Two cysteines control Tse1 secretion by H1-T6SS in Pseudomonas aeruginosa

Type Six Secretion Systems (T6SS) are molecular machines that export toxic effector proteins into bacterial competitors or eukaryotic cells. Pseudomonas aeruginosa's H1-T6SS secretes Tse1, which contains a disulfide bond between cysteines at positions 7 and 148, linking its N- and C-terminal regions. The role of this disulfide bond in Tse1 activity and mechanism of action during bacterial competition is unknown. In this study, we investigated the role of the C7-C148 disulfide bond within Tse1. First, NMR spectroscopy experiments suggest a redox-active instead of a structural disulfide bond. Moreover, while the presence of this bond did not alter Tse1's amidase activity or toxicity in Escherichia coli, substituting cysteines C7 or C148 in P. aeruginosa strains affected the bacterium's capacity to lyse prey cells. Secretome analysis showed that the Tse1C148S variant was not secreted via the H1-T6SS, whereas the Tse1C7S variant was secreted. These findings suggest that cysteine 148 is likely important for Tse1's assembly with the T6SS machinery, while cysteine 7 appears to be involved in its disassembly, potentially through the formation of the disulfide bond. This study points to a potential redox regulation mechanism during the assembly and disassembly of Tse1 with Hcp1, consistent with a "bridge of delivery" model.