Phase Separation Of C.Elegans Polycomb Protein SOP-2 Is Modulated By RNA And Sumoylation

Polycomb group(PcG)genes encode a highly conserved group of transcriptional repressors,which regulate a diverse of biological processes by inhibiting gene transcription.SOP-2 is a component of Polycomb related complex(PRC1)in Caenorhabditis elegans(C.elegans).Loss function of sop-2 leads to abnormal expression of Hox genes,resulting in massive homeotic transformations.SOP-2 forms distinct nuclear bodies,called "SOP-2 body",whose formation is related to its physiological function.The mechanism by which SOP-2 bodies are formed remains largely unknown.Recent studies revealed that formation of protein condensates could be mediated via liquid-liquid phase separation(LLPS).LLPS is driven by weak multivalent interactions conferred by modular domains or intrinsically disordered regions(IDRs).Phase separation is regulated by multiple factors affecting the intermolecular interaction strength such as temperature,ion concentration,the composition and concentration of key molecules.Post-translational modifications(PTMs),including phosphorylation,methylation,acylation ubiquitination and sumoylation,also regulate protein-protein or protein-RNA interaction strengths.Protein condensates with liquid properties gradually transition into gel-like and eventually into solid aggregates.In this study we demonstrated that SOP-2 bodies in the nucleus possess liquid-like properties.SOP-2 contains an IDR region and a SAM domain.We found that the IDR region of SOP-2(referred to as SOP-2(IDR)hereafter)undergo LLPS.Phase-separated SOP-2(IDR)droplets exhibit fusion and wetting phenotypes and their formation are sensitive to ionic strength,temperature and other factors.SOP-2(IDR)condensates transit into gel-like structures over time through FRAP assay observation.RNA and SAM domain modulates LLPS of SOP-2(IDR).We also found that sumoylation promotes LLPS of SOP-2 in vitro and the formation of SOP-2 bodies in vivo.Our study indicates that the PcG protein SOP-2 undergoes phase separation and LLPS of SOP-2 is regulated by SAM domain-mediated dimerization,post-translational sumoylation and corresponding RNA.