Comparison of expressed human and mouse sodium/iodide symporters reveals differences in transport properties and subcellular localization.

The active transportof iodidefrom the bloodstream into thyroid follicular cellsis mediated by the Na C /I K symporter(NIS). We studied mouse NIS (mNIS) and found that it catalyzes iodidetransport into transfected cells more efficiently than human NIS (hNIS). To further characterize this difference, we compared 125 I uptake in the transiently transfected human embryonic kidney ( HEK) 293 cells. We found that the Vmax for mNIS was four times higher than that for hNIS, and that the iodidetransport constant (Km )w as 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 w40% 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 Vmax values. We reasoned that the observed difference could also be caused by a higher turnover numberfor iodidetransport in the mouse protein. We then expressed and analyzed chimeric proteins. The data obtained with these constructs suggest that the iodiderecognition site could be located in the region extending from the N-terminus to transmembrane domain8, and that the region between transmembrane domain5 and the C-terminus could play a role in the subcellular localization of the protein.