A scrapie-like unfolding intermediate of the prion protein domain PrP(121–231) induced by acidic pH

The infectious agent of transmissible spongiform encephalopathies is believed to consist of an oligomeric isoform, PrP(Sc), of the monomeric cellular prion protein, PrP(C). The conversion of PrP(C) to PrP(Sc) is characterized by a decrease in α-helical structure, an increase in β-sheet content, and the formation of PrP(Sc) amyloid. Whereas the N-terminal part of PrP(C) comprising residues 23–120 is flexibly disordered, its C-terminal part, PrP(121–231), forms a globular domain with three α-helices and a small β-sheet. Because the segment of residues 90–231 is protease-resistant in PrP(Sc), it is most likely structured in the PrP(Sc) form. The conformational change of the segment containing residues 90–120 thus constitutes the minimal structural difference between PrP(C) and a PrP(Sc) monomer. To test whether PrP(121–231) is also capable to undergo conformational transitions, we analyzed its urea-dependent unfolding transitions at neutral and acidic pH. We identified an equilibrium unfolding intermediate of PrP(121–231) that is exclusively populated at acidic pH and shows spectral characteristics of a β-sheet protein. The intermediate is in rapid equilibrium with native PrP(121–231), significantly populated in the absence of urea at pH 4.0, and may have important implications for the presumed formation of PrP(Sc) during endocytosis.