Yeast Chaperone Hsp70-Ssb Modulates a Variety of Protein-Based Heritable Elements.

Prions are transmissible self-perpetuating protein isoforms associated with diseases and heritable traits. Yeast prions and non-transmissible protein aggregates (mnemons) are frequently based on cross-β ordered fibrous aggregates (amyloids). The formation and propagation of yeast prions are controlled by chaperone machinery. Ribosome-associated chaperone Hsp70-Ssb is known (and confirmed here) to modulate formation and propagation of the prion form of the Sup35 protein [ PSI + ]. Our new data show that both formation and mitotic transmission of the stress-inducible prion form of the Lsb2 protein ([ LSB + ]) are also significantly increased in the absence of Ssb. Notably, heat stress leads to a massive accumulation of [ LSB + ] cells in the absence of Ssb, implicating Ssb as a major downregulator of the [ LSB + ]-dependent memory of stress. Moreover, the aggregated form of Gγ subunit Ste18, [ STE + ], behaving as a non-heritable mnemon in the wild-type strain, is generated more efficiently and becomes heritable in the absence of Ssb. Lack of Ssb also facilitates mitotic transmission, while lack of the Ssb cochaperone Hsp40-Zuo1 facilitates both spontaneous formation and mitotic transmission of the Ure2 prion, [ URE3 ]. These results demonstrate that Ssb is a general modulator of cytosolic amyloid aggregation, whose effect is not restricted only to [ PSI + ].