Sugawara M, Nakanishi T, Fei Y J, Martindale R G, Ganapathy M E, Leibach F H, Ganapathy V
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912-2100, USA.
Biochim Biophys Acta. 2000 Dec 20;1509(1-2):7-13. doi: 10.1016/s0005-2736(00)00349-7.
To date, two different transporters that are capable of transporting alpha-(methylamino)isobutyric acid, the specific substrate for amino acid transport system A, have been cloned. These two transporters are known as ATA1 and ATA2. We have cloned a third transporter that is able to transport the system A-specific substrate. This new transporter, cloned from rat skeletal muscle and designated rATA3, consists of 547 amino acids and has a high degree of homology to rat ATA1 (47% identity) and rat ATA2 (57% identity). rATA3 mRNA is present only in the liver and skeletal muscle. When expressed in Xenopus laevis oocytes, rATA3 mediates the transport of alpha-(14)Cisobutyric acid and [(3)H]alanine. With the two-microelectrode voltage clamp technique, we have shown that exposure of rATA3-expressing oocytes to neutral, short-chain aliphatic amino acids induces inward currents. The amino acid-induced current is Na(+)-dependent and pH-dependent. Analysis of the currents with alanine as the substrate has shown that the K(0. 5) for alanine (i.e., concentration of the amino acid yielding half-maximal current) is 4.2+/-0.1 mM and that the Na(+):alanine stoichiometry is 1:1.
迄今为止，已经克隆出两种能够转运α-（甲基氨基）异丁酸（氨基酸转运系统A的特异性底物）的不同转运体。这两种转运体分别称为ATA1和ATA2。我们克隆了第三种能够转运系统A特异性底物的转运体。这种从大鼠骨骼肌中克隆出来并命名为rATA3的新转运体由547个氨基酸组成，与大鼠ATA1（同一性为47%）和大鼠ATA2（同一性为57%）具有高度同源性。rATA3 mRNA仅存在于肝脏和骨骼肌中。当在非洲爪蟾卵母细胞中表达时，rATA3介导α-[（14）C]（甲基氨基）异丁酸和[（3）H]丙氨酸的转运。运用双微电极电压钳技术，我们已经表明，将表达rATA3的卵母细胞暴露于中性、短链脂肪族氨基酸会诱导内向电流。氨基酸诱导的电流是Na+依赖性和pH依赖性的。以丙氨酸为底物对电流进行分析表明，丙氨酸的K（0.5）（即产生最大电流一半时的氨基酸浓度）为4.2±0.1 mM，并且Na+与丙氨酸的化学计量比为1:1。
