Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
Plant Cell Physiol. 2010 Jun;51(6):1019-28. doi: 10.1093/pcp/pcq053. Epub 2010 Apr 21.
The transfer of lipids between the endoplasmic reticulum (ER) and the plastid in Arabidopsis involves the TRIGALACTOSYLDIACYLGLYCEROL (TGD) proteins. Lipid exchange is thought to be bidirectional based on the presence of specific lipid molecular species in Arabidopsis mutants impaired in the desaturation of fatty acids of membrane lipids in the ER and plastid. However, it was unclear whether TGD proteins were required for lipid trafficking in both directions. This question was addressed through the analysis of double mutants of tgd1-1 or tgd4-3 in genetic mutant backgrounds leading to a defect in lipid fatty acid desaturation either in the ER (fad2) or the plastid (fad6). The fad6 tgd1-1 and fad6 tgd4-3 double mutants showed drastic reductions in the relative levels of polyunsaturated fatty acids and of galactolipids. The growth of these plants and the development of photosynthetic membrane systems were severely compromised, suggesting a disruption in the import of polyunsaturated fatty acid-containing lipid species from the ER. Furthermore, a forward-genetic screen in the tgd1-2 dgd1 mutant background led to the isolation of a new fad6-2 allele with a marked reduction in the amount of digalactosyldiacylglycerol. In contrast, the introduction of fad2, affecting fatty acid desaturation of lipids in the ER, into the two tgd mutant backgrounds did not further decrease the level of fatty acid desaturation in lipids of extraplastidic membranes. These results suggest that the role of TGD proteins is limited to plastid lipid import, but does not extend to lipid export from the plastid to extraplastidic membranes.
质体与内质网之间的脂类转运涉及到 TRIGALACTOSYLDIACYLGLYCEROL（TGD）蛋白。根据在质体和内质网膜脂脂肪酸去饱和作用缺陷的拟南芥突变体中存在特定的脂类分子种类，推测脂质交换是双向的。然而，TGD 蛋白是否是质体和内质网中脂类双向转运所必需的，这一点并不清楚。通过分析 fad2 或 fad6 基因缺失突变导致质体或内质网膜脂脂肪酸去饱和作用缺陷的遗传突变背景下的 tgd1-1 或 tgd4-3 双突变体，解决了这个问题。fad6 tgd1-1 和 fad6 tgd4-3 双突变体中多不饱和脂肪酸和半乳糖脂的相对含量明显降低。这些植物的生长和光合膜系统的发育受到严重影响，这表明多不饱和脂肪酸脂类从内质网的输入受到了破坏。此外，在 tgd1-2 dgd1 突变体背景下进行正向遗传筛选，导致分离到一个 fad6-2 新等位基因，该等位基因中二半乳糖基二酰基甘油的含量明显减少。相反，将 fad2（影响内质网膜脂的脂肪酸去饱和作用）导入到两个 tgd 突变体背景中，并不会进一步降低质体外膜脂的脂肪酸去饱和作用水平。这些结果表明，TGD 蛋白的作用仅限于质体脂质的输入，但不扩展到质体向质体外膜的脂质输出。
