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 modulate protein subcellular localization, stability or activity. Upon oxidative stress, conjugates to various anti-oxidant enzymes, chaperones, and stress defense proteins. May also conjugate to NFKBIA, TFAP2A and FOS, negatively regulating their transcriptional activity, and to NR3C1, positively regulating its transcriptional activity. Covalent attachment to its substrates requires prior activation by the E1 complex SAE1-SAE2 and linkage to the E2 enzyme UBE2I. Interacts with SAE2. Covalently attached to a number of proteins (Probable). Expressed mainly in adult and embryonic kidney. Expressed at various levels in immune tissues, with the highest expression in the lymph node and spleen. In contrast to SUMO1, SUMO2 and SUMO3, seems to be insensitive to sentrin-specific proteases due to the presence of Pro-90. This may impair processing to mature form and conjugation to substrates. Disease susceptibility may be associated with variants affecting the gene represented in this entry. Valine at position 55 results in greater NFKB1 transcriptional activity and IL12B expression and may be associated with susceptibility to insulin-dependent diabetes mellitus. Belongs to the ubiquitin family. SUMO subfamily. SUMO protein entry