Prion propagation and molecular chaperones

1. Introduction 3102. Protein-only hypothesis 3123. The scrapie prion protein PrPSc3133.1 Purification of PrP 27–30 3133.2 Proteinase K resistance 3143.3 Scrapie-associated fibrils 3143.4 Smallest infectious unit 3163.5 Conformational properties 3163.6 Dissociation and stability 3194. The cellular prion protein PrPC3214.1 Prnp expression 3214.2 Biosynthetic pathway 3224.3 NMR structures 3244.4 Copper binding 3265. Post-translational PrP conversion 3275.1 Conformational isoforms 3275.2 Location of propagation 3295.3 Minimal PrP sequence 3305.4 Prion species barrier 3315.5 Prion strains 3326. Effect of familial TSE mutations 3336.1 Thermodynamic stability of PrPC 3346.2 De novo synthesis of PrPSc 3356.3 Transmembrane PrP forms 3377. Physical properties of synthetic PrP 3377.1 Amyloidogenic peptides 3377.2 Folding intermediates 3398. Hypothetical protein X 3408.1 Two species-specific epitopes 3408.2 Mapping the protein X epitope 3419. Chaperone-mediated PrP conversion 3439.1 Hsp60 and Hsp10 chaperonins 3439.2 GroEL promoted PrP-res formation 3459.3 Membrane-associated chaperonins 3459.4 Preference of GroEL for positive charges 3479.5 Potential GroEL/Hsp60 epitopes on PrP 3479.6 Conformations of chaperonin-bound PrP 3499.7 Conserved Hsp60 substrate binding sites 3499.8 Requirement of ATP-hydrolysis 3519.9 Hsp60-mediated prion propagation 35410. Template-assisted annealing model 35511. Acknowledgments 35712. References 357Although the central paradigm of protein folding (Anfinsen, 1973), that the unique three-dimensional structure of a protein is encoded in its amino acid sequence, is well established, its generality has been questioned due to two recent developments in molecular biology, the ‘prion’ and ‘molecular chaperone’. Biochemical characterization of infectious scrapie material causing central nervous system (CNS) degeneration indicates that the necessary component for disease propagation is proteinaceous (Prusiner, 1982), as first outlined by Griffith (1967) in general terms, and involves a conversion from a cellular prion protein, denoted PrPC, into a toxic scrapie form, PrPSc, which is facilitated by PrPSc acting as a template for PrPC to form new PrPSc molecules (Prusiner, 1987). The ‘protein-only’ hypothesis implies that the same polypeptide sequence, in the absence of any post-translational modifications, can adopt two considerably different stable protein conformations (Fig. 1). Thus, in the case of prions it is possible, although not proven, that they violate the central paradigm of protein folding. There is some indirect evidence that another factor, provisionally named ‘protein X’, might be involved in the conformational conversion process (Prusiner et al. 1998), which includes a dramatic change from α-helical into β-sheet secondary structure (Fig. 1). This factor has not been identified yet, but it has been proposed that protein X may act as a molecular chaperone. The idea that molecular chaperones play a critical role in the generation of PrPSc is appealing also from a theoretical point of view, because PrPSc formation involves changes in protein folding and possibly intermolecular aggregation (Fig. 1), processes in which chaperones are known to participate (Musgrove & Ellis, 1986). The discovery and functional analysis of more than a dozen molecular chaperones made it clear that these proteins do not complement folding information that is not already contained in the genetic code (Ellis et al. 1989); rather they facilitate the folding and assembly of proteins by preventing misfolding and refolding misfolded proteins (Hartl, 1996). Whether a molecular chaperone or another type of macromolecule is identified as the conversion factor, therefore, the molecular chaperone concept is likely to contribute to the understanding of the molecular nature of PrPC to PrPSc conversion.In this review I consider the prion concept from the view of a structural biologist whose main interest focuses on spontaneous and chaperone-mediated conformational changes in proteins.