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 (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.