The relationship between visible intracellular aggregates that appear after overexpression of Sup35 and the yeast prion-like elements [PSI⁺] and [PIN⁺]

Overproduced fusions of Sup35 or its prion domain with green fluorescent protein (GFP) have previously been shown to form frequent dots in [PSI⁺] cells. Rare foci seen in [psi^-] cells were hypothesized to indicate the de novo induction of [PSI⁺] caused by the overproduced prion domain. Here, we describe novel ring-type aggregates that also appear in [psi^-] cultures upon Sup35 overproduction and show directly that dot and ring aggregates only appear in cells that have become [PSI⁺]. The formation of either type of aggregate requires [PIN⁺], an element needed for the induction of [PSI⁺]. Although aggregates are visible predominantly in stationary-phase cultures, [PSI⁺] induction starts in exponential phase, suggesting that much smaller aggregates can also propagate [PSI⁺]. Such small aggregates are probably present in [PSI⁺] cells and, upon Sup35-GFP overproduction, facilitate the frequent formation of dot aggregates, but only the occasional appearance of ring aggregates. In contrast, rings are very frequent when [PSI⁺] cultures, including those lacking [PIN⁺], are grown in the presence of GuHCl or excess Hsp104 while overexpressing Sup35-GFP. Thus, intermediates formed during [PSI⁺] curing seem to facilitate ring formation. Surprisingly, GuHCl and excess Hsp104, which are known to promote loss of [PSI⁺], did not prevent the de novo induction of [PSI⁺] by excess Sup35 in [psi^-][PIN⁺] strains.