Structural Analysis of Peptide Fragment 71–93 of Transthyretin by NMR Spectroscopy and Electron Microscopy: Insight into Amyloid Fibril Formation

A peptide corresponding to the amino acid region 71–93 of the plasma protein transthyretin (TTR) has been synthesized to investigate its role in the native folding of the molecule and the possible relationship between mutations in this region and amyloid formation of TTR. In the native structure this fragment includes a β-strand followed by a short helix and turns back on itself to form part of an antiparallel β-sheet. Electron microscopy has shown that the peptide is not intrinsically amyloidogenic. NMR spectroscopy has been used to investigate the conformational dependency of the peptide on the solution conditions. Minor populations of peptide showing partial turns were apparent in deuterated dimethyl sulfoxide (DMSO-d₆). Some indication of nascent helix between residues 5 and 12 was observed in water, and upon the addition of 20% trifluoroethano) (TFE) the span of helix was confirmed. The intrinsic tendency to form a helical structure between residues 5 and 12 in solution suggests that the helical region, also present in the native crystallographically determined TTR structure at corresponding residues 75–82, is an important folding initiation site. In contrast, the β-sheet motif observed in the native structure was not detected in solution. It is proposed that mutations in TTR occurring in the helical region result in subtle changes in the TTR structure leading to amyloid fibril formation.