Sequence Composition Dictates Condensate Miscibility

Numerous biomolecular condensates coexist within cells, yet the mechanisms governing their mixing and demixing remain elusive. To investigate how the amino acid composition of intrinsically disordered regions (IDRs) affects condensate coexistence, we paired and tested 28 IDRs in 378 different combinations. Our results reveal that IDRs enriched in serine or aromatic amino acids tend to form miscible condensates. Conversely, IDRs with high charge levels form immiscible condensates, even when the serine levels are artificially boosted. Additionally, phosphorylation acts as a switch, modulating condensate miscibility. We also observed that the miscibility between transcription factor condensates and Pol II condensates profoundly influences transcription. We engineered transcription factors by increasing serine content, enhancing both their miscibility with Pol II and their transcriptional activity. However, introducing high levels of charge had the opposite effect. These findings shed light on the fundamental mechanisms controlling condensate miscibility and offer insights for designing specific functional condensates.