Journal:FEBS Open Bio:2

Structural evidence for Arabidopsis glutathione transferase AtGSTF2 functioning as a transporter of small organic ligands

Laziana Ahmad, Elizabeth L. Rylott, Neil C. Bruce, Robert Edwards and Gideon Grogan ^[1]

Molecular Tour
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, AtGSTF2, revealed by X-ray crystallography. Ligands used in this study: 1 = Indole-3-aldehyde; 2 = Camalexin; 3 = Quercetrin; 4 = Quercetin. For comparison was used another GST ligand S-hexyl glutathione from (1gnw^[2]). Complexes of the AtGSTF2 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. Structure of AtGSTF2 dimer. 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 AtGSTF2 and showing location of ligands in binding sites L1 and L2: α11

• I Complex with Indole-3-aldehyde 1, ligand was found found at both L1 and L2 sites, 5a4u;
• II Complex with Camalexin 2, ligand was found only at the L1 sites, 5a5k;
• III Complex with Quercetrin 3, ligand was found found at both L1 and L2 sites, 5a4w;
• IV Complex with Quercetin 4, ligand was found only at the L2 site, 5a4v;
• V Complex with two molecules of S-hexyl glutathione ‘GSX’, showing the GSH conjugation site, 1gnw.
• Click here to see summary animation how different ligands bind in L1 and L2 sites.

Please pause animation before continuation:

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 AtGSTF2 based on isothermal calorimetry experiments (Dixon et al. (2011)^[3]) and suggest a mode of ligand binding in GSTs commensurate with a possible role in ligand transport.

Electrostatic surface views of AtGSTF2 (Anionic (-) / Cationic (+) / Histidine (+) / White Neutral):

• Same view as in scene with complex with two molecules of S-hexyl glutathione (1gnw).
• In complex with quercetrin 3, rotated 90°, and revealing ligand-binding site L1 (5a4w).
• In complex with quercetrin 3, rotated 180°, and revealing ligand-binding site L2 (5a4w).

Ligand binding in the L1 site:

• Indole-3-aldehyde 1.
• Camalexin 2.
• Quercetrin 3.
• Click here to see summary animation how different ligands bind in the L1 site.

Please pause animation before continuation:

Ligand binding in the L2 site:

• Indole-3-aldehyde 1.
• Quercetrin 3.
• Quercetin 4.
• Click here to see summary animation how different ligands bind in the L1 site.

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.