Cyt1Aa toxin: crystal structure reveals implications for its membrane-perforating function.

Area of Science:

• Structural biology
• Biochemistry
• Molecular toxicology

Background:

• Bacillus thuringiensis subsp. israelensis produces mosquito larvicidal protein complexes.
• Cytolytic (Cyt) toxins are key components of these complexes, with Cyt1Aa being the most potent.

Purpose of the Study:

• To determine the crystal structure of the activated monomeric form of Cyt1Aa.
• To elucidate the structural basis for Cyt1Aa's toxicity and mechanism of action.

Main Methods:

• Isolation and crystallization of activated monomeric Cyt1Aa.
• X-ray crystallography at 2.2 Å resolution.
• Comparative structural analysis with Cyt2Aa and sequence-based analysis of Cyt1Ca.

Main Results:

• The first crystal structure of Cyt1Aa was determined, revealing a typical cytolysin fold with β-sheet and α-helical layers.
• A key β-strand segment absent in Cyt2Aa was identified as essential for dimer formation and N-terminal activation.
• A putative lipid-binding pocket was found, supporting a membrane insertion and pore-formation mechanism.
• Structural flexibility and conformational changes were proposed as crucial for membrane perforation and toxicity.

Conclusions:

• The Cyt1Aa structure provides insights into the mechanism of cytolysin toxicity, involving conformational changes for membrane insertion.
• Differences in flexibility, as seen in Cyt1Ca, may explain the lack of activity in related toxins.
• The findings support a pore-forming model for Cyt1Aa, distinct from detergent action, highlighting the importance of the cytolysin fold.