Prion protein amyloid: separation of scrapie infectivity from PrP polymers

The prion protein (PrP) undergoes a profound conformational change when the cellular isoform (PrPc) is converted into the scrapie form (PrPSc). Limited proteolysis of PrPSc produces PrP27-30 which readily polymerizes into amyloid. To study the structure of PrP amyloid, we employed organic solvents that perturb protein conformation. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP), which promotes alpha-helix formation, modified the ultrastructure of rod-shaped PrP amyloids, producing flattened ribbons with a more regular substructure. As the concentration of HFIP was increased, the beta-sheet content and proteinase K resistance of PrP27-30 as well as prion infectivity diminished. HFIP reversibly decreased the binding of Congo red dye to the rods, whereas inactivation of prion infectivity was irreversible. In contrast to 10% HFIP, 1,1,1-trifluoro-2-propanol (TFIP) did not inactivate prion infectivity but, similarly to HFIP, TFIP did alter the morphology of the rods and abolished Congo red binding. Our studies separate prion infectivity from the amyloid properties of PrP27-30 and underscore the dependence of prion infectivity on PrPSc conformation. Our results also demonstrate that the specific beta-sheet-rich structures required for prion infectivity are different from those needed for amyloid formation.