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 []. A small region, Swi1, with a unique amino acid composition of low complexity, acts as a prion domain and supports [] propagation. Here, we further examine Swi1 through site-directed mutagenesis. We found that mutations of the two phenylalanine residues or the threonine tract inhibit Swi1 aggregation. In addition, mutating both phenylalanines can abolish prion formation by Swi1, 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 Swi1 aggregates. We also show that Swi1 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 Swi1 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.