Selective inhibition of binding of Bacillus thuringiensis Cry1Ab toxin to cadherin-like and aminopeptidase proteins in brush-border membranes and dissociated epithelial cells from Bombyx mori

Binding analyses with denatured epithelial membrane proteins from Bt (Bacillus thuringiensis) demonstrated at least two kinds of proteins, APNs (aminopeptidases N) and cadherin-like proteins, as possible receptors for the Cry1A class of Bt toxins. Two alternative models have been proposed, both based on initial toxin binding to a cadherin-like protein, but one involving APN and the other not. We have used two Bombyx mori strains (J65 and Kin), which are highly susceptible to Cry1Ab, to study the role of these two types of receptors on Cry1Ab toxin binding and cytotoxicity by means of the inhibitory effect of antibodies. BBMVs (brush-border membrane vesicles) of strain J65 incubated with labelled 125I-Cry1Ab revealed a marked reduction in reversible and irreversible binding when anti-BtR175 (a cadherin-like protein) was used for BBMV pre-treatment. By contrast, the anti-APN1 antibody specifically affected the irreversible binding, while the reversible binding component was not affected. This is the first time that binding of Cry1Ab to APN1 and to a cadherin-like protein from BBMVs in solution has been shown. Dissociated epithelial cells from the Kin strain were used to test the inhibitory effect of the antibodies on the cytotoxicity of Cry1Ab. Pre-incubation of the cells with the anti-BtR175 antibody conferred protection against Cry1Ab, but not the anti-APN1 antibody. Therefore our results seem to support the two models of the mode of action of Cry1Ab in Lepidoptera, depending on whether BBMVs or intact dissociated cells are used, suggesting that both pathways may co-operate for the toxicity of Cry1A toxins in vivo.