N546 in β18–β19 loop is important for binding and toxicity of the Bacillus thuringiensis Cry1Ac toxin

Our previous mutagenic analysis showed that the unique residue N546 in the apex of β18–β19 loop of Bacillus thuringiensis Cry1Ac toxin is important for its toxicity. In this study, trypsin digestion susceptibility, binding to BBMV and oligomer formation activity was therefore analyzed to determine the mechanism of toxicity change of these mutant toxins. The results showed that residue N546 was not involved in toxin oligomerisation and maintaining the stability of toxin, the enhanced toxicity of mutant N546A was just because of increased binding to BBMV, and reduction in toxicity of other mutants were caused by reduction in initial or irreversible binding to BBMV. This is the first report that revealed N546 in Cry1Ac domain III played an essential role in its insecticidal activity and binding to insect BBMV.