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<StructureSection load='5a4u_ab' size='450' side='right' scene='76/763766/Cv1/1' caption=''>
===  Structural evidence for Arabidopsis glutathione transferase AtGSTF2 functioning as a transporter of small organic ligands ===
<big>Laziana Ahmad, Elizabeth L. Rylott, Neil C. Bruce, Robert Edwards and Gideon Grogan</big> <ref>pmid 28174680 </ref>
<hr/>
<b>Molecular Tour</b><br>
Glutathione transferases ('''GSTs''') are involved in many processes in plant biochemistry, with their best characterised role being the detoxification of xenobiotics through their conjugation with glutathione. GSTs have also been implicated in noncatalytic roles, including the binding and transport of small heterocyclic ligands such as indole hormones, phytoalexins and flavonoids. Although evidence for ligand binding and transport has been obtained using gene deletions and ligand binding studies on purified GSTs, there has been no structural evidence for the binding of relevant ligands in noncatalytic sites. Here we provide evidence of noncatalytic ligand-binding sites in the phi class GST from the model plant ''Arabidopsis thaliana'', ''At''GSTF2, revealed by X-ray crystallography. Ligands used in this study: 1 = <scene name='76/763766/Cv2/4'>Indole-3-aldehyde</scene>; 2 = <scene name='76/763766/Cv2/3'>Camalexin</scene>; 3 = <scene name='76/763766/Cv2/5'>Quercetrin</scene>; 4 = <scene name='76/763766/Cv2/6'>Quercetin</scene>. For comparison was used another GST ligand <scene name='76/763766/Cv2/7'>S-hexyl glutathione</scene> from ([[1gnw]]<ref>pmid 8551521 </ref>). Complexes of the ''At''GSTF2 dimer were obtained with indole-3-aldehyde, camalexin, the flavonoid quercetrin and its non-rhamnosylated analogue quercetin, at resolutions of 2.00, 2.77, 2.25 and 2.38 Å respectively. Two symmetry-equivalent-binding sites ('''L1''') were identified at the periphery of the dimer, and one more ('''L2''') at the dimer interface. <scene name='76/763766/Cv1/12'>Structure of AtGSTF2 dimer</scene>. The figure is derived using the complex with indole-3-aldehyde and shows location of ligand-binding sites '''L1''' and '''L2''' labelled for ease of reference. In the complexes, indole-3-aldehyde and quercetrin were found at both '''L1''' and '''L2''' sites, but camalexin was found only at the '''L1''' sites and quercetin only at the '''L2''' site. Structure of dimers ‘A/B’ from ligand complex structures of ''At''GSTF2 and showing location of ligands in binding sites '''L1''' and '''L2''':
α11
*'''I''' <scene name='76/763766/Cv1/11'>Complex with Indole-3-aldehyde 1</scene>, ligand was found found at both '''L1''' and '''L2''' sites, [[5a4u]]; 
*'''II''' <scene name='76/763766/Cv1/17'>Complex with Camalexin 2</scene>, ligand was found only at the '''L1''' sites, [[5a5k]]; 
*'''III''' <scene name='76/763766/Cv1/18'>Complex with Quercetrin 3</scene>, ligand was found found at both '''L1''' and '''L2''' sites, [[5a4w]]; 
*'''IV''' <scene name='76/763766/Cv1/19'>Complex with Quercetin 4</scene>, ligand was found only at the '''L2''' site, [[5a4v]]; 
*'''V''' <scene name='76/763766/Cv1/20'>Complex with two molecules of S-hexyl glutathione</scene> ‘GSX’, showing the '''GSH conjugation site''', [[1gnw]].
*<scene name='76/763766/Cv1/22'>Click here to see summary animation how different ligands bind in L1 and L2 sites</scene>.
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Ligand binding at each site appeared to be largely determined through hydrophobic interactions. The crystallographic studies support previous conclusions made on ligand binding in noncatalytic sites by ''At''GSTF2 based on isothermal calorimetry experiments (Dixon ''et al''. (2011)<ref>pmid 21631432 </ref>) and suggest a mode of ligand binding in GSTs commensurate with a possible role in ligand transport.

Electrostatic surface views of AtGSTF2 ({{Template:ColorKey_Charge_Anionic}} / {{Template:ColorKey_Charge_Cationic}} / <font color='powderblue'><b>Histidine (+)</b></font> / White Neutral): 
*<scene name='76/763766/Cv1/23'>Same view as in scene with complex with two molecules of S-hexyl glutathione</scene> ([[1gnw]]).
*<scene name='76/763766/Cv1/26'>In complex with quercetrin 3, rotated 90°, and revealing ligand-binding site L1</scene> ([[5a4w]]).
*<scene name='76/763766/Cv1/27'>In complex with quercetrin 3, rotated 180°, and revealing ligand-binding site L2</scene> ([[5a4w]]).

Ligand binding in the L1 site:
*<scene name='76/763766/Cv2/14'>Indole-3-aldehyde 1</scene>.
*<scene name='76/763766/Cv2/15'>Camalexin 2</scene>. 
*<scene name='76/763766/Cv2/16'>Quercetrin 3</scene>.
*<scene name='76/763766/Cv2/17'>Click here to see summary animation how different ligands bind in the L1 site</scene>.

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Ligand binding in the L2 site:
*<scene name='76/763766/Cv2/20'>Indole-3-aldehyde 1</scene>.
*<scene name='76/763766/Cv2/21'>Quercetrin 3</scene>.
*<scene name='76/763766/Cv2/22'>Quercetin 4</scene>.
*<scene name='76/763766/Cv2/23'>Click here to see summary animation how different ligands bind in the L1 site</scene>.
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'''PDB reference:''' AtGSTF2 from ''Arabidopsis thaliana'' in complex with indole-3-aldehyde, [[5a4u]]; AtGSTF2 from ''Arabidopsis thaliana'' in complex with quercetin, [[5a4v]]; AtGSTF2 from ''Arabidopsis thaliana'' in complex with quercetrin, [[5a4w]]; AtGSTF2 from Arabidopsis thaliana in complex with camalexin, [[5a5k]]. 

</StructureSection>

<references/>
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