FIGURE 4.

Chloride independence of the Q291X/S331E mutants. The mutants Q291A, Q291G, and Q291S in the background of S331E (QASE, QGSE, and QSSE, respectively), as well as WT and S331E GAT-1, were transiently expressed, and [³H]GABA transport was measured for 3 min, as described under “Experimental Procedures,” in the presence or absence of chloride (150 mm NaCl or sodium gluconate in the external medium, respectively). The data are given as the ratio of transport in the chloride-free medium to that in chloride-containing medium, and the values are the means ± S.E. of at least three separate experiments performed in quadruplicate.