Chaperonin-mediated de novo generation of prion protein aggregates

Abstract

The infectious prion protein, PrP Sc, a predominantly β-sheet aggregate, is derived from PrP C, the largely α-helical cellular isoform of PrP. Conformational conversion of PrP C into PrP Sc has been suggested to involve a chaperone-like factor. Here we report that the bacterial chaperonin GroEL, a close homolog of eukaryotic Hsp60, can catalyze the aggregation of chemically denatured and of folded, recombinant PrP in a model reaction in vitro. Aggregates form upon ATP-dependent release of PrP from chaperonin and have certain properties of PrP Sc, including a high β-sheet content, the ability to bind the dye Congo red, detergent-insolubility and increased protease-resistance. A conserved sequence segment of PrP (residues 90-121), critical for PrP Sc generation in vivo, is also required for chaperonin-mediated aggregate formation in vitro. Initial binding of refolded, α-helical PrP to chaperonin is mediated by the unstructured N-terminal segment of PrP (residues 23-121) and is followed by a rearrangement of the globular PrP core-domain. These results show that chaperonins of the Hsp60 class can, in principle, mediate PrP aggregation de novo, i.e. independently of a pre-existent PrP Sc template.