Channel mutations in Hsp104 hexamer distinctively affect thermotolerance and prion-specific propagation

The yeast prion [PSI(+)] represents an aggregated state of the translation termination factor Sup35 resulting in the tendency of ribosomes to readthrough stop codons. In this study, we constructed an auxotrophic chromosomal marker, ura3-197 (nonsense allele), applicable to selection for loss of [PSI(+)] to [psi(-)]. Unlike [psi(-)] yeast strains, [PSI(+)] yeast strains exhibit nonsense suppression of the ura3-197 allele and are not viable in the presence of 5-fluoroorotic acid (5-FOA) that is converted to a toxic material by the readthrough product of Ura3. We selected 20 5-FOA-resistant, loss-of-[PSI(+)], mutants spontaneously or by transposon-mediated mutagenesis from ura3-197[PSI(+)] cells. All of the 20 [psi(-)] isolates were affected in Hsp104, a protein-remodelling factor. Although most of them were disabled in a normal Hsp104 function for thermotolerance, three single mutants, L462R, P557L and D704N, remained thermotolerant. Importantly, L462R and D704N also eliminate other yeast prions [URE3] and [PIN(+)], while P557L does not, suggesting that Hsp104 harbours a unique activity to prion propagation independent of its function in thermotolerance. The mutations that are specific to prion propagation are clustered around the lateral channel of the Hsp104 hexamer, suggesting a crucial and specific role of this channel for prion propagation.