Cholera toxin

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Contents

Introduction

Cholera Toxin (CT) is a protein secreted by the bacterium Vibrio choleraeUNIQ679ca1de39e88129-nowiki-00000001-QINU1UNIQ679ca1de39e88129-nowiki-00000002-QINU. is spread by dirty water and characterised by its ability to cause severe diarrhoea, often leading to dehydration[2].

Structure of the Toxin 

CT is a member of the AB5 family of toxins and consists of subunits, 2 Alpha and 5 Beta. The Beta subunits, weighing 11kDa each, binds to five ganglioslide GM1 receptors on the plasma membrane and trigers endocytosis of the toxin, whilst the Alpha subunit is made up of an A1 and A2 chain. A1 is enzymatic and does most of the work, once inside the cell. A2 is an extended alpha helix, connceting the alpha subunit to the Beta subunit. The alpha subunit is cleaved apart once inside the cell, allowing the A1 chain to dissociate and begin its catalytic activity, leaving the A2 domain which had been anchoring the A1 chain to the Beta subunit[3].  

How CT Causes Disease

Following the toxins entry into the cell and Alpha protomer cleavage, the A1 is trafficked to the endoplasmic reticulum (ER) where it associates with disulphide isomerase (PDI). This triggers the unfolding of the A1 chain and consequently allows it to hijack the ER association degradation mechanism to enter the cytosol by retro-translocation. Normally, proteins which enter the cytosol in this manner are ubiquitinated and then degraded by the proteasome, however, the A1 subunit avoids this fate by rapid refolding[4].

The A1 can now ribosylates the alpha subunit of a G-protein associated with an intestinal epithelial cell, preventing the hydrolysis of GTP bound to the same G-protein. This locks the G-protein in an active state, leading to the indefinite stimulation of adenylyl cyclase, causing a rise in cAMP concentration. This causes phosphorylation and activation of the cystic fibrosis transmembrane conductance regulator (CFTR protein) leading to  the ATP mediated efflux of chloride ions pumped out of the enterocyte. [5]This then causes water, sodium and potassium to leave the cell and enter the intestinal lumen, leading to the severe diarrhoea[6]. This is due to the water potential being dratically lowered in the lumen due to the eflux of ions. Water therefore flows out of the epithelial cells surrounding the small intestine, and into the lumen. 

References

  1. Vanden Broeck D, H. C. D. W. M., 2007. Vibrio cholerae: cholera toxin. Int J Biochem Cell Biol, 39(10), pp. 1771-1775.
  2. Dilip Kumar Biswas, Rama Bhunia, Dipankar Maji, and Palash Das, “Contaminated Pond Water Favors Cholera Outbreak at Haibatpur Village, Purba Medinipur District, West Bengal, India,” Journal of Tropical Medicine, vol. 2014, Article ID 764530, 5 pages, 2014. doi:10.1155/2014/764530
  3. Daniel J.-F. Chinnapen, H. C. D. S. W. I. L., 2006. Rafting with cholera toxin: endocytosis and trafficking from plasma membrane to ER. FEMS Microbiology Letters, 266(2), pp. 129-137.
  4. Naomi L. B. Wernick, D. J.-F. C. A. C. a. W. I. L., 2010. Cholera Toxin: An Intracellular Journey into the Cytosol by Way of the Endoplasmic Reticulum. MDPI, 2(3), pp. 310-325.
  5. Wayne I. Lencer, T. R. H. R. K. H., 1999. Membrane traffic and the cellular uptake of cholera toxin. Biochimica et Biophysica Acta (BBA), 1450(3), pp. 177-190.
  6. Bruce Alberts, A. J. J. L. M. R. K. R. a. P. W., 2007. Molecular Biology of the Cell, 5th edition. In: Molecular Biology of the Cell, 5th edition. New York: Garland Science, p. 906.
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