Vibrio cholerae type 6 secretion system effector trafficking in target bacterial cells [Microbiology]

The type 6 secretion system (T6SS) is used by many Gram-negative bacterial species to deliver toxic effector proteins into nearby bacteria prey cells to kill or inhibit their growth. VgrG proteins are core conserved secretion substrates of the T6SS and one subset of T6SS effectors consists of VgrG proteins with C-terminal extension domains carrying various enzymatic activities. In Vibrio cholerae, VgrG3 has a hydrolase extension domain and degrades peptidoglycan in the periplasm of target bacteria. In this study, we replaced this domain with a nuclease domain from Salmonella enterica subsp. arizonae. This modified V. cholerae strain was able to kill its parent using its T6SS. This result also demonstrated that V. cholerae T6SS is capable of delivering effectors that could attack substrates found either in the periplasm or cytosol of target bacteria. Additionally, we found that effectors VgrG3 and TseL, despite lacking a classical Sec or TAT secretion signal, were able to reach the periplasm when expressed in the bacterial cytosol. This effector trafficking likely represents an evolutionary strategy for T6SS effectors to reach their intended substrates regardless of which subcellular compartment in the target cell they happen to be delivered to by the T6SS apparatus.