Tuning Biomolecular Condensates: Microscopic Insights into Salt-Driven Liquid-Liquid Phase Separation

Liquid–liquid phase separation (LLPS) of intrinsically disordered proteins (IDPs) exhibits complex behavior that remains poorly understood at the microscopic level. We investigate the phase behavior of the low-complexity domain of the RNA-binding protein FUsed in Sarcoma (FUS-LC) as a function of salt concentration using explicit-solvent coarse-grained molecular dynamics simulations and analytical theory. Here, we uncover a distinct physical mechanism governing the observed salt concentration-driven reentrant LLPS of uncharged IDPs, such as FUS-LC. FUS exhibits LLPS at physiological salt concentrations (~0.15M), dissolves at higher salt concentrations (~1M), and again phase separates at even higher salt concentrations (~3M). We discover that, at low salt, correlated ionic fluctuations generate long-range effective attractions, enriching the ion population inside condensates. As salt-ions increase, reduced correlation length leads to dissolution of the condensate, while a second LLPS emerges at high salt due to entropic effects resulting in ion expulsion from the condensate. The mechanism, particularly in the low salt regime, is quite different from that of charged (but net neutral) IDPs. These findings provide a unified physical framework for salt-dependent LLPS in IDPs.