Density and viscosity measurements on the liquid condensates of FUS protein low-complexity domain

Recent discoveries have revealed in living cells the formation of liquid droplets consisting of proteins, RNA & DNA. Changes in interactions among these biomolecules may result in devastating diseases, such as amyotrophic lateral sclerosis (ALS). Thus, investigating the physical properties of these liquid droplets in connection with liquid-liquid phase transition is relevant to biomolecular functions and potential therapeutic interventions. We report experiments on the dense liquid droplets formed by FUS (Fused in Sarcoma) low-complexity domain, a section of a protein involved in the development of ALS. By applying a ball drop method under a microscope, we found the density of the protein droplets greater than that of water by 10-15%, indicative of the protein concentration ~200mg/ml. The viscosity of the dilute phase containing FUS droplets is similar to that of water, but viscosity within FUS droplets is very high, ~4000 that of water. Additionally, we found FUS droplets wet solid surface poorly based on the contact angle measurement. These results confirm extremely high protein concentration and strong intermolecular cohesiveness within the condensate state.