The structure and function of Bacillus thuringiensis delta-endotoxins and prospects for biopesticide improvement

The spore-forming bacterium Bacillus thuringiensis produces novel and highly specific insecticidal proteins (delta-endotoxins) grouped into two main families - Cry and Cyt toxins by amino acid sequence similarity. Both types of delta-endotoxin bind to insect-specific receptors on the surface of gut epithelial cells and in a second irreversible step insert into the cell membrane to form leakage channels that result in cell death by colloid osmotic lysis. The protein nature of these toxins coupled with genetic engineering offers great potential for pesticide improvement and resistance management and has allowed them to be expressed in plants as systemic biopesticides. The X-ray structure of the first Cry toxin revealed putative membrane insertion and receptor binding domains whose functions are being explored by intensive mutagenesis and domain swapping. The first Cyt toxin structure has now been described and is entirely different from the Cry toxins - despite their similar toxic mechanism. Current biochemical and genetic attempts to define structure-activity relationships for these toxins will be reviewed. The potential of these pesticides has been further enhanced by the recent cloning and sequencing of Cry toxin receptors, These receptors are transmembrane proteins exposed on the lumen surface of midgut epithelial cells. The structure of these receptors, their role in toxin recognition, membrane pore formation and toxin resistance will be discussed. Although the best Bt toxins are insecticidal at concentrations typical of potent chemical pesticides, many of them are ten or even 1,000 times less potent. Recent work suggests that additional virulence factors may contribute to the insecticidal potency of Bacillus thuringiensis. Strategies to enhance toxin potency for improved spray formulations and to allow them to be expressed at realistic levels in transgenic plants will be discussed.