Essential Role of Non-Conserved α4-His <sup>178</sup> in Stabilizing the α4-α5 Hairpin and Biotoxicity of the Cry4Aa Mosquitocidal Protein
Bibliographic record
Abstract
Background: Bacillus thuringiensis Cry toxins are well known for their insecticidal properties, primarily through the formation of ion-leakage pores via α4-α5 hairpins. His178 in helix 4 of the Cry4Aa mosquito-active toxin has been suggested to play a crucial role in its biotoxicity. Objective: This study aimed to investigate the functional importance of Cry4Aa-His178 through experimental and computational analyses. Methods: Ten His178-substituted Cry4Aa mutants (H178D, H178E, H178K, H178R, H178G, H178F, H178Y, H178S, H178C, and H178Q) were generated via site-directed mutagenesis and expressed in Escherichia coli. Toxin solubility was assessed in carbonate buffer (pH 10.0), and biotoxicity was tested against Aedes aegypti larvae. Trypsin-treated toxins were evaluated using fluorescent dye-release assays. Ion channel formation was studied in planar lipid bilayers (PLBs), and structural analysis was performed via MD simulations and sequence alignments with known Cry toxins. Results: All His178-substituted mutants were overexpressed as 130-kDa protoxin inclusions at levels comparable to the wild-type (WT). Replacing His178 with nonpolar or bulky polar residues reduced Cry4Aa biotoxicity to less than 10%, while substitutions with small, moderately polar, or negatively charged residues retained 50-85% activity, consistent with their in vitro solubility. Selected bioactive mutants, H178C and H178D, retained membrane-perturbing ability, like trypsin- activated WT, while the bioinactive H178Y mutant exhibited decreased membrane permeability. All tested mutants, including WT, induced cation-selective channels in PLBs with ~130-pS conductance. Sequence-structure analysis indicated that Cry4Aa-His178 likely forms a hydrogen bond with His217, a conserved His residue in helix 5. Discussion: Specific physicochemical properties of residue 178 are critical for optimal larvicidal activity, making it a promising target for engineering more potent mosquito-control toxins. Conclusion: His178 in Cry4Aa-α4 potentially forms a stabilizing hydrogen bond with α5-His217, which maintains the structural integrity of the α4-α5 hairpin. This structural stability is essential for efficient membrane insertion and optimal larvicidal activity.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".