Analysis of [ SWI + ] formation and propagation events

Summary The budding yeast, Saccharomyces cerevisiae , harbors several prions that are transmitted as altered, heritable protein conformations. [ SWI + ] is one such prion whose determinant is Swi1, a subunit of the evolutionarily conserved chromatin‐remodeling complex SWI/SNF. Despite the importance of Swi1, the molecular events that lead to [ SWI + ] prionogenesis remain poorly understood. In this study, we have constructed floccullin‐promoter‐based URA3 reporters for [ SWI + ] identification. Using these reporters, we show that the spontaneous formation frequency of [ SWI + ] is significantly higher than that of [ PSI + ] (prion form of Sup35). We also show that preexisting [ PSI + ] or [ PIN + ] (prion form of Rnq1), or overproduction of Swi1 prion‐domain (PrD) can considerably promote Swi1 prionogenesis. Moreover, our data suggest a strain‐specific effect of overproduction of Sse1 – a nucleotide exchange factor of the molecular chaperone Hsp70, and its interaction with another molecular chaperone Hsp104 on [ SWI + ] maintenance. Additionally, we show that Swi1 aggregates are initially ring/ribbon‐like then become dot‐like in mature [ SWI + ] cells. In the presence of [ PSI + ] or [ PIN + ], Swi1 ring/ribbon‐like aggregates predominantly colocalize with the Sup35 or Rnq1 aggregates; without a preexisting prion, however, such colocalizations are rarely seen during Swi1‐PrD overproduction‐promoted Swi1 prionogenesis. We have thus demonstrated a complex interacting mechanism of yeast prionogenesis.