Full restoration of specific infectivity and strain properties from pure mammalian prion protein

The protein-only hypothesis predicts that infectious mammalian prions are composed solely of PrP(Sc), a misfolded conformer of the normal prion protein, PrP(C). However, protein-only PrP(Sc) preparations lack significant levels of prion infectivity, leading to the alternative hypothesis that cofactor molecules are required to form infectious prions. Here, we show that prions with parental strain properties and full specific infectivity can be restored from protein-only PrP(Sc) in vitro. The restoration reaction is rapid, potent, and requires bank vole PrP(C) substrate, post-translational modifications, and cofactor molecules. To our knowledge, this represents the first report in which the essential properties of an infectious mammalian prion have been restored from pure PrP without adaptation. These findings provide evidence for a unified hypothesis of prion infectivity in which the global structure of protein-only PrP(Sc) accurately stores latent infectious and strain information, but cofactor molecules control a reversible switch that unmasks biological infectivity.