Decoupling phase separation and fibrillization preserves activity of biomolecular condensates

Age-dependent conversion of metastable, liquid-like protein condensates into amyloid fibrils is observed across many protein systems linked to neurodegeneration. A critical question is whether the thermodynamic forces governing reversible phase separation can be decoupled from those driving irreversible amyloid formation. This study addresses this question using an engineered variant of the microtubule-associated protein Tau that forms biochemically active condensates. Under quiescent, cofactor-free conditions, these protein condensates undergo rapid aging into amyloid fibrils, with the condensate interface serving as a site for fibril nucleation. This accompanies loss of condensate activity in tubulin recruitment and microtubule assembly. The small-molecule metabolite L-arginine selectively suppresses the condensate-to-fibril transition without disrupting phase separation, acting in a valence- and chemistry-dependent manner. By increasing condensate viscoelasticity, L-arginine mitigates age-dependent functional decline of Tau condensates, demonstrating that small molecules can stabilize condensate metastability and preserve function.