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J. Biol. Chem., Vol. 281, Issue 23, 15625-15635, June 9, 2006

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Mitochondrial Localization and Putative Signaling Function of Sucrose Synthase in Maize^*

Chalivendra C. Subbaiah¹, Ashok Palaniappan^¶, Kateri Duncan, David M. Rhoads^||, Steven C. Huber^**, and Martin M. Sachs

From the Department of Crop Sciences, University of Illinois, Urbana, Illinois 61801, the ^¶Center for Biophysics and Computational Biology, University of Illinois, Urbana, Illinois 61801, the Department of Plant Biology, University of Illinois, Urbana, Illinois 61801, the ^**United States Department of Agriculture/Agricultural Research Service/Midwest Area Photosynthesis Research Unit, Urbana, Illinois 61801, the ^|| School of Life Sciences, Arizona State University, Tempe, Arizona 85287, and the United States Department of Agriculture/ARS/MWA Soybean/Maize Germplasm, Pathology, and Genetics Research Unit, University of Illinois, Urbana, Illinois 61801

In many organisms, an increasing number of proteins seem to play two or more unrelated roles. Here we report that maize sucrose synthase (SUS) is distributed in organelles not involved in sucrose metabolism and may have novel roles beyond sucrose degradation. Bioinformatics analysis predicts that among the three maize SUS isoforms, SH1 protein has a putative mitochondrial targeting peptide (mTP). We validated this prediction by the immunodetection of SUS in mitochondria. Analysis with isoform-specific antisera revealed that both SH1 and SUS1 are represented in mitochondria, although the latter lacks a canonical mTP. The SUS2 isoform is not detectable in mitochondria, despite its presence in the cytosol. In maize primary roots, the mitochondrion-associated SUS (mtSUS; which includes SH1 and SUS1) is present mostly in the root tip, indicating tissue-specific regulation of SUS compartmentation. Unlike the glycolytic enzymes that occur attached to the outside of mitochondria, SH1 and SUS1 are intramitochondrial. The low abundance of SUS in mitochondria, its high K_m value for sucrose, and the lack of sucrose in mitochondria suggest that mtSUS plays a non-sucrolytic role. Co-immunoprecipitation studies indicate that SUS interacts with the voltage-dependent anion channel in an isoform-specific and anoxia-enhanced manner and may be involved in the regulation of solute fluxes into and out of mitochondria. In several plant species, at least one of the SUS proteins possesses a putative mTP, indicating the conservation of the noncatalytic function across plant species. Taken together, these observations suggest that SUS has a novel noncatalytic function in plant cells.

Received for publication, January 13, 2006 , and in revised form, March 29, 2006.

^* This work was supported by United States Department of Agriculture/Agricultural Research Service and the United States Department of Energy Grant DE-AI05-91ER20031 (to S. C. H.). 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 To whom correspondence should be addressed: Box 874501, School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501. Tel.: 480-965-2583; Fax: 480-965-6899; E-mail: Subbaiah.Chalivendra{at}asu.edu.

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