Structure of the mosquitocidal toxin from Bacillus sphaericus

Area of Science:

• Structural biology
• Biochemistry
• Molecular toxicology

Background:

• Mosquitocidal toxins are crucial for vector control.
• Understanding their structure-function relationship is key to developing new insecticides.
• ADP-ribosylating enzymes share conserved structural motifs.

Purpose of the Study:

• To determine the three-dimensional structure of a mosquitocidal toxin's catalytic domain.
• To elucidate the mechanism of toxin activation and inhibition.
• To identify the NAD(+) binding site and its role in inhibition.

Main Methods:

• X-ray crystallography at resolutions of 2.5-3.0Å.
• Multi-wavelength and single-wavelength anomalous diffraction, and molecular replacement phasing.
• Size-exclusion chromatography.

Main Results:

• Three crystal structures revealed the toxin's fold resembles ADP-ribosylating enzymes.
• The toxin forms inactive heptameric and tetradecameric structures at pH 4.3.
• A 44-residue linker occupies the NAD(+) binding site, inhibiting the toxin.
• The activation cleavage site is in a mobile loop exposed in the monomer.

Conclusions:

• The observed heptameric and tetradecameric forms represent an inactive storage state of the toxin.
• The inhibitory linker prevents self-activation within these aggregates.
• Proteolytic activation is hindered in the aggregated state due to inaccessible cleavage sites.