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Mitochondria—found in all eukaryotic cells—have transferred most of their genes to the nuclear genome. The nuclear-localized mitochondrial genes are expressed and translated in the cytoplasm and the resulting mitochondrial proteins are imported into the mitochondria. Nevertheless, a few genes remain within mitochondria in the mitochondrial genome, and these genes are translated by mitochondrial ribosomes (mitoribosomes). Amunts et al. (p. 1485; see the Perspective by Kühlbrandt) determined the structure of mitoribosomes from yeast using single-particle cryo–electron microscopy. The mitoribosome is highly diverged from the bacterial and eukaryotic ribosomes with, for example, a distinctive exit tunnel for the newly synthesized peptide, and a membrane facing protuberance that might help to anchor the mitoribosome to the mitochondrial membrane.
Mitochondria have specialized ribosomes that have diverged from their bacterial and cytoplasmic counterparts. We have solved the structure of the yeast mitoribosomal large subunit using single-particle cryo–electron microscopy. The resolution of 3.2 angstroms enabled a nearly complete atomic model to be built de novo and refined, including 39 proteins, 13 of which are unique to mitochondria, as well as expansion segments of mitoribosomal RNA. The structure reveals a new exit tunnel path and architecture, unique elements of the E site, and a putative membrane docking site.