Intramolecular Proteolytic Cleavage ofBacillus thuringiensisCry3A δ-Endotoxin May Facilitate Its Coleopteran Toxicity

The Cry3A δ-endotoxin protein inclusion synthesized byBacillus thuringiensissubsp.tenebrionisis soluble in alkaline and acid buffer solutions but the toxin precipitates when returned to neutral pH conditions. The midgut pH of susceptible beetle larvae is neutral to slightly acidic, a pH environment in which the Cry3A toxin is insoluble. To investigate this paradox we studied the Cry3A toxin after various proteolytic treatments. In many cases the toxin was cleaved into polypeptides that remained associated under nondenaturing conditions. Interestingly a chymotrypsinized Cry3A product was soluble under neutral pH conditions, retained full activity against susceptible beetle larvae, and exhibited specific binding toLeptinotarsa decemlineatamidgut membranes.