Structural studies of the mitochondrial ribosome and co-translational insertion of membrane proteins

The mitochondrion, a constituent of eukaryotic cells is important in the synthesis of ATP through a process called oxidative phosphorylation. The enzymes that make up the subunits of oxidative phosphorylation in humans and yeast are encoded by both nuclear DNA and in mitochondrial DNA. As such the mitochondrion has retained its own translational system to include mitochondrial ribosomes (mitoribosome) to translate these proteins along with other proteins which is species dependent. The mitoribosome is also involved in co-translational insertion of proteins destined for the inner mitochondrial membrane through binding of its translocon OXA1. My study has revealed the structure of the complete 75-component yeast Saccharomyces cerevisiae mitochondrial ribosome solved to 3.3 Å by single particle cryo-electron microscopy (cryo-EM). The previously unsolved small subunit of the mitoribosome was built de novo to include 34 proteins, of which 7 are specific to the yeast mitochondrial ribosome and the 15S ribosomal RNA. This mitochondrial ribosome has a distinct architecture compared to other mitochondrial ribosome and its ancestral bacterial ribosome. Elements specific to the yeast mitoribosome give it a distinct shape and architecture to include a putatively active enzyme on the periphery. An expanded messenger RNA exit channel has also been found harbouring a platform suitable for translational activator binding. In general this structure has provided insight into translation specialisation amongst species and its continued evolution. Multiple states of actively translating human mitoribosome have been elucidated using single particle cryo-EM. The human mitoribosome structures, stalled by different antibiotics, have been seen with A/A and P/P site tRNAs in situ as well as structures with an unidentified factor in the mitoribosomal factor site. In addition the human mitochondrial large subunit and stalled complete mitoribosome has been found at low resolution likely complexed to its translocon OXA1L following detergent solubilisation.