The UBA2:SAE1 complex catalyzes the formation of a thioester linkage between the C-terminal glycine of the mature SUMO and a cysteine residue (cysteine-173) in UBA2 (SAE2) (reviewed in Wang and Dasso 2009, Wilkinson and Henley 2010, Hannoun et al. 2010, Gareau and Lima 2010). During the process the C-terminal glycine residue of SUMO is reacted with ATP to yield pyrophosphate and a transient intermediate, SUMO adenylate. The SUMO adenylate then reacts with the thiol group of the cysteine residue of UBA2 (Olsen et al. 2010).