Prions are altered conformation proteins, capable of causing other natively folded proteins to change conformation. Prions are the causative agent in mammalian (including humans) diseases, termed spongiform encephalopathies. Some neurodegenerative diseases pathologies are also thought to be connected to native protein misfolding into prions.
Among unicellular organisms, both are known to form and transmit prions. To date, the best explored system for prions is in yeast model organism Saccharomyces cerevisiae, where prions were discovered before the prion term was even coined [1]. The work in this organism has yield data, that certain genes antagonize or accelerate appearance of prions. Those genes usually code for chaperones and their cofactors. In 2017, a new gene with anti-prion effect was discovered – SIW14. Siw14 is inositol polyphosphate β-pyrophosphatase. The mechanism of anti-prion effect of SIW14 is unknown [2]. A phosphatase in humans with Siw14-like activity is yet to be found.
This work explores the structural requirements of Siw14 protein to antagonize Sup35 misfolding into [PSI+] prion in yeast S. cerevisiae. Mutants, carrying SIW14 mutations (N-terminal, C-terminal/phosphatase domain, catalytically inactive Siw14 with C214S mutation in active site) were introduced in siw14Δ cells overexpressing Sup35-GFP and their ability to antagonize de novo appearance of [PSI+] prion was assessed by ade1 suppression phenotypic test and fluorescent microscopy.
The prionization tests show that functional Siw14 phosphatase domain is necessary for anti-prion activity. N-terminal domain, and catalytically inactive C214S mutant failed to prevent prion formation and had prionization rates similar to that of siw14Δ cells. Fluorescent constructs of Siw14 display diffused localization in cells, irrespective of cell's prion status.
The data shows that phosphatase activity of Siw14 is essential for prion antagonization, but mechanism of suppression is yet to be discovered. Future work should explore how Siw14 prevents prion appearance, if anti-prion system is dependent on inositol pyrophosphates or if actin defects in siw14Δ cell are affecting prion appearance rates.