Structural insight into how Pseudomonas aeruginosa peptidoglycan-hydrolase Tse1 and its immunity protein Tsi1 function

Tse1 (Tse is type VI secretion exported), an effector protein produced by Pseudomonas aeruginosa, is an amidase that hydrolyses the gamma-D-glutamyl-DAP (gamma-D-glutamyl-L-meso-diaminopimelic acid) linkage of the peptide bridge of peptidoglycan. P. aeruginosa injects Tse 1 into the periplasm of recipient cells, degrading their peptidoglycan, thereby helping itself to compete with other bacteria. Meanwhile, to protect itself from injury by Tse1, aeruginosa expresses the cognate immunity protein Tsi1 (Tsi is type VI secretion immunity) in its own periplasm to inactivate Tse1. In the present paper, we report the crystal structures of Tse1 and the Tse1-(6-148)-Tsi1-(20-end) complex at 1.4 A and 1.6 A (1 A = 0.1 nm) resolutions respectively. The Tse1 structure adopts a classical papain-like alpha + beta fold. A cysteine histidine catalytic