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.

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