T6SS bacteria employ toxic effectors to inhibit rival cells and concurrently use effector cognate immunity proteins to protect their sibling cells. The effector and immunity pairs (E-I pairs) endow the bacteria with a great advantage in niche competition. This is the C-terminal domain of Tli4. The Tle cognate immunity proteins (Tlis) can directly disable the transported Tle protein and thereby mediate the self-protection process. The Tle-Tli effector-immunity (E-I) pairs confer substantial advantage to the donor cell during interbacterial competition. Tli4 displays a two-domain structure, in which a large lobe and a small lobe form a crab claw-like conformation. Tli4 uses this crab claw to grasp the cap domain of Tle4, especially the lid2 region, which prevents the interfacial activation of Tle4 and thus causes enzymatic dysfunction of Tle4. Structural comparison indicates similarity between this C-terminal domain of Tli4 and Tsi3, which is the cognate immunity protein of the effector protein Tse3 in P. aeruginosa PDB:4n7s [1].

1: The structural basis of the Tle4-Tli4 complex reveals the self-protection mechanism of H2-T6SS in Pseudomonas aeruginosa. Lu D, Zheng Y, Liao N, Wei L, Xu B, Liu X, Liu J; Acta Crystallogr D Biol Crystallogr. 2014;70:3233-3243. PMID:25478841