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Table 1 Features of secretion pathways by which polymorphic toxins are exported

From: Polymorphic toxin systems: Comprehensive characterization of trafficking modes, processing, mechanisms of action, immunity and ecology using comparative genomics

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
  1. 1: Note only fused to toxins exported by the SEC-dependent pathway in Amoebophilus asiaticus; 2. Note only fused to toxins exported by the SEC-dependent pathway in Microscilla marina; 3: Note only fused to toxins exported by the SEC-dependent pathway in Acetivibrio cellulolyticus; 4: Note only fused to toxins exported by the SEC-dependent pathway in Caldicellulosiruptor species; 5: Note in firmicutes, the export pathway is only present in Veillonella and Selenomonas species, also referred to as the Negativicutes species; 6: Certain bacterial lineages within the β,ϵ,γ-proteobacteria, planctomycetes, verrucomicrobia, cyanobacteria and bacteroidetes have solo WXG domains that have a distinct YueA-like ATPase with 3 HerA/FtsK domains of which only the middle one is active. These appear to be mobile versions of T7SS.