Figure 1. Schematic of toxins function in eukaryotic cells. Dotted arrows indicate steps in bacterial lysis with phage/toxin release, toxin uptake by host cell and target activity. CTXphi which encodes cholera toxin (CT) does not lyse its bacterial host cell. Black solid arrows or t-bars represent activation or blocking of specified bacterial function by toxin. AB denotes a two subunit protein, subunit A is the catalytic subunit and B is the receptor binding or docking subunit on target cell. AB₅ denotes a toxin that contains one A subunit and 5 B subunits. Three examples are given; the A/B diphtheria toxin (DT) and the AB₅ toxins CT and shiga toxin (Stx). DT gains direct entry into target cell by pore formation. The second method of toxin uptake is by receptor mediated endocytosis, a mechanism used by CT and Stx. Only the A subunit of DT enters the cell cytosol and along with NAD, ADP-ribosylates Elongation factor 2 blocking protein synthesis. Both CT and Stx are retrograde trafficked to the Golgi apparatus. In CT, the A subunit is dissociated from B subunits in the endoplasmic recticulum (ER) and released into the cytosol where it activates adenylate cyclase increasing cAMP which in turn activates PKA resulting in the CFTR membrane Cl^- channel activation. The A subunit of Stx ADP- ribosylates the 28S rRNA ribosomal subunit, thus blocking protein synthesis.