title: Amyloid formation from an α-helix peptide bundle is seeded by 3(10)-helix aggregates.
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 3(10) 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 3(10) -helix aggregation triggered in this case by template-promoted helical bundling and by hydrogen-bonding glutamic acid side chains. A mechanism involving α⇌α(4) ⇌(3(10) )(4) ⇌(3(10) )(n) ⇌(β)(n) ⇋m(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.
authors.affiliation: Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, Brisbane, Qld 4072, Australia.
authors.forename: Yogendra
authors.initials: Y
authors.lastname: Singh
authors.forename: Philip C
authors.initials: PC
authors.lastname: Sharpe
authors.forename: Huy N
authors.initials: HN
authors.lastname: Hoang
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Lucke
authors.forename: Alasdair W
authors.initials: AW
authors.lastname: McDowall
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Bottomley
authors.forename: David P
authors.initials: DP
authors.lastname: Fairlie
mesh_terms: D000595:Amino Acid Sequence; D000682:Amyloid / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000737:chemistry; D010080:Oxazoles / Q000737:chemistry; D017434:Protein Structure, Tertiary
chemical_list: C555897:AEQLLQEAEQLLQEL; D000682:Amyloid; D009842:Oligopeptides; D010080:Oxazoles