Active site of Pseudomonas aeruginosa exotoxin A. Glutamic acid 553 is photolabeled by NAD and shows functional homology with glutamic acid 148 of diphtheria toxin.

Photoaffinity labeling with native NAD, a method employed earlier with diphtheria toxin (DT), was used to identify an active site residue of Pseudomonas aeruginosa exotoxin A (ETA). An enzymically active fragment (Mr 27,000), derived by partial digestion of ETA with thermolysin, was irradiated with ultraviolet light (254 nm) in the presence of various radiolabeled preparations of NAD. Label from the nicotinamide moiety was efficiently transferred to the protein (maximally 0.79 mol/mol), and the label was exclusively located at position 553. This position, like that photolabeled in DT (position 148), corresponds to glutamic acid in the native protein. Chromatographically identical photo-products were generated at these positions in the two toxins. Glu-553 lies in a cleft in domain III that is believed to represent the active site of ETA, and other evidence supports the notion that Glu-553 of ETA and Glu-148 of DT are directly involved in catalysis. When Glu-553 of ETA was aligned with Glu-148 of DT, we found similarities of local primary structure not detected earlier. These results suggest that the catalytically active domains of ETA and DT may be evolutionarily related, and they provide information that should prove useful for preparing vaccines against ETA by recombinant DNA methods.