Inhibition of Protein Synthesis by Didemnin B: How EF-1α Mediates Inhibition of Translocation

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Abstract
The antineoplastic cyclic depsipeptide didemnin B (DB) inhibits protein synthesis in cells and in vitro. The stage at which DB inhibits protein synthesis in cells is not known, although dehydrodidemnin B arrests translation at the stage of polypeptide elongation. Inhibition of protein synthesis by DB in vitro also occurs at the elongation stage, and it was shown previously that DB prevents EF-2-dependent translocation in partial reaction models of protein synthesis. This inhibition of translocation displays an absolute requirement for EF-1α; however, the dependence upon EF-1α was previously unexplained. It is shown here that DB binds only weakly to EF-1α/GTP in solution, but binds to ribosome·EF-1α complexes with a dissociation constant Kd = 4 μM. Thus, the inhibition of protein synthesis by DB appears to involve an interaction with both EF-1α and ribosomes in which all three components are required. Using diphtheria toxin-mediated ADP-ribosylation to assay for EF-2, it is demonstrated that DB blocks EF-2 binding to pre-translocative ribosome·EF-1α complexes, thus preventing ribosomal translocation. Based on this model for protein synthesis inhibition by DB, and the proposed mechanism of action of fusidic acid, evidence is presented in support of the Grasmuk model for EF-1α function in which this elongation factor does not fully depart the ribosome during polypeptide elongation.