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. (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)
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