Specific Toxicity of δ-Endotoxins from Bacillus thuringiensis to Bombyx mori

Two δ-endotoxins, CryIA(a) and CryIA(b), from Bacillus thuringiensis subsp.. aizawai were used to investigate the specificity in insecticidal activity. CryIA(a) was 17-fold more toxic to Bomhyx mori than CryIA(b). After in vitro solubilization and digestion of these δ-endotoxins, the specificity of toxicity was retained. Trypsin-activated CryIA(a) and CryIA(b) showed specific, high affinity and saturable binding to brush border membrane vesicles (BBMV) from B. mori midguts. These two toxins competed for the same binding site. Dissociation constant for CryIA(a) and CryIA(b) binding to B. mori BBMV was O.89nM and 1.46 nM, respectively. In both toxins, dissociation reaction followed a biphasic process with a fast and a very slow component, suggesting that binding of the toxins proceeds through a reversible component and an apparently irreversible component. In the CryIA(a) dissociation reaction, the irreversible component comprised a large portion of total binding. On the other hand in that of CryIA(b), the reversible component was major. These results suggest that the specific toxicity of the toxins to B. mori may depend mainly on irreversibility.