The only known difference between the cellular (PrP C ) and scrapie-specific (PrP Sc ) isoforms of the prion protein is conformational. Because disruption of PrP Sc structure decreases scrapie infectivity, restoration of the disease-specific conformation should restore infectivity. In this study, disruption of PrP Sc (as monitored by the loss of proteinase K resistance) by guanidine hydrochloride (GdnHCl) resulted in decreased infectivity. Upon dilution of the GdnHCl, protease resistance of PrP was restored and infectivity was regained. The addition of copper facilitated restoration of both infectivity and protease resistance of PrP in a subset of samples that did not renature by the simple dilution of the GdnHCl. These data demonstrate that loss of scrapie infectivity can be a reversible process and that copper can enhance this restoration of proteinase K resistance and infectivity. PrP1 is a highly conserved, host-encoded sialoglycoprotein that may play a role in normal synaptic function and circadian rhythms (1, 2). This host protein (PrP C ) is present in the brain and other tissues and is sensitive to proteinase K (PK) digestion (3). During the course of a scrapie infection, PrP C undergoes a post-translational modification to a disease-specific isoform (PrP Sc ) that has increased resistance to limited PK digestion. This modification appears to be solely conformational with PrP Sc having a higher -sheet content than PrP C (4 -7). In vitro cell culture studies have demonstrated that PrP C is the precursor to PrP Sc (8, 9). Inhibition of the migration of PrP C to the cell surface blocks the formation of PrP Sc , confirming that PrP Sc arises via alternate or misprocessing of PrP Sc (8). PrP Sc is resistant to limited PK digestion (10, 11), but upon treatment with GdnHCl, the PrP denatures (12) and becomes sensitive to protease digestion (13). The loss of PK resistance correlates well with loss of infectivity of the PrP Sc -enriched preparation. In this study, we examined the ability of GdnHCl-denatured, PrP Sc -enriched preparations to renature. Renaturation was monitored by Western blot analysis for the presence of PK-resistant PrP and animal bioassay for infectivity. The capacity of PrP to bind copper led us to determine the effect of this transition metal on the renaturation process. EXPERIMENTAL PROCEDURESDenaturation/Renaturation Reactions-Brains were removed from clinically affected hamsters infected with the 263K strain of hamsteradapted scrapie agent, flash frozen, and stored at Ϫ80°C. PrP Sc -enriched fractions were prepared to the P4 pellet step following a modification of Bolton et al. (14) that excludes the PK digestion. The P4 pellet was resuspended in buffered saline containing glycerol (10 mM Tris, pH 7.4, 130 mM NaCl, 5% glycerol), and the protein concentration was determined by the BCA protein assay (Pierce). The reaction conditions for the denaturation and subsequent renaturation were based upon the cell-free conversion reactions described by Kocisko et al. (13). PrP Sc -enriched preparat...
, the converted products selectively reproduced both characteristics, with the DY conversion product being smaller in size and less resistant to PK digestion than the HY product. These data demonstrate that non-mammalian sources of recombinant HaPrP can be converted into PKresistant form and that strain-mediated properties can be transmitted into the newly formed PrP-res.
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