Methanol-induced conformational changes in δ-endotoxin from Bacillus thuringiensis var. tenebrionis

Phospholipid vesicles and methanol were shown to similarly affect the proteolysis of insecticidal proteins from Bacillus thuringiensis. Cleavage of the Cry3A protein by pepsin acquires a different character in the presence of either liposomes or 20-25% methanol, and produces a stable 53-kDa fragment comprising the C-terminal region of the molecule from Ala-185 on. This suggests similarity of delta-endotoxin conformational changes resulting from its interaction with phospholipid vesicles and those induced by methanol. Under acidic conditions, methanol addition leads to a drastic increase of the protein sedimentation constant (from 4.2 to 11.5S), possibly due to toxin oligomerization into supramolecular units and an increasing content of beta-structure. As found by scanning microcalorimetry and circular dichroism, the structure of bound Cry3A differs much from that in aqueous solution, where Cry3A appears to be monomeric. Heat denaturation of this new structure is perfectly reversible.