Limited proteolysis of Bacillus thuringiensis CryIG and CryIVB delta-endotoxins leads to formation of active fragments that do not coincide with the structural domains.

Bacillus thuringiensis "true" toxins consist of three domains: the N-terminal, alpha-helical domain followed by two beta-structural domains. Their limited proteolysis does not proceed at the domain boundaries, but is directed to the loops within the domains. There are at least two patterns of the limited proteolysis of "true" toxins. The first pattern, observed for CryIA and CryIVD delta-endotoxins, results in the proteolysis of the loops connecting beta-strands of the second domain. The second pattern, detected for CryIG and CryIVB proteins, consists in the cleavage of the loop connecting the fifth and the sixth alpha-helices of the first domain. The choice between the routes depends on the size, sequence, and dynamics of the loop that define its accessibility to a proteinase. Bioassay of CryIG and CryIVB delta-endotoxin fragments indicates that only two alpha-helices, the sixth and the seventh within the first domain, followed by the two beta-structural domains are sufficient for the insecticidal activity.