Abstract
Bacteria share their ecological niches with other microbes. The bacterial type VI secretion system is one of the key players in microbial competition, as well as being an important virulence determinant during bacterial infections. It assembles a nano-crossbow-like structure in the cytoplasm of the attacker cell that propels an arrow made of a haemolysin co-regulated protein (Hcp) tube and a valine-glycine repeat protein G (VgrG) spike 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 type VI subunits (Tss)-TssJ, TssM and TssL-and present a structure of the fully assembled complex at 11.6 Å resolution, 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 ten arches to form a double-ring structure containing the carboxy-terminal domain of TssM (TssMct) and TssJ that is anchored in the outer membrane. The crystal structure of the TssMct-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.
Highlights
To cite this version: Eric Durand, van Son Nguyen, Abdelrahim Zoued, Laureen Logger, Gerard Pehau-Arnaudet, et al
It extends in the periplasm via 10 arches to form a double-ring structure containing the Cterminal domain of TssM (TssMct) and TssJ that is anchored in the outer membrane
We propose that the sub-volume closest to the cap corresponds to TssJ, in agreement with its location close to the outer membrane
Summary
The data presented here allows an unprecedented understanding of the biogenesis, architecture and role of the T6SS TssJLM membrane core complex This complex anchors the phage tail-like structure to the cell envelope and is thought to serve as conduit to guide the Hcp tube/VgrG spike upon sheath contraction[15,16,17]. Based on our accessibility experiments, we propose that upon assembly of other T6SS subunits with the membrane core complex, the TssM C-terminal extension (C-terminal extended stretch following helix α5 in the crystal structure and the remaining 22 non-visible amino-acids) will change its conformation and will cross the outer membrane. Further studies will be necessary to fully understand the complete assembly process of the T6SS, the trigger that releases sheath contraction and how the Hcp tube/VgrG spike crosses both bacterial and host membranes
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