TABLE 1.

Requirements for T4SS substrate-channel docking

T4SSSubstrate(s)aRecognition motif(s) (T4CP interaction)bAccessory protein (function)cReference(s)
Conjugation systems
    Gram-negative bacteria
        A. tumefaciensTraApTiC58C-terminal positive-charge tail sequence239
BID domain
        P. aeruginosaMobAR1162C-terminal positive charge and overall 2° structureMobB (binds membrane, stabilizes MobA-T4CP interaction)214
NTD recognition signal
        E. coliTraIFNDTraM (RHH DNA binding protein, tetramer, interacts with C-terminal extension of TraDF T4CP)28, 82, 180, 181
TrwCR388ND (interacts with TrwBR388 T4CP)TrwA (RHH DNA binding protein, tetramer, stimulates T4CP ATP hydrolysis)176, 192, 257
TraIRP4ND (interacts with TraGRP4 T4CP)236
MobARSF1010CT50aa suffices for translocation of Cre269
        Bordetella bronchisepticaMobpBHR1ND (interacts with TraGRP4)254
        B. fragilisMbpBpLV122ND (interacts with the T4CPs TraGRP4 and TraDF deleted of its C terminus)262
    Gram-positive bacteria
        E. faecalisPcfGpCF10ND (interacts with PcfCpCF10 T4CP)PcfF (interacts with PcfG and PcfC, spatially positions relaxosome or transfer intermediate near the T4CP)58, 59
        S. agalactiaeTraApIP501ND (interacts with the Orf10pIP501 T4CP)1
Effector translocators
    A. tumefaciensVirE2VirBCT50aa sufficient (interacts with the VirD4At T4CP, CT100aaVirE2 sufficient for the T4CP interaction)VirE1 (chaperone prevents VirE2 aggregation and DNA binding in A. tumefaciens)12, 88, 101, 246, 269, 270, 298
VirD2ND (VirD2 T strand interacts with VirD4VirB); VirD2 mediates translocation of CreVirC1 (spatial positioning), VBP1 (VirD2 binding protein recruits VirD2 to T4SS)10, 49, 119, 269
VirFVirBC-terminal positive-charge tail, CT19aa sufficient269
VirD5VirBC-terminal positive-charge tail, CT50aa sufficient269
VirE3VirBC-terminal positive-charge tail, CT50aa sufficient270
Atu6154TiC58C-terminal positive-charge tail, CT50aa sufficient269
    Agrobacterium rhizogenesGALLSVirBC-terminal pos. charge tail, CT27aa sufficient128
    L. pneumophilaRalFIcm/DotC-terminal hydrophobic tail, CT20aa sufficient; Leu-3 required for translocationIndependent of IcmS/IcmW23, 194, 203
LepAIcm/Dot, LepBIcm/DotND, presence of the C-terminal half improves but is not required for translocationIndependent of IcmS/IcmW57
AnkBIcm/DotCT Val-2 and Leu-3 required for translocationIcmS/IcmW (chaperones)3
SidGIcm/DotC-terminal hydrophobic tail, CT20aaIcmS/IcmW (chaperones)44
SidCIcm/DotC-terminal hydrophobic tail, CT100aa sufficientIcmS/IcmW (chaperones)44, 267
LegS2Icm/DotC-terminal hydrophobic tail, CT30aa sufficientIcmS/IcmW (chaperones)74
SidAIcm/Dot, SidBIcm/Dot, SidDEIcm/Dot, SidFIcm/Dot, SidHIcm/Dot, PieABCDEFGIcm/Dot, PpeABIcm/DotIcm/Dot, PpgAIcm/Dot, SdeAIcm/Dot, WipAIcm/Dot, WipBIcm/DotNDIcmS/IcmW (chaperones)23, 44, 201, 203
    C. burnetiiAnkIIcm/DotC-terminal hydrophobic tail, CT82aa sufficientIcmS (chaperone)274
AnkBIcm, AnkHIcm, AnkJIcm, AnkMIcm, AnkNIcm, OIcmNDIcmS (chaperone)274
AnkAIcm, AnkFIcm, AnkGIcm, AnkPIcmNDIndependent of IcmS274
    Brucella spp.VceAVirB and VceCVirBC-terminal hydrophobic tail, CT20aa required; CT115aa of VceC sufficient for translocation through L. pneumophila Dot/Icm (no T4CP in Brucella VirB T4S)76
    Bartonella spp.BepABCDEFGC-terminal positive-charge tail sequence BID domain239
    H. pyloriCagACagCT20aa and an intact N terminusCagF (chaperone)70, 132, 215
DNA uptake/release
    N. gonorrhoeaeTraIGGINDParA homolog (might function like VirC1At to spatially position substrate near transfer machine)122, 232
    H. pyloriComB129-131, 155
  • a For the conjugation systems, the listed proteins are relaxases that bind a cognate T4CP and are delivered to recipient cells. For the effector translocator systems, the listed proteins are effectors that play a role in the infection processes of the bacterial pathogen. TraIGGI of the N. gonorrhoeae DNA release system is a relaxase required for DNA release, but its translocation to the extracellular milieu has not been shown.

  • b The motifs listed are required for substrate translocation. In some cases, the protein or its C-terminal fragment (CT) is sufficient to mediate translocation to target cells, as shown by fusion to a reporter protein such as Cre recombinase or adenylate cyclase. Amino acids (aa) at positions listed relative to the C-terminal fragment (subscript) are required for translocation, as shown by mutational analysis. ND, not determined. Parentheses indicate that the interaction between a protein substrate and a cognate T4CP has been experimentally shown.

  • c Accessory factors required for T4SS channel docking or translocation. The proposed function in mediating substrate-T4SS channel docking is shown in parentheses.