KAAT1 is a neutral amino acid transporter activated by K⁺ or by Na⁺ (9). The protein shows significant homology with members of the Na⁺/Cl^--dependent neurotransmitter transporter super family. E59G KAAT1, expressed in Xenopus oocytes, exhibited a reduced leucine uptake (20-30% of WT), and kinetic analysis indicated that the loss of activity was due to reduction of V_max and apparent affinity for substrates. Electrophysiological analysis revealed that E59G KAAT1 has presteady-state and uncoupled currents larger than WT but no leucine-induced currents. Site-directed mutagenesis analysis showed the requirement of a negative charge in position 59 of KAAT1. The analysis of permeant and impermeant MTS effects confirmed the intracellular localization of glutamate 59. Since the MTSEA inhibition was not prevented by the presence of Na⁺ or leucine, we have concluded that E59 is not directly involved in the binding of substrates. NEM inhibition was qualitatively and quantitatively different in the two transporters, WT and E59G KAAT1, having the same cysteine residues. This indicates an altered accessibility of native cysteine residues due to a modified spatial organization of E59G KAAT1. The arginine modifier PGO effect supports this hypothesis: not only cysteine but also arginine residues become more accessible to the modifying reagents in the mutant E59G. In conclusion, the results presented suggest that glutamate 59 plays a critical role in the three-dimensional organization of KAAT1.