Channel Formation in Cry Toxins: An Alphafold-2 Perspective
Issued Date
2023-12-01
Resource Type
ISSN
16616596
eISSN
14220067
Scopus ID
2-s2.0-85179347787
Journal Title
International Journal of Molecular Sciences
Volume
24
Issue
23
Rights Holder(s)
SCOPUS
Bibliographic Citation
International Journal of Molecular Sciences Vol.24 No.23 (2023)
Suggested Citation
Torres J., Surya W., Boonserm P. Channel Formation in Cry Toxins: An Alphafold-2 Perspective. International Journal of Molecular Sciences Vol.24 No.23 (2023). doi:10.3390/ijms242316809 Retrieved from: https://repository.li.mahidol.ac.th/handle/123456789/91528
Title
Channel Formation in Cry Toxins: An Alphafold-2 Perspective
Author(s)
Author's Affiliation
Other Contributor(s)
Abstract
Bacillus thuringiensis (Bt) strains produce pore-forming toxins (PFTs) that attack insect pests. Information for pre-pore and pore structures of some of these Bt toxins is available. However, for the three-domain (I-III) crystal (Cry) toxins, the most used Bt toxins in pest control, this crucial information is still missing. In these Cry toxins, biochemical data have shown that 7-helix domain I is involved in insertion in membranes, oligomerization and formation of a channel lined mainly by helix α4, whereas helices α1 to α3 seem to have a dynamic role during insertion. In the case of Cry1Aa, toxic against Manduca sexta larvae, a tetrameric oligomer seems to precede membrane insertion. Given the experimental difficulty in the elucidation of the membrane insertion steps, we used Alphafold-2 (AF2) to shed light on possible oligomeric structural intermediates in the membrane insertion of this toxin. AF2 very accurately (<1 Å RMSD) predicted the crystal monomeric and trimeric structures of Cry1Aa and Cry4Ba. The prediction of a tetramer of Cry1Aa, but not Cry4Ba, produced an ‘extended model’ where domain I helices α3 and α2b form a continuous helix and where hydrophobic helices α1 and α2 cluster at the tip of the bundle. We hypothesize that this represents an intermediate that binds the membrane and precedes α4/α5 hairpin insertion, together with helices α6 and α7. Another Cry1Aa tetrameric model was predicted after deleting helices α1 to α3, where domain I produced a central cavity consistent with an ion channel, lined by polar and charged residues in helix α4. We propose that this second model corresponds to the ‘membrane-inserted’ structure. AF2 also predicted larger α4/α5 hairpin n-mers (14 ≤n ≤ 17) with high confidence, which formed even larger (~5 nm) pores. The plausibility of these models is discussed in the context of available experimental data and current paradigms.