The mutation R423S in the Bacillus thuringiensis hybrid toxin CryAAC increases in vitro oligomerisation and in vivo toxicity against Spodoptera frugiperda

Bacillus thuringiensis hybrid toxin CryAAC (1Ac/1Ac/1Ca) is toxic to fall armyworm (Spodoptera frugiperda). It has been previously demonstrated that a R423S mutation that increases flexibility in the CryAAC loop β7/β8 (G391-P397) had a positive effect in the toxicity of the hybrid against Mamestra brassicae. According to above mentioned results, we have investigated the toxicity of the mutant hybrid CryAAC-R423S against S. frugiperda at two stages of larval development. Bioassays demonstrated that R423S substitution in CryAAC almost doubled the toxicity to S. frugiperda neonates (LC50 165 against 288 ng/cm) and had the strongest inhibitory effect on the third-instar larvae growth [growth inhibition (G.I.) 137% against 112%]. Features of CryAAC in S. frugiperda such as, protoxin in vitro processing with gut juice and binding of FPLC-purified toxins to proteins from brush border membranes were not affected after R423S substitution. On the contrary, protoxin activation in the presence of S. frugiperda brush border membrane vesicles revealed differences in the concentration of generated oligomeric form between the hybrid and the mutant hybrid toxin that could be the reason for the increased in vivo toxicity observed for CryAAC-R423S.