Plant glutathione transferases

Abstract

The soluble glutathione transferases (GSTs, EC 2.5.1.18) are encoded by a large and diverse gene family in plants, which can be divided on the basis of sequence identity into the phi, tau, theta, zeta and lambda classes. The theta and zeta GSTs have counterparts in animals but the other classes are plant-specific and form the focus of this article. The genome of Arabidopsis thaliana contains 48 GST genes, with the tau and phi classes being the most numerous. The GST proteins have evolved by gene duplication to perform a range of functional roles using the tripeptide glutathione (GSH) as a cosubstrate or coenzyme. GSTs are predominantly expressed in the cytosol, where their GSH-dependent catalytic functions include the conjugation and resulting detoxification of herbicides, the reduction of organic hydroperoxides formed during oxidative stress and the isomerization of maleylacetoacetate to fumarylacetoacetate, a key step in the catabolism of tyrosine. GSTs also have non-catalytic roles, binding flavonoid natural products in the cytosol prior to their deposition in the vacuole. Recent studies have also implicated GSTs as components of ultraviolet-inducible cell signaling pathways and as potential regulators of apoptosis. Although sequence diversification has produced GSTs with multiple functions, the structure of these proteins has been highly conserved. The GSTs thus represent an excellent example of how protein families can diversify to fulfill multiple functions while conserving form and structure.

Figure 1

Phylogenetic tree illustrating the diversity of GSTs and the relationships between classes. All the GSTs identified from Arabidopsis are shown in black; representative GSTs from other classes and organisms are shown in red, and their names are prefixed with two letters denoting the source organism: Hs, Homo sapiens; Rr, Rattus rattus; Rn, Rattus norvegicus; Ss, Sus scrofa; An, Aspergillus nidulans; Pm, Proteus mirabilis; Ec, Escherichia coli. Branch lengths correspond to the estimated evolutionary distance between protein sequences.

Figure 2

Distribution of GST genes in the Arabidopsis genome, showing clustering of GSTs of the same classbecause of gene duplication. Chromosomes are represented by numbered gray bars; each triangle represents a single GST gene. The organization of the coding sequence of a typical gene in each class is shown below, drawn to scale; intron positions are shown as black lines.

Figure 3

Nomenclature for Arabidopsis and other plant GSTs, adapted from the mammalian GST classification system [14].

Figure 4

Ribbon representations of the structures of GST subunits. The GSTs specific to mammals (alpha, mu, pi and sigma) have a blue background; the plant-specific (phi) and bacteria-specific (beta) GSTs have yellow and white backgrounds, respectively; GSTs (theta and zeta) that have counterparts in both animals and plants have green backgrounds. The structure of a tau GST has yet to be reported. Although there is little sequence similarity between enzymes of different classes, there is significant conservation in overall structure.

Figure 5

Overview of GST dimer structure and substrate binding. (a) A ribbon/surface representation of a typical GST subunit (Z. mays GSTF1, pdb 1BYE), with the amino-terminal domain in green, the linker region in red, the carboxy-terminal domain in blue and the protein surface in gray. A glutathione conjugate of the herbicide atrazine in ball-and-stick representation is shown binding at the active site; the GSH-binding site (G site) is highlighted in yellow and the hydrophobic site (H site) is highlighted in blue. (b) A ribbon/surface representation of the ZmGSTF1 homodimer oriented with the amino-terminal domains at bottom left and top right and the subunits in blue and purple. The atrazine-glutathione conjugates are shown in ball-and-stick representation, bound at the active site of each subunit. The dimer is formed by a ball-and-socket interaction between the amino- and carboxy-terminal domains of the different subunits (see text for further details); the deep cleft between subunits is characteristic of phi GSTs.

Figure 6

Overview of known GST functions in plants. (a) In secondary metabolism, GSTs detoxify toxins by conjugation with GSH; the conjugates (toxin-SG) are then transported into the vacuole by ABC transporters (shown as circles) prior to proteolytic processing. (b) Some phi and tau class enzymes are also required for transport of flavonoid pigments to the vacuole. (c-e) Roles of GSTs in stress metabolism include acting as (c) glutathione peroxidases that can reduce cytotoxic DNA and lipid hydroperoxides; (d) in an antioxidant capacity, protecting against Bax-induced cell death; and (e) in stress signaling, playing a role in the induction of chalcone synthase following exposure to ultraviolet light. Finally, zeta GSTs (GSTZ) have a role in primary metabolism as maleylacetoacetate isomerases. Wide arrows denote an induction process; narrow arrows denote enzymatic reactions; thick lines denote inhibition of a reaction; R, an alkyl group.