Direct observation of protein folding, aggregation, and a prion-like conformational conversion.

Protein conformational transition from alpha-helices to beta-sheets precedes aggregation of proteins implicated in many diseases, including Alzheimer and prion diseases. Direct characterization of such transitions is often hindered by the complicated nature of the interaction network among amino acids. A recently engineered small protein-like peptide with a simple amino acid composition features a temperature-driven alpha-helix to beta-sheet conformational change. Here we studied the conformational transition of this peptide by molecular dynamics simulations. We observed a critical temperature, below which the peptide folds into an alpha-helical coiled-coil state and above which the peptide misfolds into beta-rich structures with a high propensity to aggregate. The structures adopted by this peptide during low temperature simulations have a backbone root mean square deviation less than 2 A from the crystal structure. At high temperatures, this peptide adopts an amyloid-like structure, which is mainly composed of coiled anti-parallel beta-sheets with the cross-beta-signature of amyloid fibrils. Most strikingly, we observed conformational conversions in which an alpha-helix is converted into a beta-strand by proximate stable beta-sheets with exposed hydrophobic surfaces and unsaturated hydrogen bonds. Our study suggested a possible generic molecular mechanism of the template-mediated aggregation process, originally proposed by Prusiner (Prusiner, S. B. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13363-13383) to account for prion infectivity.