Mechanism of action of Pseudomonas aeruginosa exotoxin Aiadenosine diphosphate-ribosylation of mammalian elongation factor 2 in vitro and in vivo

Abstract

Previous studies showed that Pseudomonas aeruginosa exotoxin A (PA toxin) catalyzes nicotinamide adenine dinucleotide (NAD)-dependent inhibition of protein synthesis in a rabbit reticulocyte lysate and transfer of radioactivity from [14C]adenine-labeled NAD to a protein having the same molecular weight as elongation factor 2 (EF-2) (B.H. Iglewski and D. Kabat, 1975). Such an inhibited protein-synthesizing lysate was restored to activity by addition of a protein from normal mouse liver which co-purifies with EF-2. In addition, EF-2 activity was almost totally absent in livers of mice which had been injected 24 h earlier with PA toxin. On the contrary, EF-2 concentrations were only partially reduced in other organs and were normal in brains of intoxicated mice. Studies using NAD labeled in various positions show that PA toxin, like fragment A of diphtheria toxin, catalyzes transfer of the adenosine 5'-diphosphate-ribosyl moiety of NAD. Furthermore, reversal occurred when the modified protein was incubated with excess concentrations of PA toxin and nicotinamide, and NAD was identified as a product of the reverse reaction. The protein modification catalyzed either by PA toxin or by fragment A of diphtheria toxin could be reversed by incubation with other toxin. These results support the proposal that these two toxins adenosine 5'-diphosphate-ribosylate and same amino acid of EF-2 in a stereochemically identical fashion. Furthermore, PA toxin inactivates EF-2 in intoxicated mice to an extent which would ultimately result in death.