Structure, function, and expression pattern of a novel sodium-coupled citrate transporter (NaCT) cloned from mammalian brain
Structure, function, and expression pattern of a novel sodium-coupled citrate transporter (NaCT) cloned from mammalian brain
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DOI：
10.1074/jbc.m207072200
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发表时间：
2002-10-18
影响因子：
4.8
通讯作者：
Ganapathy, V
中科院分区：
文献类型：
--
作者：
Inoue, K;Zhuang, L;Ganapathy, V
在哺乳动物细胞中，柠檬酸盐不仅在代谢能量的产生中，而且在脂肪酸、类异戊二烯和胆固醇的合成中起关键作用。柠檬酸盐的血浆水平在三羧酸循环的中间体中是最高的（类似于135 μ M）。在这里，我们报告的克隆和功能特性的质膜转运蛋白（NaCT的Na+耦合柠檬酸转运蛋白）从大鼠大脑介导的上坡细胞摄取柠檬酸盐耦合到电化学Na+梯度。NaCT由572个氨基酸组成，与Na+-二羧酸协同转运蛋白/Na+-硫酸盐协同转运蛋白（NaDC/NaSi）基因家族成员（包括最近鉴定的果蝇Indy）具有结构相似性。在大鼠中，NaCT的表达仅限于肝脏、睾丸和脑。当在哺乳动物细胞中异源表达时，大鼠NaCT以高亲和力（Michaelis-Menten常数，类似于20 μ M）和Na+：柠檬酸盐化学计量比4：1介导柠檬酸盐的转运。转运蛋白与其他二羧酸和三羧酸相互作用，但亲和力相当低。在小鼠脑中，NaCT mRNA的表达在大脑皮层、小脑、海马和嗅球中是明显的。NaCT代表在哺乳动物细胞中鉴定的第一种转运蛋白，其显示出对柠檬酸盐的偏好超过二羧酸盐。这种转运蛋白可能在血液中柠檬酸盐的细胞利用中发挥重要作用，用于合成脂肪酸和胆固醇（肝脏）以及产生能量（肝脏和大脑）。因此，NaCT构成了控制体重、胆固醇水平和能量稳态的潜在治疗靶点。
Citrate plays a pivotal role not only in the generation of metabolic energy but also in the synthesis of fatty acids, isoprenoids, and cholesterol in mammalian cells. Plasma levels of citrate are the highest (similar to 135 muM) among the intermediates of the tricarboxylic acid cycle. Here we report on the cloning and functional characterization of a plasma membrane transporter (NaCT for Na+-coupled citrate transporter) from rat brain that mediates uphill cellular uptake of citrate coupled to an electrochemical Na+ gradient. NaCT consists of 572 amino acids and exhibits structural similarity to the members of the Na+-dicarboxylate cotransporter/Na+-sulfate cotransporter (NaDC/NaSi) gene family including the recently identified Drosophila Indy. In rat, the expression of NaCT is restricted to liver, testis, and brain. When expressed heterologously in mammalian cells, rat NaCT mediates the transport of citrate with high affinity (Michaelis-Menten constant, similar to20 muM) and with a Na+:citrate stoichiometry of 4:1. The transporter does interact with other dicarboxylates and tricarboxylates but with considerably lower affinity. In mouse brain, the expression of NaCT mRNA is evident in the cerebral cortex, cerebellum, hippocampus, and olfactory bulb. NaCT represents the first transporter to be identified in mammalian cells that shows preference for citrate over dicarboxylates. This transporter is likely to play an important role in the cellular utilization of citrate in blood for the synthesis of fatty acids and cholesterol (liver) and for the generation of energy (liver and brain). NaCT thus constitutes a potential therapeutic target for the control of body weight, cholesterol levels, and energy homeostasis.