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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).

References associated with 1.C.36 family:

Barta, M.L., N.E. Dickenson, M. Patil, A. Keightley, G.J. Wyckoff, W.D. Picking, W.L. Picking, and B.V. Geisbrecht. (2012). The structures of coiled-coil domains from type III secretion system translocators reveal homology to pore-forming toxins. J. Mol. Biol. 417: 395-405. 22321794
Chatterjee A., Caballero-Franco C., Bakker D., Totten S. and Jardim A. (2015). Pore-forming Activity of the Escherichia coli Type III Secretion System Protein EspD. J Biol Chem. 290(42):25579-94. 26324713
Dacheux, D., J, Goure, J. Chabert, Y. Usson, and I. Attree. (2001). Pore-forming activity of type III system-secreted proteins leads to oncosis of Pseudomonas aeruginosa-infected macrophages. Mol. Microbiol. 40: 76-85. 11298277
Davis A.J., Mecsas J. (2006). Mutations in the Yersinia pseudotuberculosis type III secretion system needle protein, YscF, that specifically abrogate effector translocation into host cells. J Bacteriol. 189: 83-97. 17071752
Hueck, C.J. (1998). Type III protein secretion systems in bacterial pathogens of animals and plants. Microbiol. Mol. Biol. Rev. 62: 379-433. 9618447
Hume, P.J., E.J. McGhie, R.D. Hayward, and V. Koronakis. (2003). The purified Shigella IpaB and Salmonella SipB translocators share biochemical properties and membrane topology. Mol. Microbiol. 49: 425-439. 12828640
Lee, V.T. and O. Schneewind. (1999). Type III machines of pathogenic yersiniae secrete virulence factors into the extracellular milieu. Mol. Microbiol. 31: 1619-1629. 10209737
Luo, W. and M.S. Donnenberg. (2011). Interactions and predicted host membrane topology of the enteropathogenic Escherichia coli translocator protein EspB. J. Bacteriol. 193: 2972-2980. 21498649
McGhie, E.J., P.J. Hume, R.D. Hayward, J. Torres, and V. Koronakis. (2002). Topology of the Salmonella invasion protein SipB in a model bilayer. Mol. Microbiol. 44: 1309-1321. 12068811
Montagner, C., C. Arquint, and G.R. Cornelis. (2011). Translocators YopB and YopD from Yersinia enterocolitica form a multimeric integral membrane complex in eukaryotic cell membranes. J. Bacteriol. 193: 6923-6928. 22001511
Neyt, C. and G.R. Cornelis. (1999). Insertion of a Yop translocation pore into the macrophage plasma membrane byYersinia enterocolitica: requirement for translocators YopB and YopD, but not LcrG. Mol. Microbiol. 33: 971-981. 10476031
Nguyen, V.S., C. Jobichen, K.W. Tan, Y.W. Tan, S.L. Chan, K. Ramesh, Y. Yuan, Y. Hong, J. Seetharaman, K.Y. Leung, J. Sivaraman, and Y.K. Mok. (2015). Structure of AcrH-AopB Chaperone-Translocator Complex Reveals a Role for Membrane Hairpins in Type III Secretion System Translocon Assembly. Structure 23: 2022-2031. 26439768
Nikolaus, T., J. Deiwick, C. Rappl, J.A. Freeman, W. Schröder, S.I. Miller, and M. Hensel. (2001). SseBCD proteins are secreted by the Type III secretion system of Salmonella pathogenicity island 2 and function as a translocon. J. Bacteriol. 183: 6036-6045. 11567004
Nogawa, H., A. Kuwae, T. Matsuzawa, and A. Abe. (2004). The Type III secreted protein BopD in Bordetella bronchiseptica is complexed with BopB for pore formation on the host plasma membrane. J. Bacteriol. 186: 3806-3813. 15175294
Olsson, J., P.J. Edqvist, J.E. Bröms, A. Forsberg, H. Wolf-Watz, and M.S. Francis. (2004). The YopD translocator of Yersinia pseudotuberculosis is a multifunctional protein comprised of discrete domains. J. Bacteriol. 186: 4110-4123. 15205412
Romano, F.B., K.C. Rossi, C.G. Savva, A. Holzenburg, E.M. Clerico, and A.P. Heuck. (2011). Efficient Isolation of Pseudomonas aeruginosa Type III Secretion Translocators and Assembly of Heteromeric Transmembrane Pores in Model Membranes. Biochemistry 50: 7117-7131. 21770428
Romano, F.B., Y. Tang, K.C. Rossi, K.R. Monopoli, J.L. Ross, and A.P. Heuck. (2016). Type 3 Secretion translocators spontaneously assemble a hexadecameric transmembrane complex. J. Biol. Chem. [Epub: Ahead of Print] 26786106
Schoehn, G., A.M. Di Guilmi, D. Lemaire, I. Attree, W. Weissenhorn, and A. Dessen. (2003). Oligomerization of type III secretion proteins PopB and PopD precedes pore formation in Pseudomonas. EMBO J. 22: 4957-4967. 14517235
Solomon, R., W. Zhang, G. McCrann, J.B. Bliska, and G.I. Viboud. (2015). Random Mutagenesis Identifies a C-Terminal Region of YopD Important for Yersinia Type III Secretion Function. PLoS One 10: e0120471. 25807250
Tardy, F., F. Homblè, C. Neyt, R. Wattiez, G.R. Cornelis, J.-M. Ruysschaert, and V. Cabiaux. (1999). Yersinia enterocholitica type III secretion-translocation system: channel formation by secreted Yops. EMBO J. 18: 6793-6799. 10581252
Veenendaal, A.K., J.L. Hodgkinson, L. Schwarzer, D. Stabat, S.F. Zenk, and A.J. Blocker. (2007). The type III secretion system needle tip complex mediates host cell sensing and translocon insertion. Mol. Microbiol. 63: 1719-1730. 17367391
Wachter, C., C. Beinke, M. Mattes, and M.A. Schmidt. (1999). Insertion of EspD into epithelial target cell membranes by infecting enteropathogenic Escherichia coli. Mol. Microbiol. 31: 1695-1707. 10209743
Winans, S.C. (2004). Reciprocal regulation of bioluminescence and type III protein secretion in Vibrio harveyi and Vibrio parahaemolyticus in response to diffusible chemical signals. J. Bacteriol. 186: 3674-3676. 15175279
Zurawski, D.V. and M.A. Stein. (2004). The SP12-encoded SseA chaperone has discrete domains required for SseB stabilization and export, and binds within the C-terminus of SseB and SseD. Microbiology 150: 2055-2068. 15256549