Cryo-EM structure of ex vivo Sup35 yeast prion and the factors defining prion phenotype

Human prions and amyloids exhibit structural diversity that correlates with different pathological manifestations. Yeast prions represent a convenient model for studying fundamental properties of prions and their strain diversity. The Sup35 yeast prion has multiple structural variants, divided into "strong" and "weak" groups differing in frequency of fibril fragmentation and resulting nonsense-suppression. The molecular origin of these distinctions remains unclear. Here, we show that the different frequency of prion fragmentation is determined by their stability, rather than the efficiency of chaperone binding. The fully protease-resistant part (residues 2-32) of the Sup35 prion core is a key for the prion phenotype and maintenance, whereas partially resistant part (33-72) is less important. We established the cryo-EM structure of a strong Sup35 prion variant purified from yeast, comprising residues 2-64, with 2.7 Å resolution. Comparison of this structure with that of Sup35 amyloid formed in vitro at 4˚C revealed their significant difference.