SRRM2 phase separation drives assembly of nuclear speckle subcompartments

Nuclear speckles (NSs) are nuclear biomolecular condensates that are postulated to form by macromolecular phase separation, although the detailed underlying forces driving NS formation remain elusive. SRRM2 and SON are 2 non-redundant scaffold proteins for NSs. How each individual protein governs assembly of the NS protein network and the functional relationship between SRRM2 and SON are largely unknown. Here, we uncover immiscible multiphases of SRRM2 and SON within NSs. SRRM2 and SON are functionally independent, specifically regulating alternative splicing of subsets of mRNA targets, respectively. We further show that SRRM2 forms multicomponent liquid phases in cells to drive NS subcompartmentalization, which is reliant on homotypic interaction and heterotypic non-selective protein-RNA complex coacervation-driven phase separation. SRRM2 serine/arginine-rich (RS) domains form higher-order oligomers and can be replaced by oligomerizable synthetic modules. The serine residues within the RS domains, however, play an irreplaceable role in fine-tuning the liquidity of NSs. [Display omitted] •SRRM2 and SON form co-existing dense phases within nuclear speckles•SRRM2 undergoes phase separation to drive NS subcompartmentalization•SRRM2 RS domains form high-order oligomers to trigger NS condensation Zhang et al. uncover the spatial and functional relationships of SRRM2 and SON in nuclear speckles. SRRM2 forms multicomponent dense phases in cells to drive NS condensation, which is reliant on homotypic oligomerization and heterotypic protein-RNA complex coacervation-driven phase separation.