Characterization of Thr-354 in the human sodium/iodide symporter (NIS) by site-directed mutagenesis.

Sodium/iodide symporter (NIS) is the key molecule concentrating iodide in the thyroid gland. The first-described human NIS (hNIS) mutation to cause a complete iodide transport defect was the T354P mutation. The Thr-354 lies in the midst of the putative ninth transmembrane segment which is well-conserved within the members of the SLC5A transporter family. Here we have investigated the molecular function of Thr-354 using site-directed mutagenesis and found that T354S and T354A mutations result in significantly decreased iodide transport activity, 50 % and 2 % of wild-type hNIS. Our findings indicate that whereas Thr-354 is indispensable for the complete NIS activity, the β-hydroxyl group accounts for half, and the α-helical structure alone contributes for one-fiftieth of wild-type hNIS activity.