Decoupling Phase Separation and Fibrillization Preserves Activity of Biomolecular Condensates

Age-dependent transition of metastable, liquid-like protein condensates to amyloid fibrils is an emergent phenomenon of numerous neurodegeneration-linked protein systems. A key question is whether the thermodynamic driving forces underlying reversible phase separation and maturation to irreversible amyloids are distinct and separable. Here, we address this question using an engineered version of the microtubule-associated protein Tau, which forms biochemically active condensates. Liquid-like Tau condensates exhibit rapid aging to amyloid fibrils under quiescent, cofactor-free conditions. In particular, the Tau condensate interface promotes fibril nucleation, thereby impairing condensate activity in recruiting tubulin and catalyzing microtubule assembly. Remarkably, a small molecule metabolite, L-arginine, selectively impedes condensate-to-fibril transition without perturbing phase separation in a valence and chemistry-specific manner. By enhancing condensate viscoelasticity, L-arginine counteracts age-dependent decline in the biochemical activity of Tau condensates. These results provide a proof-of-principle demonstration that small molecule metabolites can reinforce the metastability of protein condensates against a liquid-to-amyloid transition, thereby preserving condensate function.