Identification and characterization of the nuclear localization/retention signal in the EWS proto-oncoprotein

Ewing sarcoma (EWS) protein, a member of a large family of RNA-binding proteins, contains an N-terminal transcriptional activation domain (EAD) and a C-terminal RNA-binding domain (RBD). Due to its multifunctional properties EWS protein is involved in processes such as gene expression, RNA processing and transport, and cell signaling. Chimeric EWS proteins generated by chromosomal translocations cause malignant tumors. EWS protein is located predominantly in the nucleus, but was found also in the cytosol and associated with the cell membrane. The determinants responsible for the nuclear localization of the protein were as yet unknown. We identified the nuclear localization signal of EWS protein at its C terminus (C-NLS), which is required for the nuclear import and retention of the protein. The C-NLS sequence is conserved in related proto-oncoproteins suggesting an NLS function also in these proteins. Two arginine residues, due to their positive charge, a proline residue and a tyrosine residue are essential for C-NLS function. The nuclear localization of EWS protein is independent of the regions in RBD containing numerous arginine methylation sites, RNA-recognition and zinc finger motifs. Regions in EAD guide the subnuclear partition of EWS protein and contain another but different NLS that allows nucleocytoplasmic shuttling of the N-terminal domain.