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J. Biol. Chem., Vol. 282, Issue 43, 31206-31216, October 26, 2007

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Incorporation of Newly Synthesized Fatty Acids into Cytosolic Glycerolipids in Pea Leaves Occurs via Acyl Editing^*

Philip D. Bates, John B. Ohlrogge, and Mike Pollard¹

From the Departments of Biochemistry and Molecular Biology and Plant Biology, Michigan State University, East Lansing, Michigan 48824

In expanding pea leaves, over 95% of fatty acids (FA) synthesized in the plastid are exported for assembly of eukaryotic glycerolipids. It is often assumed that the major products of plastid FA synthesis (18:1 and 16:0) are first incorporated into 16:0/18:1 and 18:1/18:1 molecular species of phosphatidic acid (PA), which are then converted to phosphatidylcholine (PC), the major eukaryotic phospholipid and site of acyl desaturation. However, by labeling lipids of pea leaves with [¹⁴C]acetate, [¹⁴C]glycerol, and [¹⁴C]carbon dioxide, we demonstrate that acyl editing is an integral component of eukaryotic glycerolipid synthesis. First, no precursor-product relationship between PA and PC [¹⁴C]acyl chains was observed at very early time points. Second, analysis of PC molecular species at these early time points showed that >90% of newly synthesized [¹⁴C]18:1 and [¹⁴C]16:0 acyl groups were incorporated into PC alongside a previously synthesized unlabeled acyl group (18:2, 18:3, or 16:0). And third, [¹⁴C]glycerol labeling produced PC molecular species highly enriched with 18:2, 18:3, and 16:0 FA, and not 18:1, the major product of plastid fatty acid synthesis. In conclusion, we propose that most newly synthesized acyl groups are not immediately utilized for PA synthesis, but instead are incorporated directly into PC through an acyl editing mechanism that operates at both sn-1 and sn-2 positions. Additionally, the acyl groups removed by acyl editing are largely used for the net synthesis of PC through glycerol 3-phosphate acylation.

Received for publication, July 3, 2007 , and in revised form, August 17, 2007.

^* This work was supported by United States Department of Energy Grant DE-FG02-87ER13729. 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 a supplemental Note and Figs. S1 and S2.

¹ To whom correspondence should be addressed: Dept. of Plant Biology, Michigan State University, Wilson Rd., East Lansing, MI 48824. Tel.: 517-353-5237; E-mail: pollard9{at}msu.edu.

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