Ferrandino Giuseppe, Nicola Juan Pablo, Sánchez Yuly E, Echeverria Ignacia, Liu Yunlong, Amzel L Mario, Carrasco Nancy
Department of Cellular and Molecular Physiology, Yale School of Medicine, New Haven, CT 06510;
Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
Proc Natl Acad Sci U S A. 2016 Sep 13;113(37):E5379-88. doi: 10.1073/pnas.1607231113. Epub 2016 Aug 25.
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 I(-) increases 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.
钠/碘同向转运体（NIS）介导甲状腺中碘离子（I⁻）的主动转运，这是甲状腺激素生物合成的第一步，其化学计量比为2个Na⁺:1个I⁻。两个Na⁺结合位点（Na1和Na2）与I⁻结合位点发生变构相互作用：当Na⁺结合到一个Na⁺位点时，NIS对另一个Na⁺和I⁻的亲和力会显著增加。在所有与NIS具有相同折叠结构的Na⁺依赖性转运体中，两个残基S353和T354（NIS编号）的侧链被确定为Na2位点的Na⁺配体。为了理解底物之间的协同作用，我们研究了Na2位点的配位情况。我们确定另外四个残基——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，但转运很少或不转运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体中发挥类似的功能。我们的研究结果还表明，当一个离子与另一个位点结合时，每个位点显示出的亲和力增加可能是由于转运体动力学的变化所致。这些机制上的见解不仅加深了我们对NIS的理解，也加深了我们对其他转运体的理解，包括许多与NIS一样具有重大医学意义的转运体。
