Mode of action of Bacillus thuringiensis Cry pesticidal proteins

Bacillus thuringiensis (Bt) produces diverse insecticidal proteins with distinct structures and modes of action. Among these, the Cry insecticidal proteins have been the most successfully used in the control of insect pests in agriculture and mosquito vectors of human diseases. Cry insecticidal proteins are produced as protoxins that are activated by the proteases found in the larval gut. To exert toxicity, Cry proteins must bind to different larval gut proteins that facilitate their oligomerization and membrane insertion, leading to pore-formation, enterocyte death and destruction of the larval midgut epithelium. Cry toxins are characterized by a three-domain structure where domain I is involved in membrane insertion, oligomerization and pore formation, while domains II and III determine the insect specificity by their capacity to bind different larval gut membrane proteins. Herein we review the structures of Cry toxins and protoxins and the recent advances on the mode of action of these proteins, including an alternative pathway for protoxin action and a new “folding-cane” model to explain Cry toxin insertion into the membrane. Also, we review the role of receptors in the mechanism of action of Cry activated toxins or protoxins, as shown by the novel editing techniques. Finally, the different strategies that have been used to improve these proteins with the aim of enhancing their toxicity, are discussed. It is foreseen that due to the current understanding of the mode of action of these proteins and the highly efficient in vitro evolution techniques, these insecticidal proteins will still be important tools for insect pest control in years to come.