Two structural domains as a general fold of the toxic fragment of the Bacillus thuringiensis δ‐endotoxins

The unfolding by guanidine hydrochloride of the toxic fragment of a Bacillus thuringiensis toxin belonging to the CryIC class reveals a two‐step denaturation under both acid and alkaline conditions. This demonstrates the existence of two structural domains as building blocks for this toxin. Protease digests performed on a CryIA(b) and CryIC B. thuringiensis toxin, under native and partially denatured conditions, confirm this conclusion. Whereas the native CryIC toxin is completely protease resistant, the CryIA(b) toxin, earlier described as consisting of two structural domains [Convents, D., Houssier, C., Lasters, I. & Lauwereys, M. (1990) J. Biol. Chem. 265 , 1369–1375], is cleaved by three proteases, resulting in at least two common fragments. This suggests that this toxin is built up of two globular units linked by a protease‐susceptible linker. The detection of a stable intermediate along the denaturation curve allows us to study and compare the consecutive unfolding of the structural domains for both toxins. By addition of a protease, under conditions where such an unfolding intermediate exists, a single denaturation phase can be assigned to a specific part of the protein. These experiments lead to the conclusion that the domain whose stability is highly dependent on pH corresponds to the N‐terminal half of both toxins.