The critical step in the pathogenesis of
transmissible spongiform encephalopathies (
prion diseases) is the conversion of a cellular
prion protein (PrP(c)) into a
protease-resistant, beta-sheet rich form (PrP(Sc)). Although the disease transmission normally requires direct interaction between exogenous PrP(Sc) and endogenous PrP(C), the pathogenic process in hereditary
prion diseases appears to develop spontaneously (i.e. not requiring
infection with exogenous PrP(Sc)). To gain insight into the molecular basis of hereditary spongiform
encephalopathies, we have characterized the biophysical properties of the recombinant human
prion protein variant containing the mutation (Phe(198) --> Ser) associated with familial
Gerstmann-Straussler-Scheinker disease. Compared with the wild-type
protein, the F198S variant shows a dramatically increased propensity to self-associate into beta-sheet-rich oligomers. In a
guanidine HCl-containing
buffer, the transition of the F198S variant from a normal alpha-helical conformation into an oligomeric beta-sheet structure is about 50 times faster than that of the wild-type
protein. Importantly, in contrast to the wild-type PrP, the
mutant protein undergoes a spontaneous conversion to oligomeric beta-sheet structure even in the absence of
guanidine HCl or any other denaturants. In addition to beta-sheet structure, the oligomeric form of the
protein is characterized by partial resistance to
proteinase K digestion, affinity for
amyloid-specific
dye,
thioflavine T, and fibrillar morphology. The increased propensity of the F198S variant to undergo a conversion to a PrP(Sc)-like form correlates with a markedly decreased thermodynamic stability of the native alpha-helical conformer of the
mutant protein. This correlation supports the notion that partially unfolded intermediates may be involved in conformational conversion of the
prion protein.