ToxT

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<u>Ligand-binding</u>. <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Barrel/1'>A nine-stranded beta sheet sandwich</scene> or "jelly-roll" with three other alpha helices (overall making up the <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/N-terminus/1'>N-terminus</scene>) contain a <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Binding_pocket/1'>binding pocket</scene>. This is largely made up of residues from the N-terminus (Y12, Y20, F22, L25, I27, K31, F33, L61, F69, L71, V81, and V83), and a few from the C-terminus (I226, K230, M259, V261, Y266, and M269). The pocket is highly hydrophobic, and has a known volume of 780.9 Angstroms.<ref name="structure">PMID: 20133655</ref> This pocket contains a ligand: <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Binding_pocket/2'>cis-palmitoleate</scene> <ref name="structure">PMID: 20133655</ref> which appears to have a negative effect on virulence when present in vitro. The <i>cis</i>-palmitoleate forms <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Salt_bridges_pam/1'>salt bridges</scene> with residues K31 and K230. This unsaturated fatty acid, like other UFAs,[http://en.wikipedia.org/wiki/Fatty_acid#Unsaturated_fatty_acids] tend to inhibit genes under the control of ToxT. The
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<u>Ligand-binding</u>. <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Barrel/1'>A nine-stranded beta sheet sandwich</scene> or "jelly-roll" with three other alpha helices (overall making up the <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/N-terminus/1'>N-terminus</scene>) contain a <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Binding_pocket/1'>binding pocket</scene>. This is largely made up of residues from the N-terminus (Y12, Y20, F22, L25, I27, K31, F33, L61, F69, L71, V81, and V83), and a few from the C-terminus (I226, K230, M259, V261, Y266, and M269). The pocket is highly hydrophobic, and has a known volume of 780.9 Angstroms.<ref name="structure">PMID: 20133655</ref> This pocket contains a ligand: <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Binding_pocket/2'>cis-palmitoleate</scene> <ref name="structure">PMID: 20133655</ref> which appears to have a negative effect on virulence when present in vitro. The <i>cis</i>-palmitoleate forms <scene name='ToxT_Transcriptional_Regulator_in_Vibrio_cholerae/Salt_bridges_pam/1'>salt bridges</scene> with residues K31 and K230 (for detail, see Figure 1B of: [http://www.pnas.org/content/107/7/2860/F1.expansion.html]). This unsaturated fatty acid, like other UFAs,[http://en.wikipedia.org/wiki/Fatty_acid#Unsaturated_fatty_acids] tend to inhibit genes under the control of ToxT. The
Specifically, the <i>cis</i>-palmitoleate appears to bind directly to ToxT, change its conformation, and thus lower the ability to bind DNA and form dimers.<ref name="structure">PMID: 20133655</ref> The presence of UFAs is associated with being in the lumen of the intestine during the bacterial infection. Here, binding of PAM brings K31 and K230 together and essentially closes off ToxT. K230 is at the end of helix seven, and binding to K31 causes pulling helix six into an unfavorable conformation that disallows DNA binding. In lower concentration of fatty acids, ie: after penetrating the intestine's mucus, the two lysine residues repel each other and give ToxT an open conformation by giving helix seven and thus six freedom, allowing DNA binding.<ref name="structure">PMID: 20133655</ref>
Specifically, the <i>cis</i>-palmitoleate appears to bind directly to ToxT, change its conformation, and thus lower the ability to bind DNA and form dimers.<ref name="structure">PMID: 20133655</ref> The presence of UFAs is associated with being in the lumen of the intestine during the bacterial infection. Here, binding of PAM brings K31 and K230 together and essentially closes off ToxT. K230 is at the end of helix seven, and binding to K31 causes pulling helix six into an unfavorable conformation that disallows DNA binding. In lower concentration of fatty acids, ie: after penetrating the intestine's mucus, the two lysine residues repel each other and give ToxT an open conformation by giving helix seven and thus six freedom, allowing DNA binding.<ref name="structure">PMID: 20133655</ref>
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Revision as of 22:12, 29 November 2011

The crystal structure of ToxT is resolved in monomeric form, after isolation from Vibrio cholerae strain O395.[1]

Introduction

ToxT is a molecule at the end of a transcriptional cascade that autoregulates the transcription of the primary virulence factors of Vibrio cholerae[3] and itself. ToxT is a cytoplasmic protein that is activated in turn by ToxR, which is itself activated by ToxS in response to environmental stimuli.[2] These two factors, cholera toxin (CT)[4] and the toxin co-regulated pilus (TCP), are instrumental in causing the disease cholera[5]. This is an intestinal infection resulting in massive water loss in the affected individual, causing extreme dehydration.[6]

ToxT, 1.9 Angstrom resolution crystal structure

Drag the structure with the mouse to rotate

Further Study

Conclusive results about what activates ToxT itself has not yet been found. The varying activity of ToxT dependent on the presence of cis-palmitoleate or other unsaturated fatty acids represents a detailed method of effective pathogenicity in humans, but may not be a reasonable target for drug treatment. By restricting transcription (and thus translation and protein production) of virulence genes until the bacterium is determined to be in a favorable location for infection, Vibrio cholerae avoids wasting energy producing virulence factors that will just be cleared by the intestine. This is a specific mechanism to ensure that the bacterium also injects CT and TCP where they will do the most damage, perpetuating the infection. [6]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Lowden MJ, Skorupski K, Pellegrini M, Chiorazzo MG, Taylor RK, Kull FJ. Structure of Vibrio cholerae ToxT reveals a mechanism for fatty acid regulation of virulence genes. Proc Natl Acad Sci U S A. 2010 Feb 16;107(7):2860-5. Epub 2010 Feb 1. PMID:20133655
  2. ↑ Kenneth Todar [1] Vibrio cholerae and Asiatic Cholera, Todar's Online Textbook of Bacteriology. Date of access: 2011-11-28.
  3. ↑ Martin RG, Rosner JL. The AraC transcriptional activators. Curr Opin Microbiol. 2001 Apr;4(2):132-7. PMID:11282467
  4. ↑ Weber GG, Klose KE. The complexity of ToxT-dependent transcription in Vibrio cholerae. Indian J Med Res. 2011 Feb;133(2):201-6. PMID:21415495
  5. ↑ Shakhnovich EA, Hung DT, Pierson E, Lee K, Mekalanos JJ. Virstatin inhibits dimerization of the transcriptional activator ToxT. Proc Natl Acad Sci U S A. 2007 Feb 13;104(7):2372-7. Epub 2007 Feb 5. PMID:17283330 doi:10.1073/pnas.0611643104
  6. ↑ Kenneth Todar [2] Vibrio cholerae and Asiatic Cholera, Todar's Online Textbook of Bacteriology. Date of access: 2011-11-28.
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