Bacillus thuringiensis CryIA(a) insecticidal toxin: crystal structure and channel formation

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Entomology

Background:

• Bacillus thuringiensis var. kurstaki HD-1 produces the CryIA(a) toxin, a key component for controlling lepidopteran pests.
• Understanding the structure-function relationship of Cry toxins is crucial for developing effective biopesticides.

Purpose of the Study:

• To determine the three-dimensional structure of the activated CryIA(a) toxin.
• To investigate its channel-forming activity in planar lipid bilayers.
• To compare its structure with other Cry toxins and elucidate structure-function relationships.

Main Methods:

• X-ray diffraction was used to determine the 3D structure of CryIA(a) toxin.
• Multiple isomorphous replacement method and refinement to 2.25 A resolution.
• Planar lipid bilayer experiments to assess channel formation.

Main Results:

• The CryIA(a) toxin possesses three distinct domains: an N-terminal alpha-helical bundle and middle/C-terminal beta-sheet domains.
• Structural comparison with coleopteran-specific CryIIIA revealed significant differences in domain II, implicating it in receptor binding.
• CryIA(a) forms cation-selective channels in lipid bilayers with lower conductance than CryIIIA.

Conclusions:

• The determined structure provides insights into the molecular basis of CryIA(a) toxicity.
• Domain II's structural variations likely play a role in the specificity of Cry toxin interactions with insect receptors.
• CryIA(a) exhibits channel-forming activity consistent with its mode of action in lepidopteran pests.