The Structural Transition of the Prion Protein into its Pathogenic Conformation is Induced by Unmasking Hydrophobic Sites

A series of structural intermediates in the putative pathway from the cellular prion protein PrPC to the pathogenic form PrPSc was established by systematic variation of low concentrations (<0.1%) of the detergent sodium dodecyl sulfate (SDS) or by the interaction with the bacterial chaperonin GroEL. Most extended studies were carried out with recombinant PrP (90-231) corresponding to the amino acid sequence of hamster prions PrP 27-30. Similar results were obtained with full-length recombinant PrP, hamster PrP 27-30 and PrPC isolated from transgenic, non-infected CHO cells. Varying the incubation conditions, i.e. the concentration of SDS, the GroEL and GroEL/ES, but always at neutral pH and room temperature, different conformations could be established. The conformations were characterized with respect to secondary structure as determined by CD spectroscopy and to molecular mass, as determined by fluorescence correlation spectroscopy and analytical ultracentrifugation: α-helical monomers, soluble α-helical dimers, soluble but β-structured oligomers of a minimal size of 12–14 PrP molecules, and insoluble multimers were observed. A high activation barrier was found between the α-helical dimers and β-structured oligomers. The numbers of SDS-molecules bound to PrP in different conformations were determined: Partially denatured, α-helical monomers bind 31 SDS molecules per PrP molecule, α-helical dimers 21, β-structured oligomers 19-20, and β-structured multimers show very strong binding of five SDS molecules per PrP molecule. Binding of only five molecules of SDS per molecule of PrP leads to fast formation of β-structures followed by irreversible aggregation. It is discussed that strongest binding of SDS has an effect identical with or similar to the interaction with GroEL thereby inducing identical or very similar transitions. The interaction with GroEL/ES stabilizes the soluble, α-helical conformation. The structure and their stabilities and particularly the induction of transitions by interaction of hydrophobic sites of PrP are discussed in respect to their biological relevance.