Membrane insertion of mitochondrial-encoded proteins regulates ribosome decoding speed

The human mitochondrial genome encodes 13 subunits of the oxidative phosphorylation system essential for energy metabolism to drive cellular activities. Translation of 11 mRNAs by membrane-bound ribosomes is coupled to insertion of the nascent polypeptides into the inner membrane aided by the OXA1L insertase. To this end, the mechanism of membrane insertion of nascent polypeptides and the functional link to the translation process are not sufficiently understood. Here, we applied ribosome profiling to assess translation dynamics in combination with cryo-electron microscopy analysis of a COX1 ribosome-nascent chain complex to visualize cotranslational folding of the nascent chain. We find that the membrane topology of the translation product impacts translation speed and that positioning of amphipathic helices in the ribosome vestibule induces structural changes, correlating with translation pausing events. Thus, our findings reveal a link between translation process and folding and membrane insertion of nascent polypeptides at the inner mitochondrial membrane.