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J. Biol. Chem., Vol. 281, Issue 27, 18707-18714, July 7, 2006

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Of Two Cytosolic Aconitases Expressed in Drosophila, Only One Functions as an Iron-regulatory Protein^*

Maria I. Lind¹², Fanis Missirlis¹³, Öjar Melefors^¶, Helge Uhrigshardt, Kim Kirby^||, John P. Phillips^||, Kenneth Söderhäll, and Tracey A. Rouault

From the Department of Comparative Physiology, Evolutionary Biology Centre, Uppsala University, S-75236 Uppsala, Sweden, Cell Biology and Metabolism Branch, NICHD, National Institutes of Health, Bethesda, Maryland 20892, ^¶Microbiology and Tumor Biology Center, Karolinska Institutet, 17177 Stockholm, Sweden, and ^||Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada

In mammalian cells, iron homeostasis is largely regulated by post-transcriptional control of gene expression through the binding of iron-regulatory proteins (IRP1 and IRP2) to iron-responsive elements (IREs) contained in the untranslated regions of target mRNAs. IRP2 is the dominant iron sensor in mammalian cells under normoxia, but IRP1 is the more ancient protein in evolutionary terms and has an additional function as a cytosolic aconitase. The Caenorhabditis elegans genome does not contain an IRP2 homolog or identifiable IREs; its IRP1 homolog has aconitase activity but does not bind to mammalian IREs. The Drosophila genome offers an evolutionary intermediate containing two IRP1-like proteins (IRP-1A and IRP-1B) and target genes with IREs. Here, we used purified recombinant IRP-1A and IRP-1B from Drosophila melanogaster and showed that only IRP-1A can bind to IREs, although both proteins possess aconitase activity. These results were also corroborated in whole-fly homogenates from transgenic flies that overexpress IRP-1A and IRP-1B in their fat bodies. Ubiquitous and muscle-specific overexpression of IRP-1A, but not of IRP-1B, resulted in pre-adult lethality, underscoring the importance of the biochemical difference between the two proteins. Domain-swap experiments showed that multiple amino acid substitutions scattered throughout the IRP1 domains are synergistically required for conferring IRE binding activity. Our data suggest that as a first step during the evolution of the IRP/IRE system, the ancient cytosolic aconitase was duplicated in insects with one variant acquiring IRE-specific binding.

Received for publication, April 7, 2006 , and in revised form, May 4, 2006.

The nucleotide sequence(s) reported in this paper has been submitted to the DDBJ/Gen-Bank^TM/EBI Data Bank with accession number(s) AJ252016 and AJ252017.

^* This work was supported by the Wenner-Gren Foundations (to M. I. L.), the Swedish Research Council (to K. S.), and the intramural program of NICHD, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Both authors contributed equally to this work.

² To whom correspondence may be addressed: Dept. of Comparative Physiology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18A, S-75236 Uppsala, Sweden. Tel.: 46-18-471-2815; Fax: 46-18-471-6425; E-mail: maria.Lind{at}ebc.uu.se. ³ To whom correspondence may be addressed: National Institutes of Health, Bldg. 18T, Rm. 101, 9000 Rockville Pike, Bethesda, MD 20892. Tel.: 301-435-8418; Fax: 301-402-0078; E-mail: missirlf{at}mail.nih.gov.

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