Differential stability of the bovine prion protein upon urea unfolding

Prion diseases, or transmissible spongiform encephalopathies, are a group of infectious neurological diseases associated with the structural conversion of an endogenous protein (PrP) in the central nervous system. There are two major forms of this protein: the native and noninfectious cellular form, PrPC; and the misfolded, infectious, and proteinase K-resistant form, PrPSc. The C-terminal domain of PrPC is mainly alpha-helical in structure, whereas PrPSc in known to aggregate into an assembly of beta-sheets, forming amyloid fibrils. To identify the regions of PrPC potentially involved in the initial steps of the conversion to the infectious conformation, we have used high-resolution NMR spectroscopy to characterize the stability and structure of bovine recombinant PrPC (residues 121 to 230) during unfolding with the denaturant urea. Analysis of the 800 MHz H-1 NMR spectra reveals region-specific information about the structural changes occurring upon unfolding. Our data suggest that the dissociation of the native beta-sheet of PrPC is a primary step in the urea-induced unfolding process, while strong hydrophobic interactions between helices alpha 1 and alpha 3, and between alpha 2 and alpha 3, stabilize these regions even at very high concentrations of urea.