SUMO-1 binds to a wide range of target proteins as part of a post-translational modification system. Unlike ubiquitin, it does not seem to target protein for degradation, but is involved in a number of cellular processes such as nuclear transport, DNA replication and repair, mitosis, apoptosis, protein stability and signal transduction. SUBUNIT: Covalently attached to a number of proteins such as PML, RANGAP1, HIPK2, SP100, p53, p73alpha, MDM2, JUN and DNMT3B. Also interacts with HIF1A, HIPK2, HIPK3, CHD3, PIAS1, EXOSC9, TDG, RAD51 and RAD52. SUBCELLULAR LOCATION: Nucleus; nuclear membrane. Nucleus; nucleoplasm; nuclear speckle. Cytoplasm. SIMILARITY: Belongs to the ubiquitin family. SMT3 subfamily. SIMILARITY: Contains 1 ubiquitin-like domain. PTM: Cleavage of the last four amino acids of the carboxy-terminus of the precursor form by SENP1 or SENP2 is necessary for function. Several pseudogenes have been reported as well as a number of alternatively spliced isoforms.