The UBA2:SAE1 complex catalyzes the formation of a thioester bond between SUMO3 and cysteine-173 of UBA2 (Tatham et al. 2001, Werner et al. 2009). ATP reacts with the C-terminal glycine residue of SUMO3 to yield pyrophosphate and a transient intermediate, SUMO3 adenylate, which then reacts with the thiol group of the cysteine residue on UBA2.
SUMO activating enzyme activity of UBA2:SAE1 [nucleoplasm]