Role of zinc binding in type A botulinum neurotoxin light chain's toxic structure

Clostridial neurotoxins are zinc endopeptidases, and each contains one Zn(2+)/molecule. To investigate the structural/functional role of Zn(2+) in botulinum neurotoxin light chain (the enzymatic subunit of the neurotoxin), the effect of the removal of zinc on protein folding and enzyme kinetics was investigated. The active site Zn(2+), which was easily displaced from the active site by ethylenediaminetetraacetate, reversibly binds to the BoNT/A light chain (LC) in a stoichiometric manner. Enzymatic activity was completely abolished in the zinc-depleted light chain (apo-LC). However, Zn(2+) replenishment partially restored the activity in the re-Zn(2+)-LC (k(cat) = 72 min(-)(1)) compared to the holo-LC (k(cat) = 140 min(-)(1)). Comparable K(m) values in the holo- and re-Zn(2+)-LC were observed (41 and 55 microM, respectively), indicating a similar substrate binding ability. We investigated the structural basis of a 3-fold difference in the catalytic efficiency of the native holo-LC and re-Zn(2+)-LC by analyzing secondary and tertiary structural parameters. Removal of the zinc causes irreversible tertiary structural change while the secondary structure remains unchanged. Zinc binding leads to enhanced thermal stability of the LC, which is not identical in the native holo-LC and re-Zn(2+)-LC.     

High-level expression, purification, and characterization of recombinant type A botulinum neurotoxin light chain

Botulinum neurotoxin light chain (BoNT LC, 50 kDa) is responsible for the zinc endopeptidase activity specific for proteins of neuroexocytosis apparatus. We describe the expression of recombinant type A BoNT LC in Escherichia coli as well as the purification and characterization of the recombinant protein. A high level of expression of BoNT/A LC was obtained by an extended postinduction time of 15 h at 30 degrees C. Recombinant BoNT/A LC was isolated from an Ni(2+) column. Due to its high pI ( approximately 8.7), purification was achieved by a single step of passing the protein through anion-exchange chromatography at pH 8.0 without the need of elution. The purified recombinant BoNT/A LC retained proteolytic activity and had a secondary structure similar to that of native LC determined by CD measurement.     

The slow folding reaction of barstar: the core tryptophan region attains tight packing before substantial secondary and tertiary structure formation and final compaction of the polypeptide chain

The slow folding of a single tryptophan-containing mutant of barstar has been studied in the presence of 2 M urea at 10 degrees C, using steady state and time-resolved fluorescence methods and far and near-UV CD measurements. The protein folds in two major phases: a fast phase, which is lost in the dead time of measurement during which the polypeptide collapses to a compact form, is followed by a slow observable phase. During the fast phase, the rotational correlation time of Trp53 increases from 2.2 ns to 7.2 ns, and its mean fluorescence lifetime increases from 2.3 ns to 3.4 ns. The fractional changes in steady-state fluorescence, far-UV CD, and near-UV CD signals, which are associated with the fast phase are, respectively, 36 %, 46 %, and 16 %. The product of the fast phase can bind the hydrophobic dye ANS. These observations together suggest that the folding intermediate accumulated at the end of the fast phase has: (a) about 20 % of the native-state secondary structure, (b) marginally formed or disordered tertiary structure, (c) a water-intruded and mobile protein interior; and (d) solvent-accessible patches of hydrophobic groups. Measurements of the anisotropy decay of Trp53 suggest that it undergoes two types of rotational motion in the intermediate: (i) fast (tau(r) approximately 1 ns) local motion of its indole side-chain, and (ii) a slower (tau(r) approximately 7.2 ns) motion corresponding to global tumbling of the entire protein molecule. The ability of the Trp53 side-chain to undergo fast local motion in the intermediate, but not in the fully folded protein where it is completely buried in the hydrophobic core, suggests that the core of the intermediate is still poorly packed. The global tumbling time of the fully folded protein is faster at 5.6 ns, suggesting that the volume of the intermediate is 25 % more than that of the fully folded protein. The rate of folding of this intermediate to the native state, measured by steady-state fluorescence, far-UV CD, and near-UV CD, is 0.07(+/-0.01) min(-1) This rate compares to a rate of folding of 0.03(+/-0.005) min(-1), determined by double-jump experiments which monitor directly formation of native protein; and to a rate of folding of 0.05 min(-1), when determined from time-resolved anisotropy measurements of the long rotational correlation time, which relaxes from an initial value of 7.2 ns to a final value of 5. 6 ns as the protein folds. On the other hand, the amplitude of the short correlation time decreases rapidly with a rate of 0.24(+/-0.06) min(-1). These results suggest that tight packing of residues in the hydrophobic core occurs relatively early during the observable slow folding reaction, before substantial secondary and tertiary structure formation and before final compaction of the protein.     

Dynamics of tryptophan in the histidine-containing phosphocarrier protein of Streptomyces coelicolor: evidence of multistate equilibrium unfolding

The nanosecond dynamics of the single tryptophan, Trp10, of HPr from Streptomyces coelicolor, HPrsc, has been monitored at different pHs. Time-resolved fluorescence methods and DOSY measurements have been used to map the compactness of the protein. At low pHs, where a molten globule-like species has been described, the correlation times from fluorescence showed an abrupt change as the pH was increased. When the protein was folded (above pH 4), two correlation times were observed, which remained practically constant up to pH 9.5. The long correlation time, around 7.5 ns, corresponds to the global rotational motion of the protein, since this value is in agreement with that determined theoretically from hydrodynamic measurements. The short correlation time, around 1.4 ns, must report on fast movements of the protein segment containing the tryptophan residue. On the other hand, fluorescence lifetimes showed the same abrupt change as the correlation times at low pH, but, in addition, a sigmoidal change with a pKa approximately 4.3 was also observed. On the basis of the modeled structure of HPrsc, this last transition could be due to the proximity of Glu12 to Trp10. The changes monitored by the fluorescence lifetimes agree with those observed previously by steady-state fluorescence, CD, and ANS binding experiments. Taken together, these data suggest a multistate equilibrium during folding of HPrsc starting from low pHs.     

Tryptophan fluorescence reveals induced folding of Vibrio harveyi acyl carrier protein upon interaction with partner enzymes

We have introduced tryptophan as a local fluorescent probe to monitor the conformation of Vibrio harveyi acyl carrier protein (ACP), a small flexible protein that is unfolded at neutral pH but must undergo reversible conformational change during the synthesis and delivery of bacterial fatty acids. Consistent with known 3D structures of ACP, steady-state fluorescence and quenching experiments indicated that Trp at positions 46, 50, and 72 are buried in the hydrophobic core upon Mg(2+)-induced ACP folding, whereas residues 25 and 45 remain in a hydrophilic environment on the protein surface. Attachment of fatty acids to the phosphopantetheine prosthetic group progressively stabilized the folded conformation of all Trp-substituted ACPs, but longer chains (14:0) were less effective than medium chains (8:0) in shielding Trp from acrylamide quenching in the L46W protein. Interaction with ACP-dependent enzymes LpxA and holo-ACP synthase also caused folding of L46W; fluorescence quenching indicated proximity of Trp-45 in helix II of ACP in LpxA binding. Our results suggest that divalent cations and fatty acylation produce differing environments in the ACP core and also reveal enzyme partner-induced folding of ACP, a key feature of "natively unfolded" proteins.     

Botulinum Neurotoxin Light Chain Refolds at Endosomal pH for its Translocation

Botulinum neurotoxins (BoNTs), the most poisonous member of class A biothreat agent, cause neuroparalysis by blocking neurotransmitter release at the neuromuscular junctions. In its mechanism of action, the catalytic domain (light chain (LC) of BoNT) is transported to the cytosol by the heavy chain (HC) in order to reach its proteolytic substrates. The BoNT HC forms a membrane channel under acidic conditions encountered in endosomes to serve as a passageway for LC to enter into cytosol. We demonstrate here that BoNT/A LC undergoes unique structural changes under the low pH conditions, and adopts a molten globule state, exposing substantial number of hydrophobic groups. The flexibility of the molten globular structure combined with retention of the secondary structure and exposure of specific residues of LC for interaction with the HC, allows its translocation through the narrow endosomal membrane channel.     

The protein conformation and a zinc-binding domain of an autoantigen from mouse seminal vesicle.

The protein conformation of a mouse seminal vesicle autoantigen was studied by circular dichroism spectroscopy. At pH 7.4, the spectrum in the UV region appears as one negative band at 217 nm and one positive band at 200 nm. This together with the predicted secondary structures indicates no helices but a mixture of beta form, beta turn, and unordered form in the protein molecule. The conformation is stable even at pH 10.5 or 3.0. The spectrum in the near-UV region consists of fine structures that are disturbed in acidic or alkaline solution. The environments around Trp2 and Trp82 of this protein were studied by intrinsic fluorescence and solute quenching. They give an emission peak at 345 nm, and about 87% of them are accessible to quenching by acrylamide. Correlating the quenching effect of CsCl and Kl on the protein fluorescence to the charged groups along the polypeptide chain suggests the difference in the "local charge" around the two tryptophan residues. The presence of ZnCl2 in the protein solution effects no change in the circular dichroism but perturbs the fluorescence due to Trp82. Analysis of the fluorescence data suggests a Zn(2+)-binding site on the protein, which cannot coordinate with both Ca2+ and Mg2+. The association constant for the complex formation is 1.35 x 10(5) +/- 0.04 x 10(5) M-1 at pH 7.4.     

Molecular mechanism of the Zn2+-induced folding of the distal CCHC finger motif of the HIV-1 nucleocapsid protein

HIV-1 nucleocapsid protein, NCp7, contains two highly conserved CCHC zinc fingers. Binding of Zn(2+) drives NCp7 from an unfolded to a highly folded structure that is critical for its functions. Using the intrinsic fluorescence of Trp(37), we investigated, by the stopped-flow technique, the folding of NCp7 distal finger through the pH dependence of its Zn(2+) association and dissociation kinetics. Zn(2+) binding was found to involve four different paths associated with the four deprotonated states of the finger. Each binding path involves the rapid formation of an intermediate complex that is subsequently rearranged and stabilized in a rate-limiting step. The equilibrium and kinetic rate constants of the full Zn(2+)-binding process have been determined. At neutral pH, the preferential pathway for the Zn(2+)-driven folding implies Zn(2+) binding to the deprotonated Cys(36) and His(44) residues, in the bidentate state of the finger. The resulting intermediate is then converted with a rate constant of 500 s(-1) into a more suitably folded form, probably through a rearrangement of the peptide backbone around Zn(2+) to optimize the binding geometry. This form then rapidly leads to the final native complex, through deprotonation of Cys(39) and Cys(49) residues and intramolecular substitution of coordinated water molecules. Zn(2+) dissociation is also characterized by a multistep process and occurs fastest via the deprotonated Zn(2+)-bound bidentate state with a rate constant of 3 s(-1). Due to their critical role in folding, the intermediates identified for the first time in this study may constitute potential targets for HIV therapy.     

Probing the cytochrome c' folding landscape

The folding kinetics of R. palustris cytochrome c' (cyt c') have been monitored by heme absorption and native Trp72 fluorescence at pH 5. The Trp72 fluorescence burst signal suggests early compaction of the polypeptide ensemble. Analysis of heme transient absorption spectra reveals deviations from two-state behavior, including a prominent slow phase that is accelerated by the prolyl isomerase cyclophilin. A nonnative proline configuration (Pro21) likely interferes with the formation of the helical bundle surrounding the heme.     

A fluorescence spectroscopy study of the calpactin I complex and its subunits p11 and p36: calcium-dependent conformation changes

A fluorescence study of the calpactin I complex, a heterotetramer composed of two molecules of p36 and two molecules of p11, and its subunits, was performed to clarify their conformation. The analysis of the fluorescence characteristics of the single Trp of p36, in the absence of Ca(2+), shows that: (i) in the complex, Trp is buried within the protein matrix and subjected to static quenching from nearby groups; (ii) for p36 the results are similar, but Trp seems even more shielded than in the complex. Adding Ca(2+) to the calpactin I complex, or to p36, shifts the Trp emission maximum wavelengths, and increases the quantum yields which reflect a conformational change, burying the Trp in a more hydrophobic environment. In the presence and even in the absence of Ca(2+), the binding of phosphatidylserine liposomes induces a conformational change, detected by fluorescence measurements. The Ca(2+) dissociation constants, as determined by fluorescence titrations, are similar for the complex and p36 (KD approximately 0.5 x 10(-3) M). The affinity is enhanced a 1000-times in the presence of negatively charged phospholipids. In p11, both Try residues are located in a hydrophobic environment and the protein fluorescence does not change upon Ca(2+) addition.     

Arg(362) and Tyr(365) of the botulinum neurotoxin type a light chain are involved in transition state stabilization

The botulinum neurotoxin type A (BoNT/A) light chain (LC) acts as zinc endopeptidase. The X-ray structure of the toxin demonstrated that Zn(2+) is coordinated by His(222) and His(226) of the Zn(2+) binding motif HisGluXXHis and Glu(261), whereas Glu(223) coordinates the water molecule required for hydrolysis as the fourth ligand. Recent analysis of a cocrystal of the BoNT/B LC and its substrate synaptobrevin 2 suggested that Arg(362) and Tyr(365) of the homologous BoNT/A may be directly involved in catalysis. Their role and that of Glu(350) which is also found in the vicinity to the active site were analyzed by site-directed mutagenesis. Various replacements of Arg(362) and substitution of Tyr(365) with Phe resulted in 79- and 34-fold lower k(cat)/K(m) values, respectively. These changes were provoked by decreased catalytic rates (k(cat)) and not by alterations of ground state substrate binding as evidenced by largely unchanged K(d) and K(m) values. None of these mutations affected the overall secondary structure or zinc content of the LC. These findings suggest that the guanidino group of Arg(362) and the hydroxyl group of Tyr(365) together accomplish transition state stabilization as was proposed for thermolysin, being the prototypical member of the gluzincin superfamily of metalloproteases. Mutation of Glu(350) dramatically diminished the hydrolytic activity which must partly be attributed to an altered active site fine structure as demonstrated by an increased sensitivity toward heat-induced denaturing and a lower Zn(2+) binding affinity. Glu(350) apparently occupies a central position in the active site and presumably positions His(222) and Arg(362).     

Probing the mechanistic role of glutamate residue in the zinc-binding motif of type A botulinum neurotoxin light chain

Type A botulinum neurotoxin (BoNT/A) is a zinc endopeptidase that contains the consensus sequence HEXXH (residues 223-227) in the toxic light chain (LC). The X-ray structure of the toxin has predicted that the two histidines of this motif are two of the three zinc-coordinating ligands and that the glutamate is a crucial amino acid involved in catalysis. The functional implication of E224 in the motif of LC was investigated by replacing the residue with glutamine and aspartate using site-directed mutagenesis. Substitution of Glu-224 with Gln (E224Q) resulted in a total loss of the endopeptidase activity, whereas substitution with Asp (E224D) retained about 1.4% of the enzymatic activity (k(cat) 140 vs 1.9 min(-1), respectively). However, K(m) values for wild-type and E224D BoNT/A LC were similar, 42 and 50 microM, respectively. Global structure, in terms of secondary structure content and topography of aromatic amino residues, Zn(2+) content, and substrate binding ability are retained in the enzymatically inactive mutants. Titration of Zn(2+) to EDTA-treated wild-type and mutant proteins indicated identical enthalpy for Zn(2+) binding. These results suggest an essential and direct role of the carboxyl group of Glu-224 in the hydrolysis of the substrate. The location of the carboxyl group at a precise position is critical for the enzymatic activity, as replacement of Glu-224 with Asp resulted in almost total loss of the activity.     

Early events during folding of wild-type staphylococcal nuclease and a single-tryptophan variant studied by ultrarapid mixing

A continuous-flow mixing device with a dead time of 100 micros coupled with intrinsic tryptophan and 1-anilinonaphthalene-8-sulfonate (ANS) fluorescence was used to monitor structure formation during early stages of the folding of staphylococcal nuclease (SNase). A variant with a unique tryptophan fluorophore in the N-terminal beta-barrel domain (Trp76 SNase) was obtained by replacing the single Trp140 in wild-type SNase with His in combination with Trp substitution of Phe76. A common background of P47G, P117G and H124L mutations was chosen in order to stabilize the protein and prevent accumulation of cis proline isomers under native conditions. In contrast to WT(*) SNase, which shows no changes in tryptophan fluorescence prior to the rate-limiting folding step ( approximately 100 ms), the F76W/W140H variant shows additional changes (enhancement) during an early folding phase with a time constant of 75 micros. Both proteins exhibit a major increase in ANS fluorescence and identical rates for this early folding event. These findings are consistent with the rapid accumulation of an ensemble of states containing a loosely packed hydrophobic core involving primarily the beta-barrel domain while the specific interactions in the alpha-helical domain involving Trp140 are formed only during the final stages of folding. The fact that both variants exhibit the same number of kinetic phases with very similar rates confirms that the folding mechanism is not perturbed by the F76W/W140H mutations. However, the Trp at position 76 reports on the rapid formation of a hydrophobic cluster in the N-terminal beta-sheet region while the wild-type Trp140 is silent during this early stage of folding. Quantitative modeling of the (un)folding kinetics and thermodynamics of these two proteins versus urea concentration revealed that the F76W/W140H mutation selectively destabilizes the native state relative to WT(*) SNase while the stability of transient intermediates remains unchanged, leading to accumulation of intermediates under equilibrium conditions at moderate denaturant concentrations.     

Organization and dynamics of tryptophan residues in tetrameric and monomeric soybean agglutinin: Studies by steady-state and time-resolved fluorescence,phosphorescence and chemical modification

We have investigated the organization and dynamics of tryptophan residues in tetrameric, monomeric and unfolded states of soybean agglutinin (SBA) by selective chemical modification, steady-state and time-resolved fluorescence, and phosphorescence. Oxidation with N-bromosuccinimide (NBS) modifies two tryptophans (Trp 60 and Trp 132) in tetramer, four (Trp 8, Trp 203 and previous two) in monomer, and all six (Trp 8, Trp 60, Trp 132, Trp 154, Trp 203 and Trp 226) in unfolded state. Utilizing wavelength-selective fluorescence approach, we have observed a red-edge excitation shift (REES) of 10 and 5 nm for tetramer and monomer, respectively. A more pronounced REES (21 nm) is observed after NBS oxidation. These results are supported by fluorescence anisotropy experiments. Acrylamide quenching shows the Stern–Volmer constant (K_SV) for tetramer, monomer and unfolded SBA being 2.2, 5.0 and 14.6 M⁻¹, respectively. Time-resolved fluorescence studies exhibit biexponential decay with the mean lifetime increasing along tetramer (1.0 ns) to monomer (1.9 ns) to unfolded (3.6 ns). Phosphorescence studies at 77 K give more structured spectra, with two (0,0) bands at 408.6 (weak) and 413.2 nm for tetramer. However, a single (0,0) band appears at 411.8 and 407.2 nm for monomer and unfolded SBA, respectively. The exposure of hydrophobic surface in SBA monomer has been examined by 8-anilino-1-naphthalenesulfonate (ANS) binding, which shows ∼20-fold increase in ANS fluorescence compared to that for tetramer. The mean lifetime of ANS also shows a large increase (12.0 ns) upon binding to monomer. These results may provide important insight into the role of tryptophans in the folding and association of SBA, and oligomeric proteins in general.     

The Translocation Domain of Botulinum Neurotoxin A Moderates the Propensity of the Catalytic Domain to Interact with Membranes at Acidic pH

Botulinum neurotoxin A (BoNT/A) is composed of three domains: a catalytic domain (LC), a translocation domain (H_N) and a receptor-binding domain (H_C). Like most bacterial toxins BoNT/A is an amphitropic protein, produced in a soluble form that is able to interact, penetrate and/or cross a membrane to achieve its toxic function. During intoxication BoNT/A is internalized by the cell by receptor-mediated endocytosis. Then, LC crosses the membrane of the endocytic compartment and reaches the cytosol. This translocation is initiated by the low pH found in this compartment. It has been suggested that LC passes in an unfolded state through a transmembrane passage formed by H_N. We report here that acidification induces no major conformational change in either secondary or tertiary structures of LC and H_N of BoNT/A in solution. GdnHCl-induced denaturation experiments showed that the stability of LC and H_N increases as pH drops, and that H_N further stabilizes LC. Unexpectedly we found that LC has a high propensity to interact with and permeabilize anionic lipid bilayers upon acidification without the help of H_N. This property is downplayed when LC is linked to H_N. H_N thus acts as a chaperone for LC by enhancing its stability but also as a moderator of the membrane interaction of LC.     

A correlation between differential structural features and the degree of endopeptidase activity of type A botulinum neurotoxin in aqueous solution

Botulinum neurotoxin type A is one of the most toxic substances known to man (LD(50) for mouse 0.1 ng/kg). It is also an effective therapeutic drug against involuntary muscle disorders and for pain management. BoNT/A is a Zn(2+) endopeptidase which selectively cleaves SNAP-25 (synaptosomal-associated protein of 25 kDa), a critical component of the exocytotic machinery. Based on nucleotide sequence, BoNT/A is a 145 kDa protein, which appears as a 145 kDa protein band on sodium dodecyl sulfate--polyacrylamide gel electrophoresis. We have examined the structure of BoNT/A in aqueous solution, and found the structure in aqueous solution differs dramatically from that resolved by X-ray crystallography, both at secondary and at quaternary levels. In terms of secondary structure, BoNT/A in aqueous solution has about 47% beta-sheet structure as revealed by infrared spectroscopy, while X-ray crystallography revealed only 17% beta-sheet structure. In terms of quaternary structure, the estimated molecular mass of the native BoNT/A in aqueous solution ranged between 230 and 314 kDa, based on results from different chemical and biophysical techniques (native gel electrophoresis, chemical cross-linking, size exclusion chromatography, and fluorescence anisotropy). These results indicate that BoNT/A exists as a dimer in aqueous solution, which contrasts with the reported monomeric structure of BoNT/A based on X-ray crystallography. The dimeric form of BoNT/A can self-dissociate into the monomeric form at a concentration lower than 50 nM. This concentration-dependent structural change has a significant impact on the endopeptidase activity of BoNT/A: the catalytic efficiency of the monomeric BoNT/A is about 4-fold higher than that of its dimeric form. This difference implies a sterically restricted catalytic site of BoNT/A in the dimeric form of BoNT/A.     

Natural domain design: enhanced thermal stability of a zinc-lacking ferredoxin isoform shows that a hydrophobic core efficiently replaces the structural metal site

Zinc centers play a key role as important structure determinants in a variety of proteins including ferredoxins (Fd). Here, we exploit the availability of two highly similar ferredoxin isoforms from the thermophile Sulfolobus metallicus, which differ in the residues involved in coordinating a His/Asp zinc site that ties together the protein core with its N-terminal extension, to investigate the effect of the absence of this site on ferredoxin folding. The conformational properties of the zinc-containing (FdA) and zinc-lacking (FdB) isoforms were investigated using visible absorption and tryptophan fluorescence emission. Fluorescence quenching studies, together with comparative modeling and molecular dynamics simulations, indicate that the FdB N-terminal extension assumes a fold identical to that of the Zn(2+)-containing isoform. The thermal stability of the isoforms was investigated in a broad pH range (2 < pH < 10), and at physiological pH conditions, both proteins unfold above 100 degrees C. Surprisingly, the Zn(2+)-lacking isoform was always found to be more stable than its Zn(2+)-containing counterpart: a DeltaT(m) approximately 9 degrees C is determined at pH 7, a difference that becomes even more significant at extreme pH values, reaching a DeltaT(m) approximately 24 degrees C at pH 2 and 10. The contribution of the Zn(2+) site to ferredoxin stability was further resolved using selective metal chelators. During thermal unfolding, the zinc scavenger TPEN significantly lowers the T(m) in FdA ( approximately 10 degrees C), whereas it has no effect in FdB. This shows that the Zn(2+) site contributes to ferredoxin stability but that FdB has devised a structural strategy that accounts for an enhanced stability without using a metal cross-linker. An analysis of the FdB sequence and structural model leads us to propose that the higher stability of the zinc-containing ferredoxin results from van der Waals contacts formed between the residues that occupy the same spatial region where the zinc ligands are found in FdA. These favor the formation of a novel local stabilizing hydrophobic core and illustrate a strategy of natural fold design.     

Spectroscopic analysis of low pH and lipid-induced structural changes in type A botulinum neurotoxin relevant to membrane channel formation and translocation

Botulinum neurotoxin (BoNT) is an extremely toxic protein to animals and humans. In its mode of action, one of its subunits mediates its translocation by integrating itself into the membrane bilayer. We have examined the membrane channel activity of type A BoNT (BoNT/A) and its heavy (H) chain in planar lipid membrane under various pH conditions to understand the possible role of the channel activity in the translocation of the BoNT/A light (L) chain under physiological conditions. Only BoNT/A H chain, and not the BoNT/A, exhibited membrane channel activity for translocation of ions. The H chain-induced increase in conductance did not require a pH gradient across the lipid membrane, although it was enhanced by a pH gradient. To understand the molecular basis of the membrane channel activity and the translocation of the L chain, the secondary structure of BoNT/A and its H and L chains were analyzed using circular dichroism (CD) and Fourier-transform infrared (FT-IR) spectroscopy at different pH values. BoNT/A showed no structural alternation upon acidifying the buffer pH. However, an increase in beta-sheet content of BoNT/A H chain at low pH was noted when examined by FT-IR. The L chain structure significantly changed with decrease in pH, and the change was mostly reversible. In addition, the neurotoxin and its subunit chains induced a partially reversible aggregation of liposomes at low pH, which indicated their integration into the lipid bilayer. Temperature-induced denaturation studies of BoNT/A H chain indicated major structural reorganization upon its interaction with membrane, especially at low pH.     

Effect of the N-terminal hydrophobic sequence of hepatitis B virus surface antigen on the folding and assembly of hybrid beta-galactosidase in Escherichia coli

To investigate the mechanism of inclusion body formation and the effect of a hydrophobic sequence on the in vivo polypeptide folding, the aggregation caused by recombinant fusion beta-galactosidase in Escherichia coli was examined. Two plasmids were constructed: pTBG(H-) carried only the preS2 sequence of the hepatitis B virus surface antigen (HBsAg) in front of the beta-galactosidase gene (lacZ) while pTBG(H+) carried an additional sequence encoding the amino-terminal hydrophobic sequence of the S region of HBsAg between preS2 and lacZ. Unlike cells expressing the fusion protein not containing the hydrophobic sequence, E. coli JM109/pTBG(H+) exhibited temperature-sensitive production of beta-galactosidase. As the culture temperature increased the activity decreased dramatically. This decrease in activity was not due to a decrease in fusion polypeptide production, but rather the fusion polypeptides containing the hydrophobic sequence aggregated within the cells at high temperature. However once the fusion polypeptides folded into proper conformation at low temperature, they maintained the activity even at high temperature. The results indicate that aggregation is a consequence of incorrect folding and assembly of the polypeptides, and is not derived from the native structure. The aggregates of the pTBG(H+)-encoded fusion polypeptides did not revert to active form when the culture temperature was lowered.     

Expression, purification, and characterization of Clostridium botulinum type B light chain

A full-length synthetic gene encoding the light chain of botulinum neurotoxin serotype B, approximately 50 kDa (BoNT/B LC), has been cloned into a bacterial expression vector pET24a+. BoNT/B LC was expressed in Escherichia coli BL21.DE3.pLysS and isolated from the soluble fraction. The resultant protein was purified to homogeneity by cation chromatography and was determined to be >98% pure as assessed by SDS-polyacrylamide gel stained with SilverXpress and analyzed by densitometry. Mass spectroscopic analysis indicated the protein to be 50.8 kDa, which equaled the theoretically expected mass. N-terminal sequencing of the purified protein showed the sequence corresponded to the known reported sequence. The recombinant BoNT/B light chain was found to be highly stable, catalytically active, and has been used to prepare antisera that neutralizes against BoNT/B challenge. Characterization of the protein including pH, temperature, and the stability of the protein in the presence or absence of zinc is described within. The influence of pH differences, buffer, and added zinc on secondary and tertiary structure of BoNT/B light chain was analyzed by circular dichroism and tryptophan fluorescence measurements. Optimal conditions for obtaining maximum metalloprotease activity and stabilizing the protein for long term storage were determined. We further analyzed the thermal denaturation of BoNT/B LC as a function of temperature to probe the pH and added zinc effects on light chain stability. The synthetic BoNT/B LC has been found to be highly active on its substrate (vesicle associated membrane protein-2) and, therefore, can serve as a useful reagent for BoNT/B research.