Structural Analysis of Alzheimer's β(1–40) Amyloid: Protofilament Assembly of Tubular Fibrils

AbstractDetailed structural studies of amyloid fibrils can elucidate the way in which their constituent polypeptides are folded and self-assemble, and exert their neurotoxic effects in Alzheimer's disease (AD). We have previously reported that when aqueous solutions of the N-terminal hydrophilic peptides of AD β-amyloid (Aβ) are gradually dried in a 2-Tesla magnetic field, they form highly oriented fibrils that are well suited to x-ray fiber diffraction. The longer, more physiologically relevant sequences such as Aβ(1–40) have not been amenable to such analysis, owing to their strong propensity to polymerize and aggregate before orientation is achieved. In seeking an efficient and inexpensive method for rapid screening of conditions that could lead to improved orientation of fibrils assembled from the longer peptides, we report here that the birefringence of a small drop of peptide solution can supply information related to the cooperative packing of amyloid fibers and their capacity for magnetic orientation. The samples were examined by electron microscopy (negative and positive staining) and x-ray diffraction. Negative staining showed a mixture of straight and twisted fibers. The average width of both types was ∼70Å, and the helical pitch of the latter was ∼460Å. Cross sections of plastic-embedded samples showed a ∼60-Å-wide tubular structure. X-ray diffraction from these samples indicated a cross-β fiber pattern, characterized by a strong meridional reflection at 4.74Å and a broad equatorial reflection at 8.9Å. Modeling studies suggested that tilted arrays of β-strands constitute tubular, 30-Å-diameter protofilaments, and that three to five of these protofilaments constitute the Aβ fiber. This type of structure—a multimeric array of protofilaments organized as a tubular fibril—resembles that formed by the shorter Aβ fragments (e.g., Aβ(6–25), Aβ(11–25), Aβ(1–28)), suggesting a common structural motif in AD amyloid fibril organization