The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc complex or complex III, a central component of the mitochondrial energy conversion system.
Highlights d Proximity labeling of proteins shows connectivity of mitochondrial gene expression d Interactors at the mRNA channel entrance and exit support translation initiation d Polypeptide tunnel exit serves as a platform to organize OXPHOS assembly factors d Synthesis of Cox1 is directly coupled to co-factor acquisition and early assembly
The mitochondrial genome almost exclusively encodes a handful of transmembrane constituents of the oxidative phosphorylation (OXPHOS) system. Coordinated expression of these genes ensures the correct stoichiometry of the system’s components. Translation initiation in mitochondria is assisted by two general initiation factors mIF2 and mIF3, orthologues of which in bacteria are indispensible for protein synthesis and viability. mIF3 was thought to be absent in Saccharomyces cerevisiae until we recently identified mitochondrial protein Aim23 as the missing orthologue. Here we show that, surprisingly, loss of mIF3/Aim23 in S. cerevisiae does not indiscriminately abrogate mitochondrial translation but rather causes an imbalance in protein production: the rate of synthesis of the Atp9 subunit of F1F0 ATP synthase (complex V) is increased, while expression of Cox1, Cox2 and Cox3 subunits of cytochrome c oxidase (complex IV) is repressed. Our results provide one more example of deviation of mitochondrial translation from its bacterial origins.
Highlights d Translational feedback regulation in mitochondria involves a molecular rheostat d Alternate binding of two translational activators to the ribosomal tunnel exit d Ribosomal binding of Cbs1 sequesters COB mRNA to repress translation d Cbp3-Cbp6 liberated during assembly replaces Cbs1 for translational activation
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