The sodium╱iodide symporter (NIS) is a key plasma membrane glycoprotein that mediates the active iodide transport in the thyroid and other tissues. Our previous study found that a novel exon 6 deletion (234 a.a. to 260 a.a.) in NIS losses the iodide uptake activity. In this study, we tried to determine whether His-226 is involved in iodide uptake of NIS. His-226 was then replaced with Ala, Asp, Glu, or Lys by site-directed mutagenesis, and the NIS mutants were transfected into cells to evaluate the iodide uptake ability. Our data showed that all NIS mutants yielded defects in iodide uptake activities. Furthermore, all NIS mutants were located on the membrane by immunofluorescent analysis, suggesting that the defects of mutants in iodide uptake did not result from the loss of protein targeting. Further analysis on the kinetics indicated that all mutants displayed a lower Vmax and a higher Km than wild type. These results suggested that His-226 of NIS was involved in the binding of iodide.
