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

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.

Effector loading onto the VgrG carrier activates type VI secretion system assembly

“…Cargo effector loading onto its cognate VgrG is critical for T6SS assembly and ejecting toxin effectors to kill competitor bacterial cells. Previous studies provided evidence that VgrG interacts with components of the T6SS baseplate and the interactions are critical for assembly of the T6SS [7,8,26,27]. However, this mechanism may have been overlooked in many other species of bacteria, because of the lack of mutants deleted of all effector-encoding genes.…”

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…The baseplate then serves as a nucleation platform for polymerization of the VipA/VipB (TssB/TssC) sheath and the inner Hcp tube (Zoued et al , 2016). Additionally, while one class of TssA proteins is required for baseplate assembly, another class of TssA proteins was shown to first bind to the baseplate and then stay associated with the distal end of the sheath during assembly (Planamente et al , 2016; Zoued et al , 2016). …”

The type VI secretion system sheath assembles at the end distal from the membrane anchor

“…Interestingly, the sheath assembly in Escherichia coli is dependent on the presence of TssA forming a dodecameric structure colocalizing with the end of a polymerizing sheath, which is distal from the assembly initiation16. However, TssA1 in Pseudomonas aeruginosa was shown to interact with TssK1 and TssF1 in the baseplate17. This is likely due to the fact that TssAs of E. coli and P. aeruginosa share little homology and belong to two different subfamilies17.…”

“…However, TssA1 in Pseudomonas aeruginosa was shown to interact with TssK1 and TssF1 in the baseplate17. This is likely due to the fact that TssAs of E. coli and P. aeruginosa share little homology and belong to two different subfamilies17.…”