Delta endotoxins are pore-forming toxins produced by Bacillus thuringiensis species of bacteria. They are useful for their insecticidal action and are the primary toxin produced by Bt corn. During spore formation the bacteria producecrystals of such proteins that are also known as parasporal bodies, next to the endospores; as a result some members are known as a parasporin. The Cyt toxin group is a group of delta-endotoxins different from the Cry group.
Mechanism of action
When an insect ingests these proteins, they are activated by proteolytic cleavage. The N-terminus is cleaved in all of the proteins and a C-terminal extension is cleaved in some members. Once activated, the endotoxin binds to the gutepithelium and causes cell lysis by the formation of cation-selective channels, which leads to death.
Structure
The activated region of the delta toxin is composed of three distinct structural domains: an N-terminal helical bundle domain involved in membraneinsertion and pore formation; a beta-sheet central domain involved in receptor binding; and a C-terminal beta-sandwich domain that interacts with the N-terminal domain to form a channel.
Types
B. thuringiensis encodes many proteins of the delta endotoxin family, with some strains encoding multiple types simultaneously. A gene mostly found on plasmids, delta-entotoxins sometimes show up in genomes of other species, albeit at a lower proportion than those found in B. Thuringiensis. The gene names looks like Cry3Bb, which in this case indicates a Cry toxin of superfamily 3 family B subfamily b. Cry proteins that are interesting to cancer research are listed under a parasporin nomenclature in addition to the Cry nomenclature. They do not kill insects, but instead kill leukemia cells. The Cyt toxins tend to form their own group distinct from Cry toxins. Not all Cry -- crystal-form -- toxins directly share a common root. Examples of non-three-domain toxins that nevertheless have a Cry name include Cry34/35Ab1 and related beta-sandwich binary toxins, Cry6Aa, and many beta-sandwich parasporins. Specific delta-endotoxins that has been used for genetic engineering include Cry3Bb1 found in MON 863 and Cry1Ab found in MON 810, both of which are corn species. Cry3Bb1 is particularly useful because it kills the coleopteran insects such as the corn rootworm, an activity not seen in other Cry proteins. Other common toxins include Cry2Ab and Cry1F in cotton and corn. In addition, Cry1Ac is effective as a vaccine adjuvant in humans. Some insects populations have started to develop resistance towards delta endotoxin, with five resistant species found as of 2013. Plants with two kinds of delta endotoxins tend to make resistance happen slower, as the insects have to evolve to overcome both toxins at once. Planting non-Bt plants with the resistant plants will reduce the selection pressure for developing the toxin. Finally, two-toxin plants should not be planted with one-toxin plants, as one-toxin plants act as a stepping stone for adaption in this case.
