Concealment of epitope by reduction and alkylation in prion protein

Conversion of the cellular prion protein (PrPC) into its pathological isoform (PrPSc), the key molecular event in the pathogenesis of prion diseases, is accompanied by a conformational transition of α-helix into β-sheet structures involving α-helix 1 (α1) domain from residues 144 to 154 of the protein. Reduction and alkylation of PrPC have been found to inhibit the conversion of PrPC into PrPSc in vitro. Here we report that while antibody affinity of epitopes in the N- and C-terminal domains remained unchanged, reduction and alkylation of the PrP molecule induced complete concealment of an epitope in α1 for anti-PrP antibody 6H4 that is able to cure prion infection in the cell model. Mass spectrometric analysis of recombinant PrP showed that the alkylation reaction takes place at reduced cysteines but no modification was observed in this cryptic epitope. Our study suggests that reduction and alkylation result in local or global rearrangement of PrP tertiary structure that is maintained in both liquid and solid phases. The implications in the conversion of PrPC into PrPSc and the therapeutics of prion diseases are discussed.