Ubiquitin-like protein that can be covalently attached to proteins as a monomer or as a lysine-linked polymer. Covalent attachment via an isopeptide bond to its substrates requires prior activation by the E1 complex SAE1-SAE2 and linkage to the E2 enzyme UBE2I, and can be promoted by an E3 ligase such as PIAS1-4, RANBP2 or CBX4. This post-translational modification on lysine residues of proteins plays a crucial role in a number of cellular processes such as nuclear transport, DNA replication and repair, mitosis and signal transduction. Polymeric SUMO2 chains are also susceptible to polyubiquitination which functions as a signal for proteasomal degradation of modified proteins. Homotrimer (Potential). Crystal packing analysis suggests a possible trimeric assembly, of which the biological significance remains to be determined. Interacts with SAE2 and UBE2I. Covalently attached to a number of proteins. Interacts with PELP1. Interacts with USP25; the interaction sumoylates USP25. Broadly expressed. Belongs to the ubiquitin family. SUMO subfamily. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.