Structure of Amyloid Peptide Ribbons Characterized by Electron Microscopy, Atomic Force Microscopy, and Solid-State Nuclear Magnetic Resonance

Polypeptides often self-assemble to form amyloid fibrils, which contain cross-β structural motifs and are typically 5–15 nm in width and micrometers in length. In many cases, short segments of longer amyloid-forming protein or peptide sequences also form cross-β assemblies but with distinctive ribbon-like morphologies that are characterized by a well-defined thickness (on the order of 5 nm) in one lateral dimension and a variable width (typically 10–100 nm) in the other. Here, we use a novel combination of data from solid-state nuclear magnetic resonance (ssNMR), dark-field transmission electron microscopy (TEM), atomic force microscopy (AFM), and cryogenic electron microscopy (cryoEM) to investigate the structures within amyloid ribbons formed by residues 14–23 and residues 11–25 of the Alzheimer’s disease-associated amyloid-β peptide (Aβ_14–23 and Aβ_11–25). The ssNMR data indicate antiparallel β-sheets with specific registries of intermolecular hydrogen bonds. Mass-per-area values are derived from dark-field TEM data. The ribbon thickness is determined from AFM images. For Aβ_14–23 ribbons, averaged cryoEM images show a periodic spacing of β-sheets. The combined data support structures in which the amyloid ribbon growth direction is the direction of intermolecular hydrogen bonds between β-strands, the ribbon thickness corresponds to the width of one β-sheet (i.e., approximately the length of one molecule), and the variable ribbon width is a variable multiple of the thickness of one β-sheet (i.e., a multiple of the repeat distance in a stack of β-sheets). This architecture for a cross-β assembly may generally exist within amyloid ribbons.