Article

Mutation of the TGD1 chloroplast envelope protein affects phosphatidate metabolism in Arabidopsis. Plant Cell

Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan 48824, USA.
The Plant Cell (Impact Factor: 9.34). 12/2005; 17(11):3094-110. DOI: 10.1105/tpc.105.035592
Source: PubMed

ABSTRACT

Phosphatidate (PA) is a central metabolite of lipid metabolism and a signaling molecule in many eukaryotes, including plants. Mutations in a permease-like protein, TRIGALACTOSYLDIACYLGLYCEROL1 (TGD1), in Arabidopsis thaliana caused the accumulation of triacylglycerols, oligogalactolipids, and PA. Chloroplast lipids were altered in their fatty acid composition consistent with an impairment of lipid trafficking from the endoplasmic reticulum (ER) to the chloroplast and a disruption of thylakoid lipid biosynthesis from ER-derived precursors. The process mediated by TGD1 appears to be essential as mutation of the protein caused a high incidence of embryo abortion. Isolated tgd1 mutant chloroplasts showed a decreased ability to incorporate PA into galactolipids. The TGD1 protein was localized to the inner chloroplast envelope and appears to be a component of a lipid transporter. As even partial disruption of TGD1 function has drastic consequences on central lipid metabolism, the tgd1 mutant provides a tool to explore regulatory mechanisms governing lipid homeostasis and lipid trafficking in plants.

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Available from: John Froehlich, May 27, 2014
    • "The Arabidopsis tgd mutants, e.g. tgd1–1, also accumulate TAG in the cytosol of leaves (Xu et al., 2005 ); however, the TAG accumulating in the Arabidopsis tgd1–1 mutant has an acyl profile more similar to that of PtdCho, but not MGDG, as in the case of Chlamydomonas tgd2. It was concluded that the accumulation of TAG in the tgd1–1 mutant was the result of increased conversion of lipid precursors accumulating at the ER because of impaired ER to chloroplast lipid trafficking . "
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    ABSTRACT: In plants, lipids of the photosynthetic membrane are synthesized by parallel pathways associated with the endoplasmic reticulum (ER) and the chloroplast envelope membranes. Lipids derived from the two pathways are distinguished by their acyl-constituents. Following this plant paradigm, the prevalent acyl composition of chloroplast lipids suggests that Chlamydomonas does not use the ER pathway. However, the Chlamydomonas genome encodes presumed plant orthologues of a chloroplast lipid transporter consisting of TGD (TRIGALACTOSYLDIACYLGLYCEROL) proteins that are required for ER-to-chloroplast lipid trafficking in plants. To resolve this conundrum, we identified a mutant of Chlamydomonas deleted in the TGD2 gene and characterized the respective protein, CrTGD2. Notably, the mutant's viability was reduced showing the importance of CrTGD2. Galactoglycerolipid metabolism was altered in the tgd2 mutant with monogalactosyldiacylglycerol (MGDG) synthase activity being strongly stimulated. We hypothesize this to be a result of phosphatidic acid accumulation in the chloroplast outer envelope membrane, the location of MGDG synthase in Chlamydomonas. Concomitantly, increased conversion of MGDG into triacylglycerol (TAG) was observed. This TAG accumulated in lipid droplets in the tgd2 mutant under normal growth conditions. Labeling kinetics indicate that Chlamydomonas can import lipid precursors from the ER, a process that is impaired in the tgd2 mutant. This article is protected by copyright. All rights reserved.
    No preview · Article · Oct 2015 · The Plant Journal
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    • "One of the diagnostic phenotypes for all tgd mutants is the accumulation of oligogalactolipids including TGDG due to the activation of a galactolipid : galactolipid galactosyltransferase ( GGGT ) ( Xu et al . , 2003 , 2005 ) . This enzyme is localized to the outer chloroplast envelope membrane ( Xu et al . , 2003 ) and catalyzes the processive transfer of the galactosyl residue of MGDG to various galactolipids , giving rise to oligogalactolipids and DAG . A recent study showed that GGGT is identical to SENSITIVE TO FREEZING2 ( SFR2 ) ( Moellering et al . "
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    ABSTRACT: The biogenesis of photosynthetic membranes in the plastids of higher plants requires an extensive supply of lipid precursors from the endoplasmic reticulum (ER). Four TRIGALACTOSYLDIACYLGLYCEROL (TGD) proteins (TGD1,2,3,4) have thus far been implicated in this lipid transfer process. While TGD1, TGD2, and TGD3 constitute an ATP binding cassette transporter complex residing in the plastid inner envelope, TGD4 is a transmembrane lipid transfer protein present in the outer envelope. These observations raise questions regarding how lipids transit across the aqueous intermembrane space. Here, we describe the isolation and characterization of a novel Arabidopsis thaliana gene, TGD5. Disruption of TGD5 results in similar phenotypic effects as previously described in tgd1,2,3,4 mutants, including deficiency of ER-derived thylakoid lipids, accumulation of oligogalactolipids, and triacylglycerol. Genetic analysis indicates that TGD4 is epistatic to TGD5 in ER-to-plastid lipid trafficking, whereas double mutants of a null tgd5 allele with tgd1-1 or tgd2-1 show a synergistic embryo-lethal phenotype. TGD5 encodes a small glycine-rich protein that is localized in the envelope membranes of chloroplasts. Coimmunoprecipitation assays show that TGD5 physically interacts with TGD1, TGD2, TGD3, and TGD4. Collectively, these results suggest that TGD5 facilitates lipid transfer from the outer to the inner plastid envelope by bridging TGD4 with the TGD1,2,3 transporter complex.
    Full-text · Article · Sep 2015 · The Plant Cell
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    • "In vivo pulse chase studies indicate that PC (detected in envelope/chloroplast ) synthesized in the ER could be the immediate precursor for galactolipid synthesis in chloroplasts [15] [16] [17] [30] [31]. In addition, diacylglycerol (DAG) [36], phosphatic acid (PA) [41] and lyso-PC [23] have been suggested to be galactolipid precursors transported from the ER. Since chloroplasts lack capacity to synthesize the head group of PC, and both the outer envelope and the ER contain a significant amount of PC it seems likely that PC is transported from the ER whether or not it is the major precursor for galactolipids. "
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    ABSTRACT: Chloroplasts maintain their lipid balance through a tight interplay with the endoplasmic reticulum (ER). The outer envelope membrane of chloroplasts contains a large proportion of the phospholipid phospatidylcholine (PC), which is synthesized in the ER and also a possible precursor for thylakoid galactolipids. The mechanism for PC transport from the ER to chloroplasts is not known. Using isolated chloroplasts and liposomes containing radiolabelled PC we investigated non-vesicular transport of PC in vitro. PC uptake in chloroplasts was time- and temperature-dependent, but nucleotide-independent. Increased radius of liposomes stimulated PC uptake, and protease treatment of the chloroplasts impaired PC uptake. This implies that the chloroplast outer envelopes contains an exposed proteinaceous machinery for the uptake of PC from closely apposed membranes. Copyright © 2014. Published by Elsevier B.V.
    Full-text · Article · Dec 2014 · FEBS Letters
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