Pseudomonas aeruginosa exotoxin A(PE)contains three structure function domains. The amino-terminal domain Ⅰ is involved in binding to target cell-surface receptors through the active site Lys57.The central domain Ⅱ is responsible for the translocation of PE across membranes,Arg276 and Arg279 are the active key positions of the domain Ⅱ.The protease cleaves PE between Arg279 and Gly280 into 28 000 and 37 000 fragments. The carboxyl-terminal domain Ⅲ is located in 37 000 fragment which is directed by the REDLK sequence at the carboxyl end of domain Ⅲ to the endoplasmic reticulum and translocated to the cytosol, and then ADP-ribosylates the EF-2 (elongation factor 2) by binding NAD⁺ through the key site Glu553 to result in the inhibition of cell protein synthesis and death of target cells. It is a practical prospect that the recombinant immunotoxins are made by fusing DNA fragments encoding recognition proteins to the modified PE genes.