Mutable yeast prion variants are stabilized by a defective Hsp104 chaperone.

Gorkovskiy et al. observed that many [PSI ] prion isolates, obtained in yeast with the mutant Hsp104 chaperone, propagate poorly in wild-type cells and suggested that Hsp104 is part of the cellular anti-prion system, curing many nascent [PSI ] variants. Here, we argue that the concept may require reassessment. We induced [PSI ] variants in both the wild-type and the mutant background. Three new variants were isolated in the T160M background. They exhibited lower thermostability, possessed novel structural features, and were inherently mutable, changing to well-characterized VH, VK, and VL variants in wild-type cells. In contrast, VH, VK, and VL of the wild-type background, could not change freely and were lost in the mutant, due to insufficient chaperone activity. Thus, mutant Hsp104 can impose as much restriction against emerging prion variants as the wild-type protein. Such restriction conserved the transmutable variants in the T160M background, since new structures mis-templated from them could not gain a foothold. We further demonstrate excess Hsp104 or Hsp104 can eliminate nearly all of the [PSI ] variants in their native background. This finding contradicts the generally held belief that Hsp104-induced [PSI ] curing requires its N-terminal domain, and may help settling the current contention regarding how excess Hsp104 cures [PSI ].