X-ray diffraction studies of fibrils formed from peptide fragments of transthyretin

Two synthetic peptide fragments of the plasma protein transthyretin (TTR), previously shown to form fibrillar structures in vitro, have been examined using electron microscopy and X-ray diffraction. The fibrils displayed all characteristics of cross beta-sheet conformation with antiparallel strand spacing of 4.7 A and intersheet spacings of 8-10 A as well as reflections indicating further lateral repeating units. A third peptide containing a substitution equivalent to a mutation in TTR known to increase the propensity of TTR to form amyloid was also examined. It also formed fibrils and showed similar cross beta-sheet structure, but with closer intersheet packing than its native equivalent.