A conserved quality-control pathway that mediates degradation of unassembled ribosomal proteins.

Min-Kyung Sung,Justin M Reitsma,Raymond J Deshaies,Ferdinand M Huber,Michael J Sweredoski,André Hoelz,Tanya R Porras-Yakushi,Sonja Hess

doi:10.7554/elife.19105

Min-Kyung Sung, Justin M Reitsma + Show 6 more

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https://doi.org/10.7554/elife.19105

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Abstract

Overproduced yeast ribosomal protein (RP) Rpl26 fails to assemble into ribosomes and is degraded in the nucleus/nucleolus by a ubiquitin-proteasome system quality control pathway comprising the E2 enzymes Ubc4/Ubc5 and the ubiquitin ligase Tom1. tom1 cells show reduced ubiquitination of multiple RPs, exceptional accumulation of detergent-insoluble proteins including multiple RPs, and hypersensitivity to imbalances in production of RPs and rRNA, indicative of a profound perturbation to proteostasis. Tom1 directly ubiquitinates unassembled RPs primarily via residues that are concealed in mature ribosomes. Together, these data point to an important role for Tom1 in normal physiology and prompt us to refer to this pathway as ERISQ, for excess ribosomal protein quality control. A similar pathway, mediated by the Tom1 homolog Huwe1, restricts accumulation of overexpressed hRpl26 in human cells. We propose that ERISQ is a key element of the quality control machinery that sustains protein homeostasis and cellular fitness in eukaryotes.

Highlights

Protein quality control (PQC) has emerged as a major mechanism for maintaining protein homeostasis and cellular fitness
We evaluated 115 mutant yeast strains, each lacking a different non-essential ubiquitin-proteasome system (UPS) gene, for those that accumulated non-essential ribosomal protein Rpl26a tagged with a FLAG epitope (Rpl26aFLAG) upon its overexpression from the GAL10 promoter
Accumulation of Rpl26aFLAG in most mutants was similar to wild type (WT) and well below the level detected in rpl26aDrpl26bD (Figure 1—figure supplement 1A and B), which accumulated overexpressed Rpl26aFLAG due to lack of competition from endogenous Rpl26 (Sung et al, 2016)

Summary

Introduction

Protein quality control (PQC) has emerged as a major mechanism for maintaining protein homeostasis and cellular fitness. Defects in the cellular machinery that governs PQC cause multiple human diseases including multisystem proteinopathy (Brandmeir et al, 2008; Watts et al, 2004) and Amyotrophic Lateral Sclerosis (ALS) (Johnson et al, 2010; Kabashi and Durham, 2006). Other diseases, such as cancer, can exhibit heightened dependency on PQC pathways, which underlies the hypersensitivity of multiple myeloma cells to proteasome inhibitors (Cenci et al, 2012; Deshaies, 2014). Newly-synthesized human ribosomal proteins are subject to degradation by the proteasome in the

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