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.
Mitochondria contain their own genetic system that provides subunits of the complexes driving oxidative phosphorylation. A quarter of the mitochondrial proteome participates in gene expression, but how all these factors are orchestrated and spatially organized is currently unknown. Here, we established a method to purify and analyze native and intact complexes of mitochondrial ribosomes. Quantitative mass spectrometry revealed extensive interactions of ribosomes with factors involved in all the steps of posttranscriptional gene expression. These interactions result in large expressosome-like assemblies that we termed mitochondrial organization of gene expression (MIOREX) complexes. Superresolution microscopy revealed that most MIOREX complexes are evenly distributed throughout the mitochondrial network, whereas a subset is present as nucleoid-MIOREX complexes that unite the whole spectrum of organellar gene expression. Our work therefore provides a conceptual framework for the spatial organization of mitochondrial protein synthesis that likely developed to facilitate gene expression in the organelle.
Here we report that Mdm38 and Mba1 display overlapping functions in mitochondrial protein expression. Both Mdm38 and Mba1 interact with mitochondrial ribosomes and are required for translation of COX1 and cytochrome b mRNAs.
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