1.C.36 The Bacterial Type III-Target Cell Pore (IIITCP) Family

The Type III secretory pathway (IIISP, TC #3.A.6) allows bacteria to pass proteins directly from the bacterial cytoplasm into the host animal or plant cell cytoplasm. In order for this to occur, the bacteria construct a complex secretory apparatus allowing a direct pathway for the passage of proteins. One bacterial protein of each type IIISP system is secreted directly into the host cell membrane where it oligomerizes to form a transmembrane translocation pore at the site of bacterial contact. These homologous proteins comprise the IIITCP family. They are very divergent in sequence but have a uniform topology with two conserved transmembrane spanners (TMSs) and one conserved coiled coil region. The best characterized members of the IIITCP family include EspD for enteropathogenic E. coli, YopB for Yersinia species, PopB and PepB for Pseudomonas species, IpaB for Shigella species and SipB for Salmonella species. The three clusters of proteins indicated in the table (1, EspD; 2, YopB, PopB and PepB; and 3, IpaA and SipB) are very divergent in sequence so that PSI-BLAST reveals only limited sequence similarity between clusters. Proteins of the third cluster, including IpaB and SipB, are also much larger than the proteins in clusters 1 and 2 due in part to a 150 amino acyl residue N-terminal extension.

YopB and YopD of Yersinia species form a complex in the bacterial cytoplasm with their chaperone, SycD, suggesting that they are secreted as a heterocomplex. They may act together in forming the pore structure in the host cell membrane. YopB contains two central hydrophobic domains and is predicted to be an integral membrane protein, while YopD has a single central hydrophobic domain. In Bordetella bronchiseptica, BopB and BopD are also complexed to form a pore in the host cell plasma membrane (Nogawa et al., 2004). It seems likely that these two proteins are required for pore formation in all of these homologous (but very distantly related) systems.

YopB and YopD of Yersinia enterocolitica have been reported to both be required for pore formation in macrophage plasma membranes. The estimated inner pore diameter is about 2.0 nm. There is disagreement as to the requirement for YopD; some suggest that YopB alone forms the pore while others suggest that the YopB-YopD complex comprise the structural components of the pore. Studies in artificial membranes suggest that YopB alone forms channels with no ionic selectivity and that YopD alters the channel (Tardy et al., 1999). Thus, YopB is probably the channel while YopD is an accessory protein. The P. aeruginosa homologues, PopB/PopD with the PcrV protein form channels (2.8-3.5 nm inner diameter) both in red blood cells and in macrophage. In the former, they induce haemolysis. By electron microscopy PopB and PopD form 80 Å wide rings that encircle a ~40 Å cavity (Schoehn et al., 2003).

The TTSS has three main structural parts: a base, a needle, and a translocon, which work together to ensure the polarized movement of Yops directly from the bacterial cytosol into the host cell cytosol. The tip of the TTSS needle interacts with the translocon. Mutations in the needle protein, YscF (87 aas; TC# 3.A.6.1.1) separate its function in secretion from its role in translocation (Davis and Mecsas, 2006). Two yscF mutants formed needles and secreted Yops normally displayed distinct translocation defects. One showed diminished pore formation, suggesting incomplete pore insertion and/or assembly. The second formed pores but showed nonpolar translocation, suggesting unstable needle-translocon interactions.

The Salmonella SipB protein has been studied topologically in model bilayers (McGhie et al., 2002). The protein can exist in soluble and membrane-inserted forms, a property typical of bacterial exotoxins. SipB inserts into the host cell membrane and has heterotypic membrane fusion activity in vitro. The membrane integrated protein (593 aas) is predominantly α-helical as is true for the soluble form. Two hydrophobic α-helices (residues 320-353 and 409-427) integrate into the membrane obliquely, allowing residues 354-408 to cross the bilayer while a C-terminal region associates with the bilayer surface. The homologous IpAB of Shigella assembles into a homotrimer via an N-terminal coiled-coil domain (Hume et al., 2003). SipB and IpaB integrate with the same membrane topology.

The generalized transport reaction that is facilitated by IIITCP family members is:

proteins (bacterial cytoplasm) proteins (host cell cytoplasm).

This family belongs to the .



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TC#NameOrganismal TypeExample

IIITCP protein complex EspB/EspD. The topology of and EspD interaction sites in EspB have been defined (Luo and Donnenberg, 2011).  EspD inserts into the membrane with its two helical hairpins traversing the membrane with the N- and C-termini on the extraluminal surface, forming 2.5 diameter pores (Chatterjee et al. 2015).

Gram-negative bacteria

EspB/EspD of E. coli
EspB (NP_290254)
EspD (NP_290255)


TC#NameOrganismal TypeExample

IIITCP protein complex, YopB/YopD (Olsson et al., 2004). Forms a multimeric integral membrane complex (Montagner et al., 2011).  Mutants have been isoated which show defects in effector translocation and pore formation, and many of these are in a C-terminal domain (Solomon et al. 2015).


YopB/YopD of Yersinia pseudotuberculosis
YopB (Q06114)
YopD (Q06131)


IIITCP protein complex, PopB/PopD. Purified PopB and PopD form pores in model membranes (Romano et al., 2011).  PopB in isolation forms a biimodal distribution of two complexes with 6 and 12 subunits while PopD forms a hexameric complex.  However when present together, they form a hexadecameric transmembrane complex (Romano et al. 2016). PopB assists with the proper insertion of PopD into cell membranes and is required for the formation of a functional translocon and host infection (Tang et al. 2018).

Gram-negative bacteria

PopB/PopD of Pseudomonas aeruginosa
PopB (AAO91773)
PopD (AAO91774)


Translocator complex AopB/AopD of 347 and 299 aas, respectively.  AopB has been crystalized and the structure determined for this protein in complex with the AcrH chaperone protein (Nguyen et al. 2015). The structure revealed unique interactions between the various interfaces of AopB and AcrH, with the N-terminal "molecular anchor" of AopB crossing into the "N-terminal arm" of AcrH. AopB adopts a novel fold, and its transmembrane regions form two pairs of helical hairpins.

AopBD of Aeromonas hydrophila


TC#NameOrganismal TypeExample

IIITCP protein complex, IpaB/IpaC/IpaD. Physical contact with host cells initiates secretion and leads to assembly of a pore, IpaB/IpaC, in the host cell membrane. The active needle tip complex of S. flexneri is composed of a tip protein, IpaD, and the two pore-forming proteins, IpaB and IpaC. The atomic structures of IpaD and a protease-stable coiled-coil fragment in the N-terminal regions of IpaB from S. flexneri and the homologous SipB from Salmonella enterica have been determined (Barta et al. 2012).  Structural comparisons revealed similarity to the coiled-coil regions of pore-forming proteins such as colicin Ia (TC# 1.C.1.1.1).  Interaction between IpaB and IpaD at the needle tip is key to host cell sensing, orchestration of IpaC secretion and its subsequent assembly at needle tips (Veenendaal et al. 2007). The N-terminus of IpaC is extracellular and the C-terminus is intracellular, and its topology has been studied (Russo et al. 2019).


IpaB/IpaD of Shigella dysenteriae
IpaB (P18011)
IpaD (P18013)
IpaC (P18012)

1.C.36.3.2IIITCP protein complex, SipB/SipD of pathogenicity island 1 (SPI1)Gram-negative bacteria SipB/SipD of Salmonella typhimurium
SipB (AAL21765)
SipD (AAL21763)

IIITCP complex, BipB/BipD (BipB, 620aas; BipD, 310aas)


BipB/BipD of Burkholderia pseudomallei
BipB (Q3JL23)
BipD (Q3JL26)


IIITCP complex, BipB/BipD (Cell invasion protein complex).


BipB/D of Protens mirablis
BipB (B4EYC8)
BipD (B4EYC6)


IIITCP complex protein, CopB of 852aas; 4TMSs


CopB of Parachlamydia acathamoebae (F8KWQ0)


TC#NameOrganismal TypeExample
1.C.36.4.1IIITCP protein complex, BopB/BopD (Nogawa et al., 2004)BacteriaBopB/BopD of Bordetella bronchiseptica
BopB (NP_888166)
BopD (NP_888165)

TC#NameOrganismal TypeExample

IIICP protein complex SseB/SseC/SseD; SseB: translocon sheath protein; SseC and SseD: translocon pore subunits of the Salmonella pathogenicity island 2 (SPI2)

Gram-negative bacteriaSseB/SseC/SseD of Salmonella typhimurium
SseB (CAA12185)
SseC (CAA12187)
SseD (CAA12188)

TC#NameOrganismal TypeExample



EspA/D of E. coli O157:H7