Abstract
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 does human NIS (hNIS). To further characterize this difference, we compared 125I uptake in transiently transfected HEK 293 cells. We found that the Vmax for mNIS was four times higher than that for hNIS, and that the iodide transport constant (Km) was 2.5 fold lower for hNIS than for 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-labelling assays, we found that the plasma membrane localization frequency of the mouse protein was only twice 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 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 the 8th transmembrane domain and that the region between transmembrane domain 5 and the C-terminus could play a role in the subcellular localization of the protein.