Characterization of an amyloidogenic intermediate of transthyretin by NMR relaxation dispersion

The aggregation pathway of transthyretin (TTR) proceeds through rate-limiting dissociation of the tetramer and partial misfolding of the monomers, which assemble into amyloid structures through a downhill polymerization mechanism. The structural features of the aggregation-prone monomeric intermediate are poorly understood. Characterization of amyloidogenic intermediates is challenging due to their propensity to aggregate at concentrations necessary for structural studies. NMR relaxation dispersion offers a unique opportunity to characterize these intermediates when they exchange on favorable timescales with NMR-visible ground states. To characterize the structural transitions associated with tetramer dissociation, we have analyzed ground-state chemical shift differences between the native tetramer and an engineered monomer in which the critical F87 side chain is replaced by glutamate. The secondary structure and overall fold of the F87E monomer is similar to that of the tetramer except for β-strand H. This strand populates two conformations, where it is either docked on the protein core or is displaced from the edge of the β-sheet formed by β-strands D, A, G, and H (DAGH β-sheet) and is dynamically disordered. Chemical shift differences derived from analysis of 1H/15N single, double and zero quantum relaxation dispersion data provide insights into the structure of a low-lying excited state that exchanges with the ground state of the F87E monomer at a rate of 3800 s-1. Disruption of the subunit interfaces of the TTR tetramer leads to destabilization of edge strands in both β-sheets of the F87E monomer. Conformational fluctuations are propagated through the entire hydrogen bonding network of the DAGH β-sheet, from the inner β-strand H, which forms the strong dimer interface in the TTR tetramer, to outer strand D which is unfolded in TTR fibrils. Fluctuations are also propagated from the AB loop in the weak dimer interface to the EF helix, which undergoes structural remodeling in fibrils. The conformational fluctuations in both regions are enhanced at acidic pH where amyloid formation is most favorable. The relaxation dispersion data provide insights into the conformational dynamics of the amyloidogenic state of monomeric TTR that predispose it for structural remodeling and progression to amyloid fibrils. ### Competing Interest Statement The authors have declared no competing interest.