# Decoupling Phase Separation and Fibrillization Preserves Activity of Biomolecular Condensates

## TL;DR

This study shows that a small molecule can prevent protein condensates from turning into harmful amyloid fibrils, preserving their function.

## Contribution

The study demonstrates that L-arginine can decouple phase separation from fibrillization in Tau condensates.

## Key findings

- Tau condensates rapidly age into amyloid fibrils under quiescent conditions.
- L-arginine inhibits fibril formation without affecting phase separation.
- L-arginine preserves the biochemical activity of Tau condensates.

## Abstract

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 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. Tau condensate interface promotes fibril nucleation, impairing their activity to recruit tubulin and catalyze 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 heightening the fibril nucleation barrier, 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 enhance the metastability of protein condensates against a liquid-to-amyloid transition, thereby preserving condensate function.