Ubiquitin-like protein which can be covalently attached to target lysines as a monomer. Does not seem to be involved in protein degradation and may function as an antagonist of ubiquitin in the degradation process (PubMed:11806825). Plays a role in a number of cellular processes such as nuclear transport, DNA replication and repair, mitosis and signal transduction (PubMed:11806825, PubMed:25475837). Covalent attachment to its substrates requires prior activation by the E1 complex aos-1-uba-2 and linkage to the E2 enzyme ubc-9, and can be promoted by an E3 ligase such as gei-17 (PubMed:15107848, PubMed:16701625). Required for embryonic development, fertility, vulval morphogenesis and inhibition of vulval cell fates (PubMed:15466489, PubMed:15689373, PubMed:15990876, PubMed:24349540). Probably by sumoylating bet-1, prevents muscle myosin depletion in aging adults probably by preventing myoblast growth factor receptor egl-15 overexpression (PubMed:24285704). Plays a role in the attenuation of the let-60/ras pathway (PubMed:24349540, PubMed:24285704). Plays a role in male tail tip morphogenesis (PubMed:21408209). Plays a role in the mitochondrial stress response with its covalent attachment to transcription factors dve-1 and afts-1 negatively regulating the mitochondrial unfolded protein response (PubMed:30642431). Covalently attached to tbx-2 (PubMed:16701625). Covalently attached to lin-1 (PubMed:15689373). Covalently attached to lin-11 (PubMed:15466489). Covalently attached to sop-2 (PubMed:15107848). Covalently attached to bet-1 (PubMed:24349540, PubMed:24285704). At the first embryonic mitotic division, enriched in the nucleus and released to the cytoplasm when the nuclear envelope breaks down at the start of mitosis. During mitosis, localizes to the metaphase plate and to the centrosomal region. Also localizes to segregating chromosomes at the beginning of anaphase. At late anaphase and telophase, observed in the spindle midzone and in the two daughter nuclei. Cleavage of precursor form by ulp-1 is necessary for function. RNAi-mediated knockdown reduces survival (PubMed:30642431). RNAi-mediated knockdown disrupts tail tip morphogenesis resulting in retention of the pointed larval tail tip in adult males (also known as the Lep phenotype) (PubMed:21408209). RNAi-mediated knockdown causes chromosome misalignment and anaphase bridges during the first embryonic mitotic division (PubMed:25475837). RNAi-mediated knockdown results in impaired locomotion in 23% of animals (PubMed:24285704). RNAi-mediated knockdown results in ectopic expression of egl-17 in multiple vulva precursor cells and a moderate increase in phosphorylation of mpk-1 (PubMed:24349540). RNAi-mediated knockdown results in impaired activation of the mitochondrial unfolded protein response following the inhibition of respiration induced by antimycin A (PubMed:30642431). RNAi-mediated knockdown results in ectopic tbx-2 expression in seam cells and in the syncytial hypodermis (PubMed:25873636). RNAi-mediated knockdown in a bet-1 mutant background results in decreased myo-3 levels in muscles and increased transcription levels of egl-15, sur-1 and let-60 (PubMed:24285704). Belongs to the ubiquitin family. SUMO subfamily.