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, and can be promoted by E3 ligases such as PIAS1-4, RANBP2 or CBX4. 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. Involved for instance in targeting RANGAP1 to the nuclear pore complex protein RANBP2. Covalently attached to the voltage-gated potassium channel KCNB1; this modulates the gating characteristics of KCNB1. Polymeric SUMO1 chains are also susceptible to polyubiquitination which functions as a signal for proteasomal degradation of modified proteins. May also regulate a network of genes involved in palate development. Covalently attached to ZFHX3. Covalently attached to KCNB1; UBE2I increases cross-linking with KCNB1 and PIAS1 decreases cross-links with KCNB1 (By similarity). Interacts with SAE2, RANBP2, PIAS1 and PIAS2 (By similarity). Interacts with PRKN (By similarity). Covalently attached to a number of proteins such as IKFZ1, PML, RANGAP1, HIPK2, SP100, p53, p73-alpha, MDM2, JUN, DNMT3B and TDG (By similarity). Also interacts with HIF1A, HIPK2, HIPK3, CHD3, EXOSC9, RAD51 and RAD52 (By similarity). Interacts with USP25 (via ts SIM domain); the interaction weakly sumoylates USP25 (By similarity). Interacts with SIMC1, CASP8AP2, RNF111 AND SOBP (via SIM domains) (By similarity). Interacts with BHLHE40/DEC1 (By similarity). Interacts with RWDD3 (By similarity). Interacts with UBE2I/UBC9 and this interaction is enhanced in the presence of RWDD3 (By similarity). Interacts with MTA1 (By similarity). Interacts with SENP2 (By similarity). Interacts with HINT1 (By similarity). Recruited by BCL11A into the nuclear body (By similarity). In the presence of ZFHX3, sequesterd to nuclear body (NB)-like dots in the nucleus some of which overlap or closely associate with PML body (By similarity). Cleavage of precursor form by SENP1 or SENP2 is necessary for function. Polymeric SUMO1 chains undergo polyubiquitination by RNF4. Belongs to the ubiquitin family. SUMO subfamily.