Kim D K, Kanai Y, Chairoungdua A, Matsuo H, Cha S H, Endou H
Department of Pharmacology and Toxicology, Kyorin University School of Medicine, 6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan.
J Biol Chem. 2001 May 18;276(20):17221-8. doi: 10.1074/jbc.M009462200. Epub 2001 Feb 20.
A cDNA was isolated from rat small intestine by expression cloning which encodes a novel Na+-independent transporter for aromatic amino acids. When expressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter 1) exhibited Na+-independent and low-affinity transport of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (Km values: approximately 5 mm), consistent with the properties of classical amino acid transport system T. TAT1 accepted some variations of aromatic side chains because it interacted with amino acid-related compounds such as l-DOPA and 3-O-methyl-DOPA. Because TAT1 accepted N-methyl- and N-acetyl-derivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions. Consistent with this, TAT1 exhibited sequence similarity (approximately 30% identity at the amino acid level) to H+/monocarboxylate transporters. Distinct from H+/monocarboxylate transporters, however, TAT1 was not coupled with the H+ transport but it mediated an electroneutral facilitated diffusion. TAT1 mRNA was strongly expressed in intestine, placenta, and liver. In rat small intestine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of aromatic amino acids. The identification of the amino acid transporter with distinct structural and functional characteristics will not only facilitate the expansion of amino acid transporter families but also provide new insights into the mechanisms of substrate recognition of organic solute transporters.
通过表达克隆从大鼠小肠中分离出一种互补DNA（cDNA），它编码一种新型的不依赖钠离子的芳香族氨基酸转运体。当在非洲爪蟾卵母细胞中表达时，编码的蛋白质命名为TAT1（T型氨基酸转运体1）表现出对色氨酸、酪氨酸和苯丙氨酸等芳香族氨基酸的不依赖钠离子且低亲和力的转运（米氏常数：约5 mM），这与经典氨基酸转运系统T的特性一致。TAT1能接受一些芳香族侧链的变体，因为它能与诸如L-多巴和3-O-甲基多巴等氨基酸相关化合物相互作用。由于TAT1能接受芳香族氨基酸的N-甲基和N-乙酰衍生物，但不接受它们的甲酯，因此推测TAT1将氨基酸底物识别为阴离子。与此一致的是，TAT1与氢离子/单羧酸转运体表现出序列相似性（氨基酸水平上约30%的同一性）。然而，与氢离子/单羧酸转运体不同的是，TAT1不与氢离子转运偶联，而是介导电中性的易化扩散。TAT1信使核糖核酸（mRNA）在小肠、胎盘和肝脏中强烈表达。在大鼠小肠中，TAT1免疫反应性在上皮细胞的基底外侧膜中被检测到，表明其在芳香族氨基酸跨上皮转运中的作用。这种具有独特结构和功能特征的氨基酸转运体的鉴定不仅将促进氨基酸转运体家族的扩展，还将为有机溶质转运体的底物识别机制提供新的见解。
