A papain fold toxin domain found in bacterial polymorphic toxin systems. In these systems they might function either as a releasing peptidase or toxin [1]. In Shigella flexneri, UniProtKB:Q8VSD5, this protein is expressed from a plasmid, and delivered into the host via the type III secretion system where it deamidates the glutamine residue at position 100 in ubiquitin-activating enzyme E2, UBC13, to a glutamic acid residue. Invasion of host cells by pathogens normally invokes an acute inflammatory response through activating the TRAF6-mediated signalling pathway. UBC13 helps to activate TRAF6. Thus deamidation of UBC13 results in the dampening of the inflammatory response. The key glutaminase deamidase activity is mediated by a cys-his-glu triad, present in all members of the family [2,3].

1: Polymorphic toxin systems: comprehensive characterization of trafficking modes, processing, mechanisms of action, immunity and ecology using comparative genomics. Zhang D, de Souza RF, Anantharaman V, Iyer LM, Aravind L; Biol Direct. 2012;7:18. PMID:22731697

2: The Shigella flexneri effector OspI deamidates UBC13 to dampen the inflammatory response. Sanada T, Kim M, Mimuro H, Suzuki M, Ogawa M, Oyama A, Ashida H, Kobayashi T, Koyama T, Nagai S, Shibata Y, Gohda J, Inoue J, Mizushima T, Sasakawa C; Nature. 2012;483:623-626. PMID:22407319

3: Structural basis for the recognition of Ubc13 by the Shigella flexneri effector OspI. Nishide A, Kim M, Takagi K, Himeno A, Sanada T, Sasakawa C, Mizushima T; J Mol Biol. 2013;425:2623-2631. PMID:23542009