Ssa1 Overexpression and [PIN] Variants Cure [PSI] by Dilution of Aggregates

Several cellular chaperones have been shown to affect the propagation of the yeast prions PSI⁺], PIN⁺] and URE3]. Ssa1 and Ssa2 are Hsp70 family chaperones that generally cause pro-PSI⁺] effects, since dominant-negative mutants of Ssa1 or Ssa2 cure PSI⁺], and overexpression of Ssa1 enhances de novo PSI⁺] appearance and prevents curing by excess Hsp104. In contrast, Ssa1 was shown to have anti-URE3] effects, since overexpression of Ssa1 cures URE3]. Here we show that excess Ssa1 or Ssa2 can also cure PSI⁺]. This curing is enhanced in the presence of PIN⁺]. During curing, Sup35-GFP fluorescent aggregates get bigger and fewer in number, which leads to their being diluted out during cell division, a phenotype that was also observed during the curing of PSI⁺] by certain variants of PIN⁺]. The sizes of the detergent-resistant PSI⁺] prion oligomers increase during PSI⁺] curing by excess Ssa1. Excess Ssa1 likewise leads to an increase in oligomer sizes of low, medium and very high PIN⁺] variants. While these phenotypes are also caused by inhibition of Hsp104 or Sis1, the overexpression of Ssa1 did not cause any change in Hsp104 or Sis1 levels.