Molecular Analysis of a Congenital Iodide Transport Defect: G543E Impairs Maturation and Trafficking of the Na+/I− Symporter

The Na+/I− symporter (NIS) is a key membrane glycoprotein that mediates active I− transport in the thyroid and other tissues. Upon isolation of the cDNA encoding NIS, 10 NIS mutations that cause congenital iodide transport defect have been identified. Three of these mutations (T354P, G395R, and Q267E) have been thoroughly characterized at the molecular level. All three NIS mutant proteins are correctly targeted to the plasma membrane; however, whereas Q267E displays minimal activity, T354P and G395R are inactive. Here, we show that in contrast to these mutants, G543E NIS matures only partially and is retained intracellularly; thus, it is not targeted properly to the cell surface, apparently because of faulty folding. These findings indicate that the G543 residue plays significant roles in NIS maturation and trafficking. Remarkably, NIS activity was rescued by small neutral amino acid substitutions (volume < 129 Å3) at this position, suggesting that G543 is in a tightly packed region of NIS.