A perchlorate sensitive iodide transporter in frogs

Nucleotide sequence comparisons have identified a gene product in the genome database of African clawed frogs (Xenopus laevis) as a probable member of the solute carrier family of membrane transporters. To confirm its identity as a putative iodide transporter, we examined the function of this sequence after heterologous expression in mammalian cells. A green monkey kidney cell line transfected with the Xenopus nucleotide sequence had significantly greater ¹²⁵I uptake than sham-transfected control cells. The uptake in carrier-transfected cells was significantly inhibited in the presence of perchlorate, a competitive inhibitor of mammalian Na⁺/iodide symporter. Tissue distributions of the sequence were also consistent with a role in iodide uptake. The mRNA encoding the carrier was found to be expressed in the thyroid gland, stomach, and kidney of tadpoles from X. laevis, as well as the bullfrog Rana catesbeiana. The ovaries of adult X. laevis also were found to express the carrier. Phylogenetic analysis suggested that the putative X. laevis iodide transporter is orthologous to vertebrate Na⁺-dependent iodide symporters. We conclude that the amphibian sequence encodes a protein that is indeed a functional Na⁺/iodide symporter in X. laevis, as well as R. catesbeiana.