Cationic Amino Acid Identity and Net Charge Influence Condensate Properties in .

Biomolecular condensates (BMCs) are central to subcellular organization, influencing processes from RNA metabolism to the stress response and amyloid pathologies. Despite their near ubiquity, we still do not fully understand how the primary sequence of biomolecules influences the formation and dynamics of condensates. Here, we examine how cationic amino acid identity shapes the properties of protein-RNA coacervates. Using engineered recombinant proteins, we find that phase boundaries depend on both amino acid identity and protein net charge. Despite identical charge at physiological pH, arginine promotes a higher salt resistance in vitro, enhanced condensate formation in , and reduced protein mobility both in vitro and in cells. Together, these results suggest that, in addition to electrostatic interactions and disorder as the main driving forces of phase separation in biological contexts, the primary sequence and side chain composition of proteins play a significant role in dictating the dynamics of coacervates.