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J. Biol. Chem., Vol. 282, Issue 31, 22481-22491, August 3, 2007

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Molecular and Biochemical Analysis of the Plastidic ADP-glucose Transporter (ZmBT1) from Zea mays^*

Simon Kirchberger, Michaela Leroch, Martijn A. Huynen, Markus Wahl, H. Ekkehard Neuhaus, and Joachim Tjaden¹

From the Abteilung Pflanzenphysiologie, Fachbereich Biologie, Technische Universität Kaiserslautern, P. O. Box 3049, D-67653 Kaiserslautern, Germany and the Center for Molecular and Biomolecular Informatics, Nijmegen, and the Centre for Molecular Life Sciences, Radboud University, Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands

Physiological studies on the Brittle1 maize mutant have provided circumstantial evidence that ZmBT1 (Zea mays Brittle1 protein) is involved in the ADP-Glc transport into maize endosperm plastids, but up to now, no direct ADP-Glc transport mediated by ZmBT1 has ever been shown. The heterologous synthesis of ZmBT1 in Escherichia coli cells leads to the functional integration of ZmBT1 into the bacterial cytoplasmic membrane. ZmBT1 transports ADP-Glc in counterexchange with ADP with apparent affinities of about 850 and 465 µM, respectively. Recently, a complete ferredoxin/thioredoxin system has been identified in cereal amyloplasts and BT1 has been proposed as a potential Trx target protein (Balmer, Y., Vensel, W. H., Cai, N., Manieri, W., Schurmann, P., Hurkman, W. J., and Buchanan, B. B. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 2988–2993). Interestingly, we revealed that the transport activity of ZmBT1 is reversibly regulated by redox reagents such as diamide and dithiothreitol. The expression of ZmBT1 is restricted to endosperm tissues during starch synthesis, whereas a recently identified BT1 maize homologue, the ZmBT1–2, exhibits a ubiquitous expression pattern in hetero- and autotrophic tissues indicating different physiological roles for both maize BT1 isoforms. BT1 homologues are present in both mono- and dicotyledonous plants. Phylogenetic analyses classify the BT1 family into two phylogenetically and biochemically distinct groups. The first group comprises BT1 orthologues restricted to cereals where they mediate the ADP-Glc transport into cereal endosperm storage plastids during starch synthesis. The second group occurs in mono- and dicotyledonous plants and is most probably involved in the export of adenine nucleotides synthesized inside plastids.

Received for publication, March 22, 2007 , and in revised form, May 8, 2007.

^* This work was supported by Deutsche Forschungsgemeinschaft Grant TJ 5/2-1 (to J. T.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table S1 and Figs. S1–S4.

¹ To whom correspondence should be addressed: Technische Universität Kaiserslautern, Pflanzenphysiologie, Postfach 3049, D-67653 Kaiserslautern, Germany. Tel.: 0631-205-3040; Fax: 0631-205-2600; E-mail: tjaden{at}rhrk.uni-kl.de.

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