Impact of Deuteration on the Assembly Kinetics of Transthyretin Monitored by Native Mass Spectrometry and Implications for Amyloidoses

Conformational Dynamics of an Amyloidogenic Intermediate of Transthyretin: Implications for Structural Remodeling and Amyloid Formation

The aggregation pathway of transthyretin (TTR) proceeds through rate-limiting dissociation of the tetramer (a dimer of dimers) and partial misfolding of the resulting monomer, which assembles into amyloid structures through a downhill polymerization mechanism. The structural features of the aggregation-prone monomeric intermediate are poorly understood. NMR relaxation dispersion offers a unique opportunity to characterize amyloidogenic intermediates when they exchange on favorable timescales with NMR-visible ground states. Here we use NMR to characterize the structure and conformational dynamics of the monomeric F87E mutant of human TTR. Chemical shifts derived from analysis of multinuclear 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-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-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.

Topological Analysis of Transthyretin Disassembly Mechanism: Surface-Induced Dissociation Reveals Hidden Reaction Pathways

The proposed mechanism of fibril formation of transthyretin (TTR) involves self-assembly of partially unfolded monomers. However, the mechanism(s) of disassembly to monomer and potential intermediates involved in this process are not fully understood. In this study, native mass spectrometry and surface-induced dissociation (SID) are used to investigate the TTR disassembly mechanism(s) and the effects of temperature and ionic strength on the kinetics of TTR complex formation. Results from the SID of hybrid tetramers formed during subunit exchange provide strong evidence for a two-step mechanism whereby the tetramer dissociates to dimers that then dissociate to monomers. Also, the SID results uncovered a hidden pathway in which a specific topology of the hybrid tetramer is directly produced by assembly of dimers in the early steps of TTR disassembly. Implementation of SID to dissect protein topology during subunit exchange provides unique opportunities to gain unparalleled insight into disassembly pathways.