Ubiquitin-like protein that can be covalently attached to proteins as a monomer or 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. 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. Interacts with SAE2, UBE2I, RANBP2, PIAS1 and PIAS2 (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.