Role of mRNA surveillance pathways during oxidative stress in Saccharomyces cerevisiae

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Abstract

Oxidative stress can result in oxidative damage to most cellular macromolecules including DNA, RNA and protein, and this damage has been implicated in ageing and cell death. Not surprisingly therefore, eukaryotic cells contain quality control systems which monitor mRNAs for errors that might cause the production of aberrant proteins. This study focuses on three translation-associated mRNA surveillance pathways which reduce the production of potentially toxic proteins in Saccharomyces cerevisiae: nonsense-mediated decay (NMD), no-go decay (NGD) and nonstop decay (NSD). The data presented in Chapters 3 and 4 indicate that factors required for the recognition of NSD substrates and components of the SKI complex are required for oxidant tolerance. An overlapping requirement for Ski7, which bridges the interaction between the SKI complex and the exosome, and NGD components (Dom34/Hbs1) which have been shown to function in both NSD and NGD was also observed. Additionally, both ski7 dom34 and ski7 hbs1 double mutants are sensitive to hydrogen peroxide stress and accumulate an NSD substrate. A model is presented to explain the generation of NSD substrates where oxidative stress causes aggregation of the Sup35 translation termination factor, which increases stop codon readthrough allowing ribosomes to translate into the 3̢۪-end of mRNAs. Consistent with this model, overexpression of Sup35 was observed to decrease stop codon readthrough and rescued oxidant tolerance in both WT and ski2 mutant strains. In general, our data revealed an unanticipated requirement for the NSD pathway during oxidative stress conditions which reduces the production of aberrant proteins from NSD mRNAs. The data presented in Chapter 5 further examines protein aggregation in mRNA quality control mutants, since aggregation has been implicated in many neurodegenerative diseases including Alzheimer̢۪s. Protein aggregation can be categorized as either disordered amorphous aggregation or highly organized amyloid aggregation. Loss of any mRNA surveillance pathway results in increased widespread protein aggregation and bioinformatic analysis indicates that increased aggregation of aggregation-prone proteins occurs in mRNA surveillance mutants. Moreover, increased [PSI+] and [PIN+] prion formations were uniquely observed in NMD mutants. In Chapter 3, the [PSI+] status improved viability when strains were exposed to hydrogen peroxide but this phenotype is not just limited to oxidative stress but includes other stress conditions such as heat or osmotic shocks as expanded in Chapter 5. Taken together, these data suggest that mRNA surveillance pathways are important for maintaining the integrity of protein production during both normal and stress conditions.

Keyword(s)

NGD; NMD; NSD; Oxidative stress; Saccharomyces cerevisiae; prion

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