Sequential Ubiquitination of Ribosomal Protein uS3 Triggers the Degradation of Non-functional 18S rRNA

Takato Sugiyama,Sihan Li,Misaki Kato,Ken Ikeuchi,Atsushi Ichimura,Yoshitaka Matsuo,Toshifumi Inada

doi:10.1016/j.celrep.2019.02.067

Abstract

18S non-functional rRNA decay (NRD) eliminates non-functional 18S rRNA with deleterious mutations in the decoding center. Dissociation of the non-functional 80S ribosome into 40S and 60S subunits is aprerequisite step for degradation of the non-functional 18S rRNA. However, the mechanisms by which the non-functional ribosome is recognized and dissociated into subunits remain elusive. Here, we report that the sequential ubiquitination of non-functional ribosomes is crucial for subunit dissociation. 18S NRD requires Mag2-mediated monoubiquitination followed by Hel2- and Rsp5-mediated K63-linked polyubiquitination of uS3 at the 212th lysine residue. Determination of the aberrant 18S rRNA levels in sucrose gradient fractions revealed that the subunit dissociation of stalled ribosomes requires sequential ubiquitination of uS3 by E3 ligases and ATPase activity of Slh1 (Rqt2), as well as Asc1 and Dom34. We propose that sequential uS3 ubiquitination of the non-functional 80S ribosome induces subunit dissociation by Slh1, leading to degradation of the non-functional 18S rRNA.

Highlights

The ribosome is a crucial platform for precise gene expression, including translation, protein folding, and targeting
Endonucleolytic cleavage of an no-go decay (NGD) reporter mRNA occurs at sites within this disome unit and is dependent on Hel2-mediated K63-linked polyubiquitination of uS10 as well as the activity of the RQT component Slh1 (Ikeuchi et al, 2019). These results suggest that NGD and ribosome-associated quality control (RQC) are coupled via this ubiquitination event
Given that RQC requires ubiquitination of uS10 at K6 and/or K8 residues on the stalled ribosome (Matsuo et al, 2017), we suspected that ubiquitination of ribosomal proteins is involved in 18S nonfunctional rRNA decay (NRD)

Summary

Introduction

The ribosome is a crucial platform for precise gene expression, including translation, protein folding, and targeting. In yeast, Cole et al (2009) and LaRiviere et al (2006) showed that nonfunctional rRNA decay (NRD) quality controls detect and eliminate mature rRNA containing individual point mutations that adversely affect ribosome function possessed by fully assembled ribosomes and ribosomal subunits. With the fundamental but differential functions of the 40S and 60S subunits in translation, NRD quality controls eliminate non-functional subunits in distinct pathways. In NRD, subunit dissociation of the non-functional 80S ribosome is a prerequisite step for degradation of the non-functional subunit. 25S NRD requires an E3 ubiquitin ligase complex, and proteins associated with a non-functional ribosome are ubiquitinated in an Rtt101-Mms1-dependent manner (Fujii et al, 2009), with Crt responsible for substrate recognition of the Rtt101Mms1-containing E3 ligase complex (Sakata et al, 2015). The mechanism whereby the E3 ubiquitin ligase complex recognizes the substrate and ubiquitinates the ribosome and the precise role of ribosome ubiquitination in subunit dissociation remain to be elucidated

Results

Discussion

Conclusion