LIMITED PROTEOLYSIS OF BACILLUS-THURINGIENSIS CRYIG AND CRYIVB DELTA-ENDOTOXINS LEADS TO FORMATION OF ACTIVE FRAGMENTS THAT DO NOT COINCIDEWITH THE STRUCTURAL DOMAINS

Bacillus thuringiensis ''true'' toxins consist of three domains: the N -terminal, alpha-helical. domain followed by two beta-structural domai ns. Their limited proteolysis does not proceed at the domain boundarie s, 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 t he proteolysis of the loops connecting beta-strands of the second doma in. The second pattern, detected for CryIG and CryIVB proteins, consis ts in the cleavage of the loop connecting the fifth and the sixth alph a-helixes of the first domain. The choice between the routes depends o n the size, sequence, and dynamics of the loop that define its accessi bility to a proteinase. Bioassay of CryIG and CryIVB delta-endotoxin f ragments indicates that only two alpha-helixes, the sixth and the seve nth within the first domain, followed by the two beta-structural domai ns are sufficient for the insecticidal activity.