FIGURE 4.

Increased occupancy of the Na⁺ binding site(s) increases the glutamate affinity of EAAC1. A, typical anion current in response to 5 mm glutamate applied to EAAC1_T101A in the exchange mode (see inset). B, normalized glutamate dose-response curves as a function of extracellular [Na⁺] (300 mm (▴), 140 mm (■), and 20 mm (□)) in exchange mode. The dotted line represents the wild type curve at 140 mm Na⁺. The currents were normalized to the response at 30 mm glutamate (20 mm Na⁺), 5 mm glutamate (140 mm Na⁺), and 2 mm glutamate (300 mm Na⁺). The extracellular solution contained MES as the anion and the indicated Na⁺ concentration supplemented with NMG⁺ to 300 mm. The composition of the internal solution was 140 mm NaSCN and 10 mm glutamate, and the transmembrane potential was 0 mV. C, [Na⁺] dependence of the K_m of EAAC1_WT (open circles) and EAAC1_T101A (closed squares) for glutamate.