Sis1 potentiates the stress response to protein aggregation and elevated temperature

Cells adapt to conditions that compromise protein conformational stability by activating various stress response pathways, but the mechanisms used in sensing misfolded proteins remain unclear. Moreover, aggregates of disease proteins often fail to induce a productive stress response. Here, using a yeast model of polyQ protein aggregation, we identified Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress. At elevated levels, Sis1 prevented the formation of dense polyQ inclusions and directed soluble polyQ oligomers towards the formation of permeable condensates. Hsp70 accumulated in a liquid-like state within this polyQ meshwork, resulting in a potent activation of the HSF1 dependent stress response. Sis1, and the homologous DnaJB6 in mammalian cells, also regulated the magnitude of the cellular heat stress response, suggesting a general role in sensing protein misfolding. Sis1/DnaJB6 functions as a limiting regulator to enable a dynamic stress response and avoid hypersensitivity to environmental changes. Transcriptome analysis via mRNA sequencing of WT yeast (YPH499) and yeast overexpressing Sis1, Htt97Q, or Sis1 and Htt97Q, in triplicate.

细胞可通过激活各类应激反应通路，适应破坏蛋白质构象稳定性的环境条件，但目前错误折叠蛋白质的感知机制仍不明确。此外，致病蛋白质的聚集体往往无法诱导有效的应激反应。本研究利用多聚谷氨酰胺（polyQ）蛋白质聚集酵母模型，鉴定出Sis1——一种不可或缺的热休克蛋白70（Hsp70）辅助分子伴侣（属于热休克蛋白40（Hsp40）家族）——作为蛋白毒性应激的关键感受器。当Sis1表达水平升高时，其可抑制致密的polyQ包涵体形成，并引导可溶性polyQ寡聚体形成可渗透的凝聚体。Hsp70以液态样状态在该polyQ网状结构中聚集，从而强效激活依赖热休克因子1（HSF1）的应激反应通路。Sis1以及哺乳动物细胞中的同源蛋白DnaJB6，同样可调控细胞热应激反应的强度，提示二者在感知蛋白质错误折叠过程中具有保守的通用功能。Sis1/DnaJB6作为限速调控因子，可介导动态应激反应并避免对环境变化产生超敏应答。本研究对野生型（wild type, WT）酵母（YPH499）、过表达Sis1的酵母、过表达Htt97Q的酵母，以及共过表达Sis1与Htt97Q的酵母进行了三次生物学重复的mRNA测序，以开展转录组分析。