The unfolding of the prion protein sheds light on the mechanisms of prion susceptibility and species barrier.

Prion diseases are a group of fatal neurodegenerative disorders that manifest as infectious, sporadic, or familial and are all associated with the misfolding of the prion protein (PrP). Disease-modulating polymorphisms in the PrP amino acid sequence can make an individual more or less susceptible to infection. One example is the presence of arginine in place of glutamine at position 171 in sheep, which confers resistance to scrapie. To investigate whether the physical folding properties of PrP are influenced by the presence of arginine at codon 171, we have introduced the mutation at the equivalent position (codon 167) in recombinant mouse PrP. We have then compared the unfolding properties of wild-type PrP and the Q167R mutant by monitoring the fluorescence and circular dichroism of folding-sensitive tryptophan mutants. For both wild-type PrP and the Q167R mutant the formation of secondary structure and tertiary structure is concurrent, which indicates that unfolding proceeds without the accumulation of an equilibrium intermediate. The major effect of the mutation is the destabilization of the protein as shown by the shift of the unfolding transition, which can be rationalized from high-resolution structures of PrP. Comparison of the unfolding pathways of mouse and hamster PrP highlights dramatic differences in the mechanisms of folding, which may contribute to the species barrier effect that is observed in the transmission of prion disease.