PLANT PHYSIOLOGY , Vol 109, Issue 1 253-260, Copyright © 1995 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Cloning and Characterization of a Glutathione S-Transferase That Can Be Photolabeled with 5-Azido-indole-3-acetic Acid
J. Bilang and A. Sturm
Friedrich Miescher-Institut, P.O. Box 2543, CH-4002 Basel, Switzerland
Previously, we identified a soluble protein from Hyoscyamus muticus that
was photolabeled by 5-azido-indole-3-acetic acid. This protein was
determined to be a glutathione S-transferase (GST; J. Bilang, H. Macdonald,
P.J. King, and A. Sturm [1993] Plant Physiol 102: 29-34). We have examined
the effect of auxin on the activity of this H. muticus GST. Auxins reduced
enzyme activity only at high concentrations, with 2,4-dichlorophenoxyacetic
acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid being more effective
than indole-3-acetic acid (IAA) and naphthylacetic acid. IAA was a
noncompetitive inhibitor, whereas inhibition by 2,4-D was competitive with
respect to 1-chloro-2,4-dinitro-benzene. We also present the sequence of a
full-length cDNA clone that codes for a GST and contains all partial amino
acid sequences of the purified protein. The auxin-binding GST was found in
high amounts in roots and stems and low amounts in leaves and flower buds.
The steady-state mRNA level was not regulated by IAA or naphthylacetic
acid, whereas 2,4-D and 2,3-dichlorophenoxyacetic acid increased mRNA
levels. We propose a model in which 2,4-D is a substrate for GST, whereas
IAA binds at a second site, known as a ligandinbinding site for the purpose
of intracellular transport.
