The Formation of a Ternary Complex between Diphtheria Toxin, Aminoacyltransferase II, and Diphosphopyridine Nucleotide

Modes of action of ADP-ribosylated elongation factor 2 in inhibiting the polypeptide elongation cycle: a modeling study

Despite the fact that ADP-ribosylation of eukaryotic elongation factor 2 (EF2) leads to inhibition of protein synthesis, the mechanism by which ADP-ribosylated EF2 (ADPR•EF2) causes this inhibition remains controversial. Here, we applied modeling approaches to investigate the consequences of various modes of ADPR•EF2 inhibitory actions on the two coupled processes, the polypeptide chain elongation and ADP-ribosylation of EF2. Modeling of experimental data indicates that ADPR•EF2 fully blocks the late-phase translocation of tRNAs; but the impairment in the translocation upstream process, mainly the GTP-dependent factor binding with the pretranslocation ribosome and/or the guanine nucleotide exchange in EF2, is responsible for the overall inhibition kinetics. The reduced ADPR•EF2-ribosome association spares the ribosome to bind and shield native EF2 against toxin attack, thereby deferring the inhibition of protein synthesis inhibition and inactivation of EF2. Minimum association with the ribosome also keeps ADPR•EF2 in an accessible state for toxins to catalyze the reverse reaction when nicotinamide becomes available. Our work underscores the importance of unveiling the interactions between ADPR•EF2 and the ribosome, and argues against that toxins inhibit protein synthesis through converting native EF2 to a competitive inhibitor to actively disable the ribosome.

Reconstitution of diphtheria toxin from two nontoxic cross-reacting mutant proteins

The isolation of a new type of mutant Corynephage beta, which carries a missense mutation in the structural gene for diphtheria toxin synthesis is described. The lysogenic C7(8)(beta(197))(tox-crm+) strain of Corynebacterium diphtheriae produces a nontoxic, extracellular protein of molecular weight 62,000. This protein is immunologically indistinguishable from toxin itself but inhibits the action of toxin on HeLa cells, probably by competing for attachment sites on the cell membrane. In contrast to fragment A derived from diphtheria toxin, fragment A(197) is unable to catalyze the inactivation of eucaryotic polypeptidyl-transfer RNA-transferase II. When mixtures of the two nontoxic mutant proteins, enzymically active crm(45) protein and enzymically inactive crm(197) protein, are subjected to mild treatment with trypsin in the presence of a thiol and then allowed to reoxidize after dialysis to remove excess thiol, "diphtheria toxin" is reconstituted in high yield.