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Abstract
Type VI secretion systems (T6SSs) are nanomachines widely used by bacteria to deliver toxic effector proteins directly into neighbouring cells. However, the modes of action of many effectors remain unknown. Here we report that Ssp6, an anti-bacterial effector delivered by a T6SS of the opportunistic pathogen Serratia marcescens, is a toxin that forms ion-selective pores. Ssp6 inhibits bacterial growth by causing depolarisation of the inner membrane in intoxicated cells, together with increased outer membrane permeability. Reconstruction of Ssp6 activity in vitro demonstrates that it forms cation-selective pores. A survey of bacterial genomes reveals that genes encoding Ssp6-like effectors are widespread in Enterobacteriaceae and often linked with T6SS genes. We conclude that Ssp6 and similar proteins represent a new family of T6SS-delivered anti-bacterial effectors.
Highlights
Type VI secretion systems (T6SSs) are nanomachines widely used by bacteria to deliver toxic effector proteins directly into neighbouring cells
According to the current model[1,3,4,5], contraction of an extended cytoplasmic sheath anchored in a trans-membrane basal complex propels a cell-puncturing structure, comprising a tube of Hcp hexamers tipped by a VgrG-PAAR spike, out of the secreting cell and towards a target cell
In the previous sections we showed that Ssp6-mediated intoxication causes depolarisation of the inner membrane of target cells
Summary
Type VI secretion systems (T6SSs) are nanomachines widely used by bacteria to deliver toxic effector proteins directly into neighbouring cells. We report that Ssp[6], an anti-bacterial effector delivered by a T6SS of the opportunistic pathogen Serratia marcescens, is a toxin that forms ion-selective pores. The opportunistic pathogen, Serratia marcescens, possesses a potent anti-bacterial T6SS, which secretes at least eight antibacterial effector proteins, in addition to two anti-fungal effectors[14,15,16]. Several of these anti-bacterial effectors are not related to previously-characterised effectors and have mechanisms that cannot be readily predicted. Whilst many anti-bacterial T6SS effectors have demonstrated or predictable functions, there remain many others whose function is unknown or not yet fully characterised and which may represent new classes of antibacterial toxins. Homologues of Ssp[6] can be found in many species of Enterobacteriaceae, Ssp[6] defines a new family of T6SS-delivered, ion-selective pore-forming toxins
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