Folding and unfolding of the protoxin from Bacillus thuringiensis: evidence that the toxic moiety is present in an active conformation

Structural Characterisation of the Insecticidal Toxin XptA1, Reveals a 1.15 MDa Tetramer with a Cage-like Structure

A recently identified class of proteins conferring insecticidal activity to several bacteria within the Enterobacteriaceae family have potential for control of commercially important insect pests. Here, we report the first purification, biophysical characterisation and 3-D structural analysis of one of the toxin components, XptA1, from Xenorhabdus nematophila PMFI296 to a resolution of 23 A. Membrane binding studies indicate that the three-component toxin system has a different mode of action from that of proteins from Bacillus thuringiensis (Bt). Biophysical characterisation of XptA1 suggests a mechanism of action of XptA1 whereby it first binds to the cell membrane forming a structure with a central cavity and forms a complex with its partners XptB1 and XptC1 producing the full insecticidal toxin. The structure of XptA1 is shown by a combination of electron microscopy, ultracentrifugation and circular dichroism spectroscopy to be a 1.15 MDa tetramer with a cage-like structure. Each of the four symmetry-related subunits has three well-defined domains and a longitudinal twist with one end narrower than the other. One third of the residues of XptA1 are alpha-helical and it is suggested the subunits associate partly via an alpha-helical coiled-coil interaction. XptA1 itself shows the same secondary structure at neutral pH and in an alkaline environment up to pH10.5. This pH tolerance indicates that the folded XptA1 can pass through the midgut of Lepidopteran insects susceptible to the insecticidal toxin complex. This implies therefore that its folded structure is important for its biological activity.

Intranasal Cry1Ac protoxin is an effective mucosal and systemic carrier and adjuvant of Streptococcus pneumoniae polysaccharides in mice

Streptococcus pneumoniae is a major respiratory pathogen in infants, children and the elderly. Available parenteral anti-pneumococcal vaccines based on type-specific capsular polysaccharides (CPSs) are useful in adults but do not elicit protective immunity in infants and young children. To enhance their immunogenicity, pneumococcal CPSs conjugated to proteins are being developed. Mucosal vaccines may induce mucosal and systemic immune responses, but their development has been hampered by the lack of effective, inexpensive innocuous mucosal adjuvants or immunogenic vaccine carriers. We have demonstrated that the recombinant Cry1Ac protoxin from Bacillus thuringiensis is highly immunogenic and has mucosal and systemic adjuvant effects on proteins coadministered in mice. In this work, we evaluated Cry1Ac as a carrier and adjuvant of S. pneumoniae CPS for the induction of mucosal and systemic antibody responses after intranasal and intraperitoneal immunization in mice. Our results demonstrate that intranasal application of pneumococcal polysaccharides either coadministered or conjugated with Cry1Ac induces higher systemic and mucosal specific antibody responses than those elicited by pneumococcal polysaccharides alone. Adjuvant effects of Cry1Ac on polysaccharides may be appropriate for vaccine design.

Unfolding affects insect cell permeabilization by Bacillus thuringiensis Cry1C toxin

Bacillus thuringiensis Cry toxins are efficient, environment-friendly biological insecticides. Their molecular mode of action on target insect cells remains largely unknown. The aim of this study was to investigate the relation between the conformational state of the Cry1C toxin and its ionophoric activity on live Sf9 cells of Spodoptera frugiperda, a target insect for this protein. Potassium ion movement induced by Cry1C across the cell membrane was measured with a fluorescent assay developed previously and the conformation of the toxin was studied using tryptophan spectroscopy. Following treatment with 4 M guanidinium hydrochloride, which resulted in the unfolding of its N-terminal half, the toxin retained its full capacity to permeabilize the cells while the fully unfolded toxin did not induce potassium leakage. Therefore, permeabilization of Sf9 cells by Cry1C requires the integrity of the C-terminal half of the toxin and may depend on an initial unfolding step provided by the acidic environment of the cells.

Differential effects of pH on the pore-forming properties of Bacillus thuringiensis insecticidal crystal toxins

The effect of pH on the pore-forming ability of two Bacillus thuringiensis toxins, Cry1Ac and Cry1C, was examined with midgut brush border membrane vesicles isolated from the tobacco hornworm, Manduca sexta, and a light-scattering assay. In the presence of Cry1Ac, membrane permeability remained high over the entire pH range tested (6.5 to 10.5) for KCl and tetramethylammonium chloride, but was much lower at pH 6.5 than at higher pHs for potassium gluconate, sucrose, and raffinose. On the other hand, the Cry1C-induced permeability to all substrates tested was much higher at pH 6.5, 7.5, and 8.5 than at pH 9.5 and 10.5. These results indicate that the pores formed by Cry1Ac are significantly smaller at pH 6.5 than under alkaline conditions, whereas the pore-forming ability of Cry1C decreases sharply above pH 8.5. The reduced activity of Cry1C at high pH correlates well with the fact that its toxicity for M. sexta is considerably weaker than that of Cry1Aa, Cry1Ab, and Cry1Ac. However, Cry1E, despite having a toxicity comparable to that of Cry1C, formed channels as efficiently as the Cry1A toxins at pH 10.5. These results strongly suggest that although pH can influence toxin activity, additional factors also modulate toxin potency in the insect midgut.

Bacillus thuringiensis crystal delta-endotoxin: role of proteases in the conversion of protoxin to toxin

The conversion of delta-endoprotoxins of Bacillus thuringiensis to active toxins is mediated by trypsin, insect gut (exogenous) and bacterial (endogenous) proteases. The biochemical aspects of exogenous and endogenous proteases involved in the conversion of protoxin to toxin are reviewed. Perhaps, these proteases also play a role in influencing the host range of toxin and in the development of resistance to toxin.

Production of chymotrypsin-resistant Bacillus thuringiensis Cry2Aa1 delta-endotoxin by protein engineering

Cleavage of the Cry2Aa1 protoxin (molecular mass, 63 kDa) from Bacillus thuringiensis by midgut juice of gypsy moth (Lymantria dispar) larvae resulted in two major protein fragments: a 58-kDa fragment which was highly toxic to the insect and a 49-kDa fragment which was not toxic. In the midgut juice, the protoxin was processed into a 58-kDa toxin within 1 min, but after digestion for 1 h, the 58-kDa fragment was further cleaved within domain I, resulting in the protease-resistant 49-kDa fragment. Both the 58-kDa and nontoxic 49-kDa fragments were also found in vivo when (125)I-labeled toxin was fed to the insects. N-terminal sequencing revealed that the protease cleavage sites are at the C termini of Tyr49 and Leu144 for the active fragment and the smaller fragment, respectively. To prevent the production of the nontoxic fragment during midgut processing, five mutant proteins were constructed by replacing Leu144 of the toxin with Asp (L144D), Ala (L144A), Gly (L144G), His (L144H), or Val (L144V) by using a pair of complementary mutagenic oligonucleotides in PCR. All of the mutant proteins were highly resistant to the midgut proteases and chymotrypsin. Digestion of the mutant proteins by insect midgut extract and chymotrypsin produced only the active 58-kDa fragment, except that L144H was partially cleaved at residue 144.

Bacillus thuringiensis and its pesticidal crystal proteins

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Processing of delta-endotoxin of Bacillus thuringiensis subsp. kurstaki HD-1 in Heliothis armigera midgut juice and the effects of protease inhibitors

Bombyx mori was found to be more sensitive to the protoxins of HD-1 than Heliothis armigera. SDS-PAGE analysis showed that a large amount of activated toxin was yielded from protoxin by B. mori gut juice while little was yielded by H. armigera. Further degradation of activated toxin was observed in H. armigera midgut juice detected by SDS-PAGE. pH influenced the proteolytic activity of the midgut juice significantly, but there was no obvious effect of pH on the degradation of activated toxin. Specific inhibitor study revealed the presence of trypsin, chymotrypsin, and elastase in the midgut juice. TLCK, TPCK, elastatinal and some general serine protease inhibitors successfully prevented the excessive degradation of protoxin in H. armigera midgut juice. Chymotrypsin inhibitors showed strong inhibitory effects against the further degradation of activated toxin, indicating that chymotrypsin played a major role in the process. It was presumed that the excessive degradation of protoxin in H. armigera midgut juice was responsible for the low sensitivity of the insect to Bt. Further study demonstrated that the excessive degradation in vitro was triggered by SDS treatment. However, all of the tested serine protease inhibitors expressed synergism with protoxin against H. armigera larvae, suggesting that the excessive degradation of protoxin may occur in vivo to some extent and may be triggered by receptor binding of activated toxin.

Removal of Adsorbed Toxin Fragments That Modify Bacillus thuringiensis CryIC δ-Endotoxin Iodination and Binding by Sodium Dodecyl Sulfate Treatment and Renaturation

We report that 10- and 25-kDa toxin fragments adhere to CryIC prepared from Bacillus thuringiensis insecticidal crystals, block iodination, and alter membrane binding. There is no apparent affect on CryIC toxicity against Spodoptera exigua. Associated peptides remained bound to CryIC in the presence of 50 mM dithiothreitol or 6 M urea. A novel detergent-renaturation procedure was developed for the purification of B. thuringiensis CryIC toxin. Sodium dodecyl sulfate (SDS) treatment followed by gel filtration chromatography yielded a homogeneous 62-kDa CryIC toxin. After removal of SDS and renaturation, the purified CryIC toxin was fully insecticidal to S. exigua larvae. 125 I-labeled CryIC bound with high affinity to brush border membrane vesicles from S. exigua larvae.

Effects of pH on conformational properties related to the toxicity of Bacillus thuringiensis delta-endotoxin

The delta-endotoxin of Bacillus thuringiensis subspecies kurstaki is an intracellular crystalline proteinaceous inclusion which, upon ingestion, is toxic to lepidopteran insects. Upon dissolution at pH > 9 it yields a protein subunit called protoxin. Under appropriate conditions, protoxin is hydrolyzed to a toxin molecule, which is responsible for killing the insect. It is known that this toxic activity decreases considerably above pH 10. In this study, circular dichroism spectroscopy has been used to examine the secondary structures of the protoxin and toxin molecules at different pH values to determine if there are detectable conformational changes associated with their pH-dependent functional properties. At pH 10, where toxic activity is approximately maximal, both the protoxin and toxin molecules were found to assume a conformation that is on an average approx. 26% alpha-helix and approx. 45% beta-structure. As the pH was increased above 10, where the insecticidal activity decreases, the magnitude of the CD spectrum at 222 nm decreased for protoxin and the calculated alpha-helix contents of both protoxin and toxin molecules decreased. The net secondary structure did not change significantly at pH values below 10. Significant conformational differences are observed between the secondary structure of the protoxin and toxin molecules at different pH values. The pH-dependent changes in secondary structure of the protoxin and toxin can be correlated with the effects of pH on the insecticidal activity of these proteins.

Bacillus thuringiensis crystal protein: effect of chemical modification of the cysteine and lysine residues

The 16 cysteine residues of reduced protoxin from Bacillus thuringiensis subsp. kurstaki HD-73 can be quantitatively reacted with: (a) iodoacetic acid, to give carboxymethyl protoxin; (b) iodoacetamide, giving carbaminomethyl protoxin and (c) N-(beta-iodoethyl)trifluoroacetamide to give aminoethyl protoxin. The carboxymethyl derivative was found to be significantly more soluble at neutral pH values where both the native protoxin and the carbaminomethyl derivative exhibit low solubilities. At the alkaline pH values (pH 9.5-10.5) normally used to solubilize the crystal protein, the native protein was slightly more soluble than either the carboxymethyl or the carbaminomethyl derivatives. The aminoethyl derivative had an extremely low solubility at all pH values. Succinic anhydride reacted with only 35% of the lysine residues in both the carboxymethyl and the carbaminomethyl protoxin derivatives. Nonetheless, these succinylated protoxins exhibited significantly increased solubilities at neutral pH values. All the derivatives were found to retain full insecticidal activity toward spruce budworm (Choristeneura fufimerana) larvae. It is concluded that all the cysteine residues and modified lysine residues are on the surface of the protein and that derivatization does not alter the conformation of the solubilized protoxin.

Crystal structure of insecticidal delta-endotoxin from Bacillus thuringiensis at 2.5 A resolution

The structure of the delta-endotoxin from Bacillus thuringiensis subsp. tenebrionis that is specifically toxic to Coleoptera insects (beetle toxin) has been determined at 2.5 A resolution. It comprises three domains which are, from the N- to C-termini, a seven-helix bundle, a three-sheet domain, and a beta sandwich. The core of the molecule encompassing all the domain interfaces is built from conserved sequence segments of the active delta-endotoxins. Therefore the structure represents the general fold of this family of insecticidal proteins. The bundle of long, hydrophobic and amphipathic helices is equipped for pore formation in the insect membrane, and regions of the three-sheet domain are probably responsible for receptor binding.

The toxic moiety of the Bacillus thuringiensis protoxin undergoes a conformational change upon activation

Proteolytic processing of the 133-kDa crystal protein (protoxin) from Bacillus thuringiensis subsp. kurstaki yields a 67-kDa insecticidal toxin. Differential scanning calorimetry was used to investigate whether the toxic moiety in the protoxin molecule has the same conformation as activated toxin. Compared to protoxin, toxin gives rise to a more complex endotherm which extends over a 10 degrees C broader temperature range and contains a component occurring at a substantially higher temperature than any unfolding transition in the protoxin endotherm. It is concluded that the toxic moiety undergoes a conformational change upon activation in which the thermal stability of at least one of its domains is significantly increased.