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. 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 (By similarity). Interacts with sae2 and ube2i. Covalently attached to a number of proteins (By similarity). Polymeric chains can be formed through Lys-11 cross-linking. Cleavage of precursor form by a sentrin-specific protease is necessary for function. Belongs to the ubiquitin family. SUMO subfamily.