Abolition of substrate‐dependent currents by tyrosine mutation in the transmembrane domain of glutamate transporter

By site-directed mutagenesis we examined the roles of tyrosine residues (Tyr127) in the putative transmembrane domain of rat glutamate transporter (GLAST). When expressed in Xenopus oocytes, Y127F mutant protein, which was localized in plasma membranes of oocytes, completely abolished glutamate uptake currents but did not affect the intrinsic substrate-independent currents. Coexpression of wild type and mutant transporters supports that the Y127F mutation did not elicit glutamate efflux. The efflux of glutamate by wild type or Y127F mutant transporters was measured under the condition of ion perturbation where transporters run in the reverse direction.