The 70‐kilodalton heat‐shock proteins of the SSA subfamily negatively modulate heat‐shock‐induced accumulation of trehalose and promote recovery from heat stress in the yeast, Saccharomyces cerevisiae

In the yeast, Saccharomyces cerevisiae , the disaccharide trehalose is a stress‐related metabolite that accumulates upon exposure of cells to heat shock or a variety of non‐heat inducers of the stress response. Here, we describe the influence of mutations in individual heat‐shock‐protein genes on trehalose metabolism. A strain mutated in three proteins of the SSA subfamily of 70‐kDa heat‐shock proteins (hsp70) overproduced trehalose during heat shock at 37°C or 40°C and showed abnormally slow degradation of trehalose upon temperature decrease from 40°C to 27°C. The mutant cells were unimpaired in the induction of thermotolerance; however, the decay of thermotolerance during recovery at 27°C was abnormally slow. Since both a high content of trehalose and induced thermotolerance are associated with the heat‐stressed state of cells, the abnormally slow decline of trehalose levels and thermotolerance in the mutant cells indicated a defect in recovery from the heatstressed state. A similar albeit minor defect, as judged from measurements of trehalose degradation during recovery, was detected in a hsp 104 mutant, but not in a strain deleted in the polyubiquitin gene, UB14 . In all our experiments, trehalose levels were closely correlated with thermotolerance, suggesting a thermoprotective function of trehalose. In contrast, heat‐shock proteins, in particular hsp70, appear to be involved in recovery from the heat‐stressed state rather that in the acquisition of thermotolerance. Cells partially depleted of hsp70 displayed an abnormally low activity of neutral trehalase when shifted to 27°C after heat shock at 40°C. Trehalase activity is known to be under positive control by cAMP‐dependent protein kinases, suggesting that hsp70 directly or indirectly stimulate these protein‐kinase activities. Alternatively, hsp70 may physically interact with neutral trehalase, thereby protecting the enzyme from thermal denaturation.