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J Bacteriol. 2019 Mar 25. pii: JB.00173-19. doi: 10.1128/JB.00173-19. [Epub ahead of print]

Role and recruitment of the TagL peptidoglycan-binding protein during Type VI secretion system biogenesis.

Author information

1
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), Aix-Marseille Université - CNRS, UMR 7255, 31 chemin Joseph Aiguier CS7071, 13402 Marseille Cedex 09, France.
2
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), Aix-Marseille Université - CNRS, UMR 7255, 31 chemin Joseph Aiguier CS7071, 13402 Marseille Cedex 09, France cascales@imm.cnrs.fr.

Abstract

The Type VI secretion system (T6SS) is an injection apparatus that uses a spring-like mechanism for effector delivery. The contractile tail is composed of a needle tipped by a sharpened spike and wrapped by the sheath that polymerizes in an extended conformation on the assembly platform or baseplate. Contraction of the sheath propels the needle and effectors associated with it into target cells. The passage of the needle through the cell envelope of the attacker is assured by a dedicated trans-envelope channel complex. This membrane complex (MC) comprises the TssJ lipoprotein, and the TssL and TssM inner membrane proteins. MC assembly is a hierarchized mechanism in which the different subunits are recruited in a specific order: TssJ, TssM and then TssL. Once assembled, the MC serves as a docking station for the baseplate. In enteroaggregative Escherichia coli, the MC is accessorized by TagL, a peptidoglycan-binding (PGB) inner membrane-anchored protein. Here we show that the PGB domain is the only functional domain of TagL, and that the N-terminal transmembrane region mediates contact with the TssL transmembrane helix. Finally, we conduct fluorescence microscopy experiments to position TagL in the T6SS biogenesis pathway, demonstrating that TagL is recruited to the membrane complex downstream TssL and is not required for baseplate docking.Importance Bacteria use weapons to deliver effector into target cells. One of these weapons, called Type VI secretion system (T6SS), could be compared to a nano-speargun using a spring-like mechanism for effector injection. By targeting bacteria and eukaryotic cells, the T6SS reshapes bacterial communities and hijacks host cell defences. In enteroaggregative Escherichia coli, the T6SS is a multiprotein machine that comprises a cytoplasmic tail and a peptidoglycan-anchored trans-envelope channel. In this work, we show that TagL comprises a N-terminal domain that mediates contact with the channel, and a peptidoglycan-binding domain that binds the cell wall. We then determine at which stage of T6SS biogenesis TagL is recruited and how TagL absence impacts the assembly pathway.

PMID:
30910811
DOI:
10.1128/JB.00173-19

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