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The Gram-negative bacteria type VI secretion system (T6SS) has been found to play an important role in interbacterial competition, biofilm formation and many other virulence-related processes. The bacteria harboring T6SS inject the effectors into their recipient’s cytoplasm or periplasm to kill them and meanwhile, to avoid inhibiting itself, the cognate immunity proteins were produced to acts as the effector inhibitor. Tae4 (type VI amidase effector 4) and Tai4 (type VI amidase immunity 4) are newly identified T6SS effector-immunity (EI) pairs. We have recently solved the structures of  St Tae4-Tai4 and  Ec Tae4-Tai4 complexes from the human pathogens  Salmonella typhimurium  and  Enterobacter cloacae , respectively. It is very interesting and important to discover whether there is cross-neutralization between  St-  and  Ec Tai4 and whether their effector inhibition mechanism is conserved. Here, we determined the crystal structure of  St Tae4 in complex with  Ec Tai4. The solution conformation study revealed it is a compact heterotetramer that consists of an  Ec Tai4 homodimer binding two  St Tae4 molecules in solution, different from that in crystal. A remarkable shift can be observed in both the flexible winding loop of  St Tae4 and protruding loop of  Ec Tai4 and disulfide bonds are formed to stabilize their overall conformations. The  in vitro  and  in vivo  interactions studies showed  Ec Tai4 can efficiently rescue the cells from the toxicity of its cognate effectors  St Tae4, but can not neutralize the toxic activities of the effectors from other families. These findings provide clear structural evidence to support the previous observation of cross-immunity within T6SS families and provide a basis for understanding their important roles in polymicrobial environments.

Insights into the Cross-Immunity Mechanism within Effector Families of Bacteria Type VI Secretion System from the Structure of StTae4-EcTai4 Complex