A predicted immunity protein with an alpha+beta fold and a conserved tryptophan residue. Proteins containing this domain are present in bacterial polymorphic toxin systems as an immediate gene neighbour of the toxin gene, usually containing a protease domain such as Tox-PL1 and Ntox40. In some instances, it is also fused to a papain-like toxin, ADP-ribosyl glycohydrolase and a S8-like peptidase [1]. Based on these associations the domain is likely to be a protease inhibitor.

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