This domain is found in ubiquitin activating E1 family and members of the bacterial ThiF/MoeB/HesA family. It is repeated in Ubiquitin-activating enzyme E1 [1-3].

1: Mechanism of ubiquitin activation revealed by the structure of a bacterial MoeB-MoaD complex. Lake MW, Wuebbens MM, Rajagopalan KV, Schindelin H; Nature. 2001;414:325-329. PMID:11713534

2: 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

3: Structural insights into E1-catalyzed ubiquitin activation and transfer to conjugating enzymes. Lee I, Schindelin H; Cell. 2008;134:268-278. PMID:18662542