Defining Key Residues of the Swi1 Prion Domain in Prion Formation and Maintenance.

Prions are self-perpetuating, alternative protein conformations associated with neurological diseases and normal cellular functions. Saccharomyces cerevisiae contains many endogenous prions, providing a powerful system to study prionization. Previously, we demonstrated that Swi1, a component of the SWI/SNF chromatin-remodeling complex, can form the prion [SWI+]. A small region, Swi11-38, with a unique amino acid composition of low complexity, acts as a prion domain and supports [SWI+] propagation. Here, we further examine Swi11-38 through site-directed mutagenesis. We found that mutations of the two phenylalanine residues or the threonine tract inhibit Swi11-38 aggregation. In addition, mutating both phenylalanines can abolish de novo prion formation by Swi11-38, whereas mutating only one phenylalanine does not. Replacement of half of or the entire eight-threonine tract with alanines has the same effect, possibly disrupting a core region of Swi11-38 aggregates. We also show that Swi11-38 and its prion-fold-maintaining mutants form high-molecular-weight, SDS-resistant aggregates, whereas the double-phenylalanine mutants eliminate these protein species. These results indicate the necessity of the large hydrophobic residues and threonine tract in Swi11-38 in prionogenesis, possibly acting as important aggregable regions. Our findings thus highlight the importance of specific amino acid residues in the Swi1 prion domain in prion formation and maintenance.