Diphtheria toxin is a 58 kDa protein secreted by lysogenic strains of Corynebacterium diphtheriae. The toxin causes the disease diphtheria in humans by gaining entry into the cell cytoplasm and inhibiting protein synthesis. The mechanism of inhibition involves transfer of the ADP-ribose group of NAD to elongation factor-2 (EF-2), rendering EF-2 inactive. The catalysed reaction is as follows: NAD + + peptide diphthamide = nicotinamide + peptide N-(ADP-D-ribosyl)diphthamide The crystal structure of the diphtheria toxin homodimer has been determined to 2.5A resolution. The structure reveals a Y-shaped molecule of 3 domains, a catalytic domain (fragment A), whose fold is of the alpha + beta type; a transmembrane (TM) domain, which consists of 9 alpha-helices, 2 pairs of which may participate in pH-triggered membrane insertion and translocation; and a receptor-binding domain, which forms a flattened beta-barrel with a jelly-roll-like topology. The TM- and receptor binding-domains together constitute fragment B.