Eukaryotic translation elongation factor 2 (eEF2) catalyzes reverse translocation of the eukaryotic ribosome

During protein synthesis, a ribosome moves along the mRNA template and, using aminoacyl-tRNAs, decodes the template nucleotide triplets to assemble a protein amino acid sequence. This movement is accompanied by shifting of mRNA-tRNA complexes within the ribosome in a process called translocation. In living cells, this process proceeds in a unidirectional manner, bringing the ribosome to the 3' end of mRNA, and is catalyzed by the GTPase translation elongation factor 2 (EF-G in prokaryotes and eEF2 in eukaryotes). Interestingly, the possibility of spontaneous backward translocation has been shown in vitro for bacterial ribosomes, suggesting a potential reversibility of this reaction. However, this possibility has not yet been tested for eukaryotic ribosomes. Here, using a reconstituted mammalian translation system, we show that the eukaryotic elongation factor eEF2 catalyzes ribosomal reverse translocation at one mRNA triplet. We found that this process requires a cognate tRNA in the ribosomal E-site and cannot occur spontaneously without eEF2. The efficiency of this reaction depended on the concentrations of eEF2 and cognate tRNAs and increased in the presence of nonhydrolyzable GTP analogues. Of note, ADP-ribosylation of eEF2 domain IV blocked reverse translocation, suggesting a crucial role of interactions of this domain with the ribosome for the catalysis of the reaction. In summary, our findings indicate that eEF2 is able to induce ribosomal translocation in forward and backward directions, highlighting the universal mechanism of tRNA-mRNA movements within the ribosome.