Molecular and Kinetic Models for Pore Formation of Bacillus thuringiensis Cry Toxin

Abstract

Cry proteins from Bacillus thuringiensis (Bt) and other bacteria are pesticidal pore-forming toxins. Since 2010, when the ABC transporter C2 (ABCC2) was identified as a Cry1Ac protein resistant gene, our understanding of the mode of action of Cry protein has progressed substantially. ABCC2 mediates high Cry1A toxicity because of its high activity for helping pore formation. With the discovery of ABCC2, the classical killing model based on pore formation and osmotic lysis became nearly conclusive. Nevertheless, we are still far from a complete understanding of how Cry proteins form pores in the cell membrane through interactions with their host gut membrane proteins, known as receptors. Why does ABCC2 mediate pore formation with high efficiency unlike other Cry1A-binding proteins? Is the “prepore” formation indispensable for pore formation? What is the mechanism underlying the synergism between ABCC2 and the 12-cadherin domain protein? We examine potential mechanisms of pore formation via receptor interactions in this paper by merging findings from prior studies on the Cry mode of action before and after the discovery of ABC transporters as Cry protein receptors. We also attempt to explain Cry toxicity using Cry–receptor binding affinities, which successfully predicts actual Cry toxicity toward cultured cells coexpressing ABC transporters and cadherin.

Integrated Biological Control Using a Mixture of Two Entomopathogenic Bacteria, Bacillus thuringiensis and Xenorhabdus hominickii, against Spodoptera exigua and Other Congeners

“…All four Bt strains potently killed their specific target insects, in which BtA was the most virulent strain against S. exigua . This virulence specificity was likely to be a specific interaction between Cry toxin and its binding site(s) in the target insect [ 41 ]. When Bt spores are consumed by the target insects, Cry toxins associated with spores are released in the midgut lumen and cleaved by insect digestive enzymes to become active, protease-insensitive toxin proteins.…”

“…However, it did not effectively control other insects. This differential toxicity of BtA+XhE might be due to a specific interaction between Bt Cry toxins and their binding site(s) on the gut epithelium of the target insects [ 41 ]. Thus, replacing BtA in the IBC agent with other Bt strains might control other insect species.…”

Cry4Aa and Cry4Ba Mosquito-Active Toxins Utilize Different Domains in Binding to a Particular Culex ALP Isoform: A Functional Toxin Receptor Implicating Differential Actions on Target Larvae

“…Our results are rather different from other studies reported previously for lepidopteran-specific Cry1A toxins that pre-binding of the toxin to cadherin-like receptors seems to be critically needed for toxin oligomerization before interacting with second membrane-bound receptors (e.g., mALPs or mAPNs) for membrane insertion and pore formation [ 7 , 55 ]. However, it has been recently claimed that the completion of the oligomeric assembly of Cry1A-ABCC2 receptor complexes could directly signify membrane insertion and pore formation without the pre-binding to cadherin receptors [ 56 ]; it is noteworthy from the cytotoxicity studies that the Cq -mALP1264-expressed Sf 9 cells were found to be much more susceptible to Cry4Aa than Cry4Ba; these in vitro cytotoxic effects are thus in good agreement with our previous biotoxicity findings that Cry4Aa is much more active than Cry4Ba against Culex larvae [ 4 ].…”

“…Plausible 3D modeled structures of Cq -mALPs were constructed based on the target-template alignment with the shrimp-ALP high-resolution (1.9Å) crystal structure (PDB: 1K7H) [ 56 ] via Phyre2 ( (accessed on 12 September 2022)) and displayed by UCSF Chimera ( chimera/ (accessed on 12 September 2022)). The local quality of both modeled structures was validated by VERIFY-3D [ 59 ] and their stereochemistry was analyzed using the Psi/Phi Ramachandran plot computed with PROCHECK [ 60 ].…”

Molecular Genetic Basis of Lab- and Field-Selected Bt Resistance in Pink Bollworm

“…Understanding the genetic basis of resistance can be useful for monitoring, managing, and countering pest resistance to Bt crops [ 15 , 16 , 17 , 18 ]. To facilitate development of effective resistance management strategies, scientists have determined the genetic basis of resistance to Bt toxins in many lab-selected strains, including strains derived from susceptible populations by either mass selection or selection of families generated for F 1 or F 2 screens [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. If lab-selected resistance and field-selected practical resistance have a similar genetic basis, the lab results can be useful for designing and proactively implementing strategies to delay resistance in the field [ 27 ].…”