CONFORMATIONAL TRANSITIONS IN PEPTIDES CONTAINING 2 PUTATIVE ALPHA-HELICES OF THE PRION PROTEIN

Prions are composed largely, if not entirely, of the scrapie isoform o f the prion protein (PrP5c). Conversion of the cellular isoform (PrPC) to PrPSc is accompanied by a diminution in the alpha-helical content and an increase in the beta-sheet structure. To investigate the struct ural basis of this transition, peptide fragments corresponding to Syri an hamster PrP residues 90 to 145 and 109 to 141, which contain the mo st conserved residues of the prion protein and the first two putative alpha-helical regions in a PrPC model, were studied using infrared spe ctroscopy and circular dichroism. The peptides could be induced to for m alpha-helical structures in aqueous solutions in the presence of org anic solvents, such as trifluoroethanol and hexafluoroisopropanol, or detergents, such as sodium dodecyl sulfate and dodecyl phosphocholine. NaCl at physiological concentration or acetonitrile induced the pepti des to acquire substantial beta-sheet. The intermolecular nature of th e beta-sheet was evident in the formation of rod-shaped polymers as de tected by electron microscopy. Resistance to hydrolysis by proteinase K and epitope mapping argue that the beta-sheet structures were formed by the interaction of residues lying between 109 and 141. A similar r ange of residues was shown by nuclear magnetic resonance spectroscopy to be capable of forming alpha-helices. The alpha-helical structures s eem to require a hydrophobic support from either intermolecular intera ctions or the hydrophobic environment provided by micelles, in agreeme nt with the predicted hydrophobic nature of the packing surface among the four putative helices of PrPC and the outer surfaces of the first two helices. Our results suggest that perturbation of the packing envi ronment of the highly conserved residues is a possible mechanism for t riggering the conversion of PrPC to PrPSc where alpha-helices appear t o be converted into beta-sheets.