Structure, Diversity, and Evolution of Protein Toxins from Spore-Forming Entomopathogenic Bacteria

Abstract

Gram-positive spore-forming entomopathogenic bacteria can utilize a large variety of protein toxins to help them invade, infect, and finally kill their hosts, through their action on the insect midgut. These toxins belong to a number of homology groups containing a diversity of protein structures and modes of action. In many cases, the toxins consist of unique folds or novel combinations of domains having known protein folds. Some of the toxins display a similar structure and mode of action to certain toxins of mammalian pathogens, suggesting a common evolutionary origin. Most of these toxins are produced in large amounts during sporulation and have the remarkable feature that they are localized in parasporal crystals. Localization of multiple toxin-encoding genes on plasmids together with mobilizable elements enables bacteria to shuffle their armory of toxins. Recombination between toxin genes and sequence divergence has resulted in a wide range of host specificities.

Bacterial strategies to overcome insect defences

“…The insecticidal proteins that are contained in the parasporal crystals are the main factors in determining the host range of B. thuringiensis. The mechanism of action of the crystal toxins has been reviewed elsewhere [106][107][108][109] . Because of the prime role of toxins in the pathogenesis of B. thuringiensis 110 , other virulence factors have not been well studied.…”

The pre-pore from Bacillus thuringiensis Cry1Ab toxin is necessary to induce insect death in Manduca sexta

“…The pore-forming toxins typically transform from soluble, monomeric proteins to oligomers that form transmembrane ionic channels [34]. The Cry toxins produced by Bacillus thuringiensis (Bt) are widely used as insecticides [10,35,39]. These proteins have been recognized as pore-forming toxins and their primary action is to lyse midgut epithelial cells in target insects by forming lytic pores in the apical microvilli membrane.…”

“…In addition, Bt strains active against Hymenoptera, Homoptera, Orthoptera and Mallophaga insect orders and to other non-insect organisms like nematodes, mites and protozoa have been reported [10,39]. The entomopathogenic activity of Bt is mainly due to Cry toxins.…”

“…More than 150 different cry genes have been isolated. This constitutes an important arsenal for the control of a wide variety of insect pests [10].…”

“…Finally, domain III is a β-sandwich of two antiparallel β-sheets. It is also implicated in insect specificity since it is involved in receptor binding [10,39] Despite the fact that Cry toxins are widely used as insecticides, their mode of action is still not completely understood. In this review, we will summarize our current knowledge on the mode of action of Cry toxins that are active against lepidopteran insects and provide some novel information regarding the role of the toxin oligomeric structure in forming ionic channels in the targeted membrane and in the overall toxicity against insect larvae.…”