Acidic pH and detergents enhance in vitro conversion of human brain PrPc to a PrPsc-like form

In the presence of a low concentration of denaturants or detergents, acidic pH triggers a conformational transition of alpha-helices into beta-sheets in recombinant prion protein (PrP), likely mimicking some aspects of the transformation of host-encoded normal cellular PrP (PrPC) into its pathogenic isoform (PrPSc). Here we observed the effects of acidic pH and guanidine hydrochloride (GdnHCl) on the physicochemical and structural properties of PrPC derived from normal human brain and determined the ability of the acid/GdnHCl-treated PrP to form a proteinase K(PK)-resistant species in the absence and presence of PrPSc template. After treatment with 1.5 M GdnHCl at pH 3.5, PrPC from normal brain homogenates was converted into a detergent-insoluble form similar to PrPSc. Unlike PrPSc, however, the treated brain PrPC was protease-sensitive and retained epitope accessibility to monoclonal antibodies 3F4 and 6H4. Brain PrPC treated with acidic pH/GdnHCl acquired partial PK resistance upon further treatment with low concentrations of sodium dodecyl sulfate (SDS). Formation of this PrPSc-like isoform was greatly enhanced by incubation with trace quantities of PrPSc from Creutzfeldt-Jakob disease brain. Acid/GdnHCl-treated brain PrP may constitute a "recruitable intermediate" in PrPSc formation. Further structural rearrangement seems essential for this species to acquire PK resistance, which can be promoted by the presence of a PrPSc template.