Monoculture breeds poor social skills.

Two studies from Jarosz et al. describe how [GAR(+)], a protein-based epigenetic determinant found mainly in wild yeast strains, can be activated by microbial cross-kingdom communication. With the aid of genetically and ecologically diverse bacteria, yeast can override an ancient regulatory mechanism of glucose repression, promoting both microbial diversity and lifespan extension.

New links between SOD1 and metabolic dysfunction from a yeast model of amyotrophic lateral sclerosis.

A number of genes have been linked to familial forms of the fatal motor neuron disease amyotrophic lateral sclerosis (ALS). Over 150 mutations within the gene encoding superoxide dismutase 1 (SOD1) have been implicated in ALS, but why such mutations lead to ALS-associated cellular dysfunction is unclear. In this study, we identify how ALS-linked SOD1 mutations lead to changes in the cellular health of the yeast Saccharomyces cerevisiae We find that it is not the accumulation of aggregates but the loss of Sod1 protein stability that drives cellular dysfunction. The toxic effect of Sod1 instability does not correlate with a loss of mitochondrial function or increased production of reactive oxygen species, but instead prevents acidification of the vacuole, perturbs metabolic regulation and promotes senescence. Central to the toxic gain-of-function seen with the SOD1 mutants examined was an inability to regulate amino acid biosynthesis. We also report that leucine supplementation results in an improvement in motor function in a Caenorhabditis elegans model of ALS. Our data suggest that metabolic dysfunction plays an important role in Sod1-mediated toxicity in both the yeast and worm models of ALS.

Oxidative stress conditions increase the frequency of de novo formation of the yeast [PSI+] prion.

Prions are self-perpetuating amyloid protein aggregates which underlie various neurodegenerative diseases in mammals and heritable traits in yeast. The molecular basis of how yeast and mammalian prions form spontaneously into infectious amyloid-like structures is poorly understood. We have explored the hypothesis that oxidative stress is a general trigger for prion formation using the yeast [PSI(+)] prion, which is the altered conformation of the Sup35 translation termination factor. We show that the frequency of [PSI(+)] prion formation is elevated under conditions of oxidative stress and in mutants lacking key antioxidants. We detect increased oxidation of Sup35 methionine residues in antioxidant mutants and show that overexpression of methionine sulphoxide reductase abrogates both the oxidation of Sup35 and its conversion to the [PSI(+)] prion. [PSI(+)] prion formation is particularly elevated in a mutant lacking the Sod1 Cu,Zn-superoxide dismutase. We have used fluorescence microscopy to show that the de novo appearance of [PSI(+)] is both rapid and increased in frequency in this mutant. Finally, electron microscopy analysis of native Sup35 reveals that similar fibrillar structures are formed in both the wild-type and antioxidant mutants. Together, our data indicate that oxidative stress is a general trigger of [PSI(+) formation, which can be alleviated by antioxidant defenses.

Quantitative Analyses of the Yeast Oxidative Protein Folding Pathway In Vitro and In Vivo.

Aims: Efficient oxidative protein folding (OPF) in the endoplasmic reticulum (ER) is a key requirement of the eukaryotic secretory pathway. In particular, protein folding linked to the formation of disulfide bonds, an activity dependent on the enzyme protein disulfide isomerase (PDI), is crucial. For the de novo formation of disulfide bonds, reduced PDI must be reoxidized by an ER-located oxidase (ERO1). Despite some knowledge of this pathway, the kinetic parameters with which these components act and the importance of specific parameters, such as PDI reoxidation by Ero1, for the overall performance of OPF in vivo remain poorly understood. Results: We established an in vitro system using purified yeast (Saccharomyces cerevisiae) PDI (Pdi1p) and ERO1 (Ero1p) to investigate OPF. This necessitated the development of a novel reduction/oxidation processing strategy to generate homogenously oxidized recombinant yeast Ero1p. This new methodology enabled the quantitative assessment of the interaction of Pdi1p and Ero1p in vitro by measuring oxygen consumption and reoxidation of reduced RNase A. The resulting quantitative data were then used to generate a simple model that can describe the oxidizing capacity of Pdi1p and Ero1p in vitro and predict the in vivo effect of modulation of the levels of these proteins. Innovation: We describe a model that can be used to explore the OPF pathway and its control in a quantitative way. Conclusion: Our study informs and provides new insights into how OPF works at a molecular level and provides a platform for the design of more efficient heterologous protein expression systems in yeast.

[PSI(+)] turns 50.

The year 2015 sees the fiftieth anniversary of the publication of a research paper that underpins much of our understanding of fungal prion biology, namely "ψ, a cytoplasmic suppressor of super-suppressor in yeast" by Brian Cox. Here we show how our understanding of the molecular nature of the [PSI(+)] determinant evolved from an 'occult' determinant to a transmissible amyloid form of a translation termination factor. We also consider the impact studies on [PSI] have had--and continue to have--on prion research. To demonstrate this, leading investigators in the yeast prion field who have made extensive use of the [PSI(+)] trait in their research, provide their own commentaries on the discovery and significance of [PSI].

Fungal prions.

For both mammalian and fungal prion proteins, conformational templating drives the phenomenon of protein-only infectivity. The conformational conversion of a protein to its transmissible prion state is associated with changes to host cellular physiology. In mammals, this change is synonymous with disease, whereas in fungi no notable detrimental effect on the host is typically observed. Instead, fungal prions can serve as epigenetic regulators of inheritance in the form of partial loss-of-function phenotypes. In the presence of environmental challenges, the prion state [PRION(+)], with its resource for phenotypic plasticity, can be associated with a growth advantage. The growing number of yeast proteins that can switch to a heritable [PRION(+)] form represents diverse and metabolically penetrating cellular functions, suggesting that the [PRION(+)] state in yeast is a functional one, albeit rarely found in nature. In this chapter, we introduce the biochemical and genetic properties of fungal prions, many of which are shared by the mammalian prion protein PrP, and then outline the major contributions that studies on fungal prions have made to prion biology.