A Unique Platform to Study the Dynamic Process of Prion Formation.

The pathogenic conversion of the cellular prion protein (PrPC) into the β-sheet-rich isoform PrPSc is the pivotal pathogenic event in prion disease, yet the molecular steps that govern this structural transition remain elusive. In this study, we introduce a new approach to monitor site-specific conformational transitions that occur during infectious prion formation. The method relies on genetically encoded substitution of a fluorescent, environmentally sensitive noncanonical amino acid, l-(7-hydroxycoumarin-4-yl)ethylglycine (7-HCAA), into recombinant PrP substrate molecules, allowing real-time monitoring of structural changes in high-efficiency in vitro PrPSc conversion reactions. As proof of principle, we show that the W99 7-HCAA recPrP substrate efficiently propagates two different PrPSc conformers (infectious cofactor PrPSc and noninfectious protein-only PrPSc). Bioassays in knock-in mice expressing bank vole PrP confirm that W99 7-HCAA cofactor PrPSc produced by serial propagation is infectious, causing scrapie with an incubation period and neuropathological profile like those induced by wild-type cofactor PrPSc. Marked differences in fluorescence intensity were observed between native, misfolded, and denatured states of W99 7-HCAA PrP, confirming that 7-HCAA reports on local changes in PrP conformation. Together, these findings establish 7-HCAA as a site-specific and sensitive probe of local PrP conformation. Moreover, the results suggest a new strategy for studying conformational dynamics in amyloid-forming proteins.