1. Introduction 310

2. Protein-only hypothesis 312

3. The scrapie prion protein PrPSc313

3.1 Purification of PrP 27–30 313

3.2 Proteinase K resistance 314

3.3 Scrapie-associated fibrils 314

3.4 Smallest infectious unit 316

3.5 Conformational properties 316

3.6 Dissociation and stability 319

4. The cellular prion protein PrPC321

4.1 Prnp expression 321

4.2 Biosynthetic pathway 322

4.3 NMR structures 324

4.4 Copper binding 326

5. Post-translational PrP conversion 327

5.1 Conformational isoforms 327

5.2 Location of propagation 329

5.3 Minimal PrP sequence 330

5.4 Prion species barrier 331

5.5 Prion strains 332

6. Effect of familial TSE mutations 333

6.1 Thermodynamic stability of PrPC 334

6.2 De novo synthesis of PrPSc 335

6.3 Transmembrane PrP forms 337

7. Physical properties of synthetic PrP 337

7.1 Amyloidogenic peptides 337

7.2 Folding intermediates 339

8. Hypothetical protein X 340

8.1 Two species-specific epitopes 340

8.2 Mapping the protein X epitope 341

9. Chaperone-mediated PrP conversion 343

9.1 Hsp60 and Hsp10 chaperonins 343

9.2 GroEL promoted PrP-res formation 345

9.3 Membrane-associated chaperonins 345

9.4 Preference of GroEL for positive charges 347

9.5 Potential GroEL/Hsp60 epitopes on PrP 347

9.6 Conformations of chaperonin-bound PrP 349

9.7 Conserved Hsp60 substrate binding sites 349

9.8 Requirement of ATP-hydrolysis 351

9.9 Hsp60-mediated prion propagation 354

10. Template-assisted annealing model 355

11. Acknowledgments 357

12. References 357

Although 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.