Diversity of Bacillus thuringiensis Crystal Toxins and Mechanism of Action

Parasporal crystals produced by Bacillus thuringiensis (Bt) bacteria are the main virulence factors underlying Bt toxicity to insects. Parasporal crystals are composed primarily of Cry and Cyt proteins that act on the midgut of susceptible insects. Cry proteins are an important component of Bt biopesticides and are vital tools for insect control via expression in transgenic crop plants. Some members of the Cry group are more distantly related including ETX/MTX and binary type toxins. Cry toxin structure and action involves critical steps in toxin activation, binding to receptors such as cadherin and then aminopeptidase or alkaline phosphatase probably in a 'sequential binding' manner. Specific Cry toxin receptor interactions are a focus of this review. Recently, the importance of midgut ATP-binding cassette proteins to Cry intoxication of insects has been demonstrated. Mechanistic details involved in 'sequential binding' and 'pore formation' models are examined. The Cyt toxin of Bt subspecies israelensis is an important and interesting component in Cry midgut interactions in mosquitoes. For some Cry toxins, Cyt serves as a receptor for docking to midgut membrane. Recent engineering: work has demonstrated that Cyt can be re-targeted generating novel toxins for insect control. Overall, we review the remarkable progress made in the past 20 years in discovering novel Cry toxins and in elucidating complex mechanisms of Cry and Cyt toxin action; subjects relevant to the long-term control of insects that damage crops and vector human disease.