Cell-free formation of protease-resistant prion protein

Abstract

THE infectious agent (or 'prion') of the transmissible spongiform encephalopathies (TSEs) such as scrapie resembles a virus in that it replicates in vivo and has distinct strains¹, but it was postulated long ago to contain only protein^2,3. More recently, PrP^Sc, a pathogenic, scrapie-associated form of the host-encoded prion protein (PrP), was identified as a possible primary TSE agent protein^4–6. PrP^Sc is defined biochemically by its insolubility and resistance to proteases⁷ and is derived post-translationally from normal, protease-sensitive PrP (PrP^c)^8,9. The conversion seems to involve conformational change rather than covalent modification^10–13. However, the conversion mechanism and the relationship of PrP^Sc formation to TSE agent replication remain unclear. Here we report the conversion of PrP^c to protease-resistant forms similar to PrPSc in a cell-free system composed of substantially purified constituents. This conversion was selective and required the presence of preexisting PrP^Sc, providing direct evidence that PrP^Sc derives from specific PrP^c–PrP^Sc interactions.