Prion diseases are associated with the conversion of the ␣-helix rich prion protein (PrP C ) into a ␤-structure-rich insoluble conformer (PrP Sc ) that is thought to be infectious. The mechanism for the PrP C 3 PrP Sc conversion and its relationship with the pathological effects of prion diseases are poorly understood, partly because of our limited knowledge of the structure of PrP Sc . In particular, the way in which mutations in the PRNP gene yield variants that confer different susceptibilities to disease needs to be clarified. We report here the 2.5-Å-resolution crystal structures of three scrapie-susceptibility ovine PrP variants complexed with an antibody that binds to PrP C and to PrP Sc ; they identify two important features of the PrP C 3 PrP Sc conversion. First, the epitope of the antibody mainly consists of the last two turns of ovine PrP second ␣-helix. We show that this is a structural invariant in the PrP C 3 PrP Sc conversion; taken together with biochemical data, this leads to a model of the conformational change in which the two PrP C Cterminal ␣-helices are conserved in PrP Sc , whereas secondary structure changes are located in the N-terminal ␣-helix. Second, comparison of the structures of scrapie-sensitivity variants defines local changes in distant parts of the protein that account for the observed differences of PrP C stability, resistant variants being destabilized compared with sensitive ones. Additive contributions of these sensitivity-modulating mutations to resistance suggest a possible causal relationship between scrapie resistance and lowered stability of the PrP protein.

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A key feature of prion encephalopathies is the accumulation of a misfolded form of the host glycoprotein PrP. Cell-free and cell culture studies have shown that the efficiency of conversion of PrP into the disease-associated form is influenced by its amino acid sequence and also by its carbohydrate moiety. Here, we characterize four novel glycoform-dependent monoclonal antibodies raised against prokaryotic recombinant sheep PrP. We demonstrate that these antibodies discriminate the PrP monoglycosylated species, since two of them recognize molecules that have the first Asn glycosylation site occupied (mono1) while the other two recognize molecules glycosylated at the second site (mono2). Remarkably, the recognition of PrP by the anti-mono2 antibodies was strongly influenced by the amino acid present at position 171, i.e., either Gln or Arg. This polymorphism is known to be the main determinant of susceptibility and resistance to scrapie in sheep. Altogether, our findings lead us to propose that each glycan chain controls the accessibility of PrP determinants located close upstream from their attachment site. The monoglycoform-assigned and the allotype-restricted antibodies described here, the first to date, should provide further opportunities to investigate the involvement of each glycan chain in PrP conversion in relation to prion strain diversity and the basis of the resistance conferred by the Arg-171 amino acid.

Choline acetyltransferase (ChAT) and choline transport are decreased after nitrosative stress. ChAT activity is altered in scrapie-infected neurons, where oxidative stress develops.

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