41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

Spodoptera litura is a major pest attacking important commercial crops like cotton in India. Commercial Bt cotton hybrids carrying Cry1Ac toxin (Bollgard I type) or Cry1Ac and Cry2Ab toxins (Bollgard II type) give adequate control of the target insect Helicoverpa armigera, however there is a common opinion among farmers that S. litura is an emerging problem in Bt cotton. There is an urgent need to screen for more toxic holotype and/or hybrid Cry proteins against S. litura to minimise the use of chemical insecticides in Bt cotton, the main objective of transgenic cotton technology. In laboratory screening experiments G27, the EEC hybrid toxin producing strain, was more toxic than other holotypes (1Ca and 1Fa) and hybrids (AbAbC and AcAcC) tested.

The Cry4Aa toxin from Bacillus thuringiensis is toxic to larvae of Culex, Anopheles, and Aedes mosquitoes, which are vectors of important human tropical diseases. In order to understand the mechanism of toxic action and design modified toxins with improved potency that could be used as effective biopesticides, we determined the structure of this toxin in its functional form by X-ray crystallogaraphy. Like other Cry toxins, the activated Cry4Aa toxin consists of three globular domains, a seven-helix bundle responsible for pore formation (domain I) and the two other domains having structural similarities with carbohydrate binding proteins: a ß-prism (domain II) and a plant lectin-like ß-sandwich (domain III). We also studied the effect on toxicity of amino acid substitutions and deletions in three loops located at the surface of the putative receptor-binding domain II of Cry4Aa. Our results indicate that one loop is an important determinant of toxicity. Moreover, a functional importance of an aromatic amino acid cluster at the surface of Cry4Aa domain II was investigated via mutational analysis. A reduction of toxicity was observed suggesting that this region plays a crucial role for the target specificity and mosquito-larvicidal activity.