inPhase — A simple, accurate and fast approach to determine phase diagrams of protein condensates

Protein phase separation has become a widely studied phenomena in biology with implications in cell metabolism and disease. The study of phase separating proteins often relies on the precise determination of their phase diagrams. These phase diagrams give information on the protein concentration required for condensate formation and the respective concentration inside the condensate at defined external conditions (temperature, salt, pH). However, it has so far often proven difficult to accurately measure phase diagrams. Here, we report a method that is based on mass and volume conservation and defined reaction volumes, which we call inPhase. We can use this method to determine accurate values for both dilute and condensed branch protein concentrations. With this information we can produce accurate phase diagrams. We compare our method to the widely used quantitative fluorescence approach and find that it underestimates the partition factor into condensates at least two-fold for FUS and PGL-3. The accessibility of our method opens the possibility for the thermodynamic assessment of entire protein families, generating sufficient quantitative data for testing theory, and for the screening of drugs in pharmaceutical research.