Selective binding of glutathione conjugates of fatty acid derivatives by plant glutathione transferases

Abstract

Proteomic studies with Arabidopsis thaliana have revealed that the plant-specific tau (U) class glutathione transferases (GSTs) are selectively retained by S-hexylglutathione affinity supports. Over-expression of members of the Arabidopsis GST superfamily in E. coli showed that 25 of the complement of 28 GSTUs caused the aberrant accumulation of acylated glutathione thioesters in vivo, a perturbation that was not observed with other GST classes. Each GSTU caused a specific group of fatty acyl derivatives to accumulate, which varied in chain length (C6 to C18), additional oxygen content (0 or 1) and desaturation (0 or 1). Thioesters bound tightly to recombinant GSTs (Kd ~ 1 micromolar), explaining their accumulation. Transient expression of GSTUs in Nicotiana benthamiana followed by recovery by Strep-tag affinity chromatography allowed the respective plant ligands to be extracted and characterized. Again, each GST showed a distinct profile of recovered metabolites, notably glutathionylated oxophytodienoic acid (OPDA) and related oxygenated fatty acids. Similarly, the expression of the major tau protein GSTU19 in the endogenous host Arabidopsis led to the selective binding of the OPDA-glutathione conjugate, with the enzyme able to catalyze the conjugation reaction. Additional ligands identified in planta included other fatty acid derivatives including divinyl ethers and glutathionylated chlorogenic acid. The strong and specific retention of various oxygenated fatty acids by each GSTU and the conservation in binding observed in the different hosts suggests that these proteins have selective roles in binding and conjugating these unstable metabolites in vivo.