Osmolyte trimethylamine N-oxide converts recombinant alpha-helical prion protein to its soluble beta-structured form at high temperature

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English. The thermal unfolding of full-length human recombinant a-helical prion protein (alpha-PrP) in neutral pH is reversible, whereas, in the presence of the osmolyte N-trimethylamine oxide (TMAO), the protein acquires a beta-sheet structure at higher temperatures and the thermal unfolding of the protein is irreversible. Lysozyme, an amyloidogenic protein similar to prion protein, regains alpha-helical structure on cooling from its thermally unfolded form in buffer and in TMAO solutions. The thermal stability of alpha-PrP decreases, whereas that of lysozyme increases in TMAO solution. Light-scattering and turbidity values indicate that beta-sheet prion protein exists as soluble oligomers that increase thioflavin T fluorescence and bind to 1-anilino 8-naphthalene sulfonic acid (ANS). The oligomers are resistant to proteinase K digestion and during incubation for long periods they form linear amyloids > 5 mu m long. The comparable fluorescence polarization of the tryptophan groups and their accessibility to acrylamide in a-PrP and oligomers indicate that the unstructured N-terminal segments of the protein, which contain the tryptophan groups, do not associate among themselves during oligomerization. Partial unfolding of U-helical prion protein in TMAO solution leads to its structural conversion to misfolded sheet form. The formation of the misfolded prion protein oligomers and their polymerization to amyloids in TMAO are unusual, since the osmolyte generally induces denatured protein to fold to a native-like state and protects proteins from thermal denaturation and aggregation. (c) 2006 Elsevier Ltd. All rights reserved.