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 th
Transporter Classification Database
Homologues are found in a variety of

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...

and

Gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

''Clostridioides difficile'' cytotoxins

''Clostridioides difficile'', the causative agent of

nosocomial A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other healthcare facility. To emphasize both hospital and nonhospital settings, it is s ...

antibiotic-associated diarrhea and

pseudomembranous colitis Colitis is swelling or inflammation of the large intestine ( colon). Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases. In a medical context, the label ''colitis'' (without qualificati ...

, 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

Clostridioides difficile ''Clostridioides difficile'' ( syn. ''Clostridium difficile'') is a bacterium known for causing serious diarrheal infections, and may also cause colon cancer. It is known also as ''C. difficile'', or ''C. diff'' (), and is a Gram-positive spec ...

includes four steps: (1) receptor-mediated

endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...

, (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. 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 o
EPNP
completely blocks toxin B function on cultured cells and has been used to identify the catalytically active protease site. 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. ''Clostridioides 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. 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 Diphtheria toxin is an exotoxin secreted mainly by '' Corynebacterium diphtheriae'' but also by ''Corynebacterium ulcerans'' and '' Corynebacterium pseudotuberculosis'', the pathogenic bacterium that causes diphtheria. The toxin gene is enco ...

translocation domain.

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

endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of the endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membra ...

s. 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). 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

(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#
1.C.57.3.2
an
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: :N-terminal catalytic domain (out) → N-terminal catalytic domain (in) :Ions and other solutes (in) → Ions and other solutes (out)

''Clostridioides difficile'' cytotoxins

Function

Structure

Transport Reaction

See also

References

Further reading