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Specific binding of normal prion protein to the scrapie form via a localized domain initiates its conversion to the protease‐resistant state

In the transmissible spongiform encephalopathies, normal prion protein (PrP‐sen) is converted to a protease‐resistant isoform, PrP‐res, by an apparent self‐propagating activity of the latter. Here we describe new, more physiological cell‐free systems for analyzing the initial binding and subsequent conversion reactions between PrP‐sen and PrP‐res. These systems allowed the use of antibodies to map the sites of interaction between PrP‐sen and PrP‐res. Binding of antibodies (α219–232) to hamster PrP‐sen residues 219–232 inhibited the binding of PrP‐sen to PrP‐res and the subsequent generation of PK‐resistant PrP. However, antibodies to several other parts of PrP‐sen did not inhibit. The α219–232 epitope itself was not required for PrP‐res binding; thus, inhibition by α219–232 was likely due to steric blocking of a binding site that is close to, but does not include the epitope in the folded PrP‐sen structure. The selectivity of the binding reaction was tested by incubating PrP‐res with cell lysates or culture supernatants. Only PrP‐sen was observed to bind PrP‐res. This highly selective binding to PrP‐res and the localized nature of the binding site on PrP‐sen support the idea that PrP‐sen serves as a critical ligand and/or receptor for PrP‐res in the course of PrP‐res propagation and pathogenesis in vivo.