Analysis of Small Critical Regions of Swi1 Conferring Prion Formation, Maintenance, and Transmission

Saccharomyces cerevisiae contains several prion elements, which are epigenetically transmitted as self-perpetuating protein conformations. One such prion is [ SWI + ], whose protein determinant is Swi1, a subunit of the SWI/SNF chromatin-remodeling complex. We previously reported that [ SWI + ] formation results in a partial loss-of-function phenotype of poor growth in nonglucose medium and abolishment of multicellular features. We also showed that the first 38 amino acids of Swi1 propagated [ SWI + ]. We show here that a region as small as the first 32 amino acids of Swi1 (Swi1 1-32 ) can decorate [ SWI + ] aggregation and stably maintain and transmit [ SWI + ] independently of full-length Swi1. Regions smaller than Swi1 1-32 are either incapable of aggregation or unstably propagate [ SWI + ]. When fused to Sup35MC, the [ PSI + ] determinant lacking its PrD, Swi1 1-31 and Swi1 1-32 can act as transferable pr ion d omains (PrDs). The resulting fusions give rise to a novel chimeric prion, [ SPS + ], exhibiting [ PSI + ]-like nonsense suppression. Thus, an NH 2 -terminal region of ∼30 amino acids of Swi1 contains all the necessary information for in vivo prion formation, maintenance, and transmission. This PrD is unique in size and composition: glutamine free, asparagine rich, and the smallest defined to date. Our findings broaden our understanding of what features allow a protein region to serve as a PrD.