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TIRO, CEA DSV-iBEB-SBTN, CAL, School of Medicine, University of Nice Sophia Antipolis, 28 Avenue de Valombrose, 06107, Nice, France¹ UMR7009 CNRS/UPMC Biologie du Développement, 06230 Villefranche-sur-Mer, France² CEA Center of Valrhô, DSV-iBEB-SBTN, 30207 Bagnols-sur-Cèze, France

(Correspondence should be addressed to T Pourcher; Email: pourcher{at}unice.fr)

^* (M Dayem and C Basquin contributed equally to this work)

The active transport of iodide from the bloodstream into thyroid follicular cells is mediated by the Na⁺/I^– symporter (NIS). We studied mouse NIS (mNIS) and found that it catalyzes iodide transport into transfected cells more efficiently than human NIS (hNIS). To further characterize this difference, we compared ¹²⁵I uptake in the transiently transfected human embryonic kidney (HEK) 293 cells. We found that the V_max for mNIS was four times higher than that for hNIS, and that the iodide transport constant (K_m) was 2.5-fold lower for hNIS than mNIS. We also performed immunocytolocalization studies and observed that the subcellular distribution of the two orthologs differed. While the mouse protein was predominantly found at the plasma membrane, its human ortholog was intracellular in 40% of the expressing cells. Using cell surface protein-labeling assays, we found that the plasma membrane localization frequency of the mouse protein was only 2.5-fold higher than that of the human protein, and therefore cannot alone account for the difference in the obtained V_max values. We reasoned that the observed difference could also be caused by a higher turnover number for iodide transport in the mouse protein. We then expressed and analyzed chimeric proteins. The data obtained with these constructs suggest that the iodide recognition site could be located in the region extending from the N-terminus to transmembrane domain 8, and that the region between transmembrane domain 5 and the C-terminus could play a role in the subcellular localization of the protein.