An onboard checking mechanism ensures effector delivery of the type VI secretion system in Vibrio cholerae.

The type VI secretion system (T6SS) is a lethal yet energetically costly weapon in gram-negative bacteria. Through contraction of a long sheath, the T6SS ejects a few copies of effectors accompanied by hundreds of structural carrier proteins per delivery. The few ejected effectors, however, dictate T6SS functions. It remains elusive how the T6SS ensures effector loading and avoids futile ejection. Here, by systemically mutating the active sites of 3 Vibrio cholerae effectors, TseL, VasX, and VgrG3, we show that the physical presence but not their activities is crucial for T6SS assembly. We constructed catalytic mutants of TseL and VgrG3 and truncated VasX mutants. These mutations abolished the killing of the effector-cognate immunity mutants. We determined that the VasX-mediated antimicrobial activity is solely dependent on the C-terminal colicin domain. Removal of the colicin domain abolished VasX secretion and reduced T6SS assembly, while deletion of the colicin internal loop abolished its toxicity but had little effect on secretion and assembly. The triple effector-inactive mutant maintains an active T6SS that is capable of delivering chimeric VgrG, PAAR, and TseL proteins fused with a cargo nuclease, indicating effector activities are not required for T6SS assembly or penetration into the cytosol of recipient cells. Therefore, by recruiting effectors as critical components for T6SS assembly, it represents an effective onboard checking mechanism that ensures effectors are loaded in place to prevent futile secretion. Our study also demonstrates a detoxified secretion platform by inactivating native effector activities that could translocate engineered cargo proteins via multiple routes.