Epitope mapping of a PrP(Sc)-specific monoclonal antibody: identification of a novel C-terminally truncated prion fragment.

Monoclonal antibodies (mAbs) against prion proteins (PrPs) are indispensable in research and diagnosis of prion diseases, however the majority of these bind both the cellular (PrP(C)) and the disease-associated (PrP(Sc)) isoforms. According to the widely accepted protein-only hypothesis the two isoforms share the same sequence, but differ in their conformation. In the present study we set to determine the critical binding residues of our PrP(Sc)-specific mAbs with the view of discerning which residues play a key role in the conformational transition between PrP(C) and PrP(Sc). Focussing on the V5B2 mAb that provided differential labelling of prion-affected tissue from individuals positive for transmissible spongiform encephalopathies, we performed alanine scanning and phage-display epitope mapping to elucidate the antigenic determinants of this mAb and gain insight into its specificity on a molecular level. We observed that instead of discriminating between the two prion protein isoforms based on conformational differences, V5B2 binds a previously uncharacterized C-terminally truncated form of PrP(Sc) that ends with the residue Y226, which we named PrP226*. The addition of a single C-terminal amino-acid residue completely abolished V5B2 binding, while Western blots using recombinant full-length PrPs and PrPs terminating at Y226 confirmed that the V5B2 mAb discriminates between the two based on their difference in length.