Hsp104 contributes to freeze-thaw tolerance by maintaining proteasomal activity in a spore clone isolated from Shirakami kodama yeast.

The supply of oven-fresh bakery products to consumers has been improved by frozen dough technology; however, freeze-thaw stress decreases the activity of yeast cells. To breed better baker's yeasts for frozen dough, it is important to understand the factors affecting freeze-thaw stress tolerance in baker's yeast. We analyzed the stress response in IB1411, a spore clone from Saccharomyces cerevisiae Shirakami kodama yeast, with an exceptionally high tolerance to freeze-thaw stress. Genes encoding trehalose-6-phosphate synthase (TPS1), catalase (CTT1), and disaggregase (HSP104) were highly expressed in IB1411 cells even under conditions of non-stress. The expression of Hsp104 protein was also higher in IB1411 cells even under non-stress conditions. Deletion of HSP104 (hsp104Δ) in IB1411 cells reduced the activity of the ubiquitin-proteasome system (UPS). By monitoring the accumulation of aggregated proteins using the ΔssCPY*-GFP fusion protein under freeze-thaw stress or treatment with proteasomal inhibitor, we found that IB1411 cells resolved aggregated proteins faster than the hsp104Δ strain. Thus, Hsp104 seems to contribute to freeze-thaw tolerance by maintaining UPS activity via the disaggregation of aggregated proteins. Lastly, we found that the IB1411 cells maintained high leavening ability in frozen dough as compared with the parental strain, Shirakami kodama yeast, and thus will be useful for making bread.