Functional, Structural and Biochemical Features of Plant Serinyl-Glutathione Transferases

Abstract

Glutathione transferases (GSTs) belong to a ubiquitous multigenic family of enzymes involved in diverse biological processes including xenobiotic detoxification and secondary metabolism. A canonical GST is formed by two domains, the N-terminal one adopting a thioredoxin (TRX) fold and the C-terminal one an all-helical structure. The most recent genomic and phylogenetic analysis based on this domain organization allowed the classification of the GST family into 14 classes in terrestrial plants. These GSTs are further distinguished based on the presence of the ancestral cysteine (Cys-GSTs) present in TRX family proteins or on its substitution by a serine (Ser-GSTs). Cys-GSTs catalyze the reduction of dehydroascorbate and deglutathionylation reactions whereas Ser-GSTs catalyze glutathione conjugation reactions and eventually have peroxidase activity, both activities being important for stress tolerance or herbicide detoxification. Through non-catalytic, so-called ligandin properties, numerous plant GSTs also participate in the binding and transport of small heterocyclic ligands such as flavonoids including anthocyanins, and polyphenols. So far, this function has likely been underestimated compared to the other documented roles of GSTs. In this review, we compiled data concerning the known enzymatic and structural properties as well as the biochemical and physiological functions associated to plant GSTs having a conserved serine in their active site.

Keywords: glutathione transferases; ligandin property; photosynthetic organisms; phylogeny; secondary metabolism; structure; xenobiotic detoxification.

FIGURE 1

Structures of Ser-GSTs from plants highlighting the location of ligand-binding sites. (A–D) schematic structure of the GmGSTU4 and AtGSTF2 dimers, respectively. (C,D) illustrate the complexes formed between AtGSTF2 and FOE (1BX9) or QCT (5A4W). The secondary structures and the location of the ligand-binding sites are labeled. The TRX domain is in cyan and the C-terminal domain is in magenta. The labeled ligands are: GTB, S-(P-Nitrobenzyl)glutathione; 4NM, 4-Nitrophenyl methanethiol; FOE, FOE-4053-glutathione conjugate; QCT, Quercetrin.

FIGURE 2

Structure-based sequence alignments of Tau class (A) and Phi class (B) GSTs from plants. The sequence alignment was generated with Chimera (Pettersen et al., 2004) and manually adjusted. Crystal structures and sequences are available at the Protein Data Bank (http://www.rcsb.org): 1GWC for TaGSTU4, 5ECS for AtGSTU20, 6E6P for AtGSTU23, 5G5A for AtGSTU25, 2VO4 for GmGSTU4, 4CHS for GmGSTU10, 5G5E for MiGSTU1, 1OYJ for OsGSTU1, 5J4U for PtGSTU30, 4J2F for RcGSTU1, 1GNW for AtGSTF2, 4RI6 for PtGSTF1, 5EY6 for PtGSTF2, 5F05 for PtGSTF5, 5F06 for PtGSTF7, 5F07 for PtGSTF8, 1AXD for ZmGSTF1, and 1AW9 for ZmGSTF3. Secondary structures are labeled and shown using arrows (β-strands) and squiggles (helices). The active site serine, the invariant proline and the quasi-invariant aspartic acid are in bold type, colored white, highlighted black, and marked with . Residues that participate in dimer stabilization via strong polar interactions are in bold and marked with ^∗. Residues involved in binding glutathione (G-site) are in bold type, highlighted yellow, and marked with . Residues of the characterized H-sites are in bold type, highlighted green, and marked with . Residues of the L1-site (GmGSTU4, 2VO4) are in bold type, highlighted red, and marked with . Residues of the L2-site (AtGSTF2, 5A4U, 5A4V, and 5A4W) are in bold type, highlighted blue, and marked with . Residues of the L3-site (AtGSTF2, 5A4K, 5A4U, and 5A4W) are in bold type, highlighted pink, and marked with .

FIGURE 3

Transcript abundance of 44 Ser-GST genes during Arabidopsis development. Microarray experimental data (generated using Affymetrix ATH1 GeneChip arrays) of Arabidopsis Development (AtGenExpress Developmental Expression Atlas) described by Schmid et al. (2005) were obtained from the National Center for Biotechnology Information (NCBI) Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo/). Intensity values of replicates were averaged and z-score transformed across the following ten developmental conditions: roots, stems, rosette leaves, developmental leaf senescence (DLS), cauline leaf, whole plant, apex, flowering stages, flower organs, and siliques/seeds. The data were then imported into The Institute for Genomic Research Multiple Experiment Viewer (MVE) and hierarchically clustered using average linkage based on Euclidean distance. Gene families of each gene are indicated using the following color key: tau, light green; phi, medium green; theta, gray; and zeta, yellow.

FIGURE 4

Hierarchical clustering of log2 fold changes of 44 Ser-GST genes in response to abiotic stresses. Microarray experimental data (generated using Affymetrix ATH1 GeneChip arrays), described by Kilian et al. (2007), were obtained from the NCBI Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo/). Intensity values of replicates were averaged and z-score transformed across nine stresses (cold, osmotic, salt, drought, genotoxic, UV, wounding, and heat) grouped as either aerial or root tissue, and further defined according to a time course of exposure in hours. Hierarchical clustering was carried out by average linkage based on Euclidean distance using in the Multiple Experiment Viewer (MEV) analysis package, resulting in the defining of 7 clusters. Gene families of each gene was indicated using the following color key: tau, light green; phi, medium green; theta, gray; and zeta, yellow.