity and is composed of a single peptide chain that includes two homologous domains: N-terminal and C-terminal domains. A truncated mutant of EW29 comprising the C-terminal domain (rC-half) has haemagglutination activity by itself. In order to clarify how rC-half recognizes ligands and shows haemagglutination activity, X-ray crystal structures of rC-half in complex with D-lactose and N-acetyl-D-galactosamine have been determined. The structure of rC-half is similar to that of the ricin B chain and consists of a beta-trefoil fold; the fold is further divided into three similar subdomains referred to as subdomains alpha, beta and gamma, which are gathered around the pseudo-threefold axis. The structures of sugar complexes demonstrated that subdomains alpha and gamma of rC-half bind terminal galactosyl and N-acetylgalactosaminyl glycans. The sugar-binding properties are common to both ligands in both subdomains and are quite similar to those of ricin B chain-lactose complexes. These results indicate that the C-terminal domain of EW29 uses these two galactose-binding sites for its function as a single-domain-type haemagglutinin.

Molecular recognition of tryptophan tRNA by tryptophanyl-tRNA synthetase from Aeropyrum pernix K1.

The identity elements of transfer RNA are the molecular basis for recognition by each cognate aminoacyl-tRNA synthetase. In the archaea system, the tryptophan tRNA identity has not been determined in detail. To investigate the molecular recognition mechanism of tryptophan tRNA by tryptophanyl-tRNA synthetase (TrpRS) from the hyperthermophilic and aerobic archaeon, Aeropyrum pernix K1, various mutant transcripts of tryptophan tRNA prepared by an in vitro transcription system were examined by overexpression of A. pernix TrpRS. Substitution of the discriminator base, A73, impaired tryptophan incorporation activity. Changing the G1-C72 base pair to other base pairs also decreased the aminoacylation activity. Substitutions of anticodon CCA revealed that the C34 and C35 mutants dramatically reduced aminoacylation with tryptophan, but the A36 mutants had the same activity as the wild type. The results indicate that the anticodon nucleotides C34, C35, discriminator base A73 and G1-C72 base pair are major recognition sites for A. pernix TrpRS.

Significance of the N-terminal histidine-rich region for the function of the human toll-like receptor 3 ectodomain.

Toll-like receptors (TLRs) are an essential component of the innate immune response to microbial pathogens. TLR3 is localized in intracellular compartments such as endosomes and signals in response to virus-derived double-stranded RNA (dsRNA). TLR3 localization within endosomes is required for ligand recognition, suggesting that acidic pH is the driving force for TLR3 ligand binding. To clarify the pH-dependent binding mechanism of TLR3 at the structural level, we focused on 3 highly conserved histidine residues clustered at the N-terminal region of the TLR3 ectodomain (ECD): H39, H60 and H108. Mutagenesis of these residues showed that H39, H60, and H108 were essential for ligand-dependent TLR3 activation in a cell-based assay. Furthermore, dsRNA binding to the recombinant TLR3 ECD depended strongly on pH and dsRNA length, and was reduced by mutations of H39, H60, and H108, demonstrating that TLR3 signaling is initiated from the endosome through a pH-dependent binding mechanism, and that a second dsRNA binding site exists in the N-terminal region of the TLR3 ECD. We propose a novel model for the formation of TLR3 ECD dimers complexed with dsRNA that incorporates this second binding site.

Isolation of RNA aptamers specific for the 3' X tail of HCV.

The 3' end of the HCV genome, designated as the 3' X tail, comprises an almost invariant 98-nucleotide sequence containing three highly conserved stem-loop structures (3' SL1, 3' SL2, and 3' SL3). Since these sequences are all critical for the initiation of negative-strand synthesis and essential for viral replication, they are attractive targets for novel anti-HCV drugs. To obtain effective RNA aptamers specific for the 3' X tail, and with the aim of developing novel inhibitors of HCV replication, we performed in vitro selection of aptamers with specificity for the 3' X tail. In vitro selection, namely SELEX (systematic evolution of ligands by exponential enrichment) is a useful strategy for isolating nucleic acid sequences from a randomized oligonucleotide pool that have a high affinity for a target molecule. After four selection cycles, a pool of the 3' X tail-specific RNA aptamers were obtained. This RNA pool included 39 clones that could be divided into three main classes (cSL1, cSL2, and cSL3) which harbor complementary sequences to the apical loops of 3' SL1, 3' SL2, and 3' SL3, respectively. Biochemical analyses are in progress to evaluate whether these RNA aptamers have the potential to block HCV replication.

Isolation and characterization of RNA aptamers specific for the HCV minus-IRES domain I.

The minus-IRES ((-)IRES), corresponding to the 3'-terminal end of the negative strand of hepatitis C virus (HCV) RNA, is well conserved among HCV subtypes. The higher order structure of (-)IRES is essential for HCV replication, because the viral RNA dependent RNA polymerase, NS5B, recognizes it as the initiation site for plus-strand synthesis of the HCV genome. To inhibit the "de novo" synthesis of plus-strand RNA molecules, we performed an in vitro selection procedure for RNA aptamers that are specific for (-)IRES domain I. Among the selected aptamers, one RNA aptamer had two binding sites for the (-)IRES domain I. We found that this aptamer inhibited plus-strand synthesis by about 50%, suggesting that both binding sites are important for binding to its target within the (-)IRES domain I.

Modulation of double-stranded RNA recognition by the N-terminal histidine-rich region of the human toll-like receptor 3.

Toll-like receptors (TLRs) are an essential component of the innate immune response to microbial pathogens. TLR3 is localized in intracellular compartments, such as endosomes, and initiates signals in response to virus-derived double-stranded RNA (dsRNA). The TLR3 ectodomain (ECD), which is implicated in dsRNA recognition, is a horseshoe-shaped solenoid composed of 23 leucine-rich repeats (LRRs). Recent mutagenesis studies on the TLR3 ECD revealed that TLR3 activation depends on a single binding site on the nonglycosylated surface in the C-terminal region, comprising H539 and several asparagines within LRR17 to -20. TLR3 localization within endosomes is required for ligand recognition, suggesting that acidic pH is the driving force for TLR3 ligand binding. To elucidate the pH-dependent binding mechanism of TLR3 at the structural level, we focused on three highly conserved histidine residues clustered at the N-terminal region of the TLR3 ECD: His39 in the N-cap region, His60 in LRR1, and His108 in LRR3. Mutagenesis of these residues showed that His39, His60, and His108 were essential for ligand-dependent TLR3 activation in a cell-based assay. Furthermore, dsRNA binding to recombinant TLR3 ECD depended strongly on pH and dsRNA length and was reduced by mutation of His39, His60, and His108, demonstrating that TLR3 signaling is initiated from the endosome through a pH-dependent binding mechanism, and that a second dsRNA binding site exists in the N-terminal region of the TLR3 ECD characteristic solenoid. We propose a novel model for the formation of TLR3 ECD dimers complexed with dsRNA, which incorporates this second binding site.

Determination of phenylalanine tRNA recognition sites by phenylalanyl-tRNA synthetase from hyperthermophilic archaeon, Aeropyrum pernix K1.

Phenylalanine tRNA identity has been determined in the bacteria and the eukaryote system, but remains unknown for the archaea system. To investigate the molecular recognition mechanism of phenylalanine tRNA by phenylalanyl-tRNA synthetase from hyperthermophilic and aerobic archaeon, Aeropyrum pernix K1, various mutant transcripts of phenylalanine tRNA prepared by an in vitro transcription system were examined by overexpressed A. pernix phenylalanyl tRNA synthetase. The results indicated that anticodon nucleotides G34, A35 and A36, discriminator base A73 and G20 in the variable pocket were base-specifically recognized by A. pernix phenylalanyl-tRNA synthetase.

Isolation of RNA aptamers specific for the HCV minus-IRES domain I.

The minus-IRES ((-)IRES), corresponding to the 3'-terminal end of the negative strand of hepatitis C virus (HCV) RNA, is well conserved among HCV subtypes. The higher order structure of (-)IRES is essential for HCV replication, because the viral RNA dependent RNA polymerase, NS5B, recognizes it as the initiation site for plus-strand synthesis of the HCV genome. To inhibit the "de novo" synthesis of plus-strand RNA molecules, we performed an in vitro selection procedure that is specific for the (-)IRES domain I. After confirming the binding convergence in the ninth RNA pool, 42 RNA clones were sequenced and analyzed. Of these, 25 clones (Family-I) had the consensus sequence, 5'-UGGAUC-3', which is complementary to the apical loop of SL-E1, an important region for NS5B recognition. Another 13 clones (Family-II) had the consensus sequence, 5'-GAGUAC-3', which is complementary to the apical loop of SL-D1. Biochemical analyses are in progress to evaluate whether these RNA aptamers have the ability to inhibit HCV replication.

N-terminal binding site in the human toll-like receptor 3 ectodomain.

Toll-like receptors (TLRs) are an essential component of the innate immune response to microbial pathogens. TLR3 is localized in intracellular compartments such as endosomes and signals in response to virus-derived dsRNA. The TLR3 ectodomain (ECD), which is implicated in dsRNA recognition, is a horseshoe-shaped solenoid composed of 23 leucine-rich repeats (LRRs). Recent mutagenesis studies on TLR3 ECD revealed that TLR3 activation depends on a single binding site on the nonglycosylated surface in the C-terminal region that includes H539 and several asparagines in LRRs 17 to 20. The localization of TLR3 within endosomes is required for ligand recognition, suggesting that acidic pH is the driving force for the ligand binding of TLR3. To clarify the pH-dependent binding mechanism of TLR3 at the structural level, we focused on some highly conserved histidine residues clustered at the N-terminal region of the TLR3 ECD. Mutagenesis approach showed that these residues were essential for the ligand-dependent activation of TLR3 in a cell-based assay. Furthermore, the binding of dsRNA to recombinant TLR3 ECD was strongly pH-dependent, and the binding was reduced by these mutations, demonstrating that TLR3 signaling is initiated from the endosome through a pH-dependent binding mechanism and that a second dsRNA binding site is present in the N-terminal region in the characteristic solenoid of the TLR3 ECD. We propose a novel model for the formation of TLR3 ECD dimers complexed with dsRNA that incorporates this second binding site.

Analysis of the interaction between human TLR3 ectodomain and nucleic acids.

Toll-like receptor 3 (TLR3) recognizes dsRNA of viral origin and polyriboinosine-polyribocytidylic acid (poly (I:C)). TLR3 mediates the activation of IRF-3 and NF-kappaB and thereby the secretion of type I interferons and inflammatory cytokines. However, the mechanism of this activation is poorly understood. To study the molecular recognition events and biochemical interactions between TLR3 and dsRNA, human TLR3 ectodomain (ECD) fused with a signal peptide at the N-terminus and 6 x His-tag at the C-terminus (TLR3-ECD) was obtained by the baculovirus expression system. To examine the various nucleic acids binding to TLR3-ECD in vitro, a filter binding assay was carried out at pH 4.2-7.6. Interaction of TLR3-ECD with various nucleic acids (particularly dsRNA, in vitro transcripts of tRNA and HCV NS3 aptamer) required an acidic pH.

Isolation of RNA aptamers against human Toll-like receptor 3 ectodomain.

Toll-like receptor 3 (TLR3) detects double-stranded RNA (dsRNA) known as a universal viral molecular pattern and activates the antiviral immune response. While TLR3 preferentially recognizes polyriboinosine-polyribocytidylic acid (poly (I:C)), a sequence-specific dsRNA has not yet been shown to activate TLR3. To determine whether TLR3 preferentially recognizes some specific sequence that acts on the signaling pathway of TLR3, in vitro selection against human TLR3 ectodomain (TLR3 ECD) was performed. After seventh selection cycle, two major classes, Family-I and -II, were emerged from 64 clones with binding constants of about 3 nM. Although these aptamers bound to TLR3 ECD with high affinity in vitro, they did not have agonist and antagonist effects on TLR3 signaling in TLR3-transfected HEK293 cells. Further analyses of the structure/function relationship of these aptamers will be carried out by mutagenesis, RNase mapping and competition assay using poly (I:C).

Rational design of dual-functional aptamers that inhibit the protease and helicase activities of HCV NS3.

The hepatitis C virus (HCV) non-structural protein 3 (NS3) is a multifunctional enzyme with protease and helicase activities. It is essential for HCV proliferation and is therefore a target for anti-HCV drugs. Previously, we obtained RNA aptamers that inhibit either the protease or helicase activity of NS3. During the present study, these aptamers were used to create advanced dual-functional (ADD) aptamers that were potentially more effective inhibitors of NS3 activity. The structural domain of the helicase aptamer, #5Delta, was conjugated via an oligo(U) tract to the 3'-end of the dual functional aptamer NEO-III-14U or the protease aptamer G9-II. The spacer length was optimized to obtain two ADD aptamers, NEO-35-s41 and G925-s50; both were more effective inhibitors of NS3 protease/helicase activity in vitro, especially the helicase, with a four- to five-fold increase in inhibition compared with #5 and NEO-III-14U. Furthermore, G925-s50 effectively inhibited NS3 protease activity in living cells and HCV replication in vitro. Overall, we have demonstrated rational RNA aptamer design based on features of both aptamer and target molecules, as well as successfully combining aptamer function and increasing NS3 inhibition.

A hepatitis C virus (HCV) internal ribosome entry site (IRES) domain III-IV-targeted aptamer inhibits translation by binding to an apical loop of domain IIId.

The hepatitis C virus (HCV) has a positive single-stranded RNA genome, and translation starts within the internal ribosome entry site (IRES) in a cap-independent manner. The IRES is well conserved among HCV subtypes and has a unique structure consisting of four domains. We used an in vitro selection procedure to isolate RNA aptamers capable of binding to the IRES domains III-IV. The aptamers that were obtained shared the consensus sequence ACCCA, which is complementary to the apical loop of domain IIId that is known to be a critical region of IRES-dependent translation. This convergence suggests that domain IIId is preferentially selected in an RNA-RNA interaction. Mutation analysis showed that the aptamer binding was sequence and structure dependent. One of the aptamers inhibited translation both in vitro and in vivo. Our results indicate that domain IIId is a suitable target site for HCV blockage and that rationally designed RNA aptamers have great potential as anti-HCV drugs.