Yeast Sup35 Prion Structure: Two Types, Four Parts, Many Variants

The yeast [PSI(+)] prion, formed by the Sup35 (eRF3) protein, has multiple structural variants differing in the strength of nonsense suppressor phenotype. Structure of [PSI(+)] and its variation are characterized poorly. Here, we mapped Sup35 amyloid cores of 26 [PSI(+)] ex vivo prions of different origin using proteinase K digestion and mass spectrometric identification of resistant peptides. In all [PSI(+)] variants 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. Two distinct digestion patterns were observed for region 2–72, which always correlated with the “strong” and “weak” [PSI(+)] nonsense suppressor phenotypes. Also, 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 with multicopy SUP35. Thus, Sup35 prion cores can be composed of up to four elements. [PSI(+)] variants can be divided into two classes reliably distinguishable basing on structure of the first element and the described assays.