Blockage of effector secretion before host cell contact is mediated, in part, by a protein that has been proposed to act as either a physical impediment to the entrance to the secretion apparatus,5 or as a gatekeeper that determines substrate hierarchy.6 Across bacterial species, this protein (known as MxiC in Shigella sp.) possesses only weak sequence homology and in some species exists as two separate polypeptide chains (e.g., in Yersinia sp. the homologue consists of YopN and TyeA).7 Despite this, distinct functional homologies can be identified across species. Functional knock-outs of members of this family have no effect on needle formation or stability but significantly reduce or abolish the secretion of translocators.8–11 In addition, in several species these mutations also result in enhanced secretion of effector proteins.9–14 This differential effect on translocator and effector secretion suggests that these proteins have a role in T3SS discrimination between secreted proteins involved in translocation and proteins that have effector function.