Biogenesis and structure of a Type VI secretion membrane core complex
Résumé
Bacteria share their ecological niches with other microbes. The bacterial Type VI secretion system is one of the key players for microbial competition, as well as an important virulence determinant during bacterial infections. It assembles a nano-crossbow-like structure that propels an arrow made of Hcp tube and VgrG spike into the cytoplasm of the attacker cell and punctures the prey's cell wall. The nano-crossbow is stably anchored to the cell envelope of the attacker by a membrane core complex. Here, we show that this complex is assembled by the sequential addition of three proteins-TssJ, TssM and TssL-and present a 11.6 Å resolution structure of the fully assembled complex, determined by negative stain electron microscopy. With overall C5 symmetry, this 1.7-megadalton complex comprises a large base in the cytoplasm. It extends in the periplasm via 10 arches to form a double-ring structure containing the C-terminal domain of TssM (TssM ct) and TssJ that is anchored in the outer membrane. The crystal structure of the TssM ct-TssJ complex coupled to whole-cell accessibility studies suggest that large conformational changes induce transient pore formation in the outer membrane allowing passage of the attacking Hcp tube/VgrG spike.
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