The ubiquitin-like SUMO-3 is conjugated to a variety of proteins in the presence of UbcH9 and the SUMO E1 activating enzyme (SAE1/SAE2 in human, or Aos1/Uba2p in yeast). This isoform is known to form poly-SUMO chains on substrates via isopeptide bond formation between the Cterminal glycine of one SUMO-3 to the lysine-ε-amino group of K11 on the preceding SUMO-3. The heterodimeric SAE1/SAE2 complex (38 and 70 kDa respectively) uses ATP to adenylate the Cterminal glycine residue of SUMO-3, forming a high-energy thiolester bond with the SAE2 subunit. The second step is the trans-esterification reaction whereby the activated SUMO-3 is transferred to Cys93 of UbcH9. UbcH9 is a member of the E2 family and is homologous to ubiquitin-conjugating enzymes, but is specific for the conjugation of SUMO to a variety of target proteins. This E2 is unusual in that it interacts directly with protein substrates that are modified by sumolyation, and may play a role in substrate recognition. Sumoylated substrates are primarily localized to the nucleus (RanGAP-1, RANBP2, PML, p53, Sp100, HIPK2) but there are also cytosolic substrates(IκBα, GLUT1, GLUT4). SUMO modification has been implicated in functions such as nuclear transport, chromosome segregation and transcriptional regulation, apoptosis and protein function and stability.