RESIDUE AT POSITION 93 OF THE NA+/IODIDE SYMPORTER (NIS) PLAYS A CRITICAL ROLE IN NA+/SUBSTRATE COUPLED TRANSPORT

MAESTRAS, M; DOHAN, O; PARODER, M; NICOLA, JP; ESKANDARI, S; CARRASCO, N

San Diego, California. USA

Congreso; Experimental Biology Society Annul Meeting; 2008

The Na+/I– symporter (NIS) mediates Na+-dependent active I– uptake in the thyroid. We report the electrophysiological characterization of G93R NIS, a clinically-identified NIS mutation that causes congenital I– transport defect, and other substitutions at position 93. Wild-type (WT) NIS and G93N, G93T, and G93R mutants were expressed in X. laevis oocytes and the two-electrode voltage clamp technique was used to study steady-state and presteady-state kinetic properties of the transporters. Similar to WT NIS, the G93N and G93T mutants exhibited electrogenic Na+-dependent I– transport; however, the mutants displayed reduced apparent affinity for I–(>500 µM vs. 30 µM). In contrast to WT NIS, G93N and G93T NIS mutants exhibited electrogenic transport of perchlorate, an environmental pollutant and well-known NIS inhibitor. Steady-state kinetics revealed the apparent affinity of the mutants for perrhenate and perchlorate to be 20–30 µM. Steady-state Na+ kinetics showed a sigmoidal dose-response relationship with all substrates examined (I–, perchlorate, perrhenate, and chlorate). The Na+ apparent affinity (~30 mM) and the Na+ Hill coefficient (~2) of the mutants were similar to those of the WT. Presteady-state currents of the mutants were similar to those of the WT. Although trafficked to the cell surface, G93R NIS was inactive. The data suggest that residue 93 may play a key role in coordinating substrate and/or Na+ in the binding pocket. NIH grants S06 GM53933 to S.E. and DK41544 and CA098390 to N.C.