Structure-activity relationships in diphtheria toxin and exotoxin A from Pseudomonas aeruginosa

Diphtheria toxin and exotoxin A from Pseudomonas aeruginosa (Pseudomonas toxin) block protein synthesis in sensitive animal cells by virtually identical mechanisms. Both toxins are proenzymes that, after activation, catalyze attachment of the adenosine diphosphate ribose (ADP-ribose) moiety of NAD to elongation factor 2 (EF-2) by covalent linkage. EF-2 is thereby inactivated. In the case of diphtheria toxin (60,000 daltons) the ADP-ribosylation of EF-2 is catalyzed by a 21,000-dalton peptide (fragment A) released after mild tryptic digestion and reduction of the toxin. The complementary B moiety of the toxin (39,000 daltons) is required for toxic activity and functions by attaching the toxin to oligosaccharide-containing cell surface receptors. In the case of the Pseudomonas toxin, the ADP-ribosylation reaction may be catalyzed either by the intact 66,000-dalton chain after reduction, or by a 26,000-dalton peptide released after mild proteolysis. Current approaches to study of the mechanisms of entry of the two toxins in active form into animal cells are reviewed.