Rocha Joana, Sarkis Joe, Thomas Aline, Pitou Laurence, Radzimanowski Jens, Audry Magali, Chazalet Valérie, de Sanctis Daniele, Palcic Monica M, Block Maryse A, Girard-Egrot Agnès, Maréchal Eric, Breton Christelle
University of Grenoble Alpes, 38400, Grenoble, France.
Centre National de la Recherche Scientifique/Centre de Recherches sur les Macromolécules Végétales, 38041, Grenoble, France.
Plant J. 2016 Mar;85(5):622-33. doi: 10.1111/tpj.13129. Epub 2016 Feb 23.
Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the major lipid components of photosynthetic membranes, and hence the most abundant lipids in the biosphere. They are essential for assembly and function of the photosynthetic apparatus. In Arabidopsis, the first step of galactolipid synthesis is catalyzed by MGDG synthase 1 (MGD1), which transfers a galactosyl residue from UDP-galactose to diacylglycerol (DAG). MGD1 is a monotopic protein that is embedded in the inner envelope membrane of chloroplasts. Once produced, MGDG is transferred to the outer envelope membrane, where DGDG synthesis occurs, and to thylakoids. Here we present two crystal structures of MGD1: one unliganded and one complexed with UDP. MGD1 has a long and flexible region (approximately 50 amino acids) that is required for DAG binding. The structures reveal critical features of the MGD1 catalytic mechanism and its membrane binding mode, tested on biomimetic Langmuir monolayers, giving insights into chloroplast membrane biogenesis. The structural plasticity of MGD1, ensuring very rapid capture and utilization of DAG, and its interaction with anionic lipids, possibly driving the construction of lipoproteic clusters, are consistent with the role of this enzyme, not only in expansion of the inner envelope membrane, but also in supplying MGDG to the outer envelope and nascent thylakoid membranes.
单半乳糖基二酰基甘油（MGDG）和双半乳糖基二酰基甘油（DGDG）是光合膜的主要脂质成分，因此也是生物圈中含量最丰富的脂质。它们对于光合装置的组装和功能至关重要。在拟南芥中，半乳糖脂合成的第一步由MGDG合酶1（MGD1）催化，该酶将来自尿苷二磷酸半乳糖的半乳糖基残基转移至二酰基甘油（DAG）。MGD1是一种单跨膜蛋白，嵌入叶绿体的内膜。MGDG一旦产生，就会转移到外膜，在那里发生DGDG的合成，并转移到类囊体。本文展示了MGD1的两种晶体结构：一种无配体结构，一种与UDP结合的复合物结构。MGD1具有一个长且灵活的区域（约50个氨基酸），这是DAG结合所必需的。这些结构揭示了MGD1催化机制及其膜结合模式的关键特征，并在仿生朗缪尔单层膜上进行了测试，为叶绿体膜生物发生提供了见解。MGD1的结构可塑性确保了对DAG的非常快速的捕获和利用，以及它与阴离子脂质的相互作用，可能驱动脂蛋白簇的构建，这与该酶的作用一致，不仅在扩大内膜，而且在向外膜和新生类囊体膜供应MGDG方面发挥作用。
