Bacillus thuringiensis Cry4A and Cry4B mosquito-larvicidal proteins: homology-based 3D model and implications for...

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Modeling

Background:

• Bacillus thuringiensis subsp. israelensis produces Cry4A and Cry4B delta-endotoxins, which are toxic to mosquito larvae.
• Understanding the structure of these toxins is crucial for elucidating their mechanism of action.

Purpose of the Study:

• To construct and analyze three-dimensional (3D) homology models of activated Cry4A and Cry4B delta-endotoxins.
• To gain structural insights into the mosquito-larvicidal activity of these proteins.

Main Methods:

• Homology modeling was employed, utilizing atomic coordinates from Cry1Aa and Cry3Aa crystal structures.
• Circular dichroism spectroscopy was used to confirm the secondary structural content of the models.
• Structural analysis focused on identifying key residues, disulfide bonds, and domain-specific differences.

Main Results:

• The 3D models exhibited a conserved three-domain organization (N-terminal helix bundle, middle and C-terminal beta-sheets).
• Identified solvent-exposed residues (Arg-235 in Cry4A, Arg-203 in Cry4B) susceptible to tryptic cleavage.
• A unique disulfide bond and proline-rich region in Cry4A's loop (alpha4-alpha5) suggest functional significance for membrane insertion.
• Significant structural variations in domain II between Cry4A and Cry4B models correlate with their distinct activity spectra.

Conclusions:

• The study provides valuable 3D structural models for Cry4A and Cry4B mosquito larvicides.
• Structural insights enhance understanding of the functional mechanisms, including potential tryptic cleavage sites and membrane insertion.
• Differences in domain II highlight structural basis for varying insecticidal activity spectra.