Elongational stalling activates mitoribosome-associated quality control

Abstract

The human mitochondrial ribosome (mitoribosome) and associated proteins regulate the synthesis of 13 essential subunits of the oxidative phosphorylation complexes. We report the discovery of a mitoribosome-associated quality control pathway that responds to interruptions during elongation, and we present structures at 3.1- to 3.3-angstrom resolution of mitoribosomal large subunits trapped during ribosome rescue. Release factor homolog C12orf65 (mtRF-R) and RNA binding protein C6orf203 (MTRES1) eject the nascent chain and peptidyl transfer RNA (tRNA), respectively, from stalled ribosomes. Recruitment of mitoribosome biogenesis factors to these quality control intermediates suggests additional roles for these factors during mitoribosome rescue. We also report related cryo–electron microscopy structures (3.7 to 4.4 angstrom resolution) of elongating mitoribosomes bound to tRNAs, nascent polypeptides, the guanosine triphosphatase elongation factors mtEF-Tu and mtEF-G1, and the Oxa1L translocase.

Release, rescue and recycling: termination of translation in mammalian mitochondria

“…In fact, C12ORF65 has only very recently been shown to cooperate with the RNA-binding protein MTRES1 (C6ORF203) in an alternative mitoribosome rescue pathway that acts on the large subunit only. (Desai et al, 2020) In combination with our data, this shows that despite similar domain architecture and related electrostatic properties, ICT1 and C12ORF65 follow distinct modes of action to recover stalled mitoribosomes for translation and it may also indicate that their interaction with the mitoribosome could be controlled by different cues. While ICT1 appears to detect truncated mRNAs on the mitoribosome, the initial trigger that commits stalled mitoribosomes for rescue by C12ORF65 binding has not been established.…”

“…We do not find C12ORF65 engaged with the mitoribosome under these conditions in line with a recent report that identified C12ORF65 (now termed mtRF-R) to team up with the mitochondrial RNA-binding protein C6ORF203 (MTRES1) in a novel translation rescue pathway under conditions when aminoacylated tRNAs become limiting. (Desai et al, 2020) In this pathway, C12ORF65 acts exclusively on the LSU instead of the 55S monosome. Collectively, this observation and our screening data argue that – despite a similar domain architecture - ICT1 and C12ORF65 rescue stalled mitoribosomes by fundamentally different mechanisms.…”

A distinct assembly pathway of the human 39S late pre-mitoribosome

“…However, due to the low local resolution, no adjustments could be further done, and, in the final model of state A1, only poly-Ala models are provided. In the states B1 and D1, the E-site tRNA did not have sufficient resolution to build a molecular model, thus a previously observed mitochondrial tRNA was used to rigid body fit into the density 9 . The position A73 was manually built in Coot.…”

Ribosome Profiling Reveals a Dichotomy Between Ribosome Occupancy of Nuclear-Encoded and Mitochondrial-Encoded OXPHOS mRNA Transcripts in a Striatal Cell Model of Huntington Disease

“…Recent studies in yeast showed that cytosolic translational regulators control the mitochondrial OXPHOS genes [56–57] . Moreover, a novel mitoribosome-associated quality control (mtRQC) pathway comprising of mtRF-R and MTES1 is shown to rescue mitoribosomes stalling [58] . Further identifying the molecular nature of cellular signals and ribosome stalling mechanisms in HD cells will provide new insights that could help to curtail mitochondrial translation defects and have a potential therapeutic impact in HD.…”