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J. Biol. Chem., Vol. 282, Issue 49, 35945-35953, December 7, 2007

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A Small ATPase Protein of Arabidopsis, TGD3, Involved in Chloroplast Lipid Import^*

Binbin Lu, Changcheng Xu, Koichiro Awai¹, A. Daniel Jones^¶, and Christoph Benning²

From the Department of Biochemistry and Molecular Biology, the ^¶Department of Chemistry, and the United States Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824-1312

Polar lipid trafficking is essential in eukaryotic cells as membranes of lipid assembly are often distinct from final destination membranes. A striking example is the biogenesis of the photosynthetic membranes (thylakoids) in plastids of plants. Lipid biosynthetic enzymes at the endoplasmic reticulum and the inner and outer plastid envelope membranes are involved. This compartmentalization requires extensive lipid trafficking. Mutants of Arabidopsis are available that are disrupted in the incorporation of endoplasmic reticulum-derived lipid precursors into thylakoid lipids. Two proteins affected in two of these mutants, trigalactosyldiacylglycerol 1 (TGD1) and TGD2, encode the permease and substrate binding component, respectively, of a proposed lipid translocator at the inner chloroplast envelope membrane. Here we describe a third protein of Arabidopsis, TGD3, a small ATPase proposed to be part of this translocator. As in the tgd1 and tgd2 mutants, triacylglycerols and trigalactolipids accumulate in a tgd3 mutant carrying a T-DNA insertion just 5' of the TGD3 coding region. The TGD3 protein shows basal ATPase activity and is localized inside the chloroplast beyond the inner chloroplast envelope membrane. Proteins orthologous to TGD1, -2, and -3 are predicted to be present in Gram-bacteria, and the respective genes are organized in operons suggesting a common biochemical role for the gene products. Based on the current analysis, it is hypothesized that TGD3 is the missing ATPase component of a lipid transporter involving TGD1 and TGD2 required for the biosynthesis of ER-derived thylakoid lipids in Arabidopsis.

Received for publication, May 16, 2007 , and in revised form, September 21, 2007.

^* This work was supported in part by United States National Science Foundation Grant MCB 0453858 and United States Department of Energy Grant DE-FG02-98ER20305. 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 Fig. S1.

¹ Current address: Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-Ku, Saitama City, Saitama 338-8570, Japan.

² To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, Michigan State University, Biochemistry 215, East Lansing, MI 48824-1319; Tel.: 517-355-1609; Fax: 517-355-9334; E-Mail: benning{at}msu.edu.

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