Structural mobility of the human prion protein probed by backbone hydrogen exchange

Prions, the causative agents of Creutzfeldt-Jacob Disease (CJD) in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals, are principally composed of PrP(Sc), a conformational isomer of cellular prion protein (PrP(C)). The propensity of PrP(C) to adopt alternative folds suggests that there maybe an unusually high proportion of alternative conformations in dynamic equilibrium with the native state. However, the rates of hydrogen/deuterium exchange demonstrate that the conformation of human PrP(C) is not abnormally plastic. The stable core of PrP(C) has extensive contributions from all three α-helices and shows protection factors equal to the equilibrium constant for the major unfolding transition. A residual, hyper-stable region is retained upon unfolding, and exchange analysis identifies this as a small nucleus of ~10 residues around the disulfide bond. These results show that the most likely route for the conversion of PrP(C) to PrP(Sc) is through a highly unfolded state that retains, at most, only this small nucleus of structure, rather than through a highly organized folding intermediate.