Exercise 3. Analysis of the ‘benign’ mouse prion protein PrP^C suggested the existence of three short alpha-helices, the first of which, H1, is unusual in that it is not amphipathic. In their Nature paper, (vol 387, 1996, pp 180-182), Wuthrich’s group suggest that the transition between the benign form and the ‘scrapie’ form of the protein, PrP^Sc  may be due to the unstable secondary structure of H1. The neighbouring four-residue beta sheet could nucleate a much more extensive beta sheet transition that could be induced to propagate through this weakly bound alpha helix and loop, yielding a conformational shift to the long-sought scrapie form of prion protein. The scrapie form might then be protease-resistant because vulnerable loop regions are now protected. You can see a good demonstration of this effect and read additional information here.