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Published on February 27, 2008; 10.1104/pp.107.113282

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Received November 15, 2007
Accepted February 20, 2008

Iron-Induced Turnover of the Arabidopsis IRT1 Metal Transporter Requires Lysine Residues

Loubna Kerkeb , Indrani Mukherjee , Iera Chatterjee , Brett Lahner , David E. Salt , and Erin L. Connolly *

Department of Biological Sciences, Coker Life Science Building, University of South Carolina, Columbia, SC 29208; Center for Plant Environmental Stress Physiology, Purdue University, 625 Agricultural Mall Drive, West Lafayette, IN 47907

* Corresponding author; email: erinc{at}biol.sc.edu.

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. IRT1 (Iron Regulated Transporter) is the major high affinity iron transporter responsible for iron uptake from the soil in Arabidopsis thaliana. Previously, we showed that IRT1 is subject to post-transcriptional regulation; when expressed from the constitutive CaMV 35S promoter, IRT1 protein accumulates only in iron-deficient roots. IRT1 contains an intracellular loop that may be critical for post-translational regulation by metals. Of particular interest are a histidine 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 histidine 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 WT. 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.






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