A widely-occurring family of pore-forming effectors broadens the impact of the Serratia Type VI secretion system

Delivery of antibacterial effector proteins into competitor cells using the Type VI secretion system (T6SS) is a widespread strategy for inter-bacterial competition. While many enzymatic T6SS effectors have been described, relatively few which form pores in target cell membranes have been reported. Here, we describe a widely-occurring family of T6SS-dependent pore-forming effectors, exemplified by Ssp4 of Serratia marcescens Db10. We show in vitro that Ssp4 forms regulated pores with high selectivity for cations, and use structural models and molecular dynamics simulations to predict how these pores conduct ions. Ssp4 has a broader phylogenetic distribution and is active against a wider range of bacterial species than Ssp6, the other pore-forming effector delivered by the same T6SS, with the two effectors displaying distinct ion selectivities and impacts on intoxicated cells. Finally, identification of Ssp4-resistant mutants revealed that a mucA mutant of Pseudomonas fluorescens is protected against T6SS attacks. We propose that deployment of two distinct T6SS-dependent pore-forming toxins is a common strategy to ensure effective de-energisation of closely- and distantly-related competitors.