Mutation K448E in the external loop 5 of rat GABA transporter rGAT1 induces pH sensitivity and alters substrate interactions

1. The effect of the mutation K448E in the rat GABA transporter rGAT1 was s tudied using heterologous expression in Xenopus oocytes and voltage clamp. 2. At neutral pH, the transport-associated current vs. voltage (I-V) relati onship of the mutated transporter was different from wild-type, and the pre -steady-state currents were shifted towards more positive potentials. The m utated transporter showed an increased apparent affinity for Na+ (e.g. 62 v s. 152 mim at -60 mV), while the opposite was true for GABA (e.g. 20 vs. 13 muM at -60 mV). 3. In both isoforms changes in [Na+](o) shifted the voltage dependence of t he pre-steady-state and of the transport-associated currents by similar amo unts. 4. In the K448E form, the moved charge and the relaxation time constant wer e shifted by increasing pH towards positive potentials. The transport-assoc iated current of the mutated transporter was strongly reduced by alkaliniza tion, while acidification slightly decreased and distorted the shape of the I-V curve, Accordingly, uptake of [H-3]GABA was strongly reduced in K448E at pH 9.0. The GABA apparent affinity of the mutated transporter was reduce d by alkalinization, while acidification had the opposite result. 5. These observations suggest that protonation of negatively charged residu es may regulate the Na+ concentration in the proximity of the transporter. Calculation of the unidirectional rate constants for charge movement shows that, in the K448E form, the inward rate constant is increased at alkaline pH, while the outward rate constant does not change, in agreement with an e ffect due to mass action law. 6. A possible explanation for the complex effect of pH on the transport-ass ociated current may be found by combining changes in local [Na+](o) with a direct action of pH on GABA concentration or affinity. Our results support the idea that the extracellular loop 5 may participate to form a vestibule to which sodium ions must have access before proceeding to the steps involv ing charge movement.