A decade has passed since SUMO (small ubiquitin-related modifier) was discovered to be a reversible post-translational protein modifier. During this time many enzymes that participate in regulated SUMO-conjugation and -deconjugation pathways have been identified and characterized. In parallel, the search for SUMO substrates has produced a long list of targets, which appear to be involved in most cellular functions. Sumoylation is a highly dynamic process and its outcomes are extremely diverse, ranging from changes in localization to altered activity and, in some cases, stability of the modified protein. At first glance, these effects have nothing in common; however, it seems that they all result from changes in the molecular interactions of the sumoylated proteins.

Posttranslational modification with small ubiquitin-related modifier (SUMO) proteins is now established as one of the key regulatory protein modifications in eukaryotic cells. Hundreds of proteins involved in processes such as chromatin organization, transcription, DNA repair, macromolecular assembly, protein homeostasis, trafficking, and signal transduction are subject to reversible sumoylation. Hence, it is not surprising that disease links are beginning to emerge and that interference with sumoylation is being considered for intervention. Here, we summarize basic mechanisms and highlight recent developments in the physiology of sumoylation.

We have found that the mammalian Ran GTPase-activating protein RanGAP1 is highly concentrated at the cytoplasmic periphery of the nuclear pore complex (NPC), where it associates with the 358-kDa Ran-GTP-binding protein RanBP2. This interaction requires the ATP-dependent posttranslational conjugation of RanGAP1 with SUMO-1 (for small ubiquitin-related modifier), a novel protein of 101 amino acids that contains low but significant homology to ubiquitin. SUMO-1 appears to represent the prototype for a novel family of ubiquitin-related protein modifiers. Inhibition of nuclear protein import resulting from antibodies directed at NPC-associated RanGAP1 cannot be overcome by soluble cytosolic RanGAP1, indicating that GTP hydrolysis by Ran at RanBP2 is required for nuclear protein import.

SUMO (small ubiquitin-related modifier) is the best-characterized member of a growing family of ubiquitin-related proteins. It resembles ubiquitin in its structure, its ability to be ligated to other proteins, as well as in the mechanism of ligation. However, in contrast to ubiquitination-often the first step on a one-way road to protein degradation-SUMOlation does not seem to mark proteins for degradation. In fact, SUMO may even function as an antagonist of ubiquitin in the degradation of selected proteins. While most SUMO targets are still at large, available data provide compelling evidence for a role of SUMO in the regulation of protein-protein interactions and/or subcellular localization.