Table 1 Features of secretion pathways by which polymorphic toxins are exported
Secretion pathway | Signature N-terminal leader domains or pre-toxin-domains | Signature genes in neighborhood | Processing proteases/repeats in toxin proteins | Phyletic patterns | Additional Notes |
|---|---|---|---|---|---|
T2SS/Sec-dependent system | Signal peptide | - | Proteases: Caspase, HINT, MCF1-SHE, subtilisin3, ZU54 | In all bacteria | Default pathway for protein export. Might contain MAFB-N (DUF1020), MicroscillaN, APD1, APD2, Inactive transglutaminase |
Repeats: ALF, ankyrins, β-propeller, RHS, Sel11, TPR1, Tail-fiber2 | |||||
T5SS | N-terminal TpsA-like secretion domain (TPSASD) | FhaB/CdiB coding for porin-like protein | Proteases: HINT Repeats: FilH | α,β,γ,δ,ϵ-proteobacteria, acidobacteria, bacteroidetes/chlorobi, firmicutes5, fusobacteria | The TPSASD domain binds the outer-membrane FhaB/CdiB during the export of the toxin domain |
Pre-toxin domains: DUF637(PT637),DUF637-N, PT-VENN | |||||
T6SS | VgrG domain, PAAR domain, Hcp1 | ClpV-like AAA + Atpase, MOG1/PspB-like, VgrG, Hcp1, Phage tail/base-plate related proteins | Repeats: RHS | All proteobacteria, acidobacteria, bacteroidetes/chlorobi, firmicutes | Complete T6SS delivered toxins are often typified by a N-terminal PAAR domain |
Photorhabdus virulence cassette pathway (PVC) | PVC-Metallopeptidase | CDC48-like AAA + ATPase, VgrG, Phage tail/base-plate related proteins | Proteases: Metallopeptidase, Subtilisin, Caspase, MCF1-SHE | Euryarchaeota, α,β,γ,δ,ϵ-proteobacteria, acidobacteria, actinobacteria, bacteroidetes, chlorobi, chloroflexi, cyanobacteria, deinococci, firmicutes, nitrospirae, spirochaetes | |
Repeats: RHS, tail fiber | |||||
T7SS/ESX/ESAT-6 secretion system | WxG, LxG, LDxD domains | YueA-like FtsK/HerA ATPase, EsaC | Proteases: HINT, Caspase, MCF1-SHE Repeats: RHS, Tail-fiber | Firmicutes, actinobacteria, chloroflexi, other bacterial lineages6 | Toxins exported by these systems may or may not possess repeat domains |
TcdB/TcaC | A signal peptide followed by a SpvB domain coupled to a C-terminal integrin-like β-propeller domain | TcdB | Repeats : Integrin-like beta propeller, RHS, tail-fiber | Euryarchaeota, α,β,γ,δ-proteobacteria, actinobacteria, bacteroidetes | |
Chloroflexi, fibrobacteres, firmicutes, lentisphaerae, spirochaetes | |||||
Proteases: HINT, Caspase, ZU5 | |||||
PrsW | PrsW-peptidase domain | Repeats: RHS | Euryarchaeota, α,β,γ,δ-proteobacteria, actinobacteria Bacteroidetes, chloroflexi, cyanobacteria,deinococci, dictyoglomi, firmicutes, fusobacteria, gemmatimonadetes, spirochaetes, verrucomicrobia | PrsW is a transmembrane peptidase with several transmembrane helices | |
Proteases: PrsW | |||||
Phage DNA packaging system | MuF | MuF, large and small subunits of terminase | Proteases: Papain-like | Euryarchaeota, acidobacteria, α,γ,δ-proteobacteria, actinobacteria, bacteroidetes, chlorobi, firmicutes, fusobacteria, spirochaetes, caudovirales | The toxin is predicted to be packaged into the phage head as in phage transduction systems |