title: Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins.
abstract: Prions are composed largely, if not entirely, of prion protein (PrPSc in the case of scrapie). Although the formation of PrPSc from the cellular prion protein (PrPC) is a post-translational process, no candidate chemical modification was identified, suggesting that a conformational change features in PrPSc synthesis. To assess this possibility, we purified both PrPC and PrPSc by using nondenaturing procedures and determined the secondary structure of each. Fourier-transform infrared (FTIR) spectroscopy demonstrated that PrPC has a high alpha-helix content (42%) and no beta-sheet (3%), findings that were confirmed by circular dichroism measurements. In contrast, the beta-sheet content of PrPSc was 43% and the alpha-helix 30% as measured by FTIR. As determined in earlier studies, N-terminally truncated PrPSc derived by limited proteolysis, designated PrP 27-30, has an even higher beta-sheet content (54%) and a lower alpha-helix content (21%). Neither PrPC nor PrPSc formed aggregates detectable by electron microscopy, while PrP 27-30 polymerized into rod-shaped amyloids. While the foregoing findings argue that the conversion of alpha-helices into beta-sheets underlies the formation of PrPSc, we cannot eliminate the possibility that an undetected chemical modification of a small fraction of PrPSc initiates this process. Since PrPSc seems to be the only component of the "infectious" prion particle, it is likely that this conformational transition is a fundamental event in the propagation of prions.
authors.affiliation: Department of Neurology, University of California, San Francisco 94143.
authors.forename: K M
authors.initials: KM
authors.lastname: Pan
authors.forename: M
authors.initials: M
authors.lastname: Baldwin
authors.forename: J
authors.initials: J
authors.lastname: Nguyen
authors.forename: M
authors.initials: M
authors.lastname: Gasset
authors.forename: A
authors.initials: A
authors.lastname: Serban
authors.forename: D
authors.initials: D
authors.lastname: Groth
authors.forename: I
authors.initials: I
authors.lastname: Mehlhorn
authors.forename: Z
authors.initials: Z
authors.lastname: Huang
authors.forename: R J
authors.initials: RJ
authors.lastname: Fletterick
authors.forename: F E
authors.initials: FE
authors.lastname: Cohen
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D006224:Cricetinae; D007150:Immunohistochemistry; D008647:Mesocricetus; D008854:Microscopy, Electron; D018620:PrPSc Proteins; D011328:Prions / Q000737:chemistry* / Q000302:isolation & purification; D017433:Protein Structure, Secondary; D012608:Scrapie / Q000209:etiology*; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D018620:PrPSc Proteins; D011328:Prions