Structural insights into the function of type VI secretion system TssA subunits

Abstract

The type VI secretion system (T6SS) is a multi-protein complex that injects bacterial effector proteins into target cells. It is composed of a cell membrane complex anchored to a contractile bacteriophage tail-like apparatus consisting of a sharpened tube that is ejected by the contraction of a sheath against a baseplate. We present structural and biochemical studies on TssA subunits from two different T6SSs that reveal radically different quaternary structures in comparison to the dodecameric E. coli TssA that arise from differences in their C-terminal sequences. Despite this, the different TssAs retain equivalent interactions with other components of the complex and position their highly conserved N-terminal ImpA_N domain at the same radius from the centre of the sheath as a result of their distinct domain architectures, which includes additional spacer domains and highly mobile interdomain linkers. Together, these variations allow these distinct TssAs to perform a similar function in the complex.

Diverse roles of TssA‐like proteins in the assembly of bacterial type VI secretion systems

“…Instead, they show moderate degree of motion relative to the sixfold axis and degree of rotation around linker axis (Fig A and C). Similar behavior of Nt2 dimers was observed for crystal packaging of Nt2‐CTD TssA AH (Dix et al , ).…”

“…The first 688 Å 2 PISA‐predicted interface is based on three conserved residues (W454, E455, and P456) from α11–α12 linker of one subunit and helix α10 from the neighboring subunit (Fig B). Phylogenetic analyses have shown that the WEP motif is only present in TssA proteins of Class A (Dix et al , ). Second 672 Å 2 interface forms a sixfold axis between helices α9–α11 of two neighboring subunits (Fig B).…”

“…TssA2 PA localizes at sheath initiation site and on distal end during polymerization (G), dissociates before contraction (H), dissociates and re-associated during sheath extension (I), or stays attached to distal end after contraction (J). that the WEP motif is only present in TssA proteins of Class A (Dix et al, 2018). Second 672 Å 2 interface forms a sixfold axis between helices a9-a11 of two neighboring subunits ( Fig 3B).…”

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

“…These studies together demonstrated it is the effector loading rather than its enzymatic activity is the trigger in T6SS assembly. 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]. Our findings further suggest that effector loading onto VgrG spike is the key in completing T6SS 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.…”