Antagonistic interactions between yeast chaperones Hsp104 and Hsp70 in prion curing

The maintenance of [PSI], a prion-like form of the yeast release factor Sup 35, requires a specific concentration of the chaperone protein Hsp104: eith er deletion or overexpression of Hsp104 will cure cells of [PSI]. A major p uzzle of these studies was that overexpression of Hsp104 alone, from a hete rologous promoter, cures cells of [PSI] very efficiently, yet the natural i nduction of Hsp104 with heat shock, stationary-phase growth, or sporulation does not. These observations pointed to a mechanism for protecting the gen etic information carried by the [PSI] element from vicissitudes of the envi ronment. Here, we show that simultaneous overexpression of Ssa1, a protein of the Hsp70 family, protects [PSI] from curing by overexpression of Hsp104 . Ssa1 protein belongs to the Ssa subfamily, members of which are normally induced with Hsp104 during heat shock, stationary-phase growth, and sporula tion. At the molecular level, excess Ssa1 prevents a shift of Sup35 protein from the insoluble (prion) to the soluble (cellular) state in the presence of excess Hsp104. Overexpression of Ssa1 also increases nonsense suppressi on by [PSI] when Hsp104 is expressed at its normal level. In contrast, hsp1 04 deletion strains lose [PSI] even in the presence of overproduced Ssa1. O verproduction of the unrelated chaperone protein Hsp82 (Hsp90) neither cure d [PSI] nor antagonized the [PSI] -curing effect of overproduced Hsp104. Ou r results suggest it is the interplay between Hsp104 and Hsp70 that allows the maintenance of [PSI] under natural growth conditions.