Localized structural fluctuations promote amyloidogenic conformations in transthyretin

The process of transthyretin (TTR) misfolding and aggregation, including amyloid formation, appears to cause a number of degenerative diseases. During amyloid formation, the native protein undergoes a tetramer-to-folded monomer transition, followed by local unfolding of the monomer to an assembly-competent amyloidogenic intermediate. Here we use NMR relaxation dispersion to probe conformational exchange at physiological pH between native monomeric TTR (the F87M/L110M variant) and a small population of a transiently formed amyloidogenic intermediate. The dispersion experiments show that a majority of the residues in the β-sheet containing β-strands D, A, G, and H undergo conformational fluctuations on microsecond-to-millisecond timescales. Exchange broadening is greatest for residues in the outer β-strand H, which hydrogen bonds to β-strand H' of a neighboring subunit in the tetramer, but the associated structural fluctuations propagate across the entire β-sheet. Fluctuations in the other β-sheet are limited to the outer β-strand F, which packs against strand F' in the tetramer, while the B, C, and E β-strands of this sheet remain stable. The structural changes were also investigated under more forcing amyloidogenic conditions (pH6.4-3.7), where β-strand D and regions of the D-E and E-F loops were additionally destabilized, increasing the population of the amyloidogenic intermediate and accelerating amyloid formation. Strands B, C, and E appear to maintain native-like conformations in the partially unfolded, amyloidogenic state of wild-type TTR. In the case of the protective mutant T119M, the conformational fluctuations are suppressed under both physiological and mildly acidic conditions, indicating that the dynamic properties of TTR correlate well with its aggregation propensity.