GENETIC-EVIDENCE FOR A FUNCTIONAL-RELATIONSHIP BETWEEN HSP104 AND HSP70

The phenotypes of single Hsp104 and Hsp70 mutants of the budding yeast Saccharomyces cerevisiae provide no clue that these proteins are func tionally related. Mutation of the HSP104 gene severely reduces the abi lity of cells to survive short exposures to extreme temperatures (ther motolerance) but has no effect on growth rates. On the other hand, mut ations in the genes that encode Hsp70 proteins have significant effect s on growth rates but do not reduce thermotolerance. The absence of a thermotolerance defect in S. cerevisiae Hsp70 mutants is puzzling, sin ce the protein clearly plays an important role in thermotolerance in a variety of other organisms. In this report, examination of the phenot ypes of combined Hsp104 and Hsp70 mutants uncovers similarities in the functions of Hsp104 and Hsp70 not previously apparent. In the absence of the Hsp104 protein, Hsp70 is very important for thermotolerance in S. cerevisiae, particularly at very early times after a temperature u pshift. Similarly Hsp104 plays a substantial role in vegetative growth under conditions of decreased Hsp70 protein levels. These results sug gest a close functional relationship between Hsp104 and Hsp70.