Clostridial Cytotoxin family
Identifiers | |||||||||
---|---|---|---|---|---|---|---|---|---|
Symbol | CCT | ||||||||
Pfam | PF04488 | ||||||||
TCDB | 1.C.57 | ||||||||
OPM superfamily | 222 | ||||||||
OPM protein | 2vk9 | ||||||||
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The Clostridial Cytotoxin (CCT) Family (TC# 1.C.57) is a member of the RTX-toxin superfamily. There are currently 13 classified members belonging to the CCT family. A representative list of these proteins is available in the Transporter Classification Database. Homologues are found in a variety of Gram-positive and Gram-negative bacteria.[1]
Clostridial difficile cytotoxins
Clostridium difficile, the causative agent of nosocomial antibiotic-associated diarrhea and pseudomembranous colitis, possesses two main virulence factors: the large clostridial cytotoxins A (TcdA; TC# 1.C.57.1.2) and B (TcdB, TC# 1.C.57.1.1). Cleavage of toxin B and all other large clostridial cytotoxins, is an autocatalytic process dependent on host cytosolic inositolphosphate cofactors. A covalent inhibitor of aspartate proteases, 1,2-epoxy-3-(p-nitrophenoxy)propane or EPNP, completely blocks toxin B function on cultured cells and has been used to identify the catalytically active protease site.[2] The toxin uses eukaryotic signals for induced autoproteolysis to deliver its toxic domain into the cytosol of target cells. Reineke et al. (2007) present an integrated model for the uptake and inositolphosphate-induced activation of toxin B.[3]
Clostridium difficile infection, caused by the actions of the homologous toxins TcdA and TcdB on colonic epithelial cells is due to binding to target cells which triggers toxin internalization into acidified vesicles, whereupon cryptic segments from within the 1,050-aa translocation domain unfurl and insert into the bounding membrane, creating a transmembrane passageway to the cytosol.[4] Sensitive residues-clustered between amino acyl residues 1,035 and 1,107, when individually mutated, reduced cellular toxicity by >1,000-fold. Defective variants exhibit impaired pore formation in planar lipid bilayers and biological membranes, resulting in an inability to intoxicate cells through either apoptotic or necrotic pathways. The findings suggest similarities between the pore-forming 'hotspots' of TcdB and the diphtheria toxin translocation domain.[4]
Function
Proteolytically processed clostridial cytotoxins A (306 kDa; TC# 1.C.57.1.2) and B (269 kDa; TC# 1.C.57.1.1) are O-glycosyltransferases that modify small GTPases of the Rho family by glucosylation of threonine residues, thereby blocking the action of the GTPases as switches of signal processes such as those mediated by the actin cytoskeleton. The toxins thus induce redistribution of actin filaments and cause the cells to round up. The catalytic domains of CCTs probably enter the cytoplasm from acidic endosomes. The toxins form ion-permeable channels in cell membranes and artificial bilayers when exposed to acidic pH. pH-dependent channel formation has been demonstrated for C. difficile Toxin B and C as well as Sordellina lethal toxin.[5] Low pH presumably induces conformational/structural changes that promote membrane insertion and channel formation.
Structure
Cytotoxins of the CCT family are large (e.g., toxin B of C. difficile is 2366 aas long) and tripartite with the N-terminal domain being the catalytic unit, the C-terminal domain being the cellular receptor and the central hydrophobic domain being the channel-former. In this respect, they superficially resemble diphtheria toxin (DT; TC# 1.C.7) although no significant sequence similarity between CCTs and DT is observed. The E. coli toxin B protein (TC# 1.C.57.2.1) and the Chlamydial TC0437 protein (TC# 1.C.57.2.2) are of 3169 aas and 3255 aas, respectively. The distantly related ToxA toxin of Pasteurella multocida (TC# 1.C.57.3.1) is 1285 aas while the E. coli Cnf1 and 2 toxins(TC#s 1.C.57.3.2 and 1.C.57.3.3, respectively) are 1014 aas, and the RTX cytotoxin of Vibrio vulnificus (TC# 1.C.57.3.4) is 5206 aas.
Large Clostridial Toxins
Clostridium difficile toxins A and B are members of an important class of virulence factors known as large clostridial toxins (LCTs). Toxin action involves four major steps:
- receptor-mediated endocytosis
- translocation of a catalytic glucosyltransferase domain across the membrane
- release of the enzymatic moiety by autoproteolytic processing, and
- glucosyltransferase-dependent inactivation of Rho family proteins.
Pruitt et al. (2010) have imaged toxin A (TcdA) and toxin B (TcdB) holotoxins by negative stain electron microscopy to show that these molecules are similar in structure. They then determined a 3D structure for TcdA and mapped the organization of its functional domains. The molecule has a 'pincher-like' head corresponding to the delivery domain and two tails, long and short, corresponding to the receptor-binding and glucosyltransferase domains, respectively. A second structure, obtained at the acidic pH of an endosome, reveals a significant structural change in the delivery and glucosyltransferase domains, and thus provides a framework for understanding the molecular mechanism of LCT cellular intoxication.[6]
Transport Reaction
The generalized transport reactions catalyzed by CCTs are:[1]
- N-terminal catalytic domain (out) → N-terminal catalytic domain (in)
- Ions and other solutes (in) → Ions and other solutes (out)
See also
- Clostridium difficile toxin B
- Clostridium difficile colitis
- Clostridium difficile (bacteria)
- RTX toxin
- Transporter Classification Database
References
- 1 2 Saier, MH Jr. "1.C.57 The Clostridial Cytotoxin (CCT) Family". Transporter Classification Database. Saier Lab Bioinformatics Group / SDSC.
- ↑ Yu, Zhonghua; Caldera, Patricia; McPhee, Fiona; Voss, James J. De; Jones, Patrick R.; Burlingame, Alma L.; Kuntz, Irwin D.; Craik, Charles S.; Montellano, Paul R. Ortiz de (1996-06-26). "Irreversible Inhibition of the HIV-1 Protease: Targeting Alkylating Agents to the Catalytic Aspartate Groups". Journal of the American Chemical Society. 118 (25): 5846–5856. doi:10.1021/ja954069w.
- ↑ Reineke, Jessica; Tenzer, Stefan; Rupnik, Maja; Koschinski, Andreas; Hasselmayer, Oliver; Schrattenholz, André; Schild, Hansjörg; Eichel-Streiber, Christoph von (2007). "Autocatalytic cleavage of Clostridium difficile toxin B". Nature. 446 (7134): 415–419. doi:10.1038/nature05622.
- 1 2 Zhang, Zhifen; Park, Minyoung; Tam, John; Auger, Anick; Beilhartz, Greg L.; Lacy, D. Borden; Melnyk, Roman A. (2014-03-11). "Translocation domain mutations affecting cellular toxicity identify the Clostridium difficile toxin B pore". Proceedings of the National Academy of Sciences of the United States of America. 111 (10): 3721–3726. doi:10.1073/pnas.1400680111. ISSN 1091-6490. PMC 3956163. PMID 24567384.
- ↑ Voth, Daniel E.; Ballard, Jimmy D. (2005-04-01). "Clostridium difficile Toxins: Mechanism of Action and Role in Disease". Clinical Microbiology Reviews. 18 (2): 247–263. doi:10.1128/CMR.18.2.247-263.2005. ISSN 0893-8512. PMC 1082799. PMID 15831824.
- ↑ Pruitt, Rory N.; Chambers, Melissa G.; Ng, Kenneth K.-S.; Ohi, Melanie D.; Lacy, D. Borden (2010-07-27). "Structural organization of the functional domains of Clostridium difficile toxins A and B". Proceedings of the National Academy of Sciences of the United States of America. 107 (30): 13467–13472. doi:10.1073/pnas.1002199107. ISSN 1091-6490. PMC 2922184. PMID 20624955.
Further reading
- Amimoto, K., T. Noro, E. Oishi, and M. Shimizu. (2007). A novel toxin homologous to large clostridial cytotoxins found in culture supernatant of Clostridium perfringens type C. Microbiology. 153: 1198-1206. 17379729
- Baldwin, M.R., J.H. Lakey, and A.J. Lax. (2004). Identification and characterization of the Pasteurella multocida toxin translocation domain. Mol. Microbiol. 54: 239-250. 15458419
- Barth, H., G. Pfeifer, F. Hofmann, E. Maier, R. Benz, and K. Aktories. (2001). Low pH-induced formation of ion channels by Clostridium difficile toxin B in target cells. J. Biol. Chem. 276: 10670-10676. 11152463
- Belland, R.J., M.A. Scidmore, D.D. Crane, D.M. Hogan, W. Whitmire, G. McClarty, and H.D. Caldwell. (2001). Chlamydia trachomatis cytotoxicity associated with complete and partial cytotoxin genes. Proc. Natl. Acad. Sci. USA 98: 13984-13989. 11707582
- Genisyuerek, S., P. Papatheodorou, G. Guttenberg, R. Schubert, R. Benz, and K. Aktories. (2011). Structural determinants for membrane insertion, pore formation and translocation of Clostridium difficile toxin B. Mol. Microbiol. 79: 1643-1654. 21231971
- Oswald, E., M. Sugai, A. Labigne, H.C. Wu, C. Fiorentini, P. Boquet, and A.D. O'Brien. (1994). Cytotoxic necrotizing factor type 2 produced by virulent Escherichia coli modifies the small GTP-binding proteins Rho involved in assembly of actin stress fibers. Proc. Natl. Acad. Sci. USA 91: 3814-3818. 8170993
- Zhao, J.F., A.H. Sun, P. Ruan, X.H. Zhao, M.Q. Lu, and J. Yan. (2009). Vibrio vulnificus cytolysin induces apoptosis in HUVEC, SGC-7901 and SMMC-7721 cells via caspase-9/3-dependent pathway. Microb. Pathog. 46: 194-200. 19167479