Mapping interactions between disordered regions reveals promiscuity in biomolecular condensate formation

Intrinsically disordered regions (IDRs) drive intracellular phase separation and biomolecular condensate formation through interactions encoded in their sequence. Although condensates form spatially distinct assemblies in cells, the high conformational flexibility of IDRs and absence of well-defined 3D structures raise the question of how they could encode condensate specificity. To systematically characterize IDR-IDR interactions and their ability to mediate self-specific partitioning, we develop micDROP, a synthetic system of multivalent IDRs forming constitutive droplets. We examine ten natural IDRs that phase-separate in micDROP and find that their saturation concentrations in vivo correlate with total sequence stickiness. Co-expression of IDR pairs fused to distinct micDROP scaffolds reveals widespread promiscuity, whereas TDP43 and UBQ2 consistently form self-specific condensates. A short, conserved α-helical segment in the TDP43 IDR governs this self-recognition. Our results indicate that IDRs tune phase separation propensity through sequence composition, while discrete condensate identity likely requires additional structural determinants.