Bacterial translation elongation factor G (EF-G) catalyzes translocation during peptide elongation and mediates ribosomal disassembly during ribosome recycling in concert with the ribosomal recycling factor (RRF). Two homologs of EF-G have been identified in mitochondria from yeast to man, EF-G1mt and EF-G2mt. Here, we demonstrate that the dual function of bacterial EF-G is divided between EF-G1mt and EF-G2mt in human mitochondria (RRFmt). EF-G1mt specifically catalyzes translocation, whereas EF-G2mt mediates ribosome recycling with human mitochondrial RRF but lacks translocation activity. Domain swapping experiments suggest that the functional specificity for EF-G2mt resides in domains III and IV. Furthermore, GTP hydrolysis by EF-G2mt is not necessary for ribosomal splitting, in contrast to the bacterial-recycling mode. Because EF-G2mt represents a class of translational GTPase that is involved in ribosome recycling, we propose to rename this factor mitochondrial ribosome recycling factor 2 (RRF2mt).

Mammalian mitochondria synthesize a set of thirteen proteins that are essential for energy generation via oxidative phosphorylation. The genes for all of the factors required for synthesis of the mitochondrially-encoded proteins are located in the nuclear genome. A number of disease-causing mutations have been identified in these genes. In this manuscript, we have elucidated the mechanisms of translational failure for two disease states characterized by lethal mutations in mitochondrial elongation factor Ts (EF-Tsmt) and elongation factor Tu (EF-Tumt). EF-Tumt delivers the aminoacyl-tRNA (aa-tRNA) to the ribosome during the elongation phase of protein synthesis. EF-Tsmt regenerates EF-Tumt:GTP from EF-Tumt:GDP. A mutation of EF-Tsmt (R325W) leads to a two-fold reduction in its ability to stimulate the activity of EF-Tumt in poly(U)-directed polypeptide chain elongation. This loss of activity is caused by a significant reduction in the ability of EF-Tsmt R325W to bind EF-Tumt, leading to a defect in nucleotide exchange. A mutation of Arg336 to Gln in EF-Tumt causes infantile encephalopathy caused by defects in mitochondrial translation. EF-Tumt R336Q is as active as the wild-type protein in polymerization using E. coli 70S ribosomes and E. coli [14C]Phe-tRNA but is inactive in polymerization with mitochondrial [14C]Phe-tRNA and mitochondrial 55S ribosomes. The R336Q mutation causes a two-fold decrease in ternary complex formation with E. coli aa-tRNA but completely inactivates EF-Tumt for binding to mitochondrial aa-tRNA. Clearly the R336Q mutation in EF-Tumt has a far more drastic effect on its interaction with mitochondrial aa-tRNAs than bacterial aa-tRNAs.

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