Cholera toxin
Encyclopedia
Cholera toxin is a protein complex
secreted by the bacterium Vibrio cholerae
. CTX is responsible for the massive, watery diarrhea characteristic of cholera
infection.
is an oligomer
ic complex made up of six protein subunits: a single copy of the A subunit (part A, enzymatic), and five copies of the B subunit (part B, receptor binding). The three-dimensional structure of the toxin was determined using X-ray crystallography
by Zhang et al. in 1995.
The five B subunits—each weighing 12 kDa
, and all coloured blue in the accompanying figure—form a five-membered ring. The A subunit has two important segments. The A1 portion of the chain (CTA1, red) is a globular enzyme
payload that ADP-ribosylates
G protein
s, while the A2 chain (CTA2, orange) forms an extended alpha helix
which sits snugly in the central pore of the B subunit ring.
This structure is similar in shape, mechanism, and sequence to the heat-labile enterotoxin
secreted by some strains of the Escherichia coli
bacterium.
cells via receptor-mediated endocytosis
, the A subunit detaches and becomes activated by proteolytic cleavage, allowing it to catalyze the ADP ribosylation of the Gαs subunit of the heterotrimeric G protein
resulting in constitutive cAMP
production. This in turn leads to secretion of H2O
, Na+
, K+
, Cl-
, and HCO3-
into the lumen
of the small intestine resulting in rapid dehydration
and other factors associated with cholera, including a rice-water stool.
Interestingly, the pertussis toxin
(also an AB5 protein) produced by Bordetella pertussis
acts in a similar manner with the exception that it ADP-ribosylates the Gαi subunit, rendering it inactive and unable to inhibit adenylyl cyclase production of cAMP (leading to constitutive production).
. Virulent
strains of V. cholerae carry a variant of lysogenic
bacteriophage
called CTXf or CTXφ.
ganglioside
s on the surface of the host's cells. After binding takes place, the entire CTX complex is internalised by the cell and the CTA1 chain is released by the reduction of a disulfide bridge. In fact, the endosome is moved to the Golgi, where the A1 protein is recognized by the endoplasmic reticulum chaperon, protein disulfide isomerase
, unfolded and delivered to the membrane, where the ER-oxidase - Ero1 triggers the release of the A1 protein by oxidation of protein disulfide isomerase
complex. As A1 moves from the ER into the cytoplasm by the Sec61 channel, it refolds and avoids ubiquitination.
CTA1 is then free to bind with a human partner protein called ADP-ribosylation factor 6 (Arf6); binding to Arf6 drives a change in the conformation (the shape) of CTA1 which exposes its active site and enables its catalytic activity.
The CTA1 fragment catalyses ADP ribosylation from NAD to the regulatory component of adenylate cyclase
, thereby activating it. Increased adenylate cyclase
activity increases cyclic AMP (cAMP) synthesis causing massive fluid and electrolyte
efflux, resulting in diarrhea
.
GM1 gangliosides are found in lipid raft
s on the cell surface. B subunit complexes labelled with fluorescent tags or subsequently targeted with antibodies can be used to identify rafts.
Protein complex
A multiprotein complex is a group of two or more associated polypeptide chains. If the different polypeptide chains contain different protein domain, the resulting multiprotein complex can have multiple catalytic functions...
secreted by the bacterium Vibrio cholerae
Vibrio cholerae
Vibrio cholerae is a Gram-negative, comma-shaped bacterium. Some strains of V. cholerae cause the disease cholera. V. cholerae is facultatively anaerobic and has a flagella at one cell pole. V...
. CTX is responsible for the massive, watery diarrhea characteristic of cholera
Cholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...
infection.
Structure
The cholera toxinExotoxin
An exotoxin is a toxin excreted by a microorganism, like bacteria, fungi, algae, and protozoa. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host...
is an oligomer
Oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...
ic complex made up of six protein subunits: a single copy of the A subunit (part A, enzymatic), and five copies of the B subunit (part B, receptor binding). The three-dimensional structure of the toxin was determined using X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
by Zhang et al. in 1995.
The five B subunits—each weighing 12 kDa
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
, and all coloured blue in the accompanying figure—form a five-membered ring. The A subunit has two important segments. The A1 portion of the chain (CTA1, red) is a globular enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
payload that ADP-ribosylates
ADP-ribosylation
ADP-ribosylation is the addition of one or more ADP-ribose moieties to a protein. These reactions are involved in cell signaling and the control of many cell processes, including DNA repair and apoptosis.-ADP-ribosylation enzymes:...
G protein
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
s, while the A2 chain (CTA2, orange) forms an extended alpha helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
which sits snugly in the central pore of the B subunit ring.
This structure is similar in shape, mechanism, and sequence to the heat-labile enterotoxin
Heat-labile enterotoxin
Heat-labile enterotoxin is a type of labile toxin found in Escherichia coli that is inactivated at high temperatures.It acts similarly to the cholera toxin by raising cAMP levels through ADP-ribosylation of the alpha-subunit of a Gs protein leading to the constitutive activation of adenylate cyclase...
secreted by some strains of the Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
bacterium.
Pathogenesis
The toxin is an oligomeric protein composed of one A subunit and five B subunits (AB5). After entrance into intestinal epithelialIntestinal epithelium
The intestinal epithelium is the epithelium that covers the small and large intestine. It is simple columnar and nonciliated.They primarily take part in the digestive system. However, they also express TLR 4 receptors, and are thus a part of the immune system, both as a barrier and as a first-line...
cells via receptor-mediated endocytosis
Receptor-mediated endocytosis
Receptor-mediated endocytosis , also called clathrin-dependent endocytosis, is a process by which cells internalize molecules by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized.-Process:After the binding of a...
, the A subunit detaches and becomes activated by proteolytic cleavage, allowing it to catalyze the ADP ribosylation of the Gαs subunit of the heterotrimeric G protein
Heterotrimeric G protein
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha , beta and gamma subunits, the latter two referred to as the beta-gamma...
resulting in constitutive cAMP
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
production. This in turn leads to secretion of H2O
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, Na+
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, K+
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, Cl-
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, and HCO3-
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
into the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the small intestine resulting in rapid dehydration
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
and other factors associated with cholera, including a rice-water stool.
Interestingly, the pertussis toxin
Pertussis toxin
Pertussis toxin is a protein-based AB5-type exotoxin produced by the bacterium Bordetella pertussis, which causes whooping cough. PT is involved in the colonization of the respiratory tract and the establishment of infection...
(also an AB5 protein) produced by Bordetella pertussis
Bordetella pertussis
Bordetella pertussis is a Gram-negative, aerobic coccobacillus of the genus Bordetella, and the causative agent of pertussis or whooping cough. Unlike B. bronchiseptica, B. pertussis is non-motile. Its virulence factors include pertussis toxin, filamentous hæmagglutinin, pertactin, fimbria, and...
acts in a similar manner with the exception that it ADP-ribosylates the Gαi subunit, rendering it inactive and unable to inhibit adenylyl cyclase production of cAMP (leading to constitutive production).
Origin
The gene encoding the cholera toxin is introduced into V. cholerae by horizontal gene transferHorizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
. Virulent
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
strains of V. cholerae carry a variant of lysogenic
Lysogeny
Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction . Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome...
bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
called CTXf or CTXφ.
Working mechanism
Once secreted, the B subunit ring of CTX will bind to GM1GM1
GM1 the "prototype" ganglioside, is a member of the ganglio series of gangliosides which contain one sialic acid residue. GM1 has important physiological properties and impacts neuronal plasticity and repair mechanisms, and the release of neurotrophins in the brain...
ganglioside
Ganglioside
Ganglioside is a molecule composed of a glycosphingolipid with one or more sialic acids linked on the sugar chain. The 60+ known gangliosides differ mainly in the position and number of NANA residues.It is a component of the cell plasma membrane that modulates cell signal transduction events...
s on the surface of the host's cells. After binding takes place, the entire CTX complex is internalised by the cell and the CTA1 chain is released by the reduction of a disulfide bridge. In fact, the endosome is moved to the Golgi, where the A1 protein is recognized by the endoplasmic reticulum chaperon, protein disulfide isomerase
Protein disulfide isomerase
Protein disulfide isomerase or PDI is an enzyme in the endoplasmic reticulum in eukaryotes that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold...
, unfolded and delivered to the membrane, where the ER-oxidase - Ero1 triggers the release of the A1 protein by oxidation of protein disulfide isomerase
Protein disulfide isomerase
Protein disulfide isomerase or PDI is an enzyme in the endoplasmic reticulum in eukaryotes that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold...
complex. As A1 moves from the ER into the cytoplasm by the Sec61 channel, it refolds and avoids ubiquitination.
CTA1 is then free to bind with a human partner protein called ADP-ribosylation factor 6 (Arf6); binding to Arf6 drives a change in the conformation (the shape) of CTA1 which exposes its active site and enables its catalytic activity.
The CTA1 fragment catalyses ADP ribosylation from NAD to the regulatory component of adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
, thereby activating it. Increased adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
activity increases cyclic AMP (cAMP) synthesis causing massive fluid and electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
efflux, resulting in diarrhea
Diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
.
Applications
Because the B subunit appears to be relatively non-toxic, researchers have found a number of applications for it in cell and molecular biology. It is routinely used as a neuronal tracer.GM1 gangliosides are found in lipid raft
Lipid raft
The plasma membrane of cells is made of a combination of glycosphingolipids and protein receptors organized in glycolipoprotein microdomains termed lipid rafts...
s on the cell surface. B subunit complexes labelled with fluorescent tags or subsequently targeted with antibodies can be used to identify rafts.
External links
- http://www.ebi.ac.uk/interpro/potm/2005_9/Page1.htm
- Molecule of the Month