Targeting and plasticity of mitochondrial proteins revealed by proximity-specific ribosome profiling

TLDR

Nearly all mitochondrial proteins are nuclear‑encoded and are targeted to their mitochondrial destination from the cytosol. The study used proximity‑specific ribosome profiling to comprehensively measure translation at the mitochondrial surface in yeast. Proximity‑specific ribosome profiling was applied to yeast mitochondria, revealing that dual localization of proteins such as Osm1 is enabled by alternative translation initiation sites encoding distinct targeting signals. The study found that most inner‑membrane proteins are cotranslationally targeted to mitochondria, that the majority of proteins are organelle‑specific, and that Osm1 has a conserved ER isoform contributing to oxidative protein folding, highlighting the exquisite in vivo specificity of organellar targeting mechanisms.

Abstract

Nearly all mitochondrial proteins are nuclear-encoded and are targeted to their mitochondrial destination from the cytosol. Here, we used proximity-specific ribosome profiling to comprehensively measure translation at the mitochondrial surface in yeast. Most inner-membrane proteins were cotranslationally targeted to mitochondria, reminiscent of proteins entering the endoplasmic reticulum (ER). Comparison between mitochondrial and ER localization demonstrated that the vast majority of proteins were targeted to a specific organelle. A prominent exception was the fumarate reductase Osm1, known to reside in mitochondria. We identified a conserved ER isoform of Osm1, which contributes to the oxidative protein-folding capacity of the organelle. This dual localization was enabled by alternative translation initiation sites encoding distinct targeting signals. These findings highlight the exquisite in vivo specificity of organellar targeting mechanisms.