井澤 俊明

Accurate gene expression is a prerequisite for all cellular processes. Cells actively promote correct protein folding, which prevents the accumulation of abnormal and non-functional proteins. Translation elongation is the fundamental step in gene expression to ensure cellular functions, and abnormal translation arrest is recognized and removed by the quality controls. Recent studies demonstrated that ribosome plays crucial roles as a hub for gene regulation and quality controls. Ribosome-interacting factors are critical for the quality control mechanisms responding to abnormal translation arrest by targeting its products for degradation. Aberrant mRNAs are produced by errors in mRNA maturation steps and cause aberrant translation and are eliminated by the quality control system. In this review, we focus on recent progress on two quality controls, Ribosome-associated Quality Control (RQC) and No-Go Decay (NGD), for abnormal translational elongation. These quality controls recognize aberrant ribosome stalling and induce rapid degradation of aberrant polypeptides and mRNAs thereby maintaining protein homeostasis and preventing the protein aggregation.

Eukaryotic cells have evolved extensive protein quality-control mechanisms to remove faulty translation products. Here, we show that yeast cells continually produce faulty mitochondrial polypeptides that stall on the ribosome during translation but are imported into the mitochondria. The cytosolic protein Vms1, together with the E3 ligase Ltn1, protects against the mitochondrial toxicity of these proteins and maintains cell viability under respiratory conditions. In the absence of these factors, stalled polypeptides aggregate after import and sequester critical mitochondrial chaperone and translation machinery. Aggregation depends on C-terminal alanyl/threonyl sequences (CAT-tails) that are attached to stalled polypeptides on 60S ribosomes by Rqc2. Vms1 binds to 60S ribosomes at the mitochondrial surface and antagonizes Rqc2, thereby facilitating import, impeding aggregation, and directing aberrant polypeptides to intra-mitochondrial quality control. Vms1 is a key component of a rescue pathway for ribosome-stalled mitochondrial polypeptides that are inaccessible to ubiquitylation due to coupling of translation and translocation.