Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
Plant J. 2011 Jun;66(5):759-69. doi: 10.1111/j.1365-313X.2011.04536.x. Epub 2011 Mar 23.
In most plants the assembly of the photosynthetic thylakoid membrane requires lipid precursors synthesized at the endoplasmic reticulum (ER). Thus, the transport of lipids from the ER to the chloroplast is essential for biogenesis of the thylakoids. TGD2 is one of four proteins in Arabidopsis required for lipid import into the chloroplast, and was found to bind phosphatidic acid in vitro. However, the significance of phosphatidic acid binding for the function of TGD2 in vivo and TGD2 interaction with membranes remained unclear. Developing three functional assays probing how TGD2 affects lipid bilayers in vitro, we show that it perturbs membranes to the point of fusion, causes liposome leakage and redistributes lipids in the bilayer. By identifying and characterizing five new mutant alleles, we demonstrate that these functions are impaired in specific mutants with lipid phenotypes in vivo. At the structural level, we show that TGD2 is part of a protein complex larger than 500 kDa, the formation of which is disrupted in two mutant alleles, indicative of the biological relevance of this TGD2-containing complex. Based on the data presented, we propose that TGD2, as part of a larger complex, forms a lipid transport conduit between the inner and outer chloroplast envelope membranes, with its N terminus anchored in the inner membrane and its C terminus binding phosphatidic acid in the outer membrane.
在大多数植物中，光合类囊体膜的组装需要内质网（ER）合成的脂质前体。因此，脂质从 ER 向叶绿体的转运对于类囊体的生物发生是必不可少的。TGD2 是拟南芥中四种将脂质导入叶绿体所必需的蛋白之一，并且被发现能够在体外结合磷脂酸。然而，磷脂酸结合对于 TGD2 在体内的功能以及 TGD2 与膜的相互作用的意义仍不清楚。通过开发三种功能测定法来研究 TGD2 如何在体外影响脂质双层，我们表明它会使膜融合到一定程度，导致脂质体泄漏并重新分配双层中的脂质。通过鉴定和表征五个新的突变等位基因，我们证明这些功能在体内具有脂质表型的特定突变体中受到损害。在结构水平上，我们表明 TGD2 是一个大于 500 kDa 的蛋白质复合物的一部分，其形成在两个突变等位基因中受到破坏，表明这个包含 TGD2 的复合物具有生物学相关性。基于所呈现的数据，我们提出 TGD2 作为更大复合物的一部分，在叶绿体内外膜之间形成脂质转运通道，其 N 端锚定在内膜，C 端在外壳膜中结合磷脂酸。
