Conserved loop residues-Tyr270 and Asn372 near the catalytic site of the lysostaphin endopeptidase are essential for staphylolytic activity toward pentaglycine binding and catalysis

Lysostaphin endopeptidase cleaves pentaglycine cross-bridges found in staphylococcal cell-wall peptidoglycans and proves very effective in combatting methicillin-resistant Staphylococcus aureus. Here, we revealed the functional importance of two loop residues, Tyr270 in loop 1 and Asn372 in loop 4, which are highly conserved among the M23 endopeptidase family and are found close to the Zn2+-coordinating active site. Detailed analyses of the binding groove architecture together with protein-ligand docking showed that these two loop residues potentially interact with the docked ligand-pentaglycine. Ala-substituted mutants (Y270A and N372A) were generated and over-expressed in Escherichia coli as a soluble form at levels comparable to the wild type. A drastic decrease in staphylolytic activity against S. aureus was observed for both mutants, suggesting an essential role of the two loop residues in lysostaphin function. Further substitutions with an uncharged polar Gln side-chain revealed that only the Y270Q mutation caused a dramatic reduction in bioactivity. In silico predicting the effect of binding site mutations revealed that all mutations displayed a large ΔΔGbind value, signifying requirements of the two loop residues for efficient binding to pentaglycine. Additionally, MD simulations revealed that Y270A and Y270Q mutations induced large flexibility of the loop 1 region, showing markedly increased RMSF values. Further structural analysis suggested that Tyr270 conceivably participated in the oxyanion stabilization of the enzyme catalysis. Altogether, our present study disclosed that two highly conserved loop residues, loop 1-Tyr270 and loop 4-Asn372, located near the lysostaphin active site are crucially involved in staphylolytic activity toward binding and catalysis of pentaglycine cross-links.

Two Recombinant Bacteriocins, Rhamnosin and Lysostaphin, Show Synergistic Anticancer Activity Against Gemcitabine-Resistant Cholangiocarcinoma Cell Lines

Cholangiocarcinoma (CCA), a bile duct cancer with a high mortality rate, has a poor prognosis due to its highly invasive and drug-resistant phenotypes. More effective and selective therapies are urgently needed. Bacteriocins are broad-spectrum antimicrobial peptides/proteins produced by bacterial strains to compete with other bacteria. Recent studies have reported that bacteriocins exhibit anticancer properties against various cancer cell lines with minimal toxicity toward normal cells. In this study, two types of recombinant bacteriocins, rhamnosin from probiotic Lacticaseibacillus rhamnosus and lysostaphin from Staphylococcus simulans, were highly produced in Escherichia coli and subsequently purified via immobilized-Ni²⁺ affinity chromatography. When their anticancer activity was investigated against CCA cell lines, both rhamnosin and lysostaphin were found capable of inhibiting the growth of CCA cell lines in a dose-dependent fashion but were less toxic toward a normal cholangiocyte cell line. Rhamnosin and lysostaphin as single treatments could suppress the growth of gemcitabine-resistant cell lines to the same extent as or more than they suppressed the parental counterparts. A combination of both bacteriocins more strongly inhibited growth and enhanced cell apoptosis in both parental and gemcitabine-resistant cells partly through the increased expression of the proapoptotic genes BAX, and caspase-3, -8, and -9. In conclusion, this is the first report to demonstrate an anticancer property of rhamnosin and lysostaphin. Using these bacteriocins as single agents or in combination would be effective against drug-resistant CCA.

Optimized high-yield preparation of alkaline-solubilizable crystalline inclusion of the Bacillus thuringiensis Cry4Aa δ-endotoxin expressed in Escherichia coli

The native Cry4Aa δ-endotoxin produced exclusively in Bacillus thuringiensis during sporulation as a ∼130-kDa inactive protoxin is confined within the parasporal crystalline inclusion that dissolves at alkaline pH in the midgut lumen of mosquito larvae. Here, the recombinant Cry4Aa toxin over-expressed in Escherichia coli at 30 °C as an alkaline-sobubilizable inclusion was found inevitably lost during isolation from the cell lysate (pH ∼6.5) of which host cells were pre-suspended in distilled water (pH ∼5.5). When 100 mM KH₂PO₄ (pH 5.0) was used as host cell-suspending buffer, the cell lysate's pH became more acidic (pH 5.5), allowing the expressed protoxin to be entirely retained in the form of crystalline inclusion rather than a soluble form, and thus high-yield recovery of the partially purified inclusion was obtained. Upon dialysis of the alkaline-solubilized protoxin against the KH₂PO₄ buffer, the protoxin precipitate was efficiently recovered and still exhibited high toxicity to Aedes aegypti mosquito larvae. Additionally, the precipitated protoxin was completely resolubilized in 50 mM Na₂CO₃ buffer (pH 9.0) and proteolytically processed by trypsin to produce the 65-kDa activated toxin comprising ∼47- and ∼20-kDa fragments. In silico structural analysis suggested that His¹⁵⁴, His³⁸⁸, His⁵³⁶ and His⁵⁷² were involved in a dissolution of the Cry4Aa inclusion at pH 6.5, conceivably through interchain salt bridge breakage. Altogether, such an optimized protocol described herein was effective for the preparation of alkaline-solubilizable inclusions of the recombinant Cry4Aa toxin in large amounts (>25 mg per liter culture) that would pave the way for further structure-function relationship studies of different Cry toxins.

Cry4Aa and Cry4Ba Mosquito-Active Toxins Utilize Different Domains in Binding to a Particular Culex ALP Isoform: A Functional Toxin Receptor Implicating Differential Actions on Target Larvae

The three-domain Cry4Aa toxin produced from Bacillus thuringiensis subsp. israelensis was previously shown to be much more toxic to Culex mosquito larvae than its closely related toxin—Cry4Ba. The interaction of these two individual toxins with target receptors on susceptible larval midgut cells is likely to be the critical determinant in their differential toxicity. Here, two full-length membrane-bound alkaline phosphatase (mALP) isoforms from Culex quinquefasciatus larvae, Cq-mALP1263and Cq-mALP1264, predicted to be GPI-linked was cloned and functionally expressed in Spodoptera frugiperda (Sf9) cells as 57- and 61-kDa membrane-bound proteins, respectively. Bioinformatics analysis disclosed that both Cq-mALP isoforms share significant sequence similarity to Aedes aegypti-mALP—a Cry4Ba toxin receptor. In cytotoxicity assays, Sf9 cells expressing Cq-mALP1264, but not Cq-mALP1263, showed remarkably greater susceptibility to Cry4Aa than Cry4Ba, while immunolocalization studies revealed that both toxins were capable of binding to each Cq-mALP expressed on the cell membrane surface. Molecular docking of the Cq-mALP1264-modeled structure with individual Cry4 toxins revealed that Cry4Aa could bind to Cq-mALP1264 primarily through particular residues on three surface-exposed loops in the receptor-binding domain—DII, including Thr⁵¹², Tyr⁵¹³ and Lys⁵¹⁴ in the β10-β11loop. Dissimilarly, Cry4Ba appeared to utilize only certain residues in its C-terminal domain—DIII to interact with such a Culex counterpart receptor. Ala-substitutions of selected β10-β11loop residues (T512A, Y513A and K514A) revealed that only the K514A mutant displayed a drastic decrease in biotoxicity against C. quinquefasciatus larvae. Further substitution of Lys⁵¹⁴ with Asp (K514D) revealed a further decrease in larval toxicity. Furthermore, in silico calculation of the binding affinity change (ΔΔG_bind) in Cry4Aa-Cq-mALP1264 interactions upon these single-substitutions revealed that the K514D mutation displayed the largest ΔΔG_bind value as compared to three other mutations, signifying an adverse impact of a negative charge at this critical receptor-binding position. Altogether, our present study has disclosed that these two related-Cry4 mosquito-active toxins conceivably exploited different domains in functional binding to the same Culex membrane-bound ALP isoform—Cq-mALP1264 for mediating differential toxicity against Culex target larvae.

Bacillus thuringiensis Cry4Ba Insecticidal ToxinExploits Leu615 in Its C-Terminal Domain to Interact with a Target Receptor-Aedes aegypti Membrane-Bound Alkaline Phosphatase

In addition to the receptor-binding domain (DII), the C-terminal domain (DIII) of three-domain Cry insecticidal δ-endotoxins from Bacillus thuringiensis has been implicated in target insect specificity, yet its precise mechanistic role remains unclear. Here, the 21 kDa high-purity isolated DIII fragment derived from the Cry4Ba mosquito-specific toxin was achieved via optimized preparative FPLC, allowing direct rendering analyses for binding characteristics toward its target receptor—Aedes aegypti membrane-bound alkaline phosphatase (Aa-mALP). Binding analysis via dotblotting revealed that the Cry4Ba-DIII truncate was capable of specific binding to nitrocellulose-bound Aa-mALP, with a binding signal comparable to its 65 kDa Cry4Ba-R203Q full-length toxin. Further determination of binding affinity via sandwich ELISA revealed that Cry4Ba-DIII exhibited a rather weak binding to Aa-mALP with a dissociation constant (K_d) of ≈1.1 × 10⁻⁷ M as compared with the full-length toxin. Intermolecular docking between the Cry4Ba-R203Q active toxin and Aa-mALP suggested that four Cry4Ba-DIII residues, i.e., Glu⁵²², Asn⁵⁵², Asn⁵⁷⁶, and Leu⁶¹⁵, are potentially involved in such toxin–receptor interactions. Ala substitutions of each residue (E522A, N552A, N576A and L615A) revealed that only the L615A mutant displayed a drastic decrease in biotoxicity against A. aegypti larvae. Additional binding analysis revealed that the L615A-impaired toxin also exhibited a reduction in binding capability to the surface-immobilized Aa-mALP receptor, while two bio-inactive DII-mutant toxins, Y332A and F364A, which almost entirely lost their biotoxicity, apparently retained a higher degree of binding activity. Altogether, our data disclose a functional importance of the C-terminal domain of Cry4Ba for serving as a potential receptor-binding moiety in which DIII-Leu⁶¹⁵ could conceivably be exploited for the binding to Aa-mALP, highlighting its contribution to toxin interactions with such a target receptor in mediating larval toxicity.

N-Terminally Added Tag Selectively Enhances Heterologous Expression of VacA Cytotoxin Variants from Helicobacter pylori

BACKGROUND

Gastric pathogen Helicobacter pylori secretes VacA cytotoxin displaying a high degree of polymorphic variations of which the highest VacA pathogenicity correlates with m1-type variant followed by VacA-m2.

OBJECTIVE

To comparatively evaluate expression in Escherichia coli of the mature VacA variants (m1- and m2-types) and their 33- and 55/59-kDa domains fused with His₍₆₎ tag at N- or C-terminus.

METHODS

All VacA clones expressed in E. coli TOP10™ were analyzed by SDS-PAGE and Western blotting. VacA inclusions were solubilized under native conditions (~150-rpm shaking at 37°C for 2 h in 20 mM HEPES (pH7.4) and 150 mM NaCl). Membrane-perturbing and cytotoxic activities of solubilized VacA proteins were assessed via liposome-entrapped dye leakage and resazurin- based cell viability assays, respectively. VacA binding to human gastric adenocarcinoma cells was assessed by immunofluorescence microscopy. Side-chain hydrophobicity of VacA was analyzed through modeled structures constructed by homology- and ab initio-based modeling.

RESULTS

Both full-length VacA-m1 and 33-kDa domain were efficiently expressed only in the presence of N-terminal extension while its 55-kDa domain was capably expressed with either N- or Cterminal extension. Selectively enhanced expression was also observed for VacA-m2. Protein expression profiles revealed a critical period in IPTG-induced production of the 55-kDa domain with N-terminal extension unlike its C-terminal extension showing relatively stable expression. Both VacA- m1 isolated domains were able to independently bind to cultured gastric cells similar to the full- length toxin, albeit the 33-kDa domain exhibited significantly higher activity of membrane perturbation than others. Membrane-perturbing and cytotoxic activities observed for VacA-m1 appeared to be higher than those of VacA-m2. Homology-based modeling and sequence analysis suggested a potential structural impact of non-polar residues located at the N-terminus of the mature VacA toxin and its 33-kDa domain.

CONCLUSION

Our data provide molecular insights into selective influence of the N-terminally added tag on efficient expression of recombinant VacA variants, signifying biochemical and biological implications of the hydrophobic stretch within the N-terminal domain.

Molecular Insights into Zn2+ Inhibition of the Antibacterial Endopeptidase Lysostaphin from Staphylococcus simulans

BACKGROUND

Mature lysostaphin (~28-kDa Lss) from Staphylococcus simulans proves effective in killing methicillin-resistant Staphylococcus aureus (MRSA) which is endemic in hospitals worldwide. Lss is Zn²⁺-dependent endopeptidase, but its bacteriolytic activity could be affected by exogenously added Zn²⁺.

OBJECTIVE

To gain greater insights into structural and functional impacts of Zn²⁺and Ni²⁺on Lss-induced bioactivity.

METHODS

Lss purified via immobilized metal ion-affinity chromatography was assessed for bioactivity using turbidity reduction assays. Conformational change of metal ion-treated Lss was examined by circular dichroism and intrinsic fluorescence spectroscopy. Co-sedimentation assay was performed to study interactions between Zn²⁺-treated Lss and S. aureus peptidoglycans. Metal ionbinding prediction and intermolecular docking were used to locate an extraneous Zn²⁺-binding site.

RESULTS

A drastic decrease in Lss bioactivity against S. aureus and MRSA was revealed only when treated with Zn²⁺, but not Ni²⁺, albeit no negative effect of diethyldithiocarbamate-Zn²⁺-chelator on Lss-induced bioactivity. No severe conformational change was observed for Lss incubated with exogenous Zn²⁺ or Ni²⁺. Lss pre-treated with Zn²⁺ efficiently bound to S. aureus cell-wall peptidoglycans, suggesting non-interfering effect of exogenous metal ions on cell-wall targeting (CWT) activity. In silico analysis revealed that exogenous Zn²⁺, but not Ni²⁺, preferably interacted with a potential extraneous Zn²⁺-binding site (His253, Glu318 and His323) placed near the Zn²⁺-coordinating Lssactive site within the catalytic (CAT) domain.

CONCLUSION

Our present data signify the adverse influence of exogenous Zn²⁺ ions on Lss-induced staphylolytic activity through the exclusive presence within the CAT domain of an extraneous inhibitory Zn²⁺-binding site, without affecting the CWT activity.

His180 in the pore-lining α4 of the Bacillus thuringiensis Cry4Aa δ-endotoxin is crucial for structural arrangements of the α4-α5 transmembrane hairpin and hence biotoxicity

One proposed toxic mechanism of Bacillus thuringiensis Cry δ-endotoxins involves pore formation in target membranes by the α4-α5 transmembrane hairpin constituting their pore-forming domain. Here, nine selected charged and uncharged polar residues in the pore-lining α4 of the Cry4Aa mosquito-active toxin were substituted with Ala. All mutant toxins, i.e., D169A, R171A, Q173A, H178A, Y179A, H180A, Q182A, N183A and E187A, were over-expressed in Escherichia coli as 130-kDa protoxin inclusions at levels comparable to the wild-type toxin. Bioassays against Aedes aegypti larvae revealed that only H178A and H180A mutants displayed a drastic reduction in biotoxicity, albeit almost complete insolubility observed for H178A, but not for H180A inclusions. Further mutagenic analysis showed that replacements of His¹⁸⁰ with charged (Arg, Lys, Asp, Glu), small uncharged polar (Ser, Cys) or small non-polar (Gly, Val) residues severely impaired the biotoxicity, unlike substitutions with relatively large uncharged (Asn, Gln, Leu) or aromatic (Phe, Tyr, Trp) residues. Similar to the trypsin-activated wild-type toxin, both bio-active and -inactive H180 mutants were still capable of releasing entrapped calcein from lipid vesicles and producing cation-selective channels with ~130-pS maximum conductance. Analysis of the Cry4Aa structure revealed the existence of a hydrophobic cavity near the critical His¹⁸⁰ side-chain. Analysis of simulated structures revealed that His¹⁸⁰-to-smaller residue conversions create a gap disrupting such cavity's hydrophobicity and hence structural arrangements of the α4-α5 hairpin. Altogether, our data disclose a critical involvement in Cry4Aa-biotoxicity of His¹⁸⁰ exclusively present in the lumen-facing α4 for providing proper environment for the α4-α5 hairpin prior to membrane-inserted pore formation.

Importance of the Cys124-Cys128 intermolecular disulfide bonding for oligomeric assembly and hemolytic activity of the Helicobacter pylori TlyA hemolysin

Although the TlyA hemolysin from Helicobacter pylori has been implicated as a potential virulence factor involved in mediating host cell colonization and hence disease progression, its structural determinants underlying the biological activity are still largely uncertain. In this study, an important role of the formation of a particular disulfide bond for functional oligomeric assembly of the H. pylori TlyA toxin was evidently elucidated. The 27-kDa TlyA recombinant protein was overexpressed in Escherichia coli, subsequently purified to near homogeneity by cation exchange chromatography, and proven to be hemolytically active against sheep erythrocytes. Additionally, TlyA-induced hemolytic activity was significantly diminished under conditions of disulfide bond reduction with a thiol-reducing agent, dithiothreitol. When the purified TlyA protein was subjected to modified SDS-PAGE under non-reducing conditions, the presence of an oligomeric state of this protein was clearly revealed by its apparent molecular mass of ∼48 kDa. Recombinant E. coli cells expressing TlyA also displayed contact-dependent hemolysis of erythrocytes, suggesting TlyA localization at the bacterial outer membrane and thus supporting the formation of disulfide-bonded TlyA. Homology-based modeling and in silico structural assembly analysis of TlyA signified potential intermolecular, rather than intramolecular, disulfide bonding through Cys¹²⁴ and Cys¹²⁸. Subsequently, single substitution of either of these Cys residues with Ser severely affected the oligomeric assembly of both TlyA mutants and hence abolished their hemolytic activity. Altogether, our present data provide pivotal evidence that the formation of intermolecular disulfide bonding between Cys¹²⁴ and Cys¹²⁸ plays a critical role in structural assembly of a biologically active-TlyA oligomer.

Structural requirement of the hydrophobic region of the Bordetella pertussis CyaA-hemolysin for functional association with CyaC-acyltransferase in toxin acylation

Previously, we demonstrated that the ∼130-kDa CyaA-hemolysin (CyaA-Hly, Met⁴⁸²-Arg¹⁷⁰⁶) from Bordetella pertussis was palmitoylated at Lys⁹⁸³ when co-expressed with CyaC-acyltransferase in Escherichia coli, and thus activated its hemolytic activity. Here, further investigation on a possible requirement of the N-terminal hydrophobic region (HP, Met⁴⁸²-Leu⁷⁵⁰) for toxin acylation was performed. The ∼100-kDa RTX (Repeat-in-ToXin) fragment (CyaA-RTX, Ala⁷⁵¹-Arg¹⁷⁰⁶) containing the Lys⁹⁸³-acylation region (AR, Ala⁷⁵¹-Gln¹⁰⁰⁰), but lacking HP, was co-produced with CyaC in E. coli. Hemolysis assay indicated that CyaA-RTX showed no hemolytic activity. Additionally, MALDI-TOF/MS and LC-MS/MS analyses confirmed that CyaA-RTX was non-acylated, although the co-expressed CyaC-acyltransferase was able to hydrolyze its chromogenic substrate-p-nitrophenyl palmitate and acylate CyaA-Hly to become hemolytically active. Unlike CyaA-RTX, the ∼70-kDa His-tagged CyaA-HP/BI fragment which is hemolytically inactive and contains both HP and AR was constantly co-eluted with CyaC during IMAC-purification as the presence of CyaC was verified by Western blotting. Such potential interactions between the two proteins were also revealed by semi-native PAGE. Moreover, structural analysis via electrostatic potential calculations and molecular docking suggested that CyaA-HP comprising α1-α5 (Leu⁵⁰⁰-Val⁶⁹⁸) can interact with CyaC through several hydrogen and ionic bonds formed between their opposite electrostatic surfaces. Overall, our results demonstrated that the HP region of CyaA-Hly is conceivably required for not only membrane-pore formation but also functional association with CyaC-acyltransferase, and hence effective palmitoylation at Lys⁹⁸³.

Zn2+-dependent autocatalytic activity of the Bordetella pertussis CyaA-hemolysin

Proteolytic degradation of the ∼100-kDa isolated RTX (Repeat-in-ToXin) subdomain (CyaA-RTX) of the Bordetella pertussis CyaA-hemolysin (CyaA-Hly) was evidently detected upon solely-prolonged incubation. Here, a truncated CyaA-Hly fragment (CyaA-HP/BI) containing hydrophobic and acylation regions connected with the first RTX block (BI_1015-1088) was constructed as a putative precursor for investigating its potential autocatalysis. The 70-kDa His-tagged CyaA-HP/BI fragment which was over-expressed in Escherichia coli as insoluble aggregate was entirely solubilized with 4 M urea. After re-naturation in a Ni²⁺-NTA affinity column, the purified-refolded CyaA-HP/BI fragment in HEPES buffer (pH 7.4) supplemented with 2 mM CaCl₂ was completely degraded upon incubation at 37 °C for 3 h. Addition of 1,10-phenanthroline‒an inhibitor of Zn²⁺-dependent metalloproteases markedly reduced the extent of degradation for CyaA-HP/BI and CyaA-RTX, but the degradative effect was clearly enhanced by addition of 100 mM ZnCl₂. Structural analysis of a plausible CyaA-HP/BI model revealed a potential Zn²⁺-binding His-Asp cluster located between the acylation region and RTX-BI_1015-1088. Moreover, Arg⁹⁹⁷‒one of the identified cleavage sites of the CyaA-RTX fragment was located in close proximity to the Zn²⁺-binding catalytic site. Overall results demonstrated for the first time that the observed proteolysis of CyaA-HP/BI and CyaA-RTX fragments is conceivably due to their Zn²⁺-dependent autocatalytic activity.

Functional Contributions of Positive Charges in the Pore-Lining Helix 3 of the Bordetella pertussis CyaA-Hemolysin to Hemolytic Activity and Ion-Channel Opening

The Bordetella pertussis CyaA-hemolysin (CyaA-Hly) domain was previously demonstrated to be an important determinant for hemolysis against target erythrocytes and ion-channel formation in planar lipid bilayers (PLBs). Here, net-charge variations in the pore-lining helix of thirteen related RTX cytolysins including CyaA-Hly were revealed by amino acid sequence alignments, reflecting their different degrees of hemolytic activity. To analyze possible functional effects of net-charge alterations on hemolytic activity and channel formation of CyaA-Hly, specific mutations were made at Gln⁵⁷⁴ or Glu⁵⁸¹ in its pore-lining α3 of which both residues are highly conserved Lys in the three highly active RTX cytolysins (i.e., Escherichia coli α-hemolysin, Actinobacillus pleuropneumoniae toxin, and Aggregatibacter actinomycetemcomitans leukotoxin). All six constructed CyaA-Hly mutants that were over-expressed in E. coli as 126 kDa His-tagged soluble proteins were successfully purified via immobilized Ni²⁺-affinity chromatography. Both positive-charge substitutions (Q574K, Q574R, E581K, E581R) and negative-charge elimination (E581Q) appeared to increase the kinetics of toxin-induced hemolysis while the substitution with a negatively-charged side-chain (Q574E) completely abolished its hemolytic activity. When incorporated into PLBs under symmetrical conditions (1.0 M KCl, pH 7.4), all five mutant toxins with the increased hemolytic activity produced clearly-resolved single channels with higher open probability and longer lifetime than the wild-type toxin, albeit with a half decrease in their maximum conductance. Molecular dynamics simulations for 50 ns of a trimeric CyaA-Hly pore model comprising three α2-loop-α3 transmembrane hairpins revealed a significant role of the positive charge at both target positions in the structural stability and enlarged diameter of the simulated pore. Altogether, our present data have disclosed functional contributions of positively-charged side-chains substituted at positions Gln⁵⁷⁴ and Glu⁵⁸¹ in the pore-lining α3 to the enhanced hemolytic activity and ion-channel opening of CyaA-Hly that actually mimics the highly-active RTX (repeat-in-toxin) cytolysins.

Acylation of the Bordetella pertussis CyaA-hemolysin: Functional implications for efficient membrane insertion and pore formation

Previously, the ~130-kDa CyaA-hemolysin domain (CyaA-Hly) from Bordetella pertussis co-expressed with CyaC-acyltransferase in Escherichia coli was demonstrated to be palmitoylated at Lys⁹⁸³ and thus activated its hemolytic activity against target erythrocytes. Here, we report the functional importance of Lys⁹⁸³-palmitoylation for membrane insertion and pore formation of CyaA-Hly. Intrinsic fluorescence emissions of both non-acylated CyaA-Hly (NA/CyaA-Hly) and CyaA-Hly were indistinguishable, suggesting no severe conformational change upon acylation at Lys⁹⁸³. Following pre-incubation of sheep erythrocytes with NA/CyaA-Hly, there was a drastic decrease in CyaA-Hly-induced hemolysis. Direct interactions between NA/CyaA-Hly and target erythrocyte membranes were validated via membrane-binding assays along with Western blotting, suggestive of acylation-independent capability of NA/CyaA-Hly to interact with erythrocyte membranes. As compared with CyaA-Hly, NA/CyaA-Hly displayed a slower rate of incorporation into DOPC:DOPE:Ch or DiPhyPC bilayers under symmetrical conditions (1M KCl, 10mM HEPES, pH7.4) and formed channels exhibiting different conductance. Further analysis revealed that channel-open lifetime in DOPC:DOPE:Ch bilayers of NA/CyaA-Hly was much shorter than that of the acylated form, albeit slightly shorter lifetime found in DiPhyPC bilayers. Sequence alignments of the Lys⁹⁸³-containing CyaA-segment with those of related RTX-cytolysins revealed a number of highly conserved hydrophobic residues and a Lys/Arg cluster that is predicted be important for toxin-membrane interactions. Altogether, our data disclosed that the Lys⁹⁸³-linked palmitoyl group is not directly involved in either binding to target erythrocyte membranes or toxin-induced channel conductivity, but rather required for efficient membrane insertion and pore formation of the acylated CyaA-Hly domain.

Structural Characterization of Humanized Nanobodies with Neutralizing Activity against the Bordetella pertussis CyaA-Hemolysin: Implications for a Potential Epitope of Toxin-Protective Antigen

Previously, the 126-kDa CyaA-hemolysin (CyaA-Hly) fragment cloned from Bordetella pertussis—the causative agent of whooping cough—and functionally expressed in Escherichia coli was revealed as a key determinant for CyaA-mediated hemolysis against target erythrocytes. Here, phagemid-transfected E. coli clones producing nanobodies capable of binding to CyaA-Hly were selected from a humanized-camel VH/V_HH phage-display library. Subsequently verified for binding activities by indirect ELISA and Western blotting, four CyaA-Hly-specific nanobodies were obtained and designated according to the presence/absence of V_HH-hallmark amino acids as V_HH2, VH5, VH18 and V_HH37. In vitro neutralization assay revealed that all four ~17-kDa His-tagged VH/V_HH nanobodies, in particular V_HH37, which were over-expressed as inclusions and successfully unfolded-refolded, were able to effectively inhibit CyaA-Hly-mediated hemolysis. Phage-mimotope searching revealed that only peptides with sequence homologous to Linker 1 connecting Blocks I and II within the CyaA-RTX subdomain were able to bind to these four CyaA-Hly-specific nanobodies. Structural analysis of V_HH37 via homology modeling and intermolecular docking confirmed that this humanized nanobody directly interacts with CyaA-RTX/Linker 1 through multiple hydrogen and ionic bonds. Altogether, our present data demonstrate that CyaA-RTX/Linker 1 could serve as a potential epitope of CyaA-protective antigen that may be useful for development of peptide-based pertussis vaccines. Additionally, such toxin-specific nanobodies have a potential for test-driven development of a ready-to-use therapeutic in passive immunization for mitigation of disease severity.

Isolated CyaA-RTX subdomain from Bordetella pertussis: Structural and functional implications for its interaction with target erythrocyte membranes

The 126-kDa Bordetella pertussis CyaA-hemolysin (CyaA-Hly) was previously expressed in Escherichia coli as a soluble precursor that can be acylated to retain hemolytic activity. Here, we investigated structural and functional characteristics of a ∼100-kDa isolated RTX (Repeat-in-ToXin) subdomain (CyaA-RTX) of CyaA-Hly. Initially, we succeeded in producing a large amount with high purity of the His-tagged CyaA-RTX fragment and in establishing the interaction of acylated CyaA-Hly with sheep red blood cell (sRBC) membranes by immuno-localization. Following pre-incubation of sRBCs with non-acylated CyaA-Hly or with the CyaA-RTX fragment that itself produces no hemolytic activity, there was a dramatic decrease in CyaA-Hly-induced hemolysis. When CyaA-RTX was pre-incubated with anti-CyaA-RTX antisera, the capability of CyaA-RTX to neutralize the hemolytic activity of CyaA-Hly was greatly decreased. A homology-based model of the 100-kDa CyaA-RTX subdomain revealed a loop structure in Linker II sharing sequence similarity to human WW domains. Sequence alignment of Linker II with the human WW-domain family revealed highly conserved aromatic residues important for protein-protein interactions. Altogether, our present study demonstrates that the recombinant CyaA-RTX subdomain retains its functionality with respect to binding to target erythrocyte membranes and the WW-homologous region in Linker II conceivably serves as a functional segment required for receptor-binding activity.

Functional characterizations of residues Arg-158 and Tyr-170 of the mosquito-larvicidal Bacillus thuringiensis Cry4Ba

The insecticidal activity of Bacillus thuringiensis (Bt) Cry toxins involves toxin stabilization, oligomerization, passage across the peritrophic membrane (PM), binding to midgut receptors and pore-formation. The residues Arg-158 and Tyr-170 have been shown to be crucial for the toxicity of Bt Cry4Ba. We characterized the biological function of these residues. In mosquito larvae, the mutants R158A/E/Q (R158) could hardly penetrate the PM due to a significantly reduced ability to alter PM permeability; the mutant Y170A, however, could pass through the PM, but degraded in the space between the PM and the midgut epithelium. Further characterization by oligomerization demonstrated that Arg-158 mutants failed to form correctly sized high-molecular weight oligomers. This is the first report that Arg-158 plays a role in the formation of Cry4Ba oligomers, which are essential for toxin passage across the PM. Tyr-170, meanwhile, is involved in toxin stabilization in the toxic mechanism of Cry4Ba in mosquito larvae. [BMB Reports 2014; 47(10): 546-551]

Potential Prepore Trimer Formation by the Bacillus thuringiensis Mosquito-specific Toxin: MOLECULAR INSIGHTS INTO A CRITICAL PREREQUISITE OF MEMBRANE-BOUND MONOMERS

The insecticidal feature of the three-domain Cry δ-endotoxins from Bacillus thuringiensis is generally attributed to their capability to form oligomeric pores, causing lysis of target larval midgut cells. However, the molecular description of their oligomerization process has not been clearly defined. Here a stable prepore of the 65-kDa trypsin-activated Cry4Ba mosquito-specific toxin was established through membrane-mimetic environments by forming an ∼200-kDa octyl-β-D-glucoside micelle-induced trimer. The SDS-resistant trimer caused cytolysis to Sf9 insect cells expressing Aedes-mALP (a Cry4Ba receptor) and was more effective than a toxin monomer in membrane perturbation of calcein-loaded liposomes. A three-dimensional model of toxin trimer obtained by negative-stain EM in combination with single-particle reconstruction at ∼5 nm resolution showed a propeller-shaped structure with 3-fold symmetry. Fitting the three-dimensional reconstructed EM map with a 100-ns molecular dynamics-simulated Cry4Ba structure interacting with an octyl-β-D-glucoside micelle showed relative positioning of individual domains in the context of the trimeric complex with a major protrusion from the pore-forming domain. Moreover, high-speed atomic force microscopy imaging at nanometer resolution and a subsecond frame rate demonstrated conformational transitions from a propeller-like to a globularly shaped trimer upon lipid membrane interactions, implying prepore-to-pore conversion. Real-time trimeric arrangement of monomers associated with L-α-dimyristoylphosphatidylcholine/3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid bicelle membranes was also envisaged by successive high-speed atomic force microscopy imaging, depicting interactions among three individual subunits toward trimer formation. Together, our data provide the first pivotal insights into the structural requirement of membrane-induced conformational changes of Cry4Ba toxin monomers for the molecular assembly of a prepore trimer capable of inserting into target membranes to generate a lytic pore.

Functional assembly of 260-kDa oligomers required for mosquito-larvicidal activity of the Bacillus thuringiensis Cry4Ba toxin

Oligomerization has been shown to contribute to the toxicity of Bacillus thuringiensis Cry toxins. Mutations have been made in the Cry4Ba toxin and resulted in toxic to non-toxic mutants toward Aedes aegypti larvae. In this study, Cry4Ba wild type and mutants were analyzed for oligomer formation in vitro, biochemical properties and their relationships with larvicidal activity. In vitro, the Cry4Ba forms two-main types of the oligomers including (1) the 260-kDa and larger oligomers, which assembled in the carbonate buffer, pH 10.0 and completely dissociated by heating at 90°C and (2) 190-kDa oligomer, which was induced by heat, sodium-salt and detergent addition. Polar and charge residues in the toxin domain I and II may contribute to formation of the 260-kDa oligomers. A single Cys-525 in domain III was replaced with serine resulting in the C525S mutant, which exhibited a 50% reduction in larvicidal activity compared to the Cry4Ba wild-type. The mutant exhibited partial loss in larger oligomer of the 260kDa and total loss of 190-kDa oligomer. The results revealed an important role of the Cys-525 in intermolecular disulfide formation of larger oligomer as well as the 190-kDa oligomer. Despite of their formations in the receptor free condition, the 260-kDa and larger oligomers were found to strongly correspond to Cry4Ba toxicity suggesting their functional roles in the A. aegypti larvae. Also, possible roles of the 260-kDa and larger oligomers have been proposed in this report.

Membrane-Pore Forming Characteristics of the Bordetella pertussis CyaA-Hemolysin Domain

Previously, the 126-kDa Bordetella pertussis CyaA pore-forming/hemolysin (CyaA-Hly) domain was shown to retain its hemolytic activity causing lysis of susceptible erythrocytes. Here, we have succeeded in producing, at large quantity and high purity, the His-tagged CyaA-Hly domain over-expressed in Escherichia coli as a soluble hemolytically-active form. Quantitative assays of hemolysis against sheep erythrocytes revealed that the purified CyaA-Hly domain could function cooperatively by forming an oligomeric pore in the target cell membrane with a Hill coefficient of ~3. When the CyaA-Hly toxin was incorporated into planar lipid bilayers (PLBs) under symmetrical conditions at 1.0 M KCl, 10 mM HEPES buffer (pH 7.4), it produced a clearly resolved single channel with a maximum conductance of ~35 pS. PLB results also revealed that the CyaA-Hly induced channel was unidirectional and opened more frequently at higher negative membrane potentials. Altogether, our results first provide more insights into pore-forming characteristics of the CyaA-Hly domain as being the major pore-forming determinant of which the ability to induce such ion channels in receptor-free membranes could account for its cooperative hemolytic action on the target erythrocytes.

Two specific membrane-bound aminopeptidase N isoforms from Aedes aegypti larvae serve as functional receptors for the Bacillus thuringiensis Cry4Ba toxin implicating counterpart specificity

The interaction between Bacillus thuringiensis Cry toxins and their receptors on midgut cells of susceptible insect larvae is the critical determinant in toxin specificity. Besides GPI-linked alkaline phosphatase in Aedes aegypti mosquito-larval midguts, membrane-bound aminopeptidase N (AaeAPN) is widely thought to serve as a Cry4Ba receptor. Here, two full-length AaeAPN isoforms, AaeAPN2778 and AaeAPN2783, predicted to be GPI-linked were cloned and successfully expressed in Spodoptera frugiperda (Sf9) cells as 112- and 107-kDa membrane-bound proteins, respectively. In the cytotoxicity assay, Sf9 cells expressing each of the two AaeAPN isoforms showed increased sensitivity to the Cry4Ba mosquito-active toxin. Double immunolocalization revealed specific binding of Cry4Ba to each individual AaeAPN expressed on the cell membrane surface. Sequence analysis and homology-based modeling placed these two AaeAPNs to the M1 aminopeptidase family as they showed similar four-domain structures, with the most conserved domain II being the catalytic component. Additionally, the most variable domain IV containing negatively charged surface patches observed only in dipteran APNs could be involved in insect specificity. Overall results demonstrated that these two membrane-bound APN isoforms were responsible for mediating Cry4Ba toxicity against AaeAPN-expressed Sf9 cells, suggesting their important role as functional receptors for the toxin counterpart in A. aegypti mosquito larvae.

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Two novel Aedes GPI-APN isoforms were functionally expressed in Sf9 cells.

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Cells expressing each AaeAPN were more sensitive to Cry4Ba toxin cytolysis.

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Specific binding of Cry4Ba to individual AaeAPN-expressing Sf9 cells was demonstrated.

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These two AaeAPNs mediating Cry4Ba cytotoxicity serve as receptors in Aedes larvae.

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AaeAPN models reveal four-domain organization with implications for toxin-counterpart specificity.

Single-reversal charge in the β10-β11 receptor-binding loop of Bacillus thuringiensis Cry4Aa and Cry4Ba toxins reflects their different toxicity against Culex spp. larvae

Bacillus thuringiensis Cry4Aa toxin was previously shown to be much more toxic to Culex mosquito-larvae than its closely related toxin - Cry4Ba, conceivably due to their sequence differences within the β10-β11 receptor-binding loop. Here, single-Ala substitutions of five residues (Pro(510), Thr(512), Tyr(513), Lys(514) and Thr(515)) within the Cry4Aa β10-β11 loop revealed that only Lys(514) corresponding to the relative position of Cry4Ba-Asp(454) is crucial for toxicity against Culex quinquefasciatus larvae. Interestingly, charge-reversal mutations at Cry4Ba-Asp(454) (D454R and D454K) revealed a marked increase in toxicity against such less-susceptible larvae. In situ binding analyses revealed that both Cry4Ba-D454R and D454K mutants exhibited a significant increase in binding to apical microvilli of Culex larval midguts, albeit at lower-binding activity when compared with Cry4Aa. Altogether, our present data suggest that a positively charged side-chain near the tip of the β10-β11 loop plays a critical role in determining target specificity of Cry4Aa against Culex spp., and hence a great increase in the Culex larval toxicity of Cry4Ba was obtained toward an opposite-charge conversion of the corresponding Asp(454).

Modulation of enzymatic activity of dengue virus nonstructural protein NS3 nucleoside triphosphatase/helicase by poly(U)

The nonstructural protein 3 (NS3) appears to be the most promising target for anti-flavivirus therapy because of its multiple enzymatic activities that are indispensable for virus replication. NS3 of dengue virus type 2 (DEN2) is composed of two domains, a serine protease in the N-terminal domain (NS3pro) and RNA-stimulated nucleoside triphosphatase (NTPase)/RNA helicase at the C-terminus (NS3h). NS3 plays an important role in viral replication and the coordinated regulation of all the catalytic activities in the full-length NS3 protein. In this study, a plasmid harboring the NS3 helicase domain (NS3h) was constructed by PCR. The 56.5 kDa NS3h protein was purified by metal-chelate affinity chromatography followed by renaturation, mediated by artificial chaperone-assisted refolding, which yielded the active helicase. NTPase activity was assayed with Malachite Green. The NTPase activity in the presence of poly(U) showed a higher turnover number (kcat) and a lower Km value than without poly(U). The activity increased approximately fourfold in the presence of polynucleotides. This indicates that NTPase activity of dengue NS3 can be stimulated by polynucleotides. A helicase assay based on internal fluorescence quenching was conducted using short internally quenched DNA oligonucleotides as substrates. Significant fluorescence signaling increase was observed in the absence of polynucleotides such as poly(U). No unwinding activity was observed with addition of poly(U). The approach we describe here is useful for the further characterization of substrate specificity and for the design of high-throughput assays aimed at discovery of inhibitors against NS3 NTPase/helicase activities.

A straightforward experimental approach to expression, purification, refolding, and enzymatic analysis of recombinant dengue virus NS2B(H)-NS3pro protease

Dengue virus threatens around 2.5 billion people worldwide; about 50 million become infected every year, and yet no vaccine or drug is available for prevention and/or treatment. The flaviviral NS2B-NS3pro complex is indispensable for flaviviral replication and is considered to be an important drug target. The aim of this study was to develop a simple and generally applicable experimental strategy to construct, purify, and assay a highly active recombinant NS2B(H)-NS3pro complex that would be useful for high-throughput screening of potential inhibitors. The sequence of NS2B(H)-NS3pro was generated by overlap extension PCR (SOE-PCR) and cloned into the pTrcHisA vector. Hexahistidine-tagged NS2B(H)-NS3pro complex was expressed in E. coli predominantly as insoluble protein and purified to >95% purity by single-step immobilized metal affinity chromatography. SDS-PAGE followed by immunoblotting of the purified enzyme demonstrated the presence of the NS2B(H)-NS3pro precursor and its autocleavage products, NS3pro and NS2B(H), as 37, 21, and 10 kDa bands, respectively. Kinetic parameters, Km, kcat, and kcat/Km for the fluorophore-linked protease model substrate Ac-nKRR-amc were obtained using inner-filter effect correction. The kinetic parameters Km, kcat, and kcat/Km for Ac-nKRR-amc substrate were 100 µM, 0.112 s(-1), and 1120 M(-1)·s(-1), respectively. A simplified procedure for the cloning, overexpression, and purification of the NS2B(H)-NS3pro complex was applied, and a highly active recombinant NS2B(H)-NS3pro complex was obtained that could be useful for the design of high-throughput assays aimed at flaviviral inhibitor discovery.

Bacillus thuringiensis Cry4Ba toxin employs two receptor-binding loops for synergistic interactions with Cyt2Aa2

We previously demonstrated that co-expression in Escherichia coli of Bacillus thuringiensis (Bt) subsp. israelensis Cry4Ba and Bt subsp. darmstadiensis Cyt2Aa2 shows high synergistic toxicity against target mosquito larvae. Here, further insights into synergistic interactions between these two toxins were revealed through bioactivity restoration of particular inactive Cry4Ba-mutant toxins altered within the receptor-binding domain. Specific mutations at β2-β3 (Y332A) or β4-β5 (F364A) loops, but neither at three other β-hairpin loops (β6-β7, β8-β9 and β10-β11) of Cry4Ba, adversely affect toxicity restoration by Cyt2Aa2. Binding analysis using quartz crystal microbalance verified a decrease in binding of these two bioinactive-mutant toxins (Y332A and F364A) to the immobilized Cyt2Aa2. This suggests that Cry4Ba utilizes these two critical aromatic loop-residues, Tyr(332) and Phe(364), for synergistic toxicity with its alternative receptor-Cyt2Aa2.

In vivo identification of Bacillus thuringiensis Cry4Ba toxin receptors by RNA interference knockdown of glycosylphosphatidylinositol-linked aminopeptidase N transcripts in Aedes aegypti larvae

Bacillus thuringiensis Cry4Ba toxin selectively kills Aedes aegypti mosquito larvae as it is in part due to the presence of specific membrane-bound protein receptors. In this study, using data mining approach, we initially identified three potential glycosylphosphatidylinositol-linked aminopeptidase N (GPI-APN) isoforms, APN2778, APN2783 and APN5808, which are believed to act as Cry4Ba toxin receptors. These three isoforms that are functionally expressed in the larval midgut can be sequence-specific knocked down (ranging from ∼80 % to 95 %) by soaking the Aedes aegypti larvae in buffer of long double-stranded GPI-APN RNAs (∼300-680 bp). Finally, to see the physiological effect of APN knockdowns, the larvae were fed with Escherichia coli expressing Cry4Ba toxin. The results revealed that all the three identified GPI-APN isoforms may possibly function as a Cry4Ba receptor, particularly for APN2783 as those larvae with this transcript knockdown showed a dramatic increase in resistance to Cry4Ba toxicity.

Functional significance of the highly conserved Glu(570) in the putative pore-forming helix 3 of the Bordetella pertussis haemolysin toxin

Adenylate cyclase-haemolysin toxin (CyaA) is a virulence factor secreted from the etiologic agent of whooping cough, Bordetella pertussis. Previously, the haemolysin or pore-forming domain (CyaA-PF) has been shown to cause cell lysis of sheep erythrocytes independently, and the predicted helix 3((570-593)) within the PF-hydrophobic stretch could be a pore-lining constituent. Here, a plausible involvement in haemolytic activity of polar or charged residues (Glu(570), Gln(574), Glu(581), Ser(584) and Ser(585)) lining the hydrophilic side of CyaA-PF helix 3 was investigated via single-alanine substitutions. All the 126-kDa mutant proteins over-expressed in Escherichia coli were verified for toxin acylation as the results are corresponding to the wild-type toxin. When haemolytic activity of E. coli lysates containing soluble mutant proteins was tested against sheep erythrocytes, the importance of Glu(570), which is highly conserved among the pore-forming RTX cytotoxin family, was revealed for pore formation, conceivably for a general pore-lining residue involved in ion conduction.

Functional expression in insect cells of glycosylphosphatidylinositol-linked alkaline phosphatase from Aedes aegypti larval midgut: a Bacillus thuringiensis Cry4Ba toxin receptor

Bacillus thuringiensis produces insecticidal crystal (Cry) proteins which bind to cell surface receptors on the brush border membrane of susceptible midgut larvae. The toxin-receptor interaction generates pores in midgut epithelial cells resulting in cell lysis. Here, a cDNA encoding membrane-bound alkaline phosphatase from Aedes aegypti (Aa-mALP) midgut larvae, based on the sequence identity hit to Bombyx mori membrane-bound ALP, was amplified by RT-PCR and transiently expressed in Spodoptera frugiperda (Sf9) insect cells as a 58-kDa membrane-bound protein via the baculovirus expression system and confirmed by digestion with phosphatidylinositol-specific phospholipase C and LC-MS/MS analysis. Immunolocalization results showed that Cry4Ba is able to bind to only Sf9 cells-expressing Aa-mALP. Moreover, these cells were shown to undergo cell lysis in the presence of 100 μg/ml trypsin-treated toxin. Finally, trypan blue exclusion assay also demonstrated an increase in cell death in recombinant cells treated with Cry4Ba. Overall results indicated that Aa-mALP protein was responsible for mediating Cry4Ba toxicity against Sf9 cells, suggesting its role as a receptor for Cry4Ba toxin in A. aegypti mosquito larvae.

Structure-guided mutagenesis of active site residues in the dengue virus two-component protease NS2B-NS3

Background

The dengue virus two-component protease NS2B/NS3 mediates processing of the viral polyprotein precursor and is therefore an important determinant of virus replication. The enzyme is now intensively studied with a view to the structure-based development of antiviral inhibitors. Although 3-dimensional structures have now been elucidated for a number of flaviviral proteases, enzyme-substrate interactions are characterized only to a limited extend. The high selectivity of the dengue virus protease for the polyprotein precursor offers the distinct advantage of designing inhibitors with exquisite specificity for the viral enzyme. To identify important determinants of substrate binding and catalysis in the active site of the dengue virus NS3 protease, nine residues, L115, D129, G133, T134, Y150, G151, N152, S163 and I165, located within the S1 and S2 pockets of the enzyme were targeted by alanine substitution mutagenesis and effects on enzyme activity were fluorometrically assayed.

Methods

Alanine substitutions were introduced by site-directed mutagenesis at residues L115, D129, G133, T134, Y150, G151, N152, S163 and I165 and recombinant proteins were purified from overexpressing E. coli. Effects of these substitutions on enzymatic activity of the NS3 protease were assayed by fluorescence release from the synthetic model substrate GRR-amc and kinetic parameters K_m, k_cat and k_cat/K_m were determined.

Results

Kinetic data for mutant derivatives in the active site of the dengue virus NS3 protease were essentially in agreement with a functional role of the selected residues for substrate binding and/or catalysis. Only the L115A mutant displayed activity comparable to the wild-type enzyme, whereas mutation of residues Y150 and G151 to alanine completely abrogated enzyme activity. A G133A mutant had an approximately 10-fold reduced catalytic efficiency thus suggesting a critical role for this residue seemingly as part of the oxyanion binding hole.

Conclusions

Kinetic data obtained for mutants in the NS3 protease have confirmed predictions for the conformation of the active site S1 and S2 pockets based on earlier observations. The data presented herein will be useful to further explore structure-activity relationships of the flaviviral proteases important for the structure-guided design of novel antiviral therapeutics.

Bordetella pertussis CyaA-RTX subdomain requires calcium ions for structural stability against proteolytic degradation

Previously, the 126-kDa Bordetella pertussis CyaA pore-forming (CyaA-PF) domain expressed in Escherichia coli was shown to retain its hemolytic activity. Here, a 100-kDa RTX (Repeat-in-ToXin) subcloned fragment (CyaA-RTX) containing a number of putative calcium-binding repeats was further investigated. The recombinant CyaA-RTX protein, although expressed as a soluble form in a protease-deficient E. coli strain BL21(DE3)pLysS, was found to be highly sensitive to proteolytic degradation. Interestingly, the addition of calcium ions in a millimolar range into the CyaA-RTX preparation significantly prevented the degradation. Moreover, levels of proteolytic degradation were dependent on calcium concentrations, implying an important role for calcium-binding sites in the RTX subdomain for structural stability. Homology-based modeling of the repetitive blocks in the CyaA-RTX subdomain supports that this calcium-bound protein folds into a parallel β-roll structure with calcium ions acting as a structural stabilizing bridge.

Crystallization and preliminary X-ray crystallographic analysis of a full-length active form of the Cry4Ba toxin from Bacillus thuringiensis

To obtain a complete structure of the Bacillus thuringiensis Cry4Ba mosquito-larvicidal protein, a 65 kDa functional form of the Cry4Ba-R203Q mutant toxin was generated for crystallization by eliminating the tryptic cleavage site at Arg203. The 65 kDa trypsin-resistant fragment was purified and crystallized using the sitting-drop vapour-diffusion method. The crystals belonged to the rhombohedral space group R32, with unit-cell parameters a = b = 184.62, c = 187.36 A. Diffraction data were collected to at least 2.07 A resolution using synchrotron radiation and gave a data set with an overall R(merge) of 9.1% and a completeness of 99.9%. Preliminary analysis indicated that the asymmetric unit contained one molecule of the active full-length mutant, with a V(M) coefficient and solvent content of 4.33 A(3) Da(-1) and 71%, respectively.

Combined molecular dynamics and continuum solvent studies of the pre-pore Cry4Aa trimer suggest its stability in solution and how it may form pore

Cry4Aa toxin is one of the highly specific mosquito-larvicidal proteins produced by the bacterium Bacillus thuringiensis subspecies israelensis. It is thought to form pores in the larval midgut membrane that cause membrane leakage and subsequent insect death. Therefore, Cry4Aa and other Cry toxins have been used as efficient and safe bacterial insecticides to control the disease-carrying mosquitoes such as Aedes, Anopheles, and Culex. However, we still do not clearly understand how Cry toxins kill mosquito-larvae at molecular details. Recent electron crystallographic images of Cry4Ba toxin, another toxin closely related to Cry4Aa toxin, have suggested that the protein forms trimer in aqueous solution and in lipid monolayer. Moreover, the unit cell of X-ray crystal structure of Cry4Ba toxin has been shown to be trimeric. In this study, we constructed the first full-atom structural model of Cry4Aa trimer using the trimeric unit cell structure of Cry4Ba toxin as a template and then used the methods of molecular dynamics (MD) and molecular mechanics combined with Poisson-Boltzmann and surface area (MM-PBSA) to show that the trimeric structure of Cry4Aa toxin is stable in 150 mM KCl solution on 10 ns timescale. The results reveal that Cry4Aa toxins use polar amino acid residues on alpha-helices 3, 4, and 6 to form trimer and suggest that the proteins form trimer to reduce their non-polar interactions with surrounding water. Based on the obtained trimeric structure of Cry4Aa toxins, we propose that pore formation of Cry toxins may involve a 90 degrees -hairpin rotation during the insertion of their three alpha4-alpha5 hairpins into the membrane. This process may be mediated by water and ions.PACS Codes: 87.15.ap, 87.15.bk, 87.14.ep.

Esterase activity of Bordetella pertussis CyaC-acyltransferase against synthetic substrates: implications for catalytic mechanism in vivo

Adenylate cyclase-hemolysin toxin (CyaA) produced from the human respiratory tract pathogen Bordetella pertussis requires fatty-acyl modification by CyaC-acyltransferase to become an active toxin. Previously, the recombinant CyaA pore-forming (CyaA-PF) fragment expressed in Escherichia coli was shown to be hemolytically active upon palmitoylation in vivo by cosynthesized CyaC. Here, the 21-kDa CyaC enzyme separately expressed in E. coli as an inclusion body was solubilized in 8 M urea and successfully refolded into an enzymatically active monomer. In addition to the capability of activating CyaA-PF in vitro, CyaC showed esterase activity against p-nitrophenyl acetate (pNPA) and p-nitrophenyl palmitate (pNPP), with preferential hydrolysis toward pNPP when compared with chymotrypsin. A homology-based CyaC structure suggested a conceivable role of a catalytic triad including Ser(30), His(33) and Tyr(66) in substrate catalysis. Alanine substitutions of these individual residues caused a drastic decrease in specific activities of all three mutant enzymes (S30A, H33A and Y66A) toward pNPP, signifying that CyaC-acyltransferase shares a similar mechanism of hydrolysis with a serine esterase in which Ser(30) is part of the catalytic triad.

Correlative effect on the toxicity of three surface-exposed loops in the receptor-binding domain of the Bacillus thuringiensis Cry4Ba toxin

Surface-exposed loop residues, Pro(389) (beta(6)-beta(7) loop), Glu(417) (beta(8)-beta(9) loop), Tyr(455) and Asn(456) (beta(10)-beta(11) loop), in the receptor-binding domain of the Bacillus thuringiensis Cry4Ba toxin have been previously demonstrated to be crucial for toxicity. Herein, five combinations of two-loop mutants, P389A/E417A (beta(6)-beta(7)/beta(8)-beta(9) loops), P389A/Y455A, P389A/N456A (beta(6)-beta(7)/beta(10)-beta(11) loops), E417A/Y455A and E417A/N456A (beta(8)-beta(9)/beta(10)-beta(11) loops), were constructed as a means of examining a correlative effect of these three critical loops on Cry4Ba toxicity. All 130-kDa mutant protoxins were overexpressed as inclusion bodies in Escherichia coli with yields comparable to the wild-type toxin. In addition, all mutant toxins were structurally stable upon solubilization and trypsin activation in carbonate buffer, pH 9.0. Interestingly, E. coli cells expressing all the double-loop mutants showed an almost complete loss in toxicity against Aedes aegypti mosquito larvae while their corresponding single-loop mutants exhibited reduced activity of approximately 50%. Moreover, in situ binding analysis revealed that the 65-kDa purified toxins representing each pairwise mutant exhibited reduced binding to apical microvilli of A. aegypti larval midgut when compared with the single mutants. Altogether, the data demonstrate for the first time that all these three surface-exposed loops of the Cry4Ba toxin are equally involved in receptor binding and hence toxicity.

Ion channels formed in planar lipid bilayers by the dipteran-specific Cry4BBacillus thuringiensistoxin and its α1–α5 fragment

Trypsin activation of Cry4B, a 130-kDa Bacillus thuringiensis (Bt) protein, produces a 65-kDa toxin active against mosquito larvae. The active toxin is made of two protease resistant-products of ca. 45 kDa and ca. 20 kDa. The cloned 21-kDa fragment consisting of the N-terminal region of the toxin was previously shown to be capable of permeabilizing liposomes. The present study was designed to test the following hypotheses: (1) Cry4B, like several other Bt toxins, is a channel-forming toxin in plannar lipid bilayers; and (2) the 21-kDa N-terminal region, which maps for the first five helices (alpha1-alpha5) of domain 1 in other Cry toxins, and which putatively shares a similar tri-dimensional structure, is sufficient to account for the ion channel activity of the whole toxin. Using circular dichroism spectroscopy and planar lipid bilayers, we showed that the 21-kDa polypeptide existed as an alpha-helical structure and that both Cry4B and its alpha1-alpha5 fragment formed ion channels of 248 +/- 44 pS and 207 +/- 23 pS, respectively. The channels were cation-selective with a potassium-to-chloride permeability ratio of 6.7 for Cry4B and 4.5 for its fragment. However, contrary to the full-length toxin, the alpha1-alpha5 region formed channels at low dose; they tended to remain locked in their open state and displayed flickering activity bouts. Thus, like the full-length toxin, the alpha1-alpha5 region is a functional channel former. A pH-dependent, yet undefined region of the toxin may be involved in regulating the channel properties.

Lipid-induced conformation of helix 7 from the pore-forming domain of the Bacillus thuringiensis Cry4Ba toxin: implications for toxicity mechanism

Helix 7 in the Cry4Ba-pore-forming domain contains conserved Tyr(249) and Phe(264) that are crucially involved in mosquito-larvicidal activity. We have now characterized lipid-induced conformation of a 27-residue Cry4Ba-alpha7 peptide in phospholipid membranes using ATR-FTIR and hydrogen/deuterium (H(+)/D(+)) exchange experiments. ATR-FTIR results showed that conformation of this peptide is influenced by lipid composition and peptide-lipid ratio. For zwitterionic membranes, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-didecanoyl-sn-glycero-3-phosphocholine, the peptide adopted both alpha-helix and alpha-structure, but only alpha-helical conformation was observed in anionic membranes (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol). H(+)/D(+) exchange results showed protection of approximately 90% in DMPC for beta-form, while alpha-helical form was found preferentially on membrane surface with both critical aromatic residues pointing towards bilayers. Analysis of 10-ns simulations of Cry4Ba-alpha7 in DMPC supports the stability of alpha-helical and beta-conformations for membrane-associated and membrane-inserted states, respectively. We suggest that this lipid-induced conformational change of alpha7 is conceivably related to pore-forming mechanism as structural requirement for efficient membrane insertion.

Binding characteristics to mosquito-larval midgut proteins of the cloned domain II-III fragment from the Bacillus thuringiensis Cry4Ba toxin

Receptor binding plays an important role in determining host specificity of the Bacillus thuringiensis Cry delta-endotoxins. Mutations in domains II and III have suggested the participation of certain residues in receptor recognition and insect specificity. In the present study, we expressed the cloned domain II-III fragment of Cry4Ba and examined its binding characteristics to mosquito-larval midgut proteins. The 43-kDa Cry4Ba-domain II-III protein over-expressed in Escherichia coli as inclusion bodies was only soluble when carbonate buffer, pH 10.0 was supplemented with 4 M urea. After renaturation via stepwise dialysis and subsequent purification, the refolded domain II-III protein, which specifically reacts with anti Cry4Ba-domain III monoclonal antibody, predominantly exists as a beta-sheet structure determined by circular dichroism spectroscopy. In vitro binding analysis to both histological midgut tissue sections and brush border membrane proteins prepared from susceptible Aedes aegypti mosquito-larvae revealed that the isolated Cry4Ba-domain II-III protein showed binding functionality comparable to the 65-kDa full-length active toxin. Altogether, the data present the 43-kDa Cry4Ba fragment comprising domains II and III that was produced in isolation was able to retain its receptor-binding characteristics to the target larval midgut proteins.

High level of soluble expression in Escherichia coli and characterisation of the CyaA pore-forming fragment from a Bordetella pertussis Thai clinical isolate

Bordetella pertussis adenylate cyclase toxin-haemolysin (CyaA) can permeabilise erythrocytes by forming lytic pores. Here, a gene segment encoding CyaA pore-forming (CyaA-PF) domain cloned from genomic DNA of B. pertussis Thai isolate was over-expressed in Escherichia coli as a 126-kDa soluble protein which cross-reacted with anti-RTX monoclonal antibody. By co-expressing with acyltransferase CyaC, the CyaA-PF protein was found palmitoylated at Lys(983). Unlike E. coli lysate with the non-acylated form, the lysate containing acylated CyaA-PF exhibited high haemolytic activity against sheep erythrocytes. This study presents that the recombinant CyaA-PF protein comprising pore-forming domain can be expressed separately as soluble native-folded precursor that conserves at least part of its functionality.

Two conformational states of the membrane-associated Bacillus thuringiensis Cry4Ba delta-endotoxin complex revealed by electron crystallography: implications for toxin-pore formation

The insecticidal nature of Cry delta-endotoxins produced by Bacillus thuringiensis is generally believed to be caused by their ability to form lytic pores in the midgut cell membrane of susceptible insect larvae. Here we have analyzed membrane-associated structures of the 65-kDa dipteran-active Cry4Ba toxin by electron crystallography. The membrane-associated toxin complex was crystallized in the presence of DMPC via detergent dialysis. Depending upon the charge of the adsorbed surface, 2D crystals of the oligomeric toxin complex have been captured in two distinct conformations. The projection maps of those crystals have been generated at 17A resolution. Both complexes appeared to be trimeric; as in one crystal form, its projection structure revealed a symmetrical pinwheel-like shape with virtually no depression in the middle of the complex. The other form revealed a propeller-like conformation displaying an obvious hole in the center region, presumably representing the toxin-induced pore. These crystallographic data thus demonstrate for the first time that the 65-kDa activated Cry4Ba toxin in association with lipid membranes could exist in at least two different trimeric conformations, conceivably implying the closed and open states of the pore.

High level of soluble expression in Escherichia coli and characterisation of the cloned Bacillus thuringiensis Cry4Ba domain III fragment

Similar to the other known structures of Bacillus thuringiensis Cry delta-endotoxins, the crystal structure of the 65-kDa activated Cry4Ba toxin comprises three domains which are, from the N- to C-terminus, a bundle of alpha-helices, a three-beta-sheet domain, and a beta-sandwich. To investigate the properties of the C-terminal domain III in isolation from the rest of the toxin, the cloned Cry4Ba-domain III was over-expressed as a 21-kDa soluble protein in Escherichia coli, which cross-reacted with anti-Cry4Ba domain III monoclonal antibody. A highly-purified domain III was obtained in a monomeric form by ion-exchange and size-exclusion FPLC. Circular dichroism spectroscopy indicated that the isolated domain III fragment distinctly exists as a beta-sheet structure, corresponding to the domain III structure embodied in the Cry4Ba crystal structure. In vitro binding analysis via immuno-histochemical assay revealed that the Cry4Ba-domain III protein was able to bind to the apical microvilli of the susceptible Stegomyia aegypti larval midguts, albeit at lower-binding activity when compared with the full-length active toxin. These results demonstrate for the first time that the C-terminal domain III of the Cry4Ba mosquito-larvicidal protein, which can be isolated as a native folded monomer, conceivably participates in toxin-receptor recognition.

Structurally conserved aromaticity of Tyr249 and Phe264 in helix 7 is important for toxicity of the Bacillus thuringiensis Cry4Ba toxin

Functional elements of the conserved helix 7 in the poreforming domain of the Bacillus thuringiensis Cry delta- endotoxins have not yet been clearly identified. Here, we initially performed alanine substitutions of four highly conserved aromatic residues, Trp(243), Phe(246), Tyr(249) and Phe(264), in helix 7 of the Cry4Ba mosquito-larvicidal protein. All mutant toxins were overexpressed in Escherichia coli as 130-kDa protoxins at levels comparable to the wild-type. Bioassays against Stegomyia aegypti mosquito larvae revealed that only W243A, Y249A or F264A mutant toxins displayed a dramatic decrease in toxicity. Further mutagenic analysis showed that replacements with an aromatic residue particularly at Tyr(249) and Phe(264) still retained the high-level toxin activity. In addition, a nearly complete loss in larvicidal activity was found for Y249L/F264L or F264A/ Y249A double mutants, confirming the involvement in toxicity of both aromatic residues which face towards the same direction. Furthermore, the Y249L/F264L mutant was found to be structurally stable upon toxin solubilisation and trypsin digestion, albeit a small change in the circular dichroism spectrum. Altogether, the present study provides for the first time an insight into the highly conserved aromaticity of Tyr(249) and Phe(264) within helix 7 playing an important role in larvicidal activity of the Cry4Ba toxin.