The Cbp3–Cbp6 complex, which has been shown previously to promote cytochrome b synthesis and assembly, plays a key role in adjusting cytochrome b expression to the efficiency of assembly of the respiratory chain bc1 complex.

A complex specifically required for the biogenesis of the respiratory chain component cytochrome b binds to the tunnel exit of yeast mitochondrial ribosomes to coordinate protein synthesis and assembly.

Assembly of the two heme b cofactors into the respiratory chain complex III subunit cytochrome b occurs in a step-by-step process that is monitored by assembly factors and regulates additional cytochrome b synthesis through a feedback loop.

Oxidative phosphorylation in mitochondria requires the synthesis of proteins encoded in the mitochondrial DNA. The mitochondrial translation machinery differs significantly from that of the bacterial ancestor of the organelle. This is especially evident from many mitochondria-specific ribosomal proteins. An important site of the ribosome is the polypeptide tunnel exit. Here, nascent chains are exposed to an aqueous environment for the first time. Many biogenesis factors interact with the tunnel exit of pro-and eukaryotic ribosomes to help the newly synthesized proteins to mature. To date, nothing is known about the organization of the tunnel exit of mitochondrial ribosomes. We therefore undertook a comprehensive approach to determine the composition of the yeast mitochondrial ribosomal tunnel exit. Mitochondria contain homologues of the ribosomal proteins located at this site in bacterial ribosomes. Here, we identified proteins located in their proximity by chemical cross-linking and mass spectrometry. Our analysis revealed a complex network of interacting proteins including proteins and protein domains specific to mitochondrial ribosomes. This network includes Mba1, the membrane-bound ribosome receptor of the inner membrane, as well as Mrpl3, Mrpl13, and Mrpl27, which constitute ribosomal proteins exclusively found in mitochondria. This unique architecture of the tunnel exit is presumably an adaptation of the translation system to the specific requirements of the organelle.