Revealing structural changes of prion protein during conversion from α-helical monomer to β-oligomers by means of ESR and nanochannel encapsulation.

Under nondenaturing neutral pH conditions, full-length mouse recombinant prion protein lacking the only disulfide bridge can spontaneously convert from an α-helical-dominant conformer (α-state) to a β-sheet-rich conformer (β-state), which then associates into β-oligomers, and the kinetics of this spontaneous conversion depends on the properties of the buffer used. The molecular details of this structural conversion have not been reported due to the difficulty of exploring big protein aggregates. We introduced spin probes into different structural segments (three helices and the loop between strand 1 and helix 1), and employed a combined approach of ESR spectroscopy and protein encapsulation in nanochannels to reveal local structural changes during the α-to-β transition. Nanochannels provide an environment in which prion protein molecules are isolated from each other, but the α-to-β transition can still occur. By measuring dipolar interactions between spin probes during the transition, we showed that helix 1 and helix 3 retained their helicity, while helix 2 unfolded to form an extended structure. Moreover, our pulsed ESR results allowed clear discrimination between the intra- and intermolecular distances between spin labeled residues in helix 2 in the β-oligomers, making it possible to demonstrate that the unfolded helix 2 segment lies at the association interface of the β-oligomers to form cross-β structure.