Conformational variations in an infectious protein determine prion strain differences

A remarkable feature of prion biology is the strain phenomenon wherein prion particles apparently composed of the same protein lead to phenotypically distinct transmissible states(1-4). To reconcile the existence of strains with the 'protein-only' hypothesis of prion transmission, it has been proposed that a single protein can misfold into multiple distinct infectious forms, one for each different strain(1-3,5). Several studies have found correlations between strain phenotypes and conformations of prion particles(6-10); however, whether such differences cause or are simply a secondary manifestation of prion strains remains unclear, largely due to the difficulty of creating infectious material from pure protein(3,5). Here we report a high-efficiency protocol for infecting yeast with the [PSI+] prion using amyloids composed of a recombinant Sup35 fragment (Sup-NM). Using thermal stability and electron paramagnetic resonance spectroscopy, we demonstrate that Sup-NM amyloids formed at different temperatures adopt distinct, stably propagating conformations. Infection of yeast with these different amyloid conformations leads to different [PSI+] strains. These results establish that Sup-NM adopts an infectious conformation before entering the cell fulfilling a key prediction of the prion hypothesis(5)-and directly demonstrate that differences in the conformation of the infectious protein determine prion strain variation.