Tubulin transforms Tau and α-synuclein condensates from pathological to physiological.

Proteins undergo phase separation to form membraneless condensates that spatially organize biomolecular interactions. These condensates can support cellular physiology or instigate pathological protein aggregation. Tau and α-synuclein (αSyn) are neuronal proteins that form heterotypic Tau:αSyn condensates associated with physiological and pathological processes. Tau and αSyn regulate microtubules, but also misfold and co-deposit in aggregates linked to neurodegenerative disease, highlighting the ambivalent impact of Tau:αSyn condensation in health and disease. Here, we show that Tubulin modulates Tau:αSyn condensates by promoting microtubule interactions and inhibiting homotypic and heterotypic pathological oligomers. In the absence of Tubulin, Tau-driven condensation accelerates formation of pathogenic Tau:αSyn heterodimers and amyloid fibrils. Tubulin partitioning into condensates promotes microtubule polymerization and prevents Tau and αSyn oligomerization. We identify distinct Tau and αSyn structural states in pathological Tubulin-absent versus physiological Tubulin-rich condensates. In neuronal models, microtubule loss drives pathological oligomer formation and neurite loss, whereas inducible Tau condensation stabilizes microtubules.