Structural basis of subtilase cytotoxin SubAB assembly

Area of Science:

• Microbiology
• Structural Biology
• Toxicology

Background:

• Pathogenic Escherichia coli strains produce AB5 toxins, comprising a catalytic A-subunit and a B-pentamer.
• While subunit functions are known, their self-association and impact on cytotoxicity remain unclear.

Purpose of the Study:

• To determine the structure of the holo-SubAB toxin.
• To understand the interaction between SubA and SubB subunits.
• To elucidate the role of the B-pentamer in SubAB toxin assembly and trafficking.

Main Methods:

• X-ray crystallography to solve the holo-SubAB toxin structure.
• Structure-based sequence comparisons with other AB5 toxins.
• Mutagenesis studies on the SubB subunit.

Main Results:

• The SubAB toxin cleaves the endoplasmic reticulum chaperone BiP, unlike other AB5 toxins targeting the cytosol.
• SubA interacts with SubB via the A2 helix and a disulfide bond, similar to other AB5 toxins.
• The SubA active site is not occluded by the B-pentamer, and the B-pentamer does not modulate SubA activity.
• A hydrophobic patch on the SubB pentamer is crucial for binding the A-subunit.

Conclusions:

• The determined SubAB structure provides a framework for understanding AB5 toxin assembly and intracellular trafficking.
• The B-pentamer's role in binding the A-subunit is critical for SubAB toxin function.
• SubAB represents a distinct AB5 toxin mechanism by targeting the endoplasmic reticulum chaperone BiP.