AB5 toxin

Ribbon diagram of shiga toxin (Stx) from Shigella dysenteriae, showing the characteristic AB₅ structure. The A subunit is shown in orange, and the B-subunit complex, in blue. From PDB 1R4Q.

The AB5 (or AB₅) toxins are six-component protein complexes secreted by a number of pathogenic bacteria. All share a similar structure and mechanism for entering targeted host cells.^[1]

Structure and mechanism

A complete AB₅ toxin complex contains six protein units. Five – the B subunits – are similar or identical in structure; the remaining A subunit is unique.

The A subunit (or a portion thereof) of an AB₅ toxin is the portion of the complex responsible for toxicity. Typically it will have enzymatic activity inside the host cell.

The B subunits form a pentameric (five-membered) ring, into which one end of the A subunit extends and is held. This B subunit ring is also capable of binding to a receptor on the surface of the host cell.^[2] (Without the B subunits, the A subunit has no way of attaching to or entering the cell, and thus no way to exert its toxic effect.)

List of AB₅ toxins

• Campylobacter jejuni enterotoxin (from Campylobacter jejuni)
• cholera toxin (Vibrio cholerae)
• heat-labile enterotoxins (LT and LT-II) (Escherichia coli)
• pertussis toxin (Bordetella pertussis)
• shiga toxin (Shigella dysenteriae)
• shiga-like toxin (or verotoxin) (enterohemorrhagic varieties of E. coli including O157:H7)

References

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