Crystal structure of the Agrobacterium tumefaciens type VI effector–immunity complex

The type VI secretion system (T6SS) comprises needle-shaped multisubunit complexes that play a role in the microbial defense systems of Gram-negative bacteria. Some Gram-negative bacteria harboring a T6SS deliver toxic effector proteins into the cytoplasm or periplasm of competing bacteria in order to lyse and kill them. To avoid self-cell disruption, these bacteria have cognate immunity proteins that inhibit their toxic effector proteins. T6SS amidase effector protein 4 (Tae4) and T6SS amidase immunity protein 4 (Tai4) are a representative of the toxic effector–immunity pairs of the T6SS. Here, the three-dimensional structures of Tai4 and the Tae4–Tai4 complex from Agrobacterium tumefaciens are reported at 1.55 and 1.9 Å resolution, respectively. A structural comparison with other Tae4–Tai4 homologs revealed similarities and differences in the catalytic and inhibitory mechanisms among the Tae4 and Tai4 family proteins.