Structural insights into copper and zinc binding to tau protein and the impact of metal binding on amyloid aggregation

Abstract

Tau protein is a microtubule-associated protein central to the pathogenesis of Alzheimer's disease (AD) and other tauopathies. While metal ion homeostasis is disrupted in AD, the presence of copper and zinc in neurofibrillary tangles suggests a pathological role for metal–tau interactions. In this study, the metal binding properties of Tau441 were probed using a wide range of spectroscopic tools. Specifically, electron paramagnetic resonance, circular dichroism, nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy (XAS) results point to a single high-affinity Cu²⁺ binding site within the microtubule-binding domain (MTBD), coordinated by the bis-His motif in R3 (His329/His330), possibly His299 and an oxygen-based ligand. This complex can be reduced resulting in a trigonal Cu⁺–Tau441 complex involving Cys322, His299 and a third ligand (likely Cys291 or His329/330), as characterized by XAS and NMR. NMR and XAS results indicate the presence of three Zn²⁺ binding sites: one high-affinity site in the MTBD involving His299, His330, Cys322 and Asp295, and two lower-affinity sites in the N-terminal region, coordinated predominantly by carboxylate and His residues. Moreover, the impact of Cu²⁺ and Zn²⁺ ions on the amyloid aggregation of full length Tau441 was evaluated using thioflavin T fluorescence, electrophoresis and transmission electron microscopy. Both metal ions significantly accelerate aggregation, promoting the formation of amyloid fibrils with distinct morphologies. Our study provides valuable structural insights into the copper and zinc binding sites in Tau441 that provide a rational basis to understand the impact of metal ions on amyloid fibril aggregation and morphology. The current study expands the bioinorganic facet of AD and other tauopathies, and it underscores the importance of metal–tau interactions as potential therapeutic targets in these neurodegenerative diseases.