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Delta endotoxins (δ-endotoxins) are
pore-forming toxin Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthw ...
s produced by ''
Bacillus thuringiensis ''Bacillus thuringiensis'' (or Bt) is a gram-positive, soil-dwelling bacterium, the most commonly used biological pesticide worldwide. ''B. thuringiensis'' also occurs naturally in the gut of caterpillars of various types of moths and butter ...
'' species of bacteria. They are useful for their insecticidal action and are the primary toxin produced by Bt maize/corn. During
spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
formation the bacteria produce crystals of such proteins (hence the name Cry toxins) that are also known as parasporal bodies, next to the
endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., no ...
s; as a result some members are known as a parasporin. The Cyt (cytolytic) 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 gut
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercel ...
and causes cell lysis by the formation of cation-selective channels, which leads to death. For many years there was no clarity as to the relationship between aminopeptidase N and Bt toxins. Although AP-N does bind Cry proteins in vitro (reviewed by Soberón et al. 2009 and Pigott & Ellar 2007), no cases of resistance or even reduced in vitro binding due to AP-N structure alteration were known through 2002, and there was some doubt that the resistance mechanism was so straight forward. Indeed, Luo et al. 1997, Mohammed et al. 1996, and Zhu et al. 2000 ''positively'' found this to ''not'' occur in Lepidoptera examples. Subsequently, however Herrero et al. 2005 showed correlation between nonexpression and Bt resistance, and actual resistance was found in ''
Helicoverpa armigera ''Helicoverpa armigera'' is a species of Lepidoptera in the family Noctuidae. It is known as the cotton bollworm, corn earworm, Old World (African) bollworm, or scarce bordered straw (the lattermost in the UK, where it is a migrant). The larvae ...
'' by Zhang et al. 2009, in '' Ostrinia nubilalis'' by Khajuria et al. 2011, and in ''
Trichoplusia ni The cabbage looper (''Trichoplusia ni'') is a medium-sized moth in the family Noctuidae, a family commonly referred to as owlet moths. Its common name comes from its preferred host plants and distinctive crawling behavior. Cruciferous vegetable ...
'' by Baxter et al. 2011 and Tiewsiri & Wang 2011 (also all Lepidoptera). There continues to be confirmation that AP-Ns do not by themselves affect resistance in some cases, possibly due to sequential binding by the toxin being required to produce its effect. In this sequence each binding step is theoretically not indispensable, but if it occurs does contribute to the final pore formation result.


Structure

The activated region of the delta toxin is composed of three distinct
structural domain In molecular biology, a protein domain is a region of a protein's polypeptide chain that is self-stabilizing and that folds independently from the rest. Each domain forms a compact folded three-dimensional structure. Many proteins consist of ...
s: an
N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the am ...
helical bundle domain () involved in membrane insertion and pore formation; a
beta-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a ge ...
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 (PS) 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 (''Bin''-like) toxins,
Cry6Aa Cry6Aa (Pesticidal crystal protein Cry6Aa) is a toxic crystal protein generated by the bacterial family ''Bacillus thuringiensis'' during sporulation. This protein is a member of the alpha pore forming toxins family, which gives it insecticidal ...
, and many beta-sandwich parasporins. Specific delta-endotoxins that have been inserted with
genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including t ...
include Cry3Bb1 found in
MON 863 MON 863 is a genetically engineered variety of maize produced by Monsanto. It is genetically altered to express a modified version of Cry3Bb1, a delta endotoxin which originates from ''Bacillus thuringiensis''. This protects the plant from corn r ...
and Cry1Ab found in
MON 810 The MON 810 corn is a genetically modified maize used around the world. It is a ''Zea mays'' line known as YieldGard from the company Monsanto.Van Rie J. et al. 1989. Specificity of Bacillius thuringiensis delta-endotoxins. Eur J Biochem 186: 239-24 ...
, both of which are maize/corn cultivars. Cry3Bb1 is particularly useful because it kills Coleopteran insects such as the corn rootworm, an activity not seen in other Cry proteins. Other common toxins include Cry2Ab and Cry1F in
cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor pe ...
and maize/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.


References


Further reading

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External links


Cry3Bb1
at the ''
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
'' {{DEFAULTSORT:Delta Endotoxin Protein domains Peripheral membrane proteins Bacterial toxins Crystals Proteins