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 (PubMed:18408734; PubMed:18538659; PubMed:21965678; PubMed:9556629). Plays a role in the regulation of sumoylation status of SETX (PubMed:24105744). {ECO:0000269|PubMed:18408734; ECO:0000269|PubMed:18538659; ECO:0000269|PubMed:21965678; ECO:0000269|PubMed:24105744; ECO:0000269|PubMed:9556629}.