Molecular structures of Pseudomonas exotoxin (1IQK.pdb), diphtheria toxin (1TOX.pdb), and ricin toxin (2AAI.pdb). From left to right (blue to red), sequences are shown from the N-terminus to the C-terminus of each protein. To construct immunotoxins, the binding domain of each toxin is removed and replaced with an antibody or antibody fragment. The enzyme domains of PE, DT, and ricin are shown as either ADP-ribosylation or RNA N-glycosidase, as indicated. Middle domains for PE and DT are shown in yellow-green and include a furin processing site (white arrows): the furin site of DT is not ordered in the crystal structure so its approximate location is indicated. For DT, helical structures in the middle domain facilitate toxin translocation. The translocation activity of PE has not been definitively determined. Ricin apparently contains translocation sequences within the enzyme (blue) domain. The enzyme domain of ricin toxin is equivalent to deglycosylated ricin A chain. Intact ricin toxin contains tandem C-terminal binding modules (red).