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

Abstract

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 β‐sheet‐rich oligomer under acidic, mildly denaturing, high salt conditions. Here we describe a fast conformational switch from the native α‐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 α‐helical secondary structure at pH 4.1 and predominantly β‐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 α‐helical intermediate formed at pH 4.1 can convert to the β‐sheet conformation at pH 3.6 but not vice versa, and neither state can be reconverted to an α‐monomer. The presence of methionine rather than valine at codon 129 accelerates the rate of oligomer formation from the intermediate state.