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Amyloid Formation from an α-Helix Peptide Bundle Is Seeded by 310-Helix Aggregates

Authors

  • Dr. Yogendra Singh,

    1. Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
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  • Dr. Philip C. Sharpe,

    1. Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
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  • Dr. Huy N. Hoang,

    1. Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
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  • Dr. Andrew J. Lucke,

    1. Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
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  • Dr. Alasdair W. McDowall,

    1. Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125 (USA)
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  • Prof. Stephen P. Bottomley,

    1. Department of Biochemistry and Molecular Biology, Monash University, Victoria, Clayton, 3800 (Australia)
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  • Prof. David P. Fairlie

    Corresponding author
    1. Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
    • Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072 (Australia), Fax: (+61) 733462990
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Abstract

Transformation of proteins and peptides to fibrillar aggregates rich in β sheets underlies many diseases, but mechanistic details of these structural transitions are poorly understood. To simulate aggregation, four equivalents of a water-soluble, α-helical (65 %) amphipathic peptide (AEQLLQEAEQLLQEL) were assembled in parallel on an oxazole-containing macrocyclic scaffold. The resulting 4α-helix bundle is monomeric and even more α helical (85 %), but it is also unstable at pH 4 and undergoes concentration-dependent conversion to β-sheet aggregates and amyloid fibrils. Fibrils twist and grow with time, remaining flexible like rope (>1 μm long, 5–50 nm wide) with multiple strings (2 nm), before ageing to matted fibers. At pH 7 the fibrils revert back to soluble monomeric 4α-helix bundles. During α→β folding we were able to detect soluble 310 helices in solution by using 2D-NMR, CD and FTIR spectroscopy. This intermediate satisfies the need for peptide elongation, from the compressed α helix to the fully extended β strand/sheet, and is driven here by 310-helix aggregation triggered in this case by template-promoted helical bundling and by hydrogen-bonding glutamic acid side chains. A mechanism involving α⇌α4⇌(310)4⇌(310)n⇌(β)nm(β)n equilibria is plausible for this peptide and also for peptides lacking hydrogen-bonding side chains, with unfavourable equilibria slowing the α→β conversion.

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