Nucleolar Reorganization After Cellular Stress is Orchestrated by SMN Shuttling Between Nuclear Compartments

SMA is an autosomal recessive neuromuscular disease caused by mutations in the multifunctional protein SMN. Within the nucleus, SMN localizes to Cajal bodies (CBs), which have been shown to be associated with nucleoli, nuclear organelles dedicated to the first steps of ribosome biogenesis. The highly organized structure of the nucleolus can be dynamically altered by genotoxic agents. After a genotoxic stress, RNAP1, Fibrillarin (FBL) and nucleolar DNA are exported to the periphery of the nucleolus and once DNA repair is fully completed the organization of the nucleolus is restored. We found that SMN is required for the restoration of nucleolar structure after genotoxic stress. Unexpectedly, during DNA repair, SMN shuttles from the CBs to the nucleolus. This shuttling is important for the nucleolar homeostasis and relies on the presence of Coilin, FBL and the activity of PRMT1.