Mechanism of Yeast Prion Protein Aggregation and Strain Formation

Misfolding and aggregation of the prion protein (PrP) causes fatal neurodegenerative diseases in many mammalian species, including humans. Mutations in the gene encoding PrP are associated with ∼15% of the incidences, while, the vast majority of the cases are sporadic. Interestingly, prion diseases also display pathological variation, suggesting that there are multiple different strains. To elucidate the mechanism of prion protein aggregation and strain formation, I have taken advantage of the yeast prions [PSI]] and [ RNQ]] and their protein determinants Sup35p and Rnq1p, respectively. Using a Sup35-PrP chimera, I have investigated the effect of the disease associated oligopeptide repeat domain (ORD) expansions of PrP, on prion propagation and amyloid fiber formation. We previously determined that these chimeric proteins maintain the [PSI]] yeast prion phenotype. Interestingly, we noted that the repeat expanded chimeric prions maintained a stronger strain of [PSI]]. Investigations of the chimeric proteins in vitro revealed that repeat-expansions also increase aggregation propensity. However, despite the increased aggregation propensity of the repeat expanded proteins, there was no corresponding increase in the stability of the fibers. Therefore, we predicted that the repeat expansions may spontaneously convert to [ PRION]] with a much higher frequency. Contrary to our prediction, we observed that enhanced conversion of the repeat expanded chimeras only occurred in the presence of another prion, [RNQ]] prion. The [RNQ]] prion has previously been implicated in the de novo induction of the [PSI]] prion, and therefore also is referred to as [PIN]] for PSI Inducible. However, the interaction of [PSI]] and [RNQ]] prions has not been clearly defined and the physical basis for the strains of [RNQ]] has been largely unexplored. I have, for the first time, been able to create different strains of [ RNQ]] in vitro. Rnq1p prion forming domain (Rnq1p PFD) can form fibers that template the conversion of monomeric protein into amyloid fibers. Further, Rnq1p PFD has a shorter lag phase for fiber formation at 37C when compared to 25C. Surprisingly, increasing the temperature at which fibers are formed also increased the stability of these fibers. Additionally, the morphology of the fibers is dramatically altered at different temperatures. These distinct biochemical properties manifest as different distributions of [RNQ]] strains when the fibers are transformed into yeast cells. Transformants of Rnq1p PFD fibers resulted in weak, medium and strong [RNQ]] strains. The amount of aggregated protein and the ability to propagate the prion increased with the strength of the prion strain. However, the [PIN]] prion phenotype did not correlate with the [RNQ]] prion strength. The weak and strong [ RNQ]] both induced the [PSI]] prion with equally higher efficiencies when compared to the medium [RNQ]]. Coincidently, the weak [RNQ]] strain has very similar properties to the previously identified very high [PIN]] strain suggesting an incongruent relationship between [RNQ]] and [ PIN]] associated phenotypes. Determining the biochemical properties of the Rnq1p PFD fibers and their ability to induce specific distributions of prion strains has enabled us to dissect the mechanism for the [RNQ]] prion strains. Additionally, I have been able to distinguish the mechanisms involved in determining the [PIN]] prion strains from the [RNQ]] strains. Incorporating the mechanism of strain formation elucidated by investigating [RNQ]] strains, with the previously proposed model for [ PSI]] strains, has provided us a framework to understand both general and...