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J Mol Biol. 2018 Feb 2;430(3):297-309. doi: 10.1016/j.jmb.2017.11.015. Epub 2017 Dec 6.

Structure-Function Analysis of the C-Terminal Domain of the Type VI Secretion TssB Tail Sheath Subunit.

Author information

1
Architecture et Fonction des Macromolécules Biologiques (AFMB), Aix-Marseille Univ-Centre National de la Recherche Scientifique (CNRS), UMR 7257, Campus de Luminy, Case 932, 13288 Marseille Cedex 09, France.
2
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), Aix-Marseille Univ-Centre National de la Recherche Scientifique (CNRS), UMR7255, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
3
Architecture et Fonction des Macromolécules Biologiques (AFMB), Aix-Marseille Univ-Centre National de la Recherche Scientifique (CNRS), UMR 7257, Campus de Luminy, Case 932, 13288 Marseille Cedex 09, France. Electronic address: cambillau@afmb.univ-mrs.fr.
4
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), Aix-Marseille Univ-Centre National de la Recherche Scientifique (CNRS), UMR7255, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France. Electronic address: cascales@imm.cnrs.fr.

Abstract

The type VI secretion system (T6SS) is a specialized macromolecular complex dedicated to the delivery of protein effectors into both eukaryotic and bacterial cells. The general mechanism of action of the T6SS is similar to the injection of DNA by contractile bacteriophages. The cytoplasmic portion of the T6SS is evolutionarily, structurally and functionally related to the phage tail complex. It is composed of an inner tube made of stacked Hcp hexameric rings, engulfed within a sheath and built on a baseplate. This sheath undergoes cycles of extension and contraction, and the current model proposes that the sheath contraction propels the inner tube toward the target cell for effector delivery. The sheath comprises two subunits: TssB and TssC that polymerize under an extended conformation. Here, we show that isolated TssB forms trimers, and we report the crystal structure of a C-terminal fragment of TssB. This fragment comprises a long helix followed by a helical hairpin that presents surface-exposed charged residues. Site-directed mutagenesis coupled to functional assay further showed that these charges are required for proper assembly of the sheath. Positioning of these residues in the extended T6SS sheath structure suggests that they may mediate contacts with the baseplate.

KEYWORDS:

X-ray; contractile injection system; protein secretion; sheath; structure

PMID:
29223729
DOI:
10.1016/j.jmb.2017.11.015

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