Structure modeling of NIS. (A) Structure modeling viewed in the membrane plane. The three-dimensional structure of NIS was modeled using vSGLT as the reference protein. Mutated residues are represented by sticks. Positively charged and negative charged residues mutated in this study are colored as red and blue, respectively. (B) Core structure viewed from the extracellular side. Residues in the transmembrane segment IX, which have been identified to be involved in sodium/iodide co-transport, are displayed as sticks and colored as yellow. (C) Close-up view of the core structure. Residues involved in the entry or binding of iodide ions are colored as green. Residues involved in the iodide transport are colored as magenta.