Mitochondrial Protein Synthesis Adapts to Influx of Nuclear-Encoded Protein.

Mitochondrial ribosomes translate membrane integral core subunits of the oxidative phosphorylation system encoded by mtDNA. These translation products associate with nuclear-encoded, imported proteins to form enzyme complexes that produce ATP. Here, we show that human mitochondrial ribosomes display translational plasticity to cope with the supply of imported nuclear-encoded subunits. Ribosomes expressing mitochondrial-encoded COX1 mRNA selectively engage with cytochrome c oxidase assembly factors in the inner membrane. Assembly defects of the cytochrome c oxidase arrest mitochondrial translation in a ribosome nascent chain complex with a partially membrane-inserted COX1 translation product. This complex represents a primed state of the translation product that can be retrieved for assembly. These findings establish a mammalian translational plasticity pathway in mitochondria that enables adaptation of mitochondrial protein synthesis to the influx of nuclear-encoded subunits.

The mitochondrial TMEM177 associates with COX20 during COX2 biogenesis.

The three mitochondrial-encoded proteins, COX1, COX2, and COX3, form the core of the cytochrome c oxidase. Upon synthesis, COX2 engages with COX20 in the inner mitochondrial membrane, a scaffold protein that recruits metallochaperones for copper delivery to the CuA-Site of COX2. Here we identified the human protein, TMEM177 as a constituent of the COX20 interaction network. Loss or increase in the amount of TMEM177 affects COX20 abundance leading to reduced or increased COX20 levels respectively. TMEM177 associates with newly synthesized COX2 and SCO2 in a COX20-dependent manner. Our data shows that by unbalancing the amount of TMEM177, newly synthesized COX2 accumulates in a COX20-associated state. We conclude that TMEM177 promotes assembly of COX2 at the level of CuA-site formation.

COX16 promotes COX2 metallation and assembly during respiratory complex IV biogenesis.

Cytochrome c oxidase of the mitochondrial oxidative phosphorylation system reduces molecular oxygen with redox equivalent-derived electrons. The conserved mitochondrial-encoded COX1- and COX2-subunits are the heme- and copper-center containing core subunits that catalyze water formation. COX1 and COX2 initially follow independent biogenesis pathways creating assembly modules with subunit-specific, chaperone-like assembly factors that assist in redox centers formation. Here, we find that COX16, a protein required for cytochrome c oxidase assembly, interacts specifically with newly synthesized COX2 and its copper center-forming metallochaperones SCO1, SCO2, and COA6. The recruitment of SCO1 to the COX2-module is COX16- dependent and patient-mimicking mutations in SCO1 affect interaction with COX16. These findings implicate COX16 in CuA-site formation. Surprisingly, COX16 is also found in COX1-containing assembly intermediates and COX2 recruitment to COX1. We conclude that COX16 participates in merging the COX1 and COX2 assembly lines.

Ribosome-Associated Mba1 Escorts Cox2 from Insertion Machinery to Maturing Assembly Intermediates.

The three conserved core subunits of the cytochrome c oxidase are encoded by mitochondria in close to all eukaryotes. The Cox2 subunit spans the inner membrane twice, exposing the N and C termini to the intermembrane space. For this, the N terminus is exported cotranslationally by Oxa1 and subsequently undergoes proteolytic maturation in Saccharomyces cerevisiae Little is known about the translocation of the C terminus, but Cox18 has been identified to be a critical protein in this process. Here we find that the scaffold protein Cox20, which promotes processing of Cox2, is in complex with the ribosome receptor Mba1 and translating mitochondrial ribosomes in a Cox2-dependent manner. The Mba1-Cox20 complex accumulates when export of the C terminus of Cox2 is blocked by the loss of the Cox18 protein. While Cox20 engages with Cox18, Mba1 is no longer present at this stage. Our analyses indicate that Cox20 associates with nascent Cox2 and Mba1 to promote Cox2 maturation cotranslationally. We suggest that Mba1 stabilizes the Cox20-ribosome complex and supports the handover of Cox2 to the Cox18 tail export machinery.