Na+ coordination at the Na2 site of the Na+/I- symporter
Na+ coordination at the Na2 site of the Na+/I- symporter
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DOI：
10.1073/pnas.1607231113
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发表时间：
2016-09-13
影响因子：
11.1
通讯作者：
Carrasco, Nancy
中科院分区：
文献类型：
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作者：
Ferrandino, Giuseppe;Nicola, Juan Pablo;Carrasco, Nancy
钠/碘同向转运体 (NIS) 以 2 Na+: 1 I- 化学计量介导甲状腺中的主动 I- 转运（甲状腺激素生物合成的第一步）。两个 Na+ 结合位点（Na1 和 Na2）和 I- 结合位点以变构方式相互作用：当 Na+ 与一个 Na+ 位点结合时，NIS 对其他 Na+ 和 I 的亲和力显着增加。在所有与 NIS 折叠相同的 Na+ 依赖性转运蛋白中，两个残基 S353 和 T354（NIS 编号）的侧链被鉴定为 Na2 处的 Na+ 配体。为了了解底物之间的协同作用，我们研究了 Na2 位点的配位。我们确定其他四个残基 - S66、D191、Q194 和 Q263 - 也参与该位点的 Na+ 配位。全细胞实验表明，这四个残基参与 NIS 的转运：这些位置的突变导致蛋白质虽然在质膜上表达，但很少或不转运 I-。这些残基在 NIS 所属的整个 SLC5 家族中都是保守的，这表明它们在其他转运蛋白中发挥着类似的功能。我们的研究结果还表明，当离子与另一个位点结合时，每个位点表现出的亲和力增加可能是由于转运蛋白动力学的变化所致。这些机制见解不仅加深了我们对 NIS 的理解，而且加深了对其他转运蛋白的理解，包括许多像 NIS 一样具有重大医学意义的转运蛋白。
The sodium/iodide symporter (NIS) mediates active I- transport in the thyroid-the first step in thyroid hormone biosynthesis-with a 2 Na+: 1 I- stoichiometry. The two Na+ binding sites (Na1 and Na2) and the I- binding site interact allosterically: when Na+ binds to a Na+ site, the affinity of NIS for the other Na+ and for Iincreases significantly. In all Na+-dependent transporters with the same fold as NIS, the side chains of two residues, S353 and T354 (NIS numbering), were identified as the Na+ ligands at Na2. To understand the cooperativity between the substrates, we investigated the coordination at the Na2 site. We determined that four other residues-S66, D191, Q194, and Q263-are also involved in Na+ coordination at this site. Experiments in whole cells demonstrated that these four residues participate in transport by NIS: mutations at these positions result in proteins that, although expressed at the plasma membrane, transport little or no I-. These residues are conserved throughout the entire SLC5 family, to which NIS belongs, suggesting that they serve a similar function in the other transporters. Our findings also suggest that the increase in affinity that each site displays when an ion binds to another site may result from changes in the dynamics of the transporter. These mechanistic insights deepen our understanding not only of NIS but also of other transporters, including many that, like NIS, are of great medical relevance.