Studies of Sup35p : a yeast prion protein

[P S f] is a protein-based heritable phenotype of the yeast Saccharomyces cerevisiae that reflects the prion-like properties of the chromosome-encoded protein Sup35p. This protein is known to be an essential eukaryote polypeptide release factor, namely eRF3. In a /P S f] strain, the prion conformer of Sup35p exists predominantly as large oligomers, which results in the intracellular depletion of functional release factor (i.e. eRF3) and hence inefficient translation termination. Intriguingly, the prion conformer of Sup35p can be eliminated from [PSI+] strains by growth in the presence of the protein denaturant guanidine hydrochloride (GuHCl). Strains are ‘cured’ of [P S f] by millimolar concentrations of GuFICl, well below that normally required for protein denaturation. It was shown that the kinetics of GuHCl-induced curing fit a segregational model, whereby the heritable [P S f ] determinant is diluted from a culture following the total inhibition of prion replication. A hypothesis for the mechanism of curing is proposed namely that the guanidinium cation inhibits an arginine-modifying enzyme, whose action is required for the post-translational modification of Sup35p and ultimately [P S f] maintenance. The [P S f] determinant does not elicit a disease state in yeast, rather it was shown to confer a selective phenotypic advantage namely enhanced stress tolerance. Moreover, it was demonstrated that the efficiency of translation termination is regulated by environmental stress through a prion-mediated mechanism. This study has addressed the relationship between Sup35p, [PSP] and stress proteins of S.cerevisiae and revealed that prion proteins are not simply pathogenic misshapen proteins and that they may serve as a novel means to regulate many cellular processes in fungi

Factors affecting de novo formation of a yeast prion

Prions are aggregates of misfolded proteins that have acquired an amyloid-like structure and ability to propagate through recruitment of new proteins. [PSI+], a prion form of eukaryotic release factor Sup35 (eRF1) is widely used as a model for research on prion formation and propagation and in this study [PSI+] is used to explore an effect of three previously identified proteins on de novo prion formation. One mechanism proposed to affect prion formation is direct interaction of Sup35p with its binding partners and search for proteins that interact with Sup35p identified Ppq 1 p, a putative Ser/Thr protein phosphatase (M.F. Tuite and T. von der Haar). Another approach was to identify proteins that function to protect translational apparatus from environmental and . endogenous oxidative damage. and this approach identified two ribosome associated peroxiredoxins, Tsa1 p and Tsa2p (T. Sideri and C.M. Grant). The data presented here shows that the deletion of PPQ1 gene greatly increases the rate of de novo formation of [PSI+] but the mechanism by which loss of Ppq1 p affects [PSI+] formation is not known. Analysis of the distribution of fluorescently-tagged Ppq 1 P showed that the protein co-localises with mitochondria. A further line of evidence linking Ppq 1 P to mitochondria was an observed reduction in respiratory capacity of a ppq1 Δ strain. That exposure to environmental sources of oxidative stress promotes [PSI+] prion formation was previously reported (Tyedmers et al., 2008). Results presented here show that an endogenous source of oxidative stress, brought about by deleting the ribosomally- associated peroxiredoxins (Prx) encoded by genes TSA 1/2 (Trotter et al., 2008; Sideri et al., 2010), also increases the rate of de novo [PSI+]formation. This result provides a direct link between oxidative stress and the eukaryotic release factor Sup35p