Biochem/physiol Actions
The Shiga toxins are a family of related protein toxins secreted by certain types of bacteria. Shiga toxin (Stx) is produced by Shigella dysenteriae, whereas the Shiga-like toxins, Stx1 and Stx2, with a few known isoforms, are secreted by specific strains of Escherichia coli named Shiga-toxin-producing E. coli (STEC) such as E. coli O157:H7, which causes bloody diarrhea and hemorrhagic colitis in humans, sometimes resulting in fatal systemic complications.Stx1 is identical to Stx, while the Stx2 isoforms share less sequence similarity with Stx (~60%) and are immunologically distinct. In spite of the differences in their amino acid sequence, all Stx isoforms share the same overall toxin structure and mechanism of action.Shiga toxins consist of two polypeptides: An A chain and a B chain non-covalently associated with an apparent stoichiometry of one A and five B chains, to form the holotoxin. The catalytic A subunit has a RNA N-glycosidase activity that inhibits eukaryotic protein synthesis. The B subunits form a pentamer that recognizes and binds to the functional cell-surface receptor globotriaosylceramide [Gb3; Gala(1-4)-Galb(1-4)-Glcb1-ceramide]. Gb3 is overexpressed in membranes of numerous tumor cells, therefore STxB binding to Gb3 receptors may be useful for cell-specific vectorization, labeling and imaging purposes.
