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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Iron-Induced Turnover of the Arabidopsis IRON-REGULATED TRANSPORTER1 Metal Transporter Requires Lysine Residues^1,[W],[OA]

Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208 (L.K., I.M., I.C., E.L.C.); and the Center for Plant Environmental Stress Physiology, Purdue University, West Lafayette, Indiana 47907 (B.L., D.E.S.)

Iron is an essential micronutrient but is toxic if accumulated at high levels. Thus, iron uptake and distribution in plants are controlled by precise regulatory mechanisms. IRON-REGULATED TRANSPORTER1 (IRT1) is the major high affinity iron transporter responsible for iron uptake from the soil in Arabidopsis (Arabidopsis thaliana). Previously, we showed that IRT1 is subject to posttranscriptional regulation; when expressed from the constitutive cauliflower mosaic virus 35S promoter, IRT1 protein accumulates only in iron-deficient roots. IRT1 contains an intracellular loop that may be critical for posttranslational regulation by metals. Of particular interest are a histidine (His) motif (HGHGHGH) that might bind metals and two lysine residues that could serve as attachment sites for ubiquitin. We constructed a set of mutant IRT1 alleles: IRT1H154Q, IRT1H156Q, IRT1H158Q, IRT1H160Q, IRT14HQ (quadruple His mutant), IRT1K146R, IRT1K171R, and a double mutant (IRT1K146R,K171R). Mutation of the His or lysine residues did not eliminate the ability of IRT1 to transport iron or zinc. Expression of each of the IRT1 variants and an IRT1intact construct in plants from the 35S promoter revealed that either K146 or K171 is required for iron-induced protein turnover, and 35S-IRT1K146R,K171R plants contain higher levels of iron as compared to 35S-IRT1 and wild type. Furthermore, accumulation of metals in 35S-IRT1K146R,K171R plants was not associated with an increase in ferric chelate reductase activity; this result indicates that, at least under conditions when iron is abundant, reduction of ferric iron may not be the rate-limiting step in iron uptake by strategy I plants such as Arabidopsis.

² Present address: Bachem Inc., 3132 Kashiwa St., Torrance, CA 90505.

³ Present address: Department of Biology, New York University, 100 Washington Square East, 766 Waverly Building, New York, NY 10003.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Erin L. Connolly (erinc{at}biol.sc.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.113282

^* Corresponding author; e-mail erinc{at}biol.sc.edu.

Received November 15, 2007; accepted February 20, 2008; published February 27, 2008.