Heat shock response (HSR) is an important element of cellular homeostasis. In yeast, HSR comprises of the heat shock proteins (Hsps) and the osmolytes trehalose and glycerol. The respective roles of trehalose and Hsp104 in regulating protein aggregation remain ambiguous. We report that trehalose and Hsp104 are important during the early stages of protein aggregation, i.e. when the process is still reversible. This corroborates the earlier reported role of trehalose being an inhibitor of protein folding. Under in vitro conditions, trehalose is able to restore the GdHCl-induced loss of ATPase activity of recombinant Hsp104 to almost its original level. As the saturation phase of aggregation approaches, neither of the two components is able to exert any effect. Inactivation of Hsp104 at the stage when oligomers have already been formed increases the rate of formation of aggregates by inhibiting disaggregation of oligomers. In the absence of an active disaggregase, the oligomers are converted to mature irreversible aggregates, accelerating their formation. Our results suggest that the disaccharide may have a marginally stronger influence than Hsp104 in inhibiting protein aggregation in yeast cells.

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Chronic stress and prolonged activation of defence pathways have deleterious consequences for the cell. Dietary restriction is believed to be beneficial as it induces the cellular stress response machinery. We report here that although the phenomenon is beneficial in a wild-type cell, dietary restriction leads to an inconsistent response in a cell that is already under proteotoxicity-induced stress. Using a yeast model of Huntington’s disease, we show that contrary to expectation, aggregation of mutant huntingtin is exacerbated and activation of the unfolded protein response pathway is dampened under dietary restriction. Global proteomic analysis shows that when exposed to a single stress, either protein aggregation or dietary restriction, the expression of foldases like peptidyl-prolyl isomerase, is strongly upregulated. However, under combinatorial stress, this lead is lost, which results in enhanced protein aggregation and reduced cell survival. Successful designing of aggregation-targeted therapeutics will need to take additional stressors into account.

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Inhibition of endoribonuclease/kinase Ire1 has shown beneficial effects in many proteotoxicity‐induced pathology models. The mechanism by which this occurs has not been elucidated completely. Using a proteotoxic yeast model of Huntington's disease, we show that the deletion of Ire1 led to lower protein aggregation at longer time points. The rate of protein degradation was higher in ΔIre1 cells. We monitored the two major protein degradation mechanisms in the cell. The increase in expression of Rpn4, coding for the transcription factor controlling proteasome biogenesis, was higher in ΔIre1 cells. The chymotrypsin‐like proteasomal activity was also significantly enhanced in these cells at later time points of aggregation. The gene and protein expression levels of the autophagy gene Atg8 were higher in ΔIre1 than in wild‐type cells. Significant increase in autophagy flux was also seen in ΔIre1 cells at later time points of aggregation. The results suggest that the deletion of Ire1 activates UPR‐independent arms of the proteostasis network, especially under conditions of aggravated stress. Thus, the inhibition of Ire1 may regulate UPR‐independent cellular stress‐response pathways under prolonged stress.