Opposing Regulation of Tau Condensation by Phosphorylation Levels and Molecular Chaperone Interactions

Aggregation of the microtubule-stabilizing protein Tau into insoluble deposits is characteristic for several neurodegenerative diseases, including Alzheimer's disease. While phosphorylated Tau (pTau) appears as a normal post-translational modification, the accumulation of hyperphosphorylated Tau aggregates represents a hallmark of these disorders. The molecular mechanisms involved in the conversion from functional Tau into pathological assemblies are still not elucidated. Here, we identified the role of phosphorylation levels on the phase separation and aggregation behavior of Tau, and how these processes can be counteracted by the molecular chaperone-like Bri2 BRICHOS domain. We found that the phosphorylation pattern modulates Tau fibrillation and enhances its droplet-formation propensity. Remarkably, Bri2 BRICHOS generally shifts Tau phase boundaries, counteracting the dense state, and inhibits Tau fibril formation, irrespectively of the degree of phosphorylation. Solution NMR results pinpointed the binding site of Bri2 BRICHOS on pTau, both in the dilute phase and droplet state, revealing an effect on specific regions and phosphorylated residues, which appear to be crucial for the formation and maintenance of the droplet state. Taken together, this study uncovered the opposing regulatory roles of phosphorylation and the anti-amyloid chaperone Bri2 BRICHOS on biomolecular condensation and fibrillation of Tau, providing detailed insights into the molecular mechanisms involved in tauopathies, which may facilitate future treatment strategies using chaperone-based inhibitors.