Vip3Aa insecticidal protein is produced from Bacillus thuringiensis and exerts a broad spectrum of toxicity against lepidopteran insect species. Although Vip3Aa has been effectively used as part of integrated pest management strategies, the mechanism of the toxin remains unclear. Here, we investigated the effect of pH in a range from 5.0 to 10.0 on the pore-forming activity of the trypsin activated Vip3Aa (actVip3Aa) by in vitro pore-forming assays. Based on calcein release assay, actVip3Aa could permeabilize the artificial neutral liposomes under all the pH tested, except pH10.0. The maximum membrane permeability of actVip3Aa was detected at pH8.0 and the permeability decreased and abolished when exposing to acidic and alkaline conditions, respectively. The planar lipid bilayer experiment revealed that actVip3Aa formed ion channels at pH5.0-8.0 but no current signals were detected at pH10.0, consistent with the observation from calcein release assay. The toxin formed ion channels with a diameter of 1.4nm at pH8.0 and pore size was gradually decreased when reducing the pH. This study provided a view of the molecular mechanism of Vip3Aa by which the pore-forming activity is regulated by pH.

Bacillus thuringiensis vegetative insecticidal proteins like Vip3A have been used for crop protection and to delay resistance to existing insecticidal Cry toxins. However, little is known about Vip3A's behavior or its mechanism of action, and a structural model is required. Herein, in an effort to facilitate future crystallization and functional studies, we have used the orthogonal biophysical techniques of light scattering and sedimentation to analyze the aggregation behavior and stability of trypsin-activated Vip3A toxin in solution. Both scattering and sedimentation data suggest that at pH 10 the toxin is monomeric and adopts an elongated shape, but after overnight incubation aggregation was observed at all pH values tested (5-12). The narrowest size distribution was observed at pH 7, but it was consistent with large oligomers of ~50 nm on average. The addition of β-D-glucopyranoside (OG) helped in achieving preparations that were stable and with a narrower particle size distribution. In this case, scattering was consistent with a 4-nm monomeric globular Vip3A form. After OG dialysis, 40-nm particles were detected, with a molecular weight consistent with homotetramers. Therefore, OG is proposed as the detergent of choice to obtain a Vip3A crystal for structural studies, either before (monomers) or after dialysis (tetramers).

Bacillus sphaericus produces mosquito-larvicidal binary toxin composed of BinA and BinB. While BinB is expected to bind to a specific receptor on the cell membrane, BinA interacts to BinB or BinB receptor complex and translocates into the cytosol to exert its activity via unknown mechanism. To investigate functional roles of aromatic cluster in BinA, amino acids at positions Y213, Y214, Y215, W222 and W226 were substituted by leucine. All mutant proteins were highly produced and their secondary structures were not affected by these substitutions. All mutants are able to insert into lipid monolayers as observed by Langmuir-Blodgett trough and could permeabilize the liposomes in a similar manner as the wild type. However, mosquito-larvicidal activity was abolished for W222L and W226L mutants suggesting that tryptophan residues at both positions play an important role in the toxicity of BinA, possibly involved in the cytopathological process after toxin entry into the cells. [BMB reports 2011; 44(10): 674-679]