It was reported that yeast proteins Ssz1 and Upf1 can cure certain [PSI⁺] variants in wild-type cells and there is a special class of variants whose propagation requires the triple mutation of ssz1∆ upf1∆ Hsp104^T160M. Attempts to isolate variants with the exact properties from the 74-D694 strain (and tested there) are not yet successful. The effort nevertheless leads to an alternative analysis about how ssz1∆ and upf1∆ mutations can help prion propagation. The cellular propagation of the yeast prion [PSI⁺] requires appropriate activities of the Hsp104 disaggregase. Many [PSI⁺] variants isolated in wild-type strains cannot propagate in cells expressing Hsp104^T160M, which has weaker activities. Yet another group of [PSI⁺] variants shows the opposite, propagating well with Hsp104^T160M but is eliminated by the wild-type protein. Deletion of SSZ1 and UPF1 genes in Hsp104^T160M cells generates a just-right environment that supports the propagation of both types of [PSI⁺] variants. The pro-prion effect is not due to the removal of active curing by Ssz1 or Upf1-such curing activity is not observed for the variants. Rather, the double deletion causes a cellular response, which enables more efficient fragmentation of prion fibers, thus remedying the weak activity of Hsp104^T160M. The "Goldilocks" conditioning seems also applicable to other yeast prions. Two [PIN⁺] variants that propagate well with wild-type Hsp104 but poorly with Hsp104^∆N, lacking residues (2-147), can however thrive with the latter if Ssz1 and Upf1 are also deleted from the cell. In this case, the double deletion results in higher Hsp104^∆N expression, leading to improved generation of prion seeds for robust propagation.

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King CY

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