Table 1.
Summary of the core components of the major classes of T6SS. Type i T6SSs represent canonical Proteobacterial T6SSs, type ii T6SSs are found in Francisella, type iii T6SSs are found in Bacteroidetes, whilst a single type iv T6SS has been reported in Amoebophilus. For T6SS components with homologues in bacteriophage T4, the name of the respective phage protein is given in the column ‘T4 phage’. The table summarises information from references throughout the main text, together with Clemens, Lee and Horwitz (2018).
| Component | Role | Type i T6SS | Type ii T6SS | Type iii T6SS | Type iv T6SS | T4 phage | Comments |
|---|---|---|---|---|---|---|---|
| TssA | Promotes sheath polymerisation and co-ordinates sheath and tube assembly | ✓ | ‘Long’ TssA (TsaC) have C-terminal VasJ domain and interact with TagA at the far side of the cell to stabilise the extended sheath. ‘Short’ TssA (TsaB) have a distinct C-terminus and may interact with different accessory proteins to stabilise the sheath from the baseplate. | ||||
| TssB | Contractile sheath (small subunit) | ✓ | IglA | ✓ | ✓ | gp18 | TssB and TssC correspond to the N-and C-terminal domains of gp18 (T4 sheath protein), respectively. |
| TssC | Contractile sheath (large subunit) | ✓ | IglB | ✓ | |||
| Hcp (TssD) | Puncturing structure (tube) | ✓ | IglC | ✓ | ✓ | gp19 | |
| TssE | Baseplate | ✓ | ✓ | ✓ | gp25 | ||
| TssF | Baseplate | ✓ | ✓ | ✓ | gp6 | ||
| TssG | Baseplate | ✓ | ✓ | gp7 | |||
| TssH (ClpV) | Sheath disassembly ATPase | ✓ | ClpB | ✓ | ClpB (Type ii) is a general chaperone | ||
| VgrG (TssI) | Puncturing structure (spike) | ✓ | VgrG + PdpA | ✓ | gp27 + gp5 | gp27 + gp5 | In Type i and iii, VgrG is a fusion of gp27- and gp5-like domains. In Type ii, a short ‘VgrG’ lacks the gp27-like domain which is likely replaced by PdpA. |
| TssJ | Membrane complex (OM lipoprotein) | ✓ | IglE | IglE appears to be a functional orthologue of TssJ | |||
| TssK | Baseplate | ✓ | IglD | ✓ | Similar to siphophage receptor binding protein | ||
| TssL | Membrane complex (integral IM protein) | ✓ | DotU | ||||
| TssM | Membrane complex (integral IM protein) | ✓ | PdpB | ||||
| PAAR | Puncturing structure (spike tip) | ✓ | IglG | ✓ | ✓ | gp5.4 | |
| TssN | Membrane complex? | ✓ | |||||
| TssO | Membrane complex? | ✓ | |||||
| TssP | Membrane complex? | ✓ | |||||
| T6SSiv baseplate | Additional baseplate components | ✓ | gp48, gp6, gp53, gp54 | ||||
| T6SSiv tail terminator | Control of sheath/ tube length? | ✓ | gp15 | ||||
| T6SSiv tape measure | Control of sheath/ tube length? | ✓ | gp29 | ||||
| T6SSiv Afp-like | Components of unknown function | ✓ | Similar to Afp extracellular contraction injection system |