Prion-based memory of heat stress in yeast

Abstract

Amyloids and amyloid-based prions are self-perpetuating protein aggregates which can spread by converting a normal protein of the same sequence into a prion form. They are associated with diseases in humans and mammals, and control heritable traits in yeast and other fungi. Some amyloids are implicated in biologically beneficial processes. As prion formation generates reproducible memory of a conformational change, prions can be considered as molecular memory devices. We have demonstrated that in yeast, stress-inducible cytoskeleton-associated protein Lsb2 forms a metastable prion in response to high temperature. This prion promotes conversion of other proteins into prions and can persist in a fraction of cells for a significant number of cell generations after stress, thus maintaining the memory of stress in a population of surviving cells. Acquisition of an amino acid substitution required for Lsb2 to form a prion coincides with acquisition of increased thermotolerance in the evolution of Saccharomyces yeast. Thus the ability to form an Lsb2 prion in response to stress coincides with yeast adaptation to growth at higher temperatures. These findings intimately connect prion formation to the cellular response to environmental stresses.

Keywords: Lsb1; Lsb2; Sup35; actin; amyloid; heat shock; prion; ubiquitin; yeast (Saccharomyces cerevisiae).

Figure 1.

Association with Las17 is required for prion induction by Lsb2. A. Lsb2 cannot induce formation of prion [PSI⁺] by overexpression of Sup35 in the strain depleted of Las17 (las17Δ). Depletion of Las17 does not affect formation of [PSI⁺] by overexpression of Sup35 in the presence of prion aggregates of Rnq1 protein ([RNQ⁺]). [PSI⁺] formation is manifested by growth on SD-Ade medium. B. Total levels of Lsb2 protein are slightly decreased in Las17 strain as detected by western blotting. Different time of Lsb2 induction from plasmid copper inducible promoter is indicated. C. Wild type Lsb2 and more stable ubiquitination deficient mutants (K80; K80R, K41R; P124A, P125A) do not form detergent resistant amyloid-like aggregates in the strain depleted of Las17 as detected by SDD-AGE. Lsb2 W91S mutant unable to bind Las17 does not form amyloid-like aggregates in the presence of Las17 (WT).

Figure 2.

Prion–inducing activity of Lsb2 coincides with yeast adaptation to higher growth temperature. Schematic shows phylogenetic relationships among some members of the Saccharomyces sensu stricto genus. The preferred growth temperature of each species is indicated. CLUSTALW-formatted multiple sequence alignment of C-terminal of Lsb1/Lsb2 is shown. Difference in amino acids is indicated in red (Lsb2) and blue (Lsb1). Residue essential for prion induction is bordered. Amyloid stretch hexapeptide is underlined.

Figure 3.

Model “Metastable prion [LSB⁺] controls stress memory.” Subpopulation of cells with [LSB⁺] maintains a memory of stress and is better adapted to it. See comments in the text.