Analytical background and discussion of the chaperone model of prion diseases

It is generally accepted that prion infection is due solely to a protein i.e. the protein-only hypothesis. The essential constituent of infectious prions is the scrapie prion protein (PrPSc) which is chemically indistinguishable from the normal, cellular protein (PrPC) but exhibits distinct secondary and tertiary structure. This very unusual feature seems to be in contradiction with a major paradigm of present structural biology stated by Anfinsen: a protein folds to the most stable conformation, this means only one structure. In order to reconcile the results obtained on prions with the biophysics of protein folding, a model is proposed. It is based on the hypothesis that a thermodynamically irreversible step is involved in protein folding. The model is then extended to chaperone-assisted protein folding. It is shown that, under certain conditions, the transitory secondary structure formed during the earlier step of folding could interact with chaperone. Analysis shows that chaperone may help the protein to find correct conformation. On the other hand, analysis reveals the possibility that more than one structure may form from a single polypeptide chain. Under these conditions, the behaviour of chaperones resembles the characteristics of prion diseases.