Unfold to refold: Tracking the initial steps of PrP^C unfolding in the context of PrP^Sc propagation

It might have been believed that elucidation of the atomistic structure of PrP^Sc would lead to an immediate understanding of the mechanism of prion propagation. However, PrP^Sc, now known to be a “simple” amyloid, can just template a previously unfolded polypeptide chain. Therefore, PrP^Sc can easily template the disordered ∼90-120 domain of an incoming PrP^C molecule, but not its ∼121-231 folded domain (FD). The FD needs to accommodate into the ∼121-230 PrP^Sc surface, an inert “procrustean bed”. Thus, a mechanism for concerted unfolding/refolding of the FD must exist, with FD unfolding as a key element. To explore how this might happen, we performed thermal unfolding of recombinant bank vole PrP^C(90-231), that is a universal PrP^Sc propagator PrP^Sc, tracking changes at the residue level with solution NMR to pinpoint early unfolding propensity. Our data suggest that a key early event is the destabilization of the short β1-β2 assembly, and that the segment contiguous to the disordered tail, ∼121-140, encompassing β1 and its adjacent coils, is the most likely region to unfold first. Spectroscopic data obtained at higher temperatures suggest that portions of alpha helix α2 are likely the last elements of the FD to unfold and refold into the PrP^Sc conformation. Molecular Dynamics simulations assisted the interpretation of these changes and suggest separation of α1 from the rest of the FD ensemble. Our data provide a conceivable timeline of the early events in PrP^Sc-assisted conversion of PrP^C and should serve as a starting framework to develop a future atomistic model of PrP^Sc propagation.