Understanding the nature of specificity of Bacillus thuringiensis Cry2A toxins against Plutella xylostella

Bacillus thuringiensis (Bt) is a gram positive spore forming bacterium that has entomopathogenic properties. It has the ability to synthesize insecticidal proteins in the sporulation phase of its life cycle and package them as parasporal crystals. These so-called Cry proteins are known to have insecticidal activity against many insect orders including Hymenoptera, nematodes, Lepidoptera, Diptera and Coleoptera. This study aimed to understand the basis of the specificity of the Cry2A family of toxins against the diamondback moth Plutella xylostella. The Cry2A toxins were characterized and bioassayed against P. xylostella, the results of which were used to inform the design of hybrid and mutant proteins. These were used to establish the relationship between toxin sequence and function with respect to their activity against this insect. The results showed that both domain I and II of the Cry2A toxins influenced specificity towards P. xylostella and that different populations of this insect show dramatically different susceptibilities to individual Cry2A toxins. Two different expression systems were compared and whilst differences in yield and purity were observed, there were no differences in the specific activity of the expressed toxins. Finally, an attempt was made to interchange the activity of Cry2Ac against P. xylostella and the activity of Cry41Aa against human cancer cells through domain swaps.