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Misfolded proteins are usually refolded to their functional conformations or degraded by quality control mechanisms. When misfolded proteins evade quality control, they can be sequestered to specific sites within cells to prevent the potential dysfunction and toxicity that arises from protein aggregation. Btn2 and Hsp42 are compartment-specific sequestrases that play key roles in the assembly of these deposition sites. Their exact intracellular functions and substrates are not well defined, particularly since heat stress sensitivity is not observed in deletion mutants. We show here that Btn2 and Hsp42 are required for tolerance to oxidative stress conditions induced by exposure to hydrogen peroxide. Btn2 and Hsp42 act to sequester oxidized proteins into defined PQC sites following ROS exposure and their absence leads to an accumulation of protein aggregates. The toxicity of protein aggregate accumulation causes oxidant sensitivity in btn2 hsp42 sequestrase mutants since overexpression of the Hsp104 disaggregase rescues oxidant tolerance. We have identified the Sup35 translation termination factor as an in vivo sequestrase substrate and show that Btn2 and Hsp42 act to suppress oxidant-induced formation of the yeast [PSI+] prion, which is the amyloid form of Sup35. [PSI+] prion formation in sequestrase mutants does not require IPOD (insoluble protein deposit) localization which is the site where amyloids are thought to undergo fragmentation and seeding to propagate their heritable prion form. Instead, both amorphous and amyloid Sup35 aggregates are increased in btn2 hsp42 mutants consistent with the idea that prion formation occurs at multiple intracellular sites during oxidative stress conditions in the absence of sequestrase activity. Taken together, our data identify protein sequestration as a key antioxidant defence mechanism that functions to mitigate the damaging consequences of protein oxidation-induced aggregation. Author summary: Protein misfolding causes protein aggregation which is the abnormal association of proteins into larger aggregate structures. Protein aggregates can be toxic to cells and have been implicated in a wide variety of diseases. Protein sequestration has emerged as a key defence strategy which protects the proteome against an overload of misfolded proteins by partitioning them and protecting against potential cytotoxic effects. Previous studies have identified sequestrase chaperones that act to triage misfolded proteins to various spatially separated deposit sites in cells. We show here that sequestrases play a key role in mitigating the toxicity of protein aggregates specifically formed in response to oxidative stress. Using a yeast model, we show that sequestrases function to suppress oxidant-induced formation of prions, which are infectious agents formed by amyloid aggregates. Localization to the normal amyloid deposition site, termed the IPOD, is not required for prion formation in sequestrase mutants. The IPOD is thought to act as a site where prion proteins form their heritable form, and we propose that the high localized concentrations of multiple non-specific aggregates increases the likelihood of prion induction. This study has broad implications for understanding the role of oxidative protein damage as a trigger of prion and more general protein aggregate formation in eukaryotic cells. [ABSTRACT FROM AUTHOR]

Prions are self-propagating, misfolded forms of proteins associated with various neurodegenerative diseases in mammals and heritable traits in yeast. How prions form spontaneously into infectious amyloid-like structures without underlying genetic changes is poorly understood. Previous studies have suggested that methionine oxidation may underlie the switch from a soluble protein to the prion form. In this current study, we have examined the role of methionine sulfoxide reductases (MXRs) in protecting against de novo formation of the yeast [PSI+] prion, which is the amyloid form of the Sup35 translation termination factor. We show that [PSI+] formation is increased during normal and oxidative stress conditions in mutants lacking either one of the yeast MXRs (Mxr1, Mxr2), which protect against methionine oxidation by reducing the two epimers of methionine-S-sulfoxide. We have identified a methionine residue (Met124) in Sup35 that is important for prion formation, confirming that direct Sup35 oxidation causes [PSI+] prion formation. [PSI+] formation was less pronounced in mutants simultaneously lacking both MXR isoenzymes, and we show that the morphology and biophysical properties of protein aggregates are altered in this mutant. Taken together, our data indicate that methionine oxidation triggers spontaneous [PSI+] prion formation, which can be alleviated by methionine sulfoxide reductases. [ABSTRACT FROM AUTHOR]

A study was conducted to examine the toxic effects of linoleic acid hydroperoxide (LoaOOH) to Saccharomyces cerevisiae using a wild-type yeast and isogenic mutants affected in known antioxidant defense systems. Results reveal the resistance of the bacteria to hydrogen peroxide upon pretreatment of LoaOOH. In addition, it was observed that the yeast's defense systems help in the direct and indirect detoxification of fatty acid hydroperoxides.

A study was conducted on the conditions that promote the survival of yeast cells subjected to freeze-thaw stress. The factors examined involve signal transduction pathways, prior stress treatments, growth phase and mitochondrial functions. Results show that relationship between viability loss and the duration of freezing, suggesting that freeze-thaw damage is due mainly to freezing.

The toxicity of malondialdehyde (MDA) on yeast cells was analyzed to determine the ability of the microorganisms to detoxify the reactive aldehyde. Exposure of wild-type Saccharomyces cerevisiae cells to nonlethal concentrations of MDA and subsequent exposure to lethal doses of the aldehyde induced cellular resistance. Furthermore, adaptation response to MDA was mediated by yAP1 which encoded a yeast transcription activator that was homologous to the mammalian AP-1 protein.