Prion protein helix1 promotes aggregation but is not converted into β-sheet

Abstract

Prion diseases are caused by the aggregation of the native α-helical prion protein PrPC into its pathological β-sheet-rich isoform PrPSc. In current models of PrPSc, helix1 is assumed to be preferentially converted into β-sheet during aggregation of PrP C. This was supported by the NMR structure of PrPC since, in contrast to the isolated helix1, helix2 and helix3 are connected by a small loop and are additionally stabilized by an interhelical disulfide bond. However, helix1 is extremely hydrophilic and has a high helix propensity. This prompted us to investigate the role of helix1 in prion aggregation using humPrP 23-159 including helix1 (144-156) compared with the C-terminal-truncated isoform humPrP23-144 corresponding to the pathological human stop mutations Q160Stop and Y145Stop, respectively. Most unexpectedly, humPrP23-159 aggregated significantly faster compared with the truncated fragment humPrP23-144, clearly demonstrating that helix1 is involved in the aggregation process. However, helix1 is not resistant to digestion with proteinase K in fibrillar humPrP23-159, suggesting that helix1 is not converted to β-sheet. This is confirmed by Fourier transformation infrared spectroscopy since there is almost no difference in β-sheet content of humPrP23-159 fibrils compared with humPrP23-144. In conclusion, we provide strong direct evidence that in contrast to earlier assumptions helix1 is not converted into β-sheet during aggregation of PrPC to PrPSc. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.