Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720;
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102.
Proc Natl Acad Sci U S A. 2020 Apr 21;117(16):9101-9111. doi: 10.1073/pnas.1916946117. Epub 2020 Apr 3.
In eukaryotic photosynthetic organisms, the conversion of solar into chemical energy occurs in thylakoid membranes in the chloroplast. How thylakoid membranes are formed and maintained is poorly understood. However, previous observations of vesicles adjacent to the stromal side of the inner envelope membrane of the chloroplast suggest a possible role of membrane transport via vesicle trafficking from the inner envelope to the thylakoids. Here we show that the model plant has a chloroplast-localized Sec14-like protein (CPSFL1) that is necessary for photoautotrophic growth and vesicle formation at the inner envelope membrane of the chloroplast. The mutants are seedling lethal, show a defect in thylakoid structure, and lack chloroplast vesicles. Sec14 domain proteins are found only in eukaryotes and have been well characterized in yeast, where they regulate vesicle budding at the -Golgi network. Like the yeast Sec14p, CPSFL1 binds phosphatidylinositol phosphates (PIPs) and phosphatidic acid (PA) and acts as a phosphatidylinositol transfer protein in vitro, and expression of CPSFL1 can complement the yeast mutation. CPSFL1 can transfer PIP into PA-rich membrane bilayers in vitro, suggesting that CPSFL1 potentially facilitates vesicle formation by trafficking PA and/or PIP, known regulators of membrane trafficking between organellar subcompartments. These results underscore the role of vesicles in thylakoid biogenesis and/or maintenance. CPSFL1 appears to be an example of a eukaryotic cytosolic protein that has been coopted for a function in the chloroplast, an organelle derived from endosymbiosis of a cyanobacterium.
在真核光合生物中，太阳能到化学能的转化发生在叶绿体的类囊体膜中。类囊体膜的形成和维持机制还不太清楚。然而，先前观察到的叶绿体基质侧的内被膜附近的小泡表明，通过小泡从内被膜向类囊体的运输进行膜运输可能具有一定作用。本文中，我们展示了模式植物 中一种质体定位的 Sec14 样蛋白（CPSFL1），它对于光自养生长和叶绿体内被膜上小泡的形成是必需的。 突变体是幼苗致死的，表现出类囊体结构缺陷，并且缺乏质体小泡。Sec14 结构域蛋白仅存在于真核生物中，并且在酵母中得到了很好的研究，酵母中它们调节着高尔基网络处的小泡出芽。与酵母 Sec14p 一样，CPSFL1 结合磷脂酰肌醇磷酸（PIPs）和磷脂酸（PA），并且在体外作为一种磷脂酰肌醇转移蛋白发挥作用，并且 CPSFL1 的表达可以互补酵母 突变体。CPSFL1 可以在体外将 PIP 转移到富含 PA 的膜双层中，这表明 CPSFL1 可以通过运输 PA 和/或 PIP 来促进小泡的形成，而 PIP 和 PA 是细胞器亚区室之间膜运输的已知调节剂。这些结果强调了小泡在类囊体生物发生和/或维持中的作用。CPSFL1 似乎是一种被真核细胞溶质蛋白被内共生起源的叶绿体招募来发挥功能的例子，叶绿体起源于蓝细菌的内共生。
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