Aggregation and fibrillization of the recombinant human prion protein huPrP90-231

According to the "protein-only" hypothesis, the critical step in the pathog enesis of pi-ion diseases is the conformational transition between the norm al (PrPC) and pathological (PrPSc) isoforms of prion protein. To gain insig ht into the mechanism of this transition, we have characterized the biophys ical properties of the recombinant protein corresponding to residues 90-231 of the human prion protein (huPrP90-231). Incubation of the protein under acidic conditions (pH 3.6-5) in the presence of 1 M guanidine-HCl resulted in a time-dependent transition from an alpha-helical conformation to a beta -sheet structure and oligomerization of huPrP90-231 into large molecular we ight aggregates, No stable monomeric beta-sheet-rich folding intermediate o f the protein could be detected in the present experiments. Kinetic analysi s of the data indicates that the formation of beta-sheet structure and prot ein oligomerization likely occur concomitantly. The beta-sheet-rich oligome rs were characterized by a markedly increased resistance to proteinase K di gestion and a fibrillar morphology (i.e., they had the essential physicoche mical properties of PrPSc), Contrary to previous suggestions, the conversio n of the recombinant prion protein into a PrP(Sc-)like form could be accomp lished under nonreducing conditions, without the need to disrupt the disulf ide bond. Experiments in urea indicate that, in addition to acidic pH, anot her critical factor controlling the transition of huPrP90-231 to an oligome ric beta-sheet structure is the presence of salt.