Thylakoid membranes contain a non-selective channel permeable to small organic molecules
Thylakoid membranes contain a non-selective channel permeable to small organic molecules
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DOI：
10.1074/jbc.ra118.002367
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发表时间：
2018-05-18
影响因子：
4.8
通讯作者：
Takahashi, Hideyuki
中科院分区：
文献类型：
--
作者：
Kojima, Seiji;Iwamoto, Masayuki;Takahashi, Hideyuki
类囊体腔是膜封闭的水室。越来越多的证据表明，类囊体腔不仅是光合作用反应过程中质子和无机离子易位的库，也是代谢活动的场所，例如代谢活动。光损伤蛋白质的蛋白水解，以维持有效的光合作用。然而，有机分子穿过类囊体膜以维持这些腔内活动的机制尚不清楚。在最近对 Cyanophora paradoxa 叶绿体（室质体）的研究中，我们偶然检测到类囊体膜中明显的扩散通道活性。在这里，使用由来自室质体和另外两种系统发育上不同的生物（蓝藻集胞藻属）的类囊体膜重建的蛋白脂质体。通过 PCC 6803 和菠菜，我们证明了类囊体膜中存在足够大的非选择性通道，足以使小有机化合物（例如 M-r < 1500 的碳水化合物和氨基酸）渗透。此外，我们纯化、鉴定并表征了一个名为 CpTPOR 的胞质体通道。渗透膨胀实验表明，CpTPOR 形成了一个估计半径约为 1.3 nm 的非选择性孔。脂质双层实验显示出可变电导通道活性，在 1 m KCl 中典型的单通道电导为 1.8 nS，很少有关闭转变。 CpTPOR 氨基酸序列与线粒体外膜电压依赖性阴离子选择性通道的氨基酸序列有一定相似性，尽管 CpTPOR 对阴离子没有表现出明显的选择性，也没有电压依赖性门控。我们认为跨膜扩散途径在类囊体膜中普遍存在，可能使得各种代谢物能够在管腔和基质之间快速转移。
The thylakoid lumen is a membrane-enclosed aqueous compartment. Growing evidence indicates that the thylakoid lumen is not only a sink for protons and inorganic ions translocated during photosynthetic reactions but also a place for metabolic activities, e.g. proteolysis of photodamaged proteins, to sustain efficient photosynthesis. However, the mechanism whereby organic molecules move across the thylakoid membranes to sustain these lumenal activities is not well understood. In a recent study of Cyanophora paradoxa chloroplasts (muroplasts), we fortuitously detected a conspicuous diffusion channel activity in the thylakoid membranes. Here, using proteoliposomes reconstituted with the thylakoid membranes from muroplasts and from two other phylogenetically distinct organisms, cyanobacterium Synechocystis sp. PCC 6803 and spinach, we demonstrated the existence of nonselective channels large enough for enabling permeation of small organic compounds (e.g. carbohydrates and amino acids with M-r < 1500) in the thylakoid membranes. Moreover, we purified, identified, and characterized a muroplast channel named here CpTPOR. Osmotic swelling experiments revealed that CpTPOR forms a nonselective pore with an estimated radius of approximate to 1.3 nm. A lipid bilayer experiment showed variable-conductance channel activity with a typical single-channel conductance of 1.8 nS in 1 m KCl with infrequent closing transitions. The CpTPOR amino acid sequence was moderately similar to that of a voltage-dependent anion-selective channel of the mitochondrial outer membrane, although CpTPOR exhibited no obvious selectivity for anions and no voltage-dependent gating. We propose that transmembrane diffusion pathways are ubiquitous in the thylakoid membranes, presumably enabling rapid transfer of various metabolites between the lumen and stroma.