Expanding the yeast prion world

Figures & data

Figure 1. A procedure of genome-wide screen for identification of novel yeast prion proteins. A Q62-Sup35 chimera yeast strain was used to identify QIN factors (upper). A Sup35 protein consists of a N-terminal prion domain (N domain, residues 1–123), a highly charged middle domain (M domain, residues 124–253) and a C-terminal translation termination domain (EF domain, residues 254–685) The Gln/Asn-rich region in the SUP35 N domain (residues 1–40) was replaced by 62 glutamine repeats (Q62). The procedure for the genome-wide screen for novel QIN factors in the [Q⁺] yeast prion system is shown (lower). Expression plasmids from a yeast ORF library were introduced into [q^-]Δrnq1 yeast cells, each yeast protein was overexpressed by galactose and de novo appearance of [Q⁺] was tested by growth on 1/4 YPD and SD-Ade plates. Color and growth phenotypes of [q^-] and [Q⁺] yeasts on 1/4 YPD and SD-Ade plates are shown as negative and positive controls, respectively. (Reproduced from ref. 24.)

Figure 2. Prion conversion as a molecular switch to respond to environmental stress. [mod⁻] yeast contains the Mod5 protein as a soluble and functional monomer while [MOD⁺] yeast contains the Mod5 protein in its aggregated non/less functional form (top, schematic illustration). Mod5-GFP exhibits nuclear and cytoplasmic diffusible localization in [mod⁻] yeast cells, while Mod5-GFP forms cytoplasmic foci in [MOD⁺] yeast cells (middle images). [mod⁻] yeast show the sensitivity to antifungal drugs such as fluconazole and ketoconazole whereas [MOD⁺] yeast are resistant against them (bottom images). Prion conversion between [mod^-] and [MOD⁺] occurs at very low frequency. However, in the presence of environmental stress such as antifungal drugs, [MOD⁺] yeast cells become dominant because of the newly acquired drug resistant phenotype. Once cells are released from the environmental stress, [mod⁻] yeast cells rapidly become dominant due to their growth advantage under normal conditions. Thus, the prion conversion of Mod5 acts as a “molecular switch” for enhanced survival under different conditions.