CONVERSION OF ALPHA-HELICES INTO BETA-SHEETS FEATURES IN THE FORMATION OF THE SCRAPIE PRION PROTEINS

Prions are composed largely, if not entirely, of prion protein (PrP(Sc ) in the case of scrapie). Although the formation of PrP(Sc) from the cellular prion protein (PrP(C)) is a post-translational process, no ca ndidate chemical modification was identified, suggesting that a confor mational change features in PrP(Sc) synthesis. To assess this possibil ity, we purified both PrP(C) and PrP(Sc) by using nondenaturing proced ures and determined the secondary structure of each. Fourier-transform infrared (FTIR) spectroscopy demonstrated that PrP(C) has a high alph a-helix content (42%) and no beta-sheet (3%), findings that were confi rmed by circular dichroism measurements. In contrast, the beta-sheet c ontent of PrP(Sc) was 43% and the alpha-helix 30% as measured by FTIR. As determined in earlier studies, N-terminally truncated PrP(Sc) deri ved by limited proteolysis, designated PrP 27-30, has an even higher b eta-sheet content (54%) and a lower alpha-helix content (21%). Neither PrP(C) nor PrP(Sc) formed aggregates detectable by electron microscop y, while PrP 27-30 polymerized into rod-shaped amyloids. While the for egoing findings argue that the conversion of alpha-helices into beta-s heets underlies the formation of PrP(Sc), we cannot eliminate the poss ibility that an undetected chemical modification of a small fraction o f PrP(Sc) initiates this process. Since PrP(Sc) seems to be the only c omponent of the ''infectious'' prion particle, it is likely that this conformational transition is a fundamental event in the propagation of prions.