FUS nanoclusters are a distinct state within the dilute phase

Liquid-liquid phase separation of Fused in sarcoma (FUS) has been extensively studied due to its critical role in various physiological and pathological processes. However, the mechanism of this process remains poorly understood. Here we show that by preparing pre-cleaved FUS, we enable kinetic measurement of phase separation from the onset. We identify a distinct state within the dilute phase using a combination of biophysical measurements including dynamic light scattering, fluorescence correlation spectroscopy, and an optimized single-cluster fluorescence imaging assay. This state is termed nanoclusters. They are hundreds of nanometers in diameter and exhibit instantaneous formation far below the saturation concentration, sharply arrested coarsening, rapid mixing, high exchange rate, and resistance to hexanediol-induced dissolution. This state exhibits properties that distinguish it from condensates. Our findings define the nanocluster as a prominent intermediate in the FUS phase separation pathway. This understanding paves the way for further experiments and theories to explore the molecular mechanisms underlying FUS and other phase-separating proteins.