Are Amino Acid Antiporters Biological Maxwell Demons?

Amino acid antiporters mediate the 1:1 exchange of groups of amino acids. Whether substrate specificity can be different for the inward and outward facing conformation has not been investigated systematically, although examples of asymmetric transport have been reported. Here we used LC–MS to detect the movement of 12C- and 13C-labelled amino acid mixtures across the plasma membrane of Xenopus laevis oocytes expressing a variety of amino acid antiporters. Differences of substrate specificity between transporter paralogs were readily observed using this method. Our results suggest that antiporters are largely symmetric, equalizing the pools of their substrate amino acids. Exceptions are the antiporters y+LAT1 and y+LAT2 where neutral amino acids are co-transported with Na+ ions, favouring their import. For the antiporters ASCT1 and ASCT2 glycine acted as a selective influx substrate, while proline was a selective influx substrate of ASCT1. These data show that antiporters can display non-canonical modes of transport.

氨基酸反向转运蛋白（amino acid antiporters）可介导1:1比例的氨基酸基团跨膜交换。尽管已有非对称转运的相关案例被报道，但针对其面向胞内与胞外的构象是否具有差异化底物特异性的系统性研究仍未展开。本研究采用液相色谱-质谱联用（LC-MS）技术，检测经12C与13C标记的氨基酸混合物在表达多种氨基酸反向转运蛋白的非洲爪蟾（Xenopus laevis）卵母细胞膜上的跨膜转运情况。借助该实验方法，我们可便捷观测到转运蛋白旁系同源物之间的底物特异性差异。研究结果显示，绝大多数反向转运蛋白呈对称转运特性，可使其底物氨基酸的胞内库浓度趋于均衡。例外情况见于y+LAT1与y+LAT2两类反向转运蛋白：它们可将中性氨基酸与钠离子协同转运，从而偏向于促进底物的胞内导入。对于ASCT1与ASCT2两类反向转运蛋白而言，甘氨酸可作为选择性内流转运底物，而脯氨酸则是ASCT1的选择性内流转运底物。上述实验数据证实，反向转运蛋白可呈现非经典的转运模式。