The infectious agent of
transmissible spongiform encephalopathies is believed to consist of an oligomeric
isoform, PrPSc, of the monomeric cellular
prion protein, PrPC. The conversion of PrPC to PrPSc is characterized by a decrease in alpha-helical structure, an increase in beta-sheet content, and the formation of PrPSc
amyloid. Whereas the N-terminal part of PrPC comprising residues 23-120 is flexibly disordered, its C-terminal part,
PrP(121-231), forms a globular domain with three alpha-helices and a small beta-sheet. Because the segment of residues 90-231 is
protease-resistant in PrPSc, it is most likely structured in the PrPSc form. The conformational change of the segment containing residues 90-120 thus constitutes the minimal structural difference between PrPC and a PrPSc 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 beta-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 PrPSc during endocytosis.