SRRM2 Phase Separation Drives Assembly of Nuclear Speckle Subcompartments

Nuclear speckles (NSs) are nuclear biomolecular condensates that are postulated to arise through liquid-liquid phase separation (LLPS), 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 NS protein network and the functional relationship between SRRM2 and SON are largely unknown. Here, we uncover immiscible multiphase of SRRM2 and SON within NSs. SRRM2 and SON are functionally independent, specifically regulating alternative splicing of subsets of mRNA targets, respectively. We further uncover that SRRM2 forms multicomponent liquid phase in cells to drive NS subcompartmentalization, which is reliant on homotypic interaction and heterotypic non-selective protein-RNA complex coacervation-driven multicomponent LLPS. SRRM2 RS domains form high-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. Compared the changes of RNA alternative splicing after SRRM2 and SON gene knockdown in HeLa cells (siSRRM2, siSON).