A molecular switch at the yeast mitoribosomal tunnel exit controls cytochrome b synthesis

Mitochondrial gene expression needs to be balanced with cytosolic translation to produce oxidative phosphorylation complexes. In yeast, translational feedback loops involving lowly expressed proteins called translational activators help to achieve this balance. Synthesis of cytochrome b (Cytb or COB), a core subunit of complex III in the respiratory chain, is controlled by three translational activators and the assembly factor Cbp3-Cbp6. However, the molecular interface between the COB translational feedback loop and complex III assembly are yet unknown. Here, using protein-proximity mapping combined with selective mitoribosome profiling, we reveal the components and dynamic interaction of the molecular switch controlling COB translation. Specifically, we demonstrate that Mrx4, a previously uncharacterized ligand of the mitoribosomal polypeptide tunnel exit, interacts with either the assembly factor Cbp3-Cbp6 or with the translational activator Cbs2. These reciprocal interactions determine whether the translational activator complex with bound COB mRNA can interact with the mRNA channel exit on the small ribosomal subunit for translation initiation. Organization of the feedback loop at the tunnel exit therefore orchestrates mitochondrial translation regulation with respiratory chain biogenesis.线粒体基因表达需与胞质翻译维持平衡，以协同合成氧化磷酸化复合物（oxidative phosphorylation complexes）。在酵母中，由低丰度蛋白——翻译激活因子（translational activators）——介导的翻译反馈环路，可助力达成该平衡。呼吸链复合物III的核心亚基细胞色素b（cytochrome b，简称Cytb或COB）的合成，受三种翻译激活因子与组装因子Cbp3-Cbp6共同调控。然而，COB翻译反馈环路与复合物III组装之间的分子界面迄今仍未明确。本研究结合蛋白质邻近图谱分析（protein-proximity mapping）与选择性线粒体核糖体测序（selective mitoribosome profiling），揭示了调控COB翻译的分子开关的组成及其动态相互作用。具体而言，本研究证实，Mrx4作为一种此前未被表征的线粒体核糖体多肽隧道出口（mitoribosomal polypeptide tunnel exit）配体，可与组装因子Cbp3-Cbp6或翻译激活因子Cbs2发生相互作用。这些双向相互作用决定了结合了COB mRNA的翻译激活因子复合物，能否与核糖体小亚基的mRNA通道出口结合以启动翻译。因此，隧道出口处的反馈环路组织模式，协调了线粒体翻译调控与呼吸链生物发生过程。