Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins.

• DOI: 10.1073/pnas.90.23.10962
• Publication Date: 2006-05-31T12:36:22Z
• Authors: K M Pan; M Baldwin; J Nguyen; M Gasset; A Serban; D Groth; I Mehlhorn; Z Huang; R J Fletterick; F E Cohen
• Keywords: 0301 basic medicine; Mesocricetus; PrPSc Proteins; Prions; Circular Dichroism; Immunohistochemistry; Protein Structure, Secondary; Microscopy, Electron; 03 medical and health sciences; Cricetinae; Spectroscopy, Fourier Transform Infrared; Animals; Scrapie

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.