Elongation factors inhibitors

Several other inhibitors of protein synthesis are notable because of their toxicity to humans and other mammals. Diphtheria toxin (Mr 58,330) catalyzes the ADP-ribosylation of a diphthamide (a modified histidine) residue of eukaryotic elongation factor eEF2, thereby inactivating it. Ricin (Afr 29,895), an extremely toxic protein of the castor bean, inactivates the 60S subunit of eukaryotic ribosomes by depurinating a specific adenosine in 23S rRNA. 

Cetin R, Krab IM, Anborgh PH, Cool RH, Watanabe T, Sugiyama T, Izaki K, Parmeggiani A (1996) Enacyloxin Ila, an Inhibitor of Protein Biosynthesis That Acts on Elongation Factor Tu and the Ribosome. EMBO J 15 2604... 

Certain inhibitors - kirromycin, pulvomycin, fusidic acid and diphtheria toxin - which block protein synthesis by interacting with either elongation factor constitute potent probes to reveal eucaryal and/or bacterial traits on archaeal factors. 

The sensitivities of ribosomes and elongation factors to selected Group l-lll inhibitors have been used by Amils and coworkers [170,171] to infer genealogical relationships between the archaea and the other two domains of life. The underlying assumption is that susceptibility to a given antibiotic reveals the presence or the absence of specific details of the (functionally essential) domain that acts as the antibiotic binding site. 

Components of the archaeal translation apparatus, however, display an intra-domain diversity, not encountered within the other two domains. This situation is exemplified by (i) the lack of a uniform compatibility between ribosomes and elongation factors from different archaeal lineages (see section 5.2) (ii) the impressive diversity of archaeal ribosomes and factors in their response to a host of protein synthesis inhibitors (see section 4) (iii) the heterogeneity in shape, mass and composition of archaeal ribosomal subunits (see sections 2.6.2, 2.6.3). Importantly, within-domain diversity is also exemplified by the different complexities of the RNA polymerase subunit patterns from the sulfur-dependent and the methanogenic-halophilic archaea (see Zillig et ah. Chapter 12 of this volume). 

Diphtheria toxin is one of the most potent toxins known a single molecule of diphtheria toxin is sufficient to inactivate enough eukaryotic elongation factor eEF-2 in a cell to cause death. Ricin is derived from castor beans and its mode of action is to inactivate eukaryotic 28S rRNA molecules by removing a single adenine residue by N-glycolytic cleavage. The other inhibitor of eukaryotic protein synthesis shown in Table 26.1 is cycloheximide, which inhibits peptidyl transferase activity of the 60S ribosome subunit. 

Many of the differences between translation in prokaryotes and eukaryotes can be seen in the response to inhibitors of protein synthesis and to toxins. The antibiotic chloramphenicol (a trade name is Ghloromycetin) binds to the A site and inhibits peptidyl transferase activity in prokaryotes, but not in eukaryotes. This property has made chloramphenicol useful in treating bacterial infections. In eukaryotes, diphtheria toxin is a protein that interferes with protein synthesis by decreasing the activity of the eukaryotic elongation factor eEF2. 

Moreover recently, mechanistic studies using purified elongation factors and ribosomes identified RA-VII as a peptidyl transferase inhibitor. Thus similar to the related natural products bouvardin (165) and RA-XII (158), RA-VII (153) appears to function by binding to eukaryotic ribosomes [88]. 

Neither ricin nor the toxic lectin modeccin are capable of inhibiting the elongation factor (EFl)-dependent binding of L-[ " C]phenylalanyl-tRNA although they are specific inhibitors of translocation. Inconsistencies in earlier reports may have arisen from an overestimation of the number of control ribosomes engaged in the binding reaction if trace amounts of EF2 contaminate the ribosomal preparations. 

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