Ubiquitin-like protein that can be covalently attached to proteins as a monomer or a lysine-linked polymer (By similarity). 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. 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 sumo1 chains are also susceptible to polyubiquitination which functions as a signal for proteasomal degradation of modified proteins (By similarity). Interacts with sae2, ube2i, ranbp2, pias1 and pias2 (By similarity). Interacts with sox9 and sox10 (By similarity). Covalently attached to a number of proteins (By similarity). Cleavage of precursor form by a sentrin-specific protease is necessary for function. Belongs to the ubiquitin family. SUMO subfamily.