Ubiquitin-like protein which can be covalently attached to target lysines either as a monomer or as a lysine-linked polymer. Does not seem to be involved in protein degradation and may function as an antagonist of ubiquitin in the degradation process. Plays a role in a number of cellular processes such as nuclear transport, DNA replication and repair, mitosis and signal transduction. Covalent attachment 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. Plays a role in the regulation of sumoylation status of SETX (By similarity). Interacts with SAE2 and UBE2I. Covalently attached to a number of proteins. Interacts with USP25 (via ts SIM domain); the interaction sumoylates USP25 and inhibits its ubiquitin hydrolyzing activity (By similarity). Interacts with ARNTL/BMAL1. Polymeric chains can be formed through Lys-11 cross-linking. Cleavage of precursor form by SENP1, SENP2 or SENP5 is necessary for function. Belongs to the ubiquitin family. SUMO subfamily.