Structural basis for effector recognition by an antibacterial type IV secretion system

Many soil-, water-, and plant-associated bacterial species from the orders Xanthomonadales, Burkholderales, and Neisseriales carry a type IV secretion system (T4SS) specialized in translocating effector proteins into other gram-negative species, leading to target cell death. These effectors, known as X-Tfes, carry a carboxyl-terminal domain of ∼120 residues, termed XVIPCD, characterized by several conserved motifs and a glutamine-rich tail. Previous studies showed that the XVIPCD is required for interaction with the T4SS coupling protein VirD4 and for T4SS-dependent translocation. However, the structural basis of the XVIPCD–VirD4 interaction is unknown. Here, we show that the XVIPCD interacts with the central all-alpha domain of VirD4 (VirD4(AAD)). We used solution NMR spectroscopy to solve the structure of the XVIPCD of X-Tfe(XAC2609) from Xanthomonas citri and to map its interaction surface with VirD4(AAD). Isothermal titration calorimetry and in vivo Xanthomonas citri versus Escherichia coli competition assays using wild-type and mutant X-Tfe(XAC2609) and X-Tfe(XAC3634) indicate that XVIPCDs can be divided into two regions with distinct functions: the well-folded N-terminal region contains specific conserved motifs that are responsible for interactions with VirD4(AAD), while both N- and carboxyl-terminal regions are required for effective X-Tfe translocation into the target cell. The conformational stability of the N-terminal region is reduced at and below pH 7.0, a property that may facilitate X-Tfe unfolding and translocation through the more acidic environment of the periplasm.