This is an E2 domain observed in certain prokaryotic Ubiquitin systems found as secondary effectors in nucleotide-activated effector conflict systems [1-2].

1: Identification of Uncharacterized Components of Prokaryotic Immune Systems and Their Diverse Eukaryotic Reformulations. Burroughs AM, Aravind L; J Bacteriol. 2020; [Epub ahead of print] PMID:32868406

2: Anatomy of the E2 ligase fold: implications for enzymology and evolution of ubiquitin/Ub-like protein conjugation. Burroughs AM, Jaffee M, Iyer LM, Aravind L; J Struct Biol. 2008;162:205-218. PMID:18276160