Lab Week 1

Exploring toxin B in cholera

Cholera toxin is the agent that causes the deadly diarrheal disease cholera (Zhang et al. 2002). After the toxin is secreted into the lumen of the gut of the human host by Vibrio cholerae, cholera toxin B subunit, which is a pentamer with five identical receptor-binding sites, will bind to the epithelial cell surface of the gut through interaction with ganglioside GM1 (Zhang et al. 2002). This targeted binding mediates the activation of G proteins on cell surface by cholera toxin A subunit, which is the catalytic portion. The following activation of the channels on the cell membrane leads to large influx of sodium ions and water into the gut lumen, resulting in life-threatening dehydration (Zhang et al. 1995).

The receptor-bonding process mediated by cholera toxin B subunit makes it an attractive target for the design of vaccine against the infection result by V. cholerae. However, there are other factors need to be taken into consideration to decide if it is a viable vaccine target. First, the susceptibility of this protein to evolutionary changes is an important parameter to measure the suitability. If the gene encodes toxin B has a high mutation rate, then it is not a desirable target because the binding motif changes rapidly. Second, whether the role of this vaccine target toxin B is critical in the pathonogenesis of cholera. Third, whether the target vaccine can elicit a strong immune response in host cells will be crucial as well.

Cholera toxin B subunit is a stable pentamer. Its amino acid sequences are listed as below:
MIKLKFGVFFTVLLSSAYAHGTPQNITDLCAEYHNTQIYTLNDKIFSYTESLAGKREMAIITFKNGAIFQVEVPGSQHIDSQKKAIERMKDTLRIAYLTEAKVK
LCVWNNKTPHAIAAISMAN
Its nucleotide sequence is as follows:
atgattaaat taaaatttgg tgtttttttt acagttttac tatcttcagc atatgcaceat ggaacacctc aaaatattac tgatttgtgt gcagaatacc acaacacaca aatatatacg ctaaatgata agatattttc gtatacagaa tctctagctg gaaaaagaga gatggctatc attactttta agaatggtgc aatttttcaa gtagaagtac caggtagtca acatatagat tcacaaaaaa aagcgattga aaggatgaag gataccctga ggattgcata tcttactgaa gctaaagtcg aaaagttatg tgtatggaat aataaaacgc ctcatgcgat tgccgcaatt agtatggcaa at (NCBI)

In V. cholerae, protein 2-methylthioadenine synthetase is evolutionarily closely related to toxin B. Based on the protein Blast data, 2-methylthioadenine synthetase is 100% identical to toxin B on amino acid sequence. It shares the conserved domain - heat-labile enterotoxin beta chain – with toxin B.

According to the Blast result, heat-labile enterotoxin B subunit (LTB) in Escherichia coli is homologous to toxin B in Vibrio cholerae. LTB is closely related to cholera toxin B with 82% amino acid sequence identity. The nucleotides sequence of cholera toxin B is 77% homologous to those of LTB. They have the heat-labile enterotoxin beta chain as conserved domain and are both pentameric subunits with five equivalent GM1 binding sites. Given the similarity between cholera toxin B and E.coli LTB on the sequence and structural level, it is not surprising that they function in a similar way. E.coli LTB is the causative agent of traveler’s diarrhea (Zhang et al. 2002). The pathology of this disease is the receptor-mediated binding to GM1 on human intestine by LTB, which is essentially identical to the role of toxin B in cholera.

The comparison between homolog LTB and cholera toxin B reveals the critical role of toxin B in the disease. A particular focus can be put on the structure-based design to exploit the pentameric structure of toxin B. Based on the number of homologs, Cholera toxin B has not been very susceptible to evolutionary changes. Thus, it is likely to be a good candidate for vaccine target.

Reference:
Zhang, Z.S., Merritt, E.A., Ahn, M., Roach, C., Hou Z., Verlinde, C., Hol, W.G. and Fan E. 2002. Solutions and crystallographic studies of branched multivalent ligands that inhibit the receptor-binding of cholera toxin. J.AM.CHEM.SOC (124): 44

Zhang, R.G., Scott, D.L., Westbrook, M.L., Nance, S., Spangler, B.D., Shipley, G.G. and Westbrook, E.M. 1995. The three-demensional crystal structure of cholera toxin. J. Mol.Biol. (251): 563-573

National Center for Biotechnology Information Feb.10th 2011.
<http://www.ncbi.nlm.nih.gov/>