| Structural highlights
Function
TCDB_CLODI Precursor of a cytotoxin that targets and disrupts the colonic epithelium, inducing the host inflammatory and innate immune responses and resulting in diarrhea and pseudomembranous colitis (PubMed:20844489, PubMed:24919149). TcdB constitutes the main toxin that mediates the pathology of C.difficile infection, an opportunistic pathogen that colonizes the colon when the normal gut microbiome is disrupted (PubMed:19252482, PubMed:20844489). Compared to TcdA, TcdB is more virulent and more important for inducing the host inflammatory and innate immune responses (PubMed:19252482, PubMed:24919149). This form constitutes the precursor of the toxin: it enters into host cells and mediates autoprocessing to release the active toxin (Glucosyltransferase TcdB) into the host cytosol (PubMed:10768933, PubMed:11152463, PubMed:12941936, PubMed:17334356, PubMed:20498856). Targets colonic epithelia by binding to the frizzled receptors FZD1, FZD2 and FZD7, and enters host cells via clathrin-mediated endocytosis (PubMed:27680706). Frizzled receptors constitute the major host receptors in the colonic epithelium, but other receptors, such as CSPG4 or NECTIN3/PVRL3, have been identified (PubMed:25547119, PubMed:26038560, PubMed:27680706). Binding to carbohydrates and sulfated glycosaminoglycans on host cell surface also contribute to entry into cells (By similarity). Once entered into host cells, acidification in the endosome promotes the membrane insertion of the translocation region and formation of a pore, leading to translocation of the GT44 and peptidase C80 domains across the endosomal membrane (PubMed:11152463, PubMed:12941936, PubMed:24567384). This activates the peptidase C80 domain and autocatalytic processing, releasing the N-terminal part (Glucosyltransferase TcdB), which constitutes the active part of the toxin, in the cytosol (PubMed:17334356, PubMed:27571750).[UniProtKB:P16154][1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] Active form of the toxin, which is released into the host cytosol following autoprocessing and inactivates small GTPases (PubMed:8144660, PubMed:7777059, PubMed:16157585, PubMed:17901056, PubMed:24905543, PubMed:24919149). Acts by mediating monoglucosylation of small GTPases of the Rho family (Rac1, RhoA, RhoB, RhoC, RhoG and Cdc42) in host cells at the conserved threonine residue located in the switch I region ('Thr-37/35'), using UDP-alpha-D-glucose as the sugar donor (PubMed:7777059, PubMed:16157585, PubMed:17901056, PubMed:24905543, PubMed:24919149). Monoglucosylation of host small GTPases completely prevents the recognition of the downstream effector, blocking the GTPases in their inactive form, leading to actin cytoskeleton disruption and cell death, resulting in the loss of colonic epithelial barrier function (PubMed:7777059, PubMed:24919149).[14] [15] [16] [17] [18] [19]
Publication Abstract from PubMed
The symptoms of C. difficile infections (CDI1) are caused by two exotoxins, TcdA and TcdB, which target host colonocytes by binding to unknown cell-surface receptors, at least in part via their combined repetitive oligopeptide (CROP) domains. A combination of the anti-TcdA antibody actoxumab and the anti-TcdB antibody bezlotoxumab is currently under development for the prevention of recurrent CDI. We demonstrate here through various biophysical approaches that bezlotoxumab binds to specific regions within the N-terminal half of the TcdB CROP domain. Based on this information, we solved the X-ray structure of the N-terminal half of the TcdB CROP domain bound to Fab fragments of bezlotoxumab. The structure reveals that the TcdB CROP domain adopts a beta-solenoid fold consisting of long- and short-repeats (LR and SR), and that bezlotoxumab binds to two homologous sites within the CROP domain, partially occluding two of the four putative carbohydrate binding pockets located in TcdB. We also show that bezlotoxumab neutralizes TcdB by blocking binding of TcdB to mammalian cells. Overall, our data are consistent with a model wherein a single molecule of bezlotoxumab neutralizes TcdB by binding via its two Fab regions to two epitopes within the N-terminal half of the TcdB CROP domain, partially blocking the carbohydrate binding pockets of the toxin and blocking toxin binding to host cells.
Mechanism of action and epitopes of the Clostridium difficile toxin B neutralizing antibody bezlotoxumab revealed by x-ray crystallography.,Orth P, Xiao L, Hernandez LD, Reichert P, Sheth P, Beaumont M, Yang X, Murgolo N, Ermakov G, DiNunzio E, Racine F, Karczewski J, Secore S, Ingram RN, Mayhood T, Strickland C, Therien AG J Biol Chem. 2014 May 12. pii: jbc.M114.560748. PMID:24821719[20]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
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- ↑ Xu H, Yang J, Gao W, Li L, Li P, Zhang L, Gong YN, Peng X, Xi JJ, Chen S, Wang F, Shao F. Innate immune sensing of bacterial modifications of Rho GTPases by the Pyrin inflammasome. Nature. 2014 Sep 11;513(7517):237-41. doi: 10.1038/nature13449. Epub 2014 Jun 11. PMID:24919149 doi:http://dx.doi.org/10.1038/nature13449
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- ↑ Orth P, Xiao L, Hernandez LD, Reichert P, Sheth P, Beaumont M, Yang X, Murgolo N, Ermakov G, DiNunzio E, Racine F, Karczewski J, Secore S, Ingram RN, Mayhood T, Strickland C, Therien AG. Mechanism of action and epitopes of the Clostridium difficile toxin B neutralizing antibody bezlotoxumab revealed by x-ray crystallography. J Biol Chem. 2014 May 12. pii: jbc.M114.560748. PMID:24821719 doi:http://dx.doi.org/10.1074/jbc.M114.560748
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