A Soluble Auxin-Binding Protein from Hyoscyamus muticus Is a Glutathione S-Transferase

We have used the photoaffinity label azido-[3H]IAA (5-N3-[7-3H]indole-3-acetic acid), a biologically active analog of indole-3-acetic acid, to identify auxin-binding proteins (ABPs) in the soluble fraction of Hyoscyamus muticus. A 25-kD polypeptide previously described (H. Macdonald, A. M. Jones, P. J. King [1991] J Biol Chem 266: 7393-7399) has now been purified to homogeneity by conventional methods. Binding of azido-[3H]IAA to the purified protein was reduced by active auxins but not by inactive indoles. Partial amino acid sequences of the purified protein showed high homology to glutathione S-transferase (GST) from tobacco (ParB) and from maize (GT32). The conclusion that the 25-kD ABP is a GST is further supported by high GST activity in fractions highly enriched in the 25-kD polypeptide and recognition of the ABP by antibodies against GST from wheat and maize. Furthermore, purification of a protein from a soluble protein extract from H. muticus by affinity chromatography on glutathione-agarose also yielded a 25-kD polypeptide that was indistinguishable in its N-terminal amino acid sequence and biochemical characteristics from the protein purified by conventional methods. Possible functions of GST in auxin action are discussed.