Dataset Information

Iodide Binding in Sodium-Coupled Cotransporters.

ABSTRACT: Several apical iodide translocation pathways have been proposed for iodide efflux out of thyroid follicular cells, including a pathway mediated by the sodium-coupled monocarboxylate transporter 1 (SMCT1), which remains controversial. Herein, we evaluate structural and functional similarities between SMCT1 and the well-studied sodium-iodide symporter (NIS) that mediates the first step of iodide entry into the thyroid. Free-energy calculations using a force field with electronic polarizability verify the presence of a conserved iodide-binding pocket between the TM2, TM3, and TM7 segments in hNIS, where iodide is coordinated by Phe67, Gln72, Cys91, and Gln94. We demonstrate the mutation of residue Gly93 of hNIS to a larger amino acid expels the side chain of a critical tryptophan residue (Trp255) into the interior of the binding pocket, partially occluding the iodide binding site and reducing iodide affinity, which is consistent with previous reports associating mutation of this residue with iodide uptake deficiency and hypothyroidism. Furthermore, we find that the position of Trp255 in this hNIS mutant mirrors that of Trp253 in wild-type hSMCT1, where a threonine (Thr91) occupies the position homologous to that occupied by glycine in wild-type hNIS (Gly93). Correspondingly, mutation of Thr91 to glycine in hSMCT1 makes the pocket structure more like that of wild-type hNIS, increasing its iodide affinity. These results suggest that wild-type hSMCT1 in the inward-facing conformation may bind iodide only very weakly, which may have implications for its ability to transport iodide.

SUBMITTER: Vergara-Jaque A

PROVIDER: S-EPMC5744185 | biostudies-literature | 2017 Dec

REPOSITORIES: biostudies-literature

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Iodide Binding in Sodium-Coupled Cotransporters.

Vergara-Jaque Ariela A Fong Peying P Comer Jeffrey J

Journal of chemical information and modeling 20171204 12

Several apical iodide translocation pathways have been proposed for iodide efflux out of thyroid follicular cells, including a pathway mediated by the sodium-coupled monocarboxylate transporter 1 (SMCT1), which remains controversial. Herein, we evaluate structural and functional similarities between SMCT1 and the well-studied sodium-iodide symporter (NIS) that mediates the first step of iodide entry into the thyroid. Free-energy calculations using a force field with electronic polarizability ver ...[more]

PMID: 29131623

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Project description:Background: The ability of thyroid follicular epithelial cells to accumulate iodide via the sodium/iodide symporter (NIS) is exploited to successfully treat most thyroid cancers, although a subset of patients lose functional NIS activity and become unresponsive to radioiodide therapy, with poor clinical outcome. Our knowledge of NIS regulation remains limited, however. While numerous membrane proteins are functionally regulated via dimerization, there is little definitive evidence of NIS dimerization, and whether this might impact upon radioiodide uptake and treatment success is entirely unknown. We hypothesized that NIS dimerizes and that dimerization is a prerequisite for iodide uptake. Methods: Coimmunoprecipitation, proximity ligation, and Förster resonance energy transfer (FRET) assays were used to assess NIS:NIS interaction. To identify residues involved in dimerization, a homology model of NIS structure was built based on the crystal structure of the dimeric bacterial protein vSGLT. Results: Abundant cellular NIS dimerization was confirmed in vitro via three discrete methodologies. FRET and proximity ligation assays demonstrated that while NIS can exist as a dimer at the plasma membrane (PM), it is also apparent in other cellular compartments. Homology modeling revealed one key potential site of dimeric interaction, with six residues <3Å apart. In particular, NIS residues Y242, T243, and Q471 were identified as critical to dimerization. Individual mutation of residues Y242 and T243 rendered NIS nonfunctional, while abrogation of Q471 did not impact radioiodide uptake. FRET data show that the putative dimerization interface can tolerate the loss of one, but not two, of these three clustered residues. Conclusions: We show for the first time that NIS dimerizes in vitro, and we identify the key residues via which this happens. We hypothesize that dimerization of NIS is critical to its trafficking to the PM and may therefore represent a new mechanism that would need to be considered in overcoming therapeutic failure in patients with thyroid cancer.

Dataset Information

Iodide Binding in Sodium-Coupled Cotransporters.

Publications

Iodide Binding in Sodium-Coupled Cotransporters.

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