An exotoxin, produced by Vibrio cholerae and Vibrio eltor, thatsleeds to the massive, watery diarrhea characteristic of choleraThe cholera toxin is a member of the AB family of toxins. Thefunction of the non-toxic B subunit is to assemble into aring-shaped, homologous pentamer and to adhere to mem-branereceptors on the intestinal epithelial cell surface (GM1gangliosides). The pathogenic effect stems from the A subunit. Thisis split into two domains, an anchor domain A2 and a toxic domainA1. The anchor domain binds the toxic A1 domain to the B subunit. Through a series of further intermediate activation steps, the A1domain ultimately causes a massive secretion of chloride ions inthe gut lumen, causing diarrhoea. It follows that the A1 subunit istoxic, but the B subunit is non-toxic.