Yeast Sup35 prion structure: two types, four parts, many variants

The yeast [PSI+] prion, formed by the Sup35 (eRF3) protein, can exist as multiple structural variants exhibiting phenotypic variation in the strength of nonsense suppression and mitotic stability. Structure of [PSI+] and its variation is only partly characterized. Here, we mapped the Sup35 proteinase K-resistant amyloid cores of 26 [PSI+] prions of different origin, isolated from yeast cells. In all cases the Sup35 amino acid residues 2-32 were fully resistant and the region up to residue 72 was partially resistant. Proteinase K-resistant structures were also found within regions 73-124, 125-153 and 154-221, but their presence differed between [PSI+] isolates. The [PSI+] phenotype depended mainly, if not solely, on the structure in region 2-72. Structures in region 73-221 were in some cases mitotically unstable and heterogenous. Two distinct digestion patterns were observed for the 2-72 fragment, which correlated with the \"strong\" and \"weak\" [PSI+] nonsense-suppressor phenotypes. All [PSI+] with a weak pattern were eliminated by multicopy HSP104 gene and were not toxic when combined with multicopy SUP35. [PSI+] with a strong pattern showed opposite properties, being resistant to multicopy HSP104 and lethal in the presence of multicopy SUP35. Thus, our data suggest existence of two distinct and reliably distinguishable structural classes of [PSI+] rather than a continuum of prions with gradually altering phenotype.\n\nImportancePrions and amyloids are relatively novel and incompletely characterized structures. To understand them better, we mapped amyloid cores of 26 isolates of the Sup35 yeast prion using proteinase K digestion and mass spectrometry. We found that these cores are composed of up to four proteinase K-resistant elements spanning almost the whole length of Sup35 region inessential for viability. However, only the N-terminal element was present in all structures. There are many variants of the Sup35 prion, and these are usually roughly combined into two groups, \"strong\" and \"weak\", based on the strength of their nonsense-suppressor phenotype. However, it was not clear whether such groups could be distinguished by any reliable qualitative criteria. Our data indicate that these groups do exist and can be reliably distinguished based on the N-terminal element digestion pattern and the effects of the multicopy SUP35 and HSP104 genes on these prion variants.