Yeast Hsp78 plays an essential role in adapting to severe ethanol stress via mild ethanol stress pretreatment in mitochondrial protein quality control

Severe ethanol stress (10 % v/v) causes the denaturation and aggregation of certain mitochondrial proteins, such as aconitase (Aco1), forming the deposits of unfolded mitochondrial proteins (DUMPs) in the budding yeast Saccharomyces cerevisiae. Pre-exposing yeast cells to mild stress often induces adaptation to subsequent severe stress. However, whether pre-exposing yeast cells to mild ethanol stress mitigates mitochondrial protein aggregation remains unclear. Therefore, in this study, we examined the effects of pre-exposing yeast cells to mild ethanol stress on the yeast mitochondrial protein quality control (mtPQC) system under severe ethanol stress. Pretreatment with 6 % (v/v) ethanol significantly mitigated the formation of DUMPs and Aco1 aggregates under subsequent 10 % ethanol stress in wild-type cells but not in hsp78∆ and mdj1∆ cells. Pretreatment with 6 % ethanol increased the protein levels of mtPQC-related factors, Hsp78, Mdj1, and Hsp10; however, hsp78∆ cells showed significantly lower levels of Ssc1 (mtHsp70) and its co-chaperone Mdj1 than wild-type cells. Moreover, intracellular reactive oxygen species levels and the frequency of respiration-deficient mutants under 10 % ethanol stress were reduced after pretreatment with 6 % ethanol in wild-type cells but not in hsp78∆ cells. Overall, this study demonstrated that pre-exposing yeast cells to mild ethanol stress mitigated ethanol-induced mitochondrial damage by activating the mtPQC system, including HSP78 expression, providing novel insights into the effects of ethanol stress on mitochondria and the corresponding responses in yeast.