This is the C-terminal domain of ubiquitin-activating enzyme and SUMO-activating enzyme 2. It is structurally similar to ubiquitin. This domain is involved in E1-SUMO-thioester transfer to the SUMO E2 conjugating protein [1].

1: Structures of the SUMO E1 provide mechanistic insights into SUMO activation and E2 recruitment to E1. Lois LM, Lima CD; EMBO J. 2005;24:439-451. PMID:15660128