Structural Insights into the Polymorphism of Amyloid-Like Fibrils Formed by Region 20−29 of Amylin Revealed by Solid-State NMR and X-ray Fiber Diffraction

Many unrelated proteins and peptides can assemble into amyloid or amyloid-like nanostructures, all of which share the cross-β motif of repeat arrays of β-strands hydrogen-bonded along the fibril axis. Yet, paradoxically, structurally polymorphic fibrils may derive from the same initial polypeptide sequence. Here, solid-state nuclear magnetic resonance (SSNMR) analysis of amyloid-like fibrils of the peptide hIAPP20−29, corresponding to the region S20NNFGAILSS29 of the human islet amyloid polypeptide amylin, reveals that the peptide assembles into two amyloid-like forms, (1) and (2), which have distinct structures at the molecular level. Rotational resonance SSNMR measurements of 13C dipolar couplings between backbone F23 and I26 of hIAPP20−29 fibrils are consistent with form (1) having parallel β-strands and form (2) having antiparallel strands within the β-sheet layers of the protofilament units. Seeding hIAPP20−29 with structurally homogeneous fibrils from a 30-residue amylin fragment (hIAPP8−37) produces morphologically homogeneous fibrils with similar NMR properties to form (1). A model for the architecture of the seeded fibrils is presented, based on the analysis of X-ray fiber diffraction data, combined with an extensive range of SSNMR constraints including chemical shifts, torsional angles, and interatomic distances. The model features a cross-β spine comprising two β-sheets with an interface defined by residues F23, A25, and L27, which form a hydrophobic zipper. We suggest that the energies of formation for fibril form containing antiparallel and parallel β-strands are similar when both configurations can be stabilized by a core of hydrophobic contacts, which has implications for the relationship between amino acid sequence and amyloid polymorphism in general.