Conformational pH dependence of intermediate states during oligomerization of the human prion protein.

Intermediate states are key to understanding the molecular mechanisms governing protein misfolding. The human prion protein (PrP) can follow various misfolding pathways, and forms a soluble beta-sheet-rich oligomer under acidic, mildly denaturing, high salt conditions. Here we describe a fast conformational switch from the native alpha-monomer to monomeric intermediate states under oligomer-forming conditions, followed by a slower oligomerization process. We observe a pH dependence of the secondary structure of these intermediate forms, with almost native-like alpha-helical secondary structure at pH 4.1 and predominantly beta-sheet characteristics at pH 3.6. NMR spectroscopy differentiates these intermediate states from the native protein and indicates dynamic rearrangements of secondary structure elements characteristic of a molten globule. The alpha-helical intermediate formed at pH 4.1 can convert to the beta-sheet conformation at pH 3.6 but not vice versa, and neither state can be reconverted to an alpha-monomer. The presence of methionine rather than valine at codon 129 accelerates the rate of oligomer formation from the intermediate state.