Shiga toxin
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Shiga toxins are a family of related toxins with two major groups, Stx1 and Stx2, whose genes are considered to be part of the genome of lambdoid prophages. The toxins are named for Kiyoshi Shiga, who first described the bacterial origin of dysentery caused by Shigella dysenteriae. The most common sources for Shiga toxin are the bacteria Shigella dysenteriae and the Shigatoxigenic group of Escherichia coli (STEC), which includes serotype O157:H7 and other enterohemorrhagic E. coli.
Shiga toxins act to inhibit protein synthesis within target cells by a mechanism similar to that of ricin toxin produced by Ricinus communis. After entering a cell, the protein functions as an N-glycosidase, cleaving several nucleobases from the RNA that comprises the ribosome, thereby halting protein synthesis.
The toxin has two subunits, A and B. The B subunit is a pentamer that binds to specific glycolipids on the host cell, specifically globotriaosylceramide (Gb3). Following this, the A subunit is internalised and cleaved into two parts. The A1 component then binds to the ribosome, disrupting protein synthesis.
The toxin requires highly specific receptors on the cells' surface in order to attach and enter the cell; species such as cattle, swine, and deer which do not carry these receptors may harbor toxigenic bacteria without any ill effect, shedding them in their feces, from where they may be spread to humans.
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