Residue-specific mobility changes in soluble oligomers of the prion protein define regions involved in aggregation

Prion (PrP) diseases are neurodegenerative diseases characterized by the formation of β-sheet rich, insoluble and protease resistant protein deposits (called PrPSc) that occur throughout the brain. Formation of synthetic or in vitro PrPSc can occur through on-pathway toxic oligomers. Similarly, toxic and infectious oligomers identified in cell and animal models of prion disease indicate that soluble oligomers are likely intermediates in the formation of insoluble PrPSc. Despite the critical role of prion oligomers in disease progression, little is known about their structure. In order, to obtain structural insight into prion oligomers, we generated oligomers by shaking-induced conversion of recombinant, monomeric prion protein PrPc (spanning residues 90-231). We then obtained two-dimensional solution NMR spectra of the PrPc monomer, a 40% converted oligomer, and a 94% converted oligomer. Heteronuclear single-quantum correlation (1H-15N) studies revealed that, in comparison to monomeric PrPc, the oligomer has intense amide peak signals in the N-terminal (residues 90-114) and C-terminal regions (residues 226-231). Furthermore, a core region with decreased mobility is revealed from residues ~127 to 225. Within this core oligomer region with decreased mobility, there is a pocket of increased amide peak signal corresponding to the middle of α-helix 2 and the loop between α-helices 2 and 3 in the PrPc monomer structure. Using high-resolution solution-state NMR, this work reveals detailed and divergent residue-specific changes in soluble oligomeric models of PrP.