A functional connection between translation elongation and protein folding at the ribosome exit tunnel in Saccharomyces cerevisiae.

Olga Rodríguez-Galán,Jesús De La Cruz,Lars M Steinmetz,Alfonso Méndez-Godoy,Iván V Rosado,Vicent Pelechano,Wu Wei,Alisa Alekseenko,Juan J García-Gómez,Dieter Kressler,Benjamin Pillet

doi:10.1093/nar/gkaa1200

Abstract

Proteostasis needs to be tightly controlled to meet the cellular demand for correctly de novo folded proteins and to avoid protein aggregation. While a coupling between translation rate and co-translational folding, likely involving an interplay between the ribosome and its associated chaperones, clearly appears to exist, the underlying mechanisms and the contribution of ribosomal proteins remain to be explored. The ribosomal protein uL3 contains a long internal loop whose tip region is in close proximity to the ribosomal peptidyl transferase center. Intriguingly, the rpl3[W255C] allele, in which the residue making the closest contact to this catalytic site is mutated, affects diverse aspects of ribosome biogenesis and function. Here, we have uncovered, by performing a synthetic lethal screen with this allele, an unexpected link between translation and the folding of nascent proteins by the ribosome-associated Ssb-RAC chaperone system. Our results reveal that uL3 and Ssb-RAC cooperate to prevent 80S ribosomes from piling up within the 5′ region of mRNAs early on during translation elongation. Together, our study provides compelling in vivo evidence for a functional connection between peptide bond formation at the peptidyl transferase center and chaperone-assisted de novo folding of nascent polypeptides at the solvent-side of the peptide exit tunnel.

Highlights

Protein homeostasis is controlled by a network of cellular mechanisms that ensures the optimal concentration and composition of correctly folded proteins within cells under normal conditions
Our results indicate that this synthetic lethality is not due to an enhancement of the ribosome biogenesis defect caused by the rpl3[W255C] mutation or a significant augmentation of the aggregation propensity associated with the chaperone mutations, but appears to be related to an impairment of early translation elongation
We have uncovered a hitherto unperceived functional link between translation elongation and nascent polypeptide folding by characterizing the negative impact elicited by a specific mutation in r-protein uL3 in combination with the simultaneous absence of individual components of the ribosomeassociated Ssb-ribosome-associated complex (RAC) chaperone triad

Summary

Introduction

Protein homeostasis (proteostasis) is controlled by a network of cellular mechanisms that ensures the optimal concentration and composition of correctly folded proteins within cells under normal conditions. Key components of this network are the molecular chaperones and the cellular protein degradation machineries. The different molecular chaperones act in diverse manners on proteins, they assist the folding of nascent proteins (de novo folding), participate in the refolding of stress-denatured proteins, promote the assembly of oligomeric protein complexes, mediate protein degradation and prevent formation or promote disassembly of protein aggregates [1,2,3]. Loss of proteostasis maintenance is an underlying cause of aging and numerous diseases associated with the accumulation and aggregation of misfolded proteins [4,5].

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