ATP promotes protein coacervation through conformational compaction.

ATP has been recognized as a hydrotrope in the phase separation process of intrinsically disordered proteins (IDPs). Surprisingly, when using the disordered RG/RGG-rich motif from HNRNPG protein as a model system, we discover a biphasic relationship between the ATP concentration and IDP phase separation. We show that at a relatively low ATP concentration, ATP dynamically interacts with the IDP, which neutralizes protein surface charges, promotes intermolecular interactions, and consequently promotes phase separation. We further demonstrate that ATP induces a compact conformation of the IDP, accounting for the reduced solvent exchange rate and lower compression ratio during phase separation. As ATP concentration increases, its hydrotropic properties emerge, leading to the dissolution of the phase-separated droplets. Our finding uncovers a complex mechanism by which ATP molecules modulate the structure, interaction, and phase separation of IDPs, and accounts for the distinct phase separation behaviors for the charge-rich RGG motif and other low-complexity IDPs.