Corynebacterium diphtheriae is a Gram-positive, bacterium which infects epithelial cells of the upper respiratory tract and produces diphtheria toxin. Diphtheria toxin is proteolytically cleaved forming a two part toxin, held together by a disulfide bridge. The amino-terminal carries the toxin's enzymatic activity, capable of ADP-ribosylation and inactivation of translation elongation factor 2 (EF-2). The carboxy-terminal domain binds to specific host receptors, the heparin-binding EGF-like growth factor (HB-EGF) on human epithelial cells and translocates the catalytic domain into the cell. After binding to the cell receptor, the diphtheria toxin is taken up by endocytosis, the pH of the endocytic vesicle drops, and the translocation region of the toxin helps guide the catalytic domain into the host cytoplasm where it is released. Within the cytoplasm, the diphtheria toxin catalytic domain ADP ribosylates EF-2, terminating protein synthesis and causing the death of the cell. Diphtheria toxin is highly potent, and as little as one catalytic domain is thought to cause cell death. In cell culture, diphtheria toxin inhibits protein synthesis and causes death in cells carrying the HB-EGF receptor. This toxin has been used to specifically eliminate receptor-expressing cells in transgenic mice.
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