Abstract
Polymerization of the amyloid beta (Abeta) peptide into protease-resistant fibrils is a significant step in the pathogenesis of Alzheimer's disease. It has not been possible to obtain detailed structural information about this process with conventional techniques because the peptide has limited solubility and does not form crystals. In this work, we present experimental results leading to a molecular level model for fibril formation. Systematically selected Abeta-fragments containing the Abeta16-20 sequence, previously shown essential for Abeta-Abeta binding, were incubated in a physiological buffer. Electron microscopy revealed that the shortest fibril-forming sequence was Abeta14-23. Substitutions in this decapeptide impaired fibril formation and deletion of the decapeptide from Abeta1-42 inhibited fibril formation completely. All studied peptides that formed fibrils also formed stable dimers and/or tetramers. Molecular modeling of Abeta14-23 oligomers in an antiparallel beta-sheet conformation displayed favorable hydrophobic interactions stabilized by salt bridges between all charged residues. We propose that this decapeptide sequence forms the core of Abeta-fibrils, with the hydrophobic C terminus folding over this core. The identification of this fundamental sequence and the implied molecular model could facilitate the design of potential inhibitors of amyloidogenesis.
Highlights
The polymerization of the amyloid -peptide into proteaseresistant fibrillar deposits in the brain parenchyma and vasculature is a significant step in the pathogenesis of Alzheimer’s disease [1]
Electron microscopy (EM), molecular modeling, and the correlations of side-chain pairs found in -sheets [14], we present a detailed structural model of fibrils formed by this sequence
Identification of the Shortest Fibril-forming Sequence in A—A set of peptides with the amyloid beta (A)16–20 sequence systematically extended at both ends, were synthesized (Fig. 1)
Summary
Materials—Synthetic A1–40 was obtained from Dr David Teplow, the Biopolymer Laboratory at Harvard University, MA. All other peptides were purchased from Research Genetics, Huntsville, AL. The peptides were purified on a Polymer Laboratories (Church Stretton, UK) PLRP-S column (150 ϫ 25 mm or 150 ϫ 7.5 mm; Polymer Labs.), using a water-acetonitrile gradient with 0.1% trifluoroacetic acid. Identity and purity were verified with electrospray mass spectrometry using a Quattro triple quadropole (Micromass, Altrincham, UK). The peptides were lyophilized and stored at Ϫ80 °C. Incubation of Peptides—The peptides were dissolved at 200 M in 50 mM Tris-buffered saline (TBS) (150 mM NaCl), pH 7.4 and incubated at 37 °C for 3 days. The peptides were initially dissolved in 2 volumes of 50 mM Tris, pH 10, to ensure starting solutions
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