Mispacking of the Phe87 Side Chain Reduces the Kinetic Stability of Human Transthyretin

Aggregation of transthyretin (TTR) causes TTR amyloidoses. The native TTR tetramer (a dimer of dimers) is stabilized by packing of phenylalanine 87 (F87) into a hydrophobic cavity of a neighboring protomer across the strong dimer interface. X-ray structures at acidic pH show that the side chain of F87 can be displaced from its binding pocket, but the resultant solution conformations remain unknown. Here we used ¹⁹F nuclear magnetic resonance (NMR) and ¹⁹F-labeled C10S-S85C TTR to characterize two local conformations of the loop containing F87. At neutral pH, F87 packs correctly into the interprotomer cavity in the dominant conformational state (93% population, T) whereas the remaining minor population is a mispacked tetramer (T*). The population of T* can be enhanced in heterotetramers by mixing C10S-S85C TTR with increasing molar ratios of A120L TTR, where a bulky leucine residue is introduced to disfavor the T state by steric hindrance. Exchange between the T and T* states in the presence of A120L is mediated by subunit exchange from the C10S-S85C tetramer. Compared to the TTR tetramer in which the dimers are correctly packed, mispacking of one or both dimer pairs leads to an increase in the urea unfolding rate of 4-fold or at least 15-fold, respectively. Consistent acid-mediated tetramer dissociation was observed by ¹⁹F NMR aggregation assays. Our results highlight the important role of the interprotomer F87 side chain packing in determining the kinetic stability of the TTR tetramer; mispacking of F87 in the T* state predisposes it for rapid dissociation and entry into the aggregation pathway.