NEMF-mediated Listerin-independent mitochondrial translational surveillance by E3 ligase Pirh2 and mitochondrial protease ClpXP

The ribosome-associated protein quality control (RQC) pathway acts as a translational surveillance mechanism to maintain proteostasis. In mammalian cells, the cytoplasmic RQC pathway involves Nuclear Export Mediator Factor (NEMF)-dependent recruitment of the E3 ligase, Listerin, to ubiquitinate ribosome-stalled nascent polypeptides on the lysine residue for degradation. However, the quality control of ribosome-stalled nuclear-encoded mitochondrial nascent polypeptide remains elusive, as these peptides can be partially imported into mitochondrion through translocon, restricting accessibility to the lysine by Listerin. Here we identified a Listerin-independent organelle-specific mitochondrial RQC pathway that acts on NEMF-mediated carboxyl terminal poly-alanine modification. In the pathway, mitochondrial proteins carrying C-end poly-Ala tails are recognized by cytosolic E3 ligase Pirh2 and by the ClpXP protease in the mitochondria, which coordinately clear ribosome-stalled mitochondrial nascent polypeptides. Defects in this elimination pathway result in NEMF-mediated aggregates and mitochondrial integrity failure, thus providing a potential molecular mechanism of RQC pathway in mitochondrial-associated human diseases.

核糖体相关蛋白质量控制（RQC）途径作为一种翻译监督机制，负责维持蛋白质稳态。在哺乳动物细胞中，细胞质RQC途径涉及依赖核输出介质因子（NEMF）的E3连接酶Listerin的募集，以泛素化赖氨酸残基上的核糖体停滞新生多肽，从而进行降解。然而，核编码的线粒体新生多肽的RQC质量监控仍然是一个未解之谜，因为这些多肽可以通过转位酶部分导入线粒体，从而限制了Listerin对赖氨酸的访问。在本研究中，我们鉴定了一种Listerin非依赖性的细胞器特异性线粒体RQC途径，该途径作用于NEMF介导的羧基末端多丙氨酸修饰。在该途径中，携带C端多丙氨酸尾的线粒体蛋白被细胞质E3连接酶Pirh2以及线粒体内的ClpXP蛋白酶识别，它们协同清除核糖体停滞的线粒体新生多肽。该消除途径的缺陷会导致NEMF介导的聚集和线粒体完整性破坏，从而为线粒体相关的人类疾病中RQC途径的潜在分子机制提供了新的见解。