College of Life and Environment Sciences, Shanghai Normal University, 100 Guilin Road, Shanghai, 200234, China.
Biochem J. 2011 Mar 1;434(2):287-96. doi: 10.1042/BJ20100507.
SNAT (sodium-coupled neutral amino acid transporter) 2 belongs to the SLC38 (solute carrier 38) family of solute transporters. Transport of one amino acid molecule into the cell is driven by the co-transport of one Na(+) ion. The functional significance of the C-terminus of SNAT2, which is predicted to be located in the extracellular space, is currently unknown. In the present paper, we removed 13 amino acid residues from the SNAT2 C-terminus and studied the effect of this deletion on transporter function. The truncation abolished amino acid transport currents at negative membrane potentials (<0 mV), as well as substrate uptake. However, transport currents were observed at positive membrane potentials demonstrating that transport was accelerated while the driving force decreased. Membrane expression levels were normal in the truncated transporter. SNAT2(Del C-ter) (13 residues deleted from the C-terminus) showed 3-fold higher apparent affinity for alanine, and 2-fold higher Na(+) affinity compared with wild-type SNAT2, suggesting that the C-terminus is not required for high-affinity substrate and Na(+) interaction with SNAT2. The pH sensitivity of amino acid transport was retained partially after the truncation. In contrast with the truncation after TM (transmembrane domain) 11, the deletion of TM11 resulted in an inactive transporter, most probably due to a defect in cell surface expression. Taken together, the results demonstrate that the C-terminal domain of SNAT2 is an important voltage regulator that is required for a normal amino acid translocation process at physiological membrane potentials. However, the C-terminus appears not to be involved in the regulation of membrane expression.
SNAT（钠耦合中性氨基酸转运体）2 属于溶质转运体 SLC38（溶质载体 38）家族。一个氨基酸分子进入细胞的转运是由一个 Na（+）离子的共转运驱动的。SNAT2 的 C 末端（预测位于细胞外空间）的功能意义目前尚不清楚。在本论文中，我们从 SNAT2 的 C 末端去除了 13 个氨基酸残基，并研究了这种缺失对转运体功能的影响。截断使负膜电位（<0 mV）下的氨基酸转运电流以及底物摄取都消失了。然而，在正膜电位下观察到转运电流，表明在驱动力降低的同时，转运速度加快。在截断的转运体中，膜表达水平正常。与野生型 SNAT2 相比，SNAT2（Del C-ter）（从 C 末端缺失 13 个残基）对丙氨酸的表观亲和力提高了 3 倍，对 Na（+）的亲和力提高了 2 倍，表明 C 末端对于 SNAT2 与高亲和力底物和 Na（+）的相互作用不是必需的。在截断后，氨基酸转运的 pH 敏感性部分保留。与 TM（跨膜域）11 后的截断相反，TM11 的缺失导致无活性的转运体，这很可能是由于细胞表面表达的缺陷。综上所述，这些结果表明，SNAT2 的 C 末端结构域是一种重要的电压调节因子，对于在生理膜电位下正常的氨基酸转运过程是必需的。然而，C 末端似乎不参与膜表达的调节。
