Transthyretin variants with improved inhibition of β-amyloid aggregation

Aggregation of beta-amyloid (A beta) is widely believed to cause neuronal dysfunction in Alzheimer's disease. Transthyretin (TTR) binds to A beta and inhibits its aggregation and neurotoxicity. TTR is a homotetrameric protein, with each monomer containing a short a-helix and two anti-parallel beta-sheets. Dimers pack into tetramers to form a hydrophobic cavity. Here we report the discovery of a TTR mutant, N98A, that was more effective at inhibiting A beta aggregation than wild-type (WT) TTR, although N98A and WT bound A beta equally. The N98A mutation is located on a flexible loop distant from the putative A beta-binding sites and does not alter secondary and tertiary structures nor prevent correct assembly into tetramers. Under non-physiological conditions, N98A tetramers were kinetically and thermodynamically less stable than WT, suggesting a difference in the tetramer folded structure. In vivo, the lone cysteine in TTR is frequently modified by S-cysteinylation or S-sulfonation. Like the N98A mutation, S-cysteinylation of TTR modestly decreased tetramer stability and increased TTR's effectiveness at inhibiting A beta aggregation. Collectively, these data indicate that a subtle change in TTR tetramer structure measurably increases TTR's ability to inhibit A beta aggregation.