Delta endotoxins explained

Symbol:Endotoxin_N
Delta endotoxin, N-terminal domain
Pfam:PF03945
Interpro:IPR005639
Scop:1dlc
Tcdb:1.C.2
Class:collapsible collapsed
Symbol:Endotoxin_mid
Delta endotoxin, middle domain, Cry2A and Cry18
Pfam:PF09131
Interpro:IPR015214
Scop:1i5p
Symbol:Endotoxin_C
Delta endotoxin, C-terminal
Pfam:PF03944
Pfam Clan:CL0202
Interpro:IPR005638
Scop:1dlc
Tcdb:1.C.2
Cdd:cd04085

Delta endotoxins (δ-endotoxins) are a family of pore-forming toxins produced by Bacillus thuringiensis species of bacteria. They are useful for their insecticidal action and are the primary toxin produced by the genetically modified (GM) Bt maize/corn and other GM crops. During spore formation the bacteria produce crystals of such proteins (hence the name Cry toxins) that are also known as parasporal bodies, next to the endospores; as a result some members are known as a parasporin. The Cyt (cytolytic) toxin group is another group of delta-endotoxins formed in the cytoplasm. VIP toxins (vegetative insecticidal proteins) are formed at other stages of the life cycle.[1]

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 and causes cell lysis by the formation of cation-selective channels, which leads to death.[2]

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[3] and Pigott & Ellar 2007[4]),[5] 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.[6] Subsequently, however Herrero et al. 2005 showed correlation between nonexpression and Bt resistance, and actual resistance was found in Helicoverpa armigera by Zhang et al. 2009,[7] in Ostrinia nubilalis by Khajuria et al. 2011, and in Trichoplusia ni 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 domains: an N-terminal helical bundle domain involved in membrane insertion 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.[8] A gene mostly found on plasmids,[9] delta-entotoxins sometimes show up in genomes of other species, albeit at a lower proportion than those found in B. thuringiensis.[10] The gene names looks like Cry3Bb, which in this case indicates a Cry toxin of superfamily 3 family B subfamily b.[11]

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.[12] [13] [14] The Cyt toxins tend to form their own group distinct from Cry toxins.[15] Not all Cry crystal-form toxins directly share a common root.[16] Examples of non-three-domain toxins that nevertheless have a Cry name include Cry34/35Ab1 and related beta-sandwich binary (Bin-like) toxins, Cry6Aa, and many beta-sandwich parasporins.[17]

Specific delta-endotoxins that have been inserted with genetic engineering include Cry3Bb1 found in MON 863 and Cry1Ab found in MON 810, 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 and maize/corn. In addition, Cry1Ac is effective as a vaccine adjuvant in humans.[18]

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.[19]

Further reading

External links

Notes and References

  1. Book: Risk assessment and management—Environment. Roger Hull. Genetically Modified Plants. second. 2021. Upon sporulation, B. thuringiensis forms proteinaceous insecticidal δ-endotoxins either in crystals (Cry toxins) or cytoplasmically (Cyt toxins), which are encoded by cry or cyt genes, respectively. When insects ingest toxin crystals, the enzymes in their digestive tract cause the toxin to become activated. The toxin binds to the insect’s gut membranes, forming a pore that results in swelling, cell lysis, and eventually killing the insect. B. thuringiensis also produces insecticidal proteins at other stages in its lifecycle, specifically the vegetative insecticidal proteins (VIPs). etal.
  2. Grochulski P, Masson L, Borisova S, Pusztai-Carey M, Schwartz JL, Brousseau R, Cygler M . Bacillus thuringiensis CryIA(a) insecticidal toxin: crystal structure and channel formation . Journal of Molecular Biology . 254 . 3 . 447–464 . December 1995 . 7490762 . 10.1006/jmbi.1995.0630 .
  3. Soberón M, Gill SS, Bravo A . Signaling versus punching hole: How do Bacillus thuringiensis toxins kill insect midgut cells? . Cellular and Molecular Life Sciences . 66 . 8 . 1337–1349 . April 2009 . 19132293 . 10.1007/s00018-008-8330-9 . . 5928827 . 11131463 .
  4. Pigott CR, Ellar DJ . Role of receptors in Bacillus thuringiensis crystal toxin activity . Microbiology and Molecular Biology Reviews . 71 . 2 . 255–281 . June 2007 . 17554045 . 10.1128/mmbr.00034-06 . . 1899880 . 13982571 .
  5. Pardo-López L, Soberón M, Bravo A . Bacillus thuringiensis insecticidal three-domain Cry toxins: mode of action, insect resistance and consequences for crop protection . FEMS Microbiology Reviews . 37 . 1 . 3–22 . January 2013 . 22540421 . 10.1111/j.1574-6976.2012.00341.x . Federation of European Microbiological Societies (OUP) . free .
  6. Ferré J, Van Rie J . Biochemistry and genetics of insect resistance to Bacillus thuringiensis . Annual Review of Entomology . 47 . 1 . 501–533 . 2002 . 11729083 . 10.1146/annurev.ento.47.091201.145234 . .
  7. Vachon V, Laprade R, Schwartz JL . Current models of the mode of action of Bacillus thuringiensis insecticidal crystal proteins: a critical review . Journal of Invertebrate Pathology . 111 . 1 . 1–12 . September 2012 . 22617276 . 10.1016/j.jip.2012.05.001 . Academic Press (Elsevier) .
  8. Web site: Pesticidal crystal protein (IPR038979) . InterPro . 12 April 2019.
  9. Dean DH . Biochemical genetics of the bacterial insect-control agent Bacillus thuringiensis: basic principles and prospects for genetic engineering . Biotechnology & Genetic Engineering Reviews . 2 . 341–363 . 1984 . 6443645 . 10.1080/02648725.1984.10647804 . free .
  10. Web site: Species: Pesticidal crystal protein (IPR038979) . InterPro.
  11. Web site: Bacillus thuringiensis Toxin Nomenclature . Bt toxin specificity database . 12 April 2019.
  12. Mizuki E, Park YS, Saitoh H, Yamashita S, Akao T, Higuchi K, Ohba M . Parasporin, a human leukemic cell-recognizing parasporal protein of Bacillus thuringiensis . Clinical and Diagnostic Laboratory Immunology . 7 . 4 . 625–634 . July 2000 . 10882663 . 95925 . 10.1128/CDLI.7.4.625-634.2000 .
  13. Ohba M, Mizuki E, Uemori A . Parasporin, a new anticancer protein group from Bacillus thuringiensis . Anticancer Research . 29 . 1 . 427–433 . January 2009 . 19331182 .
  14. Web site: List of Parasporins . Committee of Parasporin Classification and Nomenclature . Accessed Jan 4, 2013
  15. Web site: Neil . Crickmore . vanc . Other Cry Sequences . 12 April 2019.
  16. Crickmore N, Zeigler DR, Feitelson J, Schnepf E, Van Rie J, Lereclus D, Baum J, Dean DH . 6 . Revision of the nomenclature for the Bacillus thuringiensis pesticidal crystal proteins . Microbiology and Molecular Biology Reviews . 62 . 3 . 807–813 . September 1998 . 9729610 . 98935 . 10.1128/MMBR.62.3.807-813.1998 .
  17. Kelker MS, Berry C, Evans SL, Pai R, McCaskill DG, Wang NX, Russell JC, Baker MD, Yang C, Pflugrath JW, Wade M, Wess TJ, Narva KE . 6 . Structural and biophysical characterization of Bacillus thuringiensis insecticidal proteins Cry34Ab1 and Cry35Ab1 . PLOS ONE . 9 . 11 . e112555 . 2014-11-12 . 25390338 . 4229197 . 10.1371/journal.pone.0112555 . free . 2014PLoSO...9k2555K .
  18. Rodriguez-Monroy MA, Moreno-Fierros L . Striking activation of NALT and nasal passages lymphocytes induced by intranasal immunization with Cry1Ac protoxin . Scandinavian Journal of Immunology . 71 . 3 . 159–168 . March 2010 . 20415781 . 10.1111/j.1365-3083.2009.02358.x . free .
  19. Tabashnik BE, Brévault T, Carrière Y . Insect resistance to Bt crops: lessons from the first billion acres . Nature Biotechnology . 31 . 6 . 510–521 . June 2013 . 23752438 . 10.1038/nbt.2597 . 205278530 .

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