Cloning and characterization of the thyroid iodide transporter

IODIDE (I - ) is an essential constituent of the thyroid hormones T 3 and T 4 , and is accumulated by the thyroid. The transport of iodide, the first step in thyroid hormogenesis, is catalysed by the Na + /I - symporter, an intrinsic membrane protein that is crucial for the evaluation, diagnosis and treatment of thyroid disorders. Although several other important thyroid proteins involved in hormogenesis have been characterized, the Na + /I - symporter has not. Here we report the isolation of a complementary DNA clone that encodes this symporter, as a result of functional screening of a cDNA library from a rat thyroid-derived cell line (FRTL-5) in Xenopus laevis oocytes. Oocyte microinjection of an RNA transcript made in vitro from this cDNA clone elicited a more than 700-fold increase in perchlorate-sensitive Na + /I - symport activity over background. To our knowledge, this is the first iodide-transporting molecule to have its cDNA cloned, providing a missing link in the thyroid hormone biosynthetic pathway.