TssA forms a gp6-like ring attached to the type VI secretion sheath.

Sara Planamente,Paul S Freemont,Osman Salih,Eleni Manoli,Alain Filloux,David Albesa‐Jové

doi:10.15252/embj.201694024

Sara Planamente, Paul S Freemont + Show 4 more

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https://doi.org/10.15252/embj.201694024

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Abstract

The type VI secretion system (T6SS) is a supra‐molecular bacterial complex that resembles phage tails. It is a killing machine which fires toxins into target cells upon contraction of its TssBC sheath. Here, we show that TssA1 is a T6SS component forming dodecameric ring structures whose dimensions match those of the TssBC sheath and which can accommodate the inner Hcp tube. The TssA1 ring complex binds the T6SS sheath and impacts its behaviour in vivo. In the phage, the first disc of the gp18 sheath sits on a baseplate wherein gp6 is a dodecameric ring. We found remarkable sequence and structural similarities between TssA1 and gp6 C‐termini, and propose that TssA1 could be a baseplate component of the T6SS. Furthermore, we identified similarities between TssK1 and gp8, the former interacting with TssA1 while the latter is found in the outer radius of the gp6 ring. These observations, combined with similarities between TssF and gp6N‐terminus or TssG and gp53, lead us to propose a comparative model between the phage baseplate and the T6SS.

Highlights

Bacteria face fierce competition for resources in their environment or hosts
A tssA1 deletion was engineered, which abolished the secretion of Hcp1, Tse3 and VgrG proteins, all markers of P. aeruginosa T6SS activity (Fig 1C), confirming the essential role of TssA1
The crosslinked TssA1 shows a series of species consistent with dimeric (~80 kDa), trimeric (~120 kDa) and hexameric (~240 kDa) forms, together with a band corresponding to a MW higher than 315 kDa

Summary

Introduction

A molecular machine, called the type VI secretion system (T6SS), has emerged as a weapon (Filloux, 2013) that delivers toxins directly into bacterial competitors (Russell et al, 2011). T6SS-dependent bacterial confrontation plays a role in the persistence of P. aeruginosa in the lungs of cystic fibrosis patients (Mougous et al, 2006), or the establishment of Bacteroidetes as members of the gut microbiota (Russell et al, 2014). Several T6SS effectors manipulate eukaryotic cells (Hachani et al, 2016). T6SS effectors can act on the membrane of prokaryotic and eukaryotic cells, for example the PldA and PldB (Jiang et al, 2014) phospholipases produced by P. aeruginosa. The T6SS has a crucial role in mediating interactions between living cells, but its structure and mode of action has yet to be fully elucidated

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