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.