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.