Is thermotolerance of yeast dependent on trehalose accumulation?

During heat stress, trehalose concentration increases in yeast cells in parallel to thermotolerance. This parallelism suggested that trehalose mediated thermotolerance. We show in this work that, under certain conditions, trehalose accumulation and increase in thermotolerance do not go in parallel. A mutant deficient in the trehalose-degrading neutral trehalase shows, after shift from 40 degrees C to 30 degrees C, low thermotolerance in spite of a high trehalose concentration. When glucose is added to stationary yeast cells with high trehalose concentration and high thermotolerance, trehalose concentration decreases while thermotolerance remains high. A mutant deficient in ubiquitin-conjugating genes, ubc4ubc5, shows during exponential growth a low trehalose concentration, but a high thermotolerance, in contrast to wild-type cells. Because the ubc4ubc5 mutant synthesizes heat-shock proteins constitutively, it is proposed that, under these conditions, accumulation of heat-shock proteins, and not trehalose [corrected], mediates thermotolerance.