EF-G2mt Is an Exclusive Recycling Factor in Mammalian Mitochondrial Protein Synthesis

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).