Cholera toxin

Cholera toxin (sometimes abbreviated to CTX, Ctx, or CT) is a protein complex secreted by the bacterium Vibrio cholerae.^[1][2] CTX is responsible for the massive, watery diarrhea characteristic of cholera infection.

Structure

The cholera toxin is an oligomeric 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.^[3]

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 proteins, while the A2 chain (CTA2, orange) forms an extended alpha helix which sits snugly in the central pore of the B subunit ring.^[4]

This structure is similar in shape, mechanism, and sequence to the heat-labile enterotoxin secreted by some strains of the Escherichia coli bacterium.

Pathogenesis

The toxin is an oligomeric protein composed of one A subunit and five B subunits (AB₅). After entrance into intestinal epithelial cells via receptor-mediated endocytosis, the A subunit detaches and becomes activated by proteolytic cleavage, allowing it to catalyze the ADP ribosylation of the Gα_s 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 Gα_i subunit, rendering it inactive and unable to inhibit adenylyl cyclase production of cAMP (leading to constitutive production).^[5]

Origin

The gene encoding the cholera toxin is introduced into V. cholerae by horizontal gene transfer. Virulent strains of V. cholerae carry a variant of lysogenic bacteriophage called CTXf or CTXφ.^[6]

Working mechanism

Once secreted, the B subunit ring of CTX will bind to GM1 gangliosides 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.^[7]

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.

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.^[8]

GM1 gangliosides are found in lipid rafts on the cell surface. B subunit complexes labelled with fluorescent tags or subsequently targeted with antibodies can be used to identify rafts.

See also

• Enterotoxin

References

1. ^ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. p. 375. ISBN 0838585299. 
2. ^ Faruque SM; Nair GB (editors). (2008). Vibrio cholerae: Genomics and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-33-2 . http://www.horizonpress.com/vib. 
3. ^ Zhang R, Scott D, Westbrook M, Nance S, Spangler B, Shipley G, Westbrook E (1995). "The three-dimensional crystal structure of cholera toxin". J Mol Biol 251 (4): 563–73. doi:10.1006/jmbi.1995.0456. PMID 7658473. 
4. ^ De Haan L, Hirst TR (2004). "Cholera toxin: a paradigm for multi-functional engagement of cellular mechanisms (Review)". Mol. Membr. Biol. 21 (2): 77–92. doi:10.1080/09687680410001663267. PMID 15204437. 
5. ^ Boron, W. F., & Boulpaep, E. L. (2009). Medical physiology: a cellular and molecular approach (2nd ed.). Philadelphia, PA: Saunders/Elsevier.
6. ^ Davis B, Waldor M (2003). "Filamentous phages linked to virulence of Vibrio cholerae". Curr Opin Microbiol 6 (1): 35–42. doi:10.1016/S1369-5274(02)00005-X. PMID 12615217. 
7. ^ O'Neal C, Jobling M, Holmes R, Hol W (2005). "Structural basis for the activation of cholera toxin by human ARF6-GTP". Science 309 (5737): 1093–6. doi:10.1126/science.1113398. PMID 16099990. 
8. ^ Pierre-Hervé Luppi. "The Discovery of Cholera-Toxin as a Powerful Neuroanatomical Tool". http://sommeil.univ-lyon1.fr/articles/luppi/frenchcorner/sommaire.php. Retrieved 2011-03-23. 

External links

• http://www.ebi.ac.uk/interpro/potm/2005_9/Page1.htm
• Molecule of the Month
• MeSH Cholera+Toxin