Newnam G P, Wegrzyn R D, Lindquist S L, Chernoff Y O
School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA.
Mol Cell Biol. 1999 Feb;19(2):1325-33. doi: 10.1128/MCB.19.2.1325.
The maintenance of [PSI], a prion-like form of the yeast release factor Sup35, requires a specific concentration of the chaperone protein Hsp104: either deletion or overexpression of Hsp104 will cure cells of [PSI]. A major puzzle of these studies was that overexpression of Hsp104 alone, from a heterologous promoter, cures cells of [PSI] very efficiently, yet the natural induction of Hsp104 with heat shock, stationary-phase growth, or sporulation does not. These observations pointed to a mechanism for protecting the genetic information carried by the [PSI] element from vicissitudes of the environment. 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 sporulation. 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 suppression by [PSI] when Hsp104 is expressed at its normal level. In contrast, hsp104 deletion strains lose [PSI] even in the presence of overproduced Ssa1. Overproduction of the unrelated chaperone protein Hsp82 (Hsp90) neither cured [PSI] nor antagonized the [PSI]-curing effect of overproduced Hsp104. Our results suggest it is the interplay between Hsp104 and Hsp70 that allows the maintenance of [PSI] under natural growth conditions.
酵母释放因子Sup35的一种朊病毒样形式[PSI]的维持需要伴侣蛋白Hsp104的特定浓度：Hsp104的缺失或过表达都会使细胞消除[PSI]。这些研究的一个主要谜题是，从异源启动子单独过表达Hsp104能非常有效地使细胞消除[PSI]，然而热休克、稳定期生长或孢子形成对Hsp104的自然诱导却不能。这些观察结果指向了一种保护[PSI]元件携带的遗传信息免受环境变迁影响的机制。在这里，我们表明，热休克蛋白70（Hsp70）家族的一种蛋白Ssa1的同时过表达可保护[PSI]不被Hsp104的过表达消除。Ssa1蛋白属于Ssa亚家族，其成员在热休克、稳定期生长和孢子形成过程中通常会与Hsp104一起被诱导产生。在分子水平上，过量的Ssa1可防止在存在过量Hsp104的情况下Sup35蛋白从不溶性（朊病毒）状态转变为可溶性（细胞）状态。当Hsp104以其正常水平表达时，Ssa1的过表达也会增加[PSI]介导的无义抑制。相比之下，hsp104缺失菌株即使在存在过量产生的Ssa1时也会失去[PSI]。过量产生无关的伴侣蛋白Hsp82（Hsp90）既不能消除[PSI]，也不能拮抗过量产生的Hsp104对[PSI]的消除作用。我们的结果表明，正是Hsp104和Hsp70之间的相互作用使得[PSI]在自然生长条件下得以维持。
