Mechanism of anion selectivity and stoichiometry of the Na+/I- symporter (NIS)

I(-) uptake in the thyroid, the first step in thyroid hormone biosynthesis, is mediated by the Na(+)/I(-) symporter (NIS) with an electrogenic 2Na(+):1I(-) stoichiometry. We have obtained mechanistic information on NIS by characterizing the congenital I(-) transport defect-causing NIS mutant G93R. This mutant is targeted to the plasma membrane but is inactive. Substitutions at position 93 show that the longer the side chain of the neutral residue at this position, the higher the K(m) for the anion substrates. Unlike WT NIS, which mediates symport of Na(+) and the environmental pollutant perchlorate electroneutrally, G93T/N/Q/E/D NIS, strikingly, do it electrogenically with a 21 stoichiometry. Furthermore, G93E/Q NIS discriminate between anion substrates, a discovery with potential clinical relevance. A 3D homology model of NIS based on the structure of the bacterial Na(+)/galactose transporter identifies G93 as a critical player in the mechanism of the transporter: the changes from an outwardly to an inwardly open conformation during the transport cycle use G93 as a pivo

Mechanism of anion selectivity and stoichiometry of the Na+/I- symporter (NIS)

I- uptake in the thyroid, the first step in thyroid hormone biosynthesis, is mediated by the Na+/I- symporter (NIS) with an electrogenic 2Na+ : 1I- stoichiometry. We have obtained mechanistic information on NIS by characterizing the congenital I- transport defect-causing NIS mutant G93R. This mutant is targeted to the plasma membrane but is inactive. Substitutions at position 93 show that the longer the side chain of the neutral residue at this position, the higher the Km for the anion substrates. Unlike WT NIS, which mediates symport of Na+ and the environmental pollutant perchlorate electroneutrally, G93T/N/Q/E/D NIS, strikingly, do it electrogenically with a 2∶1 stoichiometry. Furthermore, G93E/Q NIS discriminate between anion substrates, a discovery with potential clinical relevance. A 3D homology model of NIS based on the structure of the bacterial Na+/galactose transporter identifies G93 as a critical player in the mechanism of the transporter: the changes from an outwardly to an inwardly open conformation during the transport cycle use G93 as a pivot

Allosteric regulation of mammalian Na+/I− symporter activity by perchlorate

The Na+/I- symporter (NIS), the plasma membrane protein that actively transports I- (stoichiometry 2Na+:1I-) in thyroid physiology and radioiodide-based thyroid cancer treatment, also transports the environmental pollutant perchlorate (stoichiometry 1Na+:1ClO4-), which competes with I- for transport. Until now, the mechanism by which NIS transports different anion substrates with different stoichiometries has remained unelucidated. We carried out transport measurements and analyzed these using a statistical thermodynamics-based equation and electrophysiological experiments to show that the different stoichiometry of ClO4- transport is due to ClO4- binding to a high-affinity non-transport allosteric site that prevents Na+ from binding to one of its two sites. Furthermore, low concentrations of ClO4- inhibit I- transport not only by competition but also, critically, by changing the stoichiometry of I- transport to 1:1, which greatly reduces the driving force. The data reveal that ClO4- pollution in drinking water is more dangerous than previously thought