The toxin responsible for the life-threatening flood of diarrhea characterizing the infection of cholera is stylistically represented here (a separate colour was assigned to each subunit). Most Americans don't fear the attack of vibrio cholerae anymore, but pandemics of this bacterium have killed millions of people. Cholera continues to strike in communities using contaminated water. The latest well-known outbreak hit Haiti last fall following the devastation of the earthquake. People infected with cholera suffer debilitating bouts of diarrhea and the ensuing dehydration and loss of electrolytes cause the victims to appear blue-black and shriveled. Without rapid treatment, death can occur in just four hours.
The culprit at the heart of the destruction is cholera toxin (CT):
Secreted by vibrio cholerae upon entry into the wall of the small intestine, the toxin acts rapidly in the intestinal lumen. It is divided into A and B subunits:
|B unit, the stabilizer|
The B unit is the stabilizer; its five identical units form a ring to anchor the complex. It binds to the epithelial cell membranes by attaching to a specific ganglioside, GM1, found in lipid rafts.
|A unit, containing the catalyzer and accessory sections|
The A unit has two main subunits:
A1 (pink, orange, and blue) separates from the rest of the complex to enter the cell and A2 (white) mediates the vital connection between A and B. A1 and A2 are connected via a disulfide bond.
The B-unit/cell membrane connection allows a portion of subunit A, the afflictor (pink), to enter the cytosol. It acts as an ADP-ribosyltransferase, catalyzing the addition of ADP-ribose to Gsα of a G-protein regulator. This ADP-ribosylation irreversibly activates Gsα, resulting in an overproduction of cAMP.
Without the capability to turn off intracellular rush of cAMP, both salt and water accumulate in the cell and the cell dies. Occurring in a massive amount of cells, this process creates the rush of diarrhea which characterizes the disease.
Gsα + NAD → ADPR-Gsα → ↑ cAMP
- The B unit anchoring process is influenced by both cholesterol and calcium signaling. The recognition of cholesterol's role gives more significance to the purpose of lipid rafts. When lipid rafts are disrupted, the functions performed by CTB are decreased. The processes of CTB binding increase intracellular concentrations of calcium, probably through the action of the ganglioside, GM1. It is suggested that CTB attachment reduces the inhibition normally acted by gangliosides on enzymes that induce exoctyosis.
- ADP-ribosylation of Gsα is irreversible because it increases the sensitivity of Gsα to its activator GTP, as well as decreasing its GTPase activity, which normally would hydrolyze GTP to GDP and dissociate the Gsα-AC complex - this can induce cAMP production enhancements of several hundred-fold!
Soo, J., Zhang, J., He, Q., Agarwal, S., Li, H., Zhang, H., & Chen, P. (2010). Surface immobilized cholera toxin B subunit (CTB) facilitates vesicle docking, trafficking and exocytosis. Integrative Biology, 2, 250-257. doi:10.1039/C0IB00006J.
Spangler, B. D. (1992, December). Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin. Microbiology Review, 56(4), 622-647. Retrieved from PubMed Central (PMC372891).
Teter, K., Jobling, M. G., Sentz, D., & Holmes, R. K. (2006, April). The Cholera Toxin A13 Subdomain Is Essential for Interaction with ADP-Ribosylation Factor 6 and Full Toxic Activity but Is Not Required for Translocation from the Endoplasmic Reticulum to the Cytosol. Infection and Immunity, 74(4), 2259-2267. Retrieved from PubMed Central (PMC1418936).