Cross-ß spine architecture of fibrils formed by the amyloidogenic segment NFGSVQFV of medin from solid-state NMR and X-ray fibre diffraction measurements

Over 30 polypeptides are known to assemble into highly ordered fibrils associated with pathological disorders known collectively as amyloidoses. Structural studies of short model peptides are beginning to reveal trends in the types of molecular interactions that drive aggregation and stabilize the packing of ß-sheet layers within fibrillar assemblies. This work investigates the molecular architecture of fibrils formed by the peptide AMed42-49 representing residues 42-49 of the 50 amino acid polypeptide medin associated with aortic medial amyloid, the most common form of senile localized amyloid. The peptide aggregates within 2 days to form bundles of microcrystalline-like needles displaying a high degree of order. Fibrils were prepared from peptides containing up to 23 13C labels, and the solid-state nuclear magnetic resonance (SSNMR) method rotational resonance (RR) was used to determine constraints on the distances between selective atomic sites within fibrils. The constraints are consistent with unbroken ß-strands hydrogen bonded in a parallel in-register arrangement within ß-sheets. Further RR measurements identify close (>6.5 Å) contacts between residues F43 and V46 and between S45 and V46, which can only occur between ß-sheet layers and which are consistent with two principal models of ß-sheet arrangements. X-ray fiber diffraction from partially aligned fibrils revealed the classical amyloid diffraction pattern, and comparison of patterns calculated from model coordinates with experimental data allowed determination of a consistent molecular model.