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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).

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

IIITCP protein complex, YopB/YopD (Olsson et al., 2004). TMS2 is essential for function, while TMS1 is partially defective for translocation, pore formation, and signaling (Ryndak et al. 2005). The system 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).

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

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)

IIITCP protein complex, SipB/SipD of pathogenicity island 1 (SPI1)
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

Chlamydiae/Verrucomicrobia group
CopB of Parachlamydia acathamoebae (F8KWQ0)

Putative channel-forming system of a bacterial type III secretion system (see family description).  The two proteins included in this system are encoded by genes that are in a gene cluster that includes a type III secretion system and a chaparone protein of 166 aas, SicA (AKM45441).

Pore-forming two component system of Burkholderia contaminans

IIITCP protein complex, BopB/BopD (Nogawa et al., 2004)
BopB/BopD of Bordetella bronchiseptica
BopB (NP_888166)
BopD (NP_888165)

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

SseB/SseC/SseD of Salmonella typhimurium
SseB (CAA12185)
SseC (CAA12187)
SseD (CAA12188)