Biology:Clostridial Cytotoxin family
| Identifiers | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Symbol | CCT | ||||||||
| Pfam | PF04488 | ||||||||
| TCDB | 1.C.57 | ||||||||
| OPM superfamily | 199 | ||||||||
| OPM protein | 2vk9 | ||||||||
| |||||||||
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]
Clostridium 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). Action by large clostridial toxins (LCTs) from Clostridium difficile includes four steps: (1) receptor-mediated endocytosis, (2) translocation of a catalytic glucosyltransferase domain across the membrane, (3) release of the enzymatic part by auto-proteolysis, and (4) inactivation of Rho family proteins.[2] 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.[3] 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 inositol phosphate-induced activation of toxin B.[4]
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.[5] 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.[5]
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 Clostridium sordellii lethal toxin (TcsL).[6] 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.
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
- ↑ 1.0 1.1 Saier, MH Jr.. "1.C.57 The Clostridial Cytotoxin (CCT) Family". Saier Lab Bioinformatics Group / SDSC. http://www.tcdb.org/search/result.php?tc=1.C.57#ref10862.
- ↑ "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): 13467–72. July 2010. doi:10.1073/pnas.1002199107. PMID 20624955. Bibcode: 2010PNAS..10713467P.
- ↑ Yu, Zhonghua; Caldera, Patricia; McPhee, Fiona; Voss, James J. De; Jones, Patrick R.; Burlingame, Alma L.; Kuntz, Irwin D.; Craik, Charles S. et al. (1996-06-26). "Irreversible Inhibition of the HIV-1 Protease: Targeting Alkylating Agents to the Catalytic Aspartate Groups" (in en). Journal of the American Chemical Society 118 (25): 5846–5856. doi:10.1021/ja954069w.
- ↑ "Autocatalytic cleavage of Clostridium difficile toxin B". Nature 446 (7134): 415–9. March 2007. doi:10.1038/nature05622. PMID 17334356. Bibcode: 2007Natur.446..415R.
- ↑ 5.0 5.1 "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): 3721–6. March 2014. doi:10.1073/pnas.1400680111. PMID 24567384. Bibcode: 2014PNAS..111.3721Z.
- ↑ "Clostridium difficile toxins: mechanism of action and role in disease". Clinical Microbiology Reviews 18 (2): 247–63. April 2005. doi:10.1128/CMR.18.2.247-263.2005. PMID 15831824.
Further reading
- "A novel toxin homologous to large clostridial cytotoxins found in culture supernatant of Clostridium perfringens type C". Microbiology 153 (Pt 4): 1198–206. April 2007. doi:10.1099/mic.0.2006/002287-0. PMID 17379729.
- "Identification and characterization of the Pasteurella multocida toxin translocation domain". Molecular Microbiology 54 (1): 239–50. October 2004. doi:10.1111/j.1365-2958.2004.04264.x. PMID 15458419.
- "Low pH-induced formation of ion channels by clostridium difficile toxin B in target cells". The Journal of Biological Chemistry 276 (14): 10670–6. April 2001. doi:10.1074/jbc.M009445200. PMID 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
- "Structural determinants for membrane insertion, pore formation and translocation of Clostridium difficile toxin B". Molecular Microbiology 79 (6): 1643–54. March 2011. doi:10.1111/j.1365-2958.2011.07549.x. PMID 21231971.
- "Cytotoxic necrotizing factor type 2 produced by virulent Escherichia coli modifies the small GTP-binding proteins Rho involved in assembly of actin stress fibers". Proceedings of the National Academy of Sciences of the United States of America 91 (9): 3814–8. April 1994. doi:10.1073/pnas.91.9.3814. PMID 8170993. Bibcode: 1994PNAS...91.3814O.
- "Vibrio vulnificus cytolysin induces apoptosis in HUVEC, SGC-7901 and SMMC-7721 cells via caspase-9/3-dependent pathway". Microbial Pathogenesis 46 (4): 194–200. April 2009. doi:10.1016/j.micpath.2008.12.005. PMID 19167479.
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