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-degrees-C or 40-degrees-C and showed abnormally slow degradation of trehalose upon temperature decrease from 40-degrees-C to 27-degrees-C. The mutant cells were unimpaired in the induction of thermotolerance; however, the decay of thermotolerance during recovery at 27-degrees-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 heat-stressed state. A similar albeit minor defect, as judged from measurements of trehalose degradation during recovery, was detected in a DELTAhsp104 mutant, but not in a strain deleted in the polyubiquitin gene, UBI4. 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 than in the acquisition of thermotolerance. Cells partially depleted of hsp70 displayed an abnormally low activity of neutral trehalase when shifted to 27-degrees-C after heat shock at 40-degrees-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.