Modulation of Biomolecular Liquid–Liquid Phase Separation by Preferential Hydration and Interaction of Small Osmolytes with Proteins

We examined the effects of trimethylamine N-oxide (TMAO) and urea (known osmolytes) on the liquid–liquid phase separation (LLPS) of fused in sarcoma (FUS) and three FUS-LLPS states: LLPS states at atmospheric pressure with low- and high-salt concentrations and a re-entrant LLPS state above 2 kbar. Temperature- and pressure-scan turbidity measurements revealed that TMAO and urea contributed to stabilizing and destabilizing LLPS, respectively. These results can be attributed to the excluded volume effect of TMAO (preferential hydration) and preferential interaction of urea with proteins. Additionally, TMAO counteracted the effects of equimolar urea on LLPS, a phenomenon not previously reported. The concept of the m-value for osmolyte-induced protein folding and unfolding can be applied to the osmolyte’s effects on LLPS. In conclusion, biomolecular LLPS can be modulated by preferential hydration and the interaction of small osmolytes with proteins, thereby facilitating LLPS formation, even in extreme environments characterized by high-salt, high-urea, and high-pressure conditions.