Early Intermediate in Human Prion Protein Folding As Evidenced by Ultrarapid Mixing Experiments

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Abstract
An important step toward understanding the mechanism of the PrPC-to-PrPSc conversion is to elucidate the folding pathway(s) of the prion protein. On the basis of stopped-flow measurements, we recently proposed that the prion protein folds via a transient intermediate formed on the submillisecond time scale, and mutations linked to familial diseases result in a pronounced increase in the population of this intermediate. Here, we have extended these studies to continuous-flow measurements using a capillary mixing system with a time resolution of ∼100 μs. This allowed us to directly observe two distinct phases in folding of the recombinant human prion protein 90-231, providing unambiguous evidence for rapid accumulation of an early intermediate (with a time constant of ∼50 μs), followed by a rate-limiting folding step (with a time constant of ∼700 μs). The present study also clearly demonstrates that the population of the intermediate is significantly increased at mildly acidic pH and in the presence of urea. A similar three-state folding behavior was observed for the Gerstmann−Straussler−Scheinker disease-associated F198S mutant, in which case the population of an intermediate was greatly increased as compared to that of the wild-type protein. Overall, the present data strongly suggest that this partially structured intermediate may be a direct monomeric precursor of the misfolded PrPSc oligomer.