A papain fold toxin domain found in bacterial polymorphic toxin systems. In these systems they might function either as a releasing peptidase or toxin. 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.