Analysis of the region for receptor binding and triggering of oligomerization on Bacillus thuringiensis Cry1Aa toxin

The determination of the receptor-binding region of Cry toxins produced by Bacillus thuringiensis is expected to facilitate an improvement in their insecticidal ability through protein engineering. We analyzed the region on Cry1Aa molecules involved in interactions with the cadherin-like protein receptor BtR175 using cysteine-substituted mutant toxins and several synthetic peptides corresponding to the loops in domain 2. In addition, the region necessary to trigger oligomerization was analyzed using these mutant toxins. The mutant toxins were modified by two types of molecule, i.e. digested fragments of the Cry1Aa precursor with an average molecular mass of 2 kDa and 5-iodoacetamidofluorescein, which has a molecular mass of 515 kDa. We examined whether these modi. cations interfere with the toxin-BtR175 interaction as a result of steric hindrance. 5-Iodoacetamidofluorescein modi. cation of R311C, N376C and G442C revealed steric hindrance effects, indicating that R311 on loop 1, N376 on loop 2 and G442 on loop 3 are on the contact face of the toxin-BtR175 interface when Cry1Aa binds to BtR175. Loop 2 is thought to interact with BtR175 directly, as a peptide corresponding to the N-terminal half of loop 2, (365) LYRRIILG(372), has the potential to bind to BtR175 fragments. Meanwhile, mutant toxins with cysteine substitutions in loops 1 and 2 were oligomerized by the binding of digested fragments in the activation process without receptor interaction, and the wild-type toxin formed oligomers by interaction with BtR175 fragments. These observations suggest that loops 1 and 2 form both a binding region and a sensor region, which triggers toxin oligomer formation.