Pathway Complexity of Prion Protein Assembly into Amyloid*

In vivo under pathological conditions, the normal cellular form of the prion protein, PrP^C (residues 23–231), misfolds to the pathogenic isoform PrP^Sc, a β-rich aggregated pathogenic multimer. Proteinase K digestion of PrP^Sc leads to a proteolytically resistant core, PrP 27–30 (residues 90–231), that can form amyloid fibrils. To study the kinetic pathways of amyloid formation in vitro, we used unglycosylated recombinant PrP corresponding to the proteinase K-resistant core of PrP^Sc and found that it can adopt two non-native abnormal isoforms, a β-oligomer and an amyloid fibril. Several lines of kinetic data suggest that the β-oligomer is not on the pathway to amyloid formation. The preferences for forming either a β-oligomer or amyloid can be dictated by experimental conditions, with acidic pH similar to that seen in endocytic vesicles favoring the β-oligomer and neutral pH favoring amyloid. Although both abnormal isoforms have high β-sheet content and bind 1-anilinonaphthalene-8-sulfonate, they are dissimilar structurally. Multiple pathways of misfolding and the formation of distinct β-sheet-rich abnormal isoforms may explain the difficulties in refolding PrP^Scin vitro, the need for a PrP^Sc template, and the significant variation in disease presentation and neuropathology.