Sordarin inhibits fungal protein synthesis by blocking translocation differently to fusidic acid

Sordarin derivatives are selective inhibitors of fungal protein synthesis, which specifically impair elongation factor 2 (EF-2) function. We have stud ied the effect of sordarin on the ribosome-dependent GTPase activity of EF- 2 from Candida albicans in the absence of any other component of the transl ation system. The effect of sordarin turned out to be dependent both on the ratio of ribosomes to EF-2 and on the nature of the ribosomes. When the am ount of EF-2 exceeded that of ribosomes sordarin inhibited the GTPase activ ity following an inverted bell-shaped dose-response curve, whereas when EF- 2 and ribosomes were in equimolar concentrations sordarin yielded a typical sigmoidal dose-dependent inhibition. However, when ricin-treated ribosomes were used, sordarin stimulated the hydrolysis of GTP. These results were c ompared with those obtained with fusidic acid, showing that both drugs act in a different manner. All these data are consistent with sordarin blocking the elongation cycle at the initial steps of translocation, prior to GTP h ydrolysis, In agreement with this conclusion, sordarin prevented the format ion of peptidyl-[H-3]puromycin on polysomes from Candida albicans.