The Rqc2/Tae2 subunit of the ribosome-associated quality control (RQC) complex marks ribosome-stalled nascent polypeptide chains for aggregation.

Ryo Yonashiro,Maria Khokhrina,John R Yates,Yu Kigoshi-Tansho,Axel Mogk,Bernd Bukau,Kai-Chun Chen,Steve A Kay,Holger Lorenz,Claudio Ap Joazeiro,Mario H Bengtson,Erich B Tahara,Elizabeth A Craig,Jeffrey N Savas

doi:10.7554/elife.11794

Abstract

Ribosome stalling during translation can potentially be harmful, and is surveyed by a conserved quality control pathway that targets the associated mRNA and nascent polypeptide chain (NC). In this pathway, the ribosome-associated quality control (RQC) complex promotes the ubiquitylation and degradation of NCs remaining stalled in the 60S subunit. NC stalling is recognized by the Rqc2/Tae2 RQC subunit, which also stabilizes binding of the E3 ligase, Listerin/Ltn1. Additionally, Rqc2 modifies stalled NCs with a carboxy-terminal, Ala- and Thr-containing extension-the 'CAT tail'. However, the function of CAT tails and fate of CAT tail-modified ('CATylated') NCs has remained unknown. Here we show that CATylation mediates formation of detergent-insoluble NC aggregates. CATylation and aggregation of NCs could be observed either by inactivating Ltn1 or by analyzing NCs with limited ubiquitylation potential, suggesting that inefficient targeting by Ltn1 favors the Rqc2-mediated reaction. These findings uncover a translational stalling-dependent protein aggregation mechanism, and provide evidence that proteins can become specifically marked for aggregation.

Highlights

Under various circumstances, translating ribosomes can halt nascent polypeptide chain (NC) elongation and become stalled, such as upon translation of mRNA templates lacking stop codons, containing sequential suboptimal codons, or encoding homopolymeric Lys tracts (Wang et al, 2015; Comyn et al, 2014; LykkeAndersen and Bennett, 2014)
To prevent these undesirable consequences from taking place, stalled ribosomes are rescued by factors that split the subunits, releasing the mRNA, the 40S subunit, and the 60S subunit stalled with a nascent peptidyl-tRNA conjugate, which is targeted by the ribosome-associated quality control (RQC) complex
The results presented so far indicate that NCs can be assembled into aggregates through a process triggered by ribosome stalling and requiring CAT tail synthesis by Rqc2

Summary

Introduction

Under various circumstances, translating ribosomes can halt NC elongation and become stalled, such as upon translation of mRNA templates lacking stop codons, containing sequential suboptimal codons, or encoding homopolymeric Lys tracts (Wang et al, 2015; Comyn et al, 2014; LykkeAndersen and Bennett, 2014). Ribosome stalling poses a problem, as it can both reduce the pool of translation-competent ribosomes and give rise to aberrant—and potentially toxic—nascent polypeptide chains (NCs). To prevent these undesirable consequences from taking place, stalled ribosomes are rescued by factors that split the subunits, releasing the mRNA (for degradation by the exosome), the 40S subunit, and the 60S subunit stalled with a nascent peptidyl-tRNA conjugate, which is targeted by the RQC complex The chain of amino acids gradually lengthens as the protein forms, yet remains attached to the ribosome until the protein is complete

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Conclusion