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First published online August 8, 2008; 10.1105/tpc.108.200811

The Plant Cell 20:2007

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IN BRIEF

An Endoplasmic Reticulum Protein Involved in Lipid Transfer to Chloroplasts

Nancy R. Hofmann

Science Editor

nhofmann{at}aspb.org

Chloroplast thylakoid membranes are predominantly made up of two galactoglycerolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). It has been hypothesized that many plants can synthesize these polar lipids via both a plastid pathway and an endoplasmic reticulum (ER) pathway (reviewed in Benning, 2008Go). Since the final steps of MGDG and DGDG synthesis are known to occur in the plastid envelope membranes, the ER pathway requires the transfer of precursors to the plastid, but the mechanism of this transfer is not understood. Xu et al. (pp. 2190–2204) present exciting new evidence on the identification of a protein that likely is directly involved in ER-to-plastid lipid trafficking.

Xu et al. characterized trigalactosyldiacylglycerol4 (tgd4) mutants, in which MGDG and DGDG have plastid-type acyl chains and are missing ER-type acyl chains, suggesting a defect in ER galactoglycerolipid synthesis. A TGD4-GFP fusion was shown to be targeted to the ER (see figure ) and embedded in the membrane. Although the sequence of TGD4 does not reveal any clues to its biochemical function, the presence of putative homologs in other species suggests that the function is conserved in plants.


Figure 1
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  		A TGD4-GFP fusion is targeted to the ER. Confocal microscopy images show the ER network visualized by an ER lumen–targeted GFP (top panel) and by a TGD4-GFP fusion (bottom panel). GFP fluorescence is in green, chlorophyll autofluorescence is in red, and the yellow color represents areas with fluorescence from both. Bars = 2 µm.

 
To examine whether the tgd4 mutants are blocked in the ER pathway of galactoglycerolipid synthesis, the authors introduced tgd4 alleles into a mutant background that is blocked in the plastid pathway. This double mutant was embryo-lethal when strong alleles of tgd4 were used. When a leaky allele was introduced, the authors recovered plants that could survive with sucrose supplementation. Interestingly, these double mutants have a plastid division defect. Whether this is due to a direct interaction between TGD4 and division machinery or is a secondary effect of lipid changes in the membrane remains to be explored.

The authors then tested for a block in the ER pathway more directly using a pulse-chase assay in excised leaves. This showed decreased label transfer from the ER to the plastid and confirmed the earlier acyl chain analysis showing that more of the MDGD in the mutants is synthesized in the chloroplast. Finally, the authors developed an assay to measure ER-to-plastid lipid transfer directly, using isolated ER membranes and chloroplasts, and again found that tgd4 had lower levels of transfer. Thus, Xu et al. provide strong evidence that TGD4 is involved in the ER pathway of galactogycerolipid synthesis and raise the tantalizing possibility that its ER membrane localization allows TGD4 to be directly involved in lipid transfer. Furthermore, the development of an in vitro assay should prove to be an invaluable step in understanding the biochemical and molecular details of ER–plastid lipid trafficking.

Footnotes

www.plantcell.org/cgi/doi/10.1105/tpc.108.200811

REFERENCES

Benning, C. (2008). A role for lipid trafficking in chloroplast biogenesis. Prog. Lipid Res. 47: 381–389.[CrossRef][ISI][Medline]

Xu, C., Fan, J., Cornish, A.J., and Benning, C. (2008). Lipid trafficking between the endoplasmic reticulum and the plastid in Arabidopsis requires the extraplastidic TGD4 protein. Plant Cell 20: 2190–2204.[Abstract/Free Full Text]


Related articles in Plant Cell:

Lipid Trafficking between the Endoplasmic Reticulum and the Plastid in Arabidopsis Requires the Extraplastidic TGD4 Protein
Changcheng Xu, Jilian Fan, Adam J. Cornish, and Christoph Benning
Plant Cell 2008 20: 2190-2204. [Abstract] [Full Text]  




This Article
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