Presteady-state and steady-state kinetics and turnover rate of the mouse γ-aminobutyric acid transporter (mGAT3)

A. Sacher, N. Nelson, J. T. Ogi, E. M. Wright, D. D.F. Loo, S. Eskandari*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

31 Scopus citations


We expressed mouse γ-aminobutyric acid (GABA) transporter (mGAT3) in Xenopus laevis oocytes and examined its steady-state and presteady-state kinetics and turnover rate by using tracer flux and electrophysiological methods. In oocytes expressing mGAT3, GABA evoked a Na+-dependent and Cl--facilitated inward current. The dependence on Na+ was absolute, whereas that for Cl- was not. At a membrane potential of -50 mV, the half-maximal concentrations for Na+, Cl-, and GABA were 14 mM, 5 mM, and 3 μM. The Hill coefficient for GABA activation and Cl- enhancement of the inward current was 1, and that for Na+ activation was ≥2. The GABA-evoked inward current was directly proportional to GABA influx (2.2 ± 0.1 charges/GABA) into cells, indicating that under these conditions, there is tight ion/GABA coupling in the transport cycle. In response to step changes in the membrane voltage and in the absence of GABA, mGAT3 exhibited presteady-state current transients (charge movements). The charge-voltage (Q-V) relation was fitted with a single Boltzmann function. The voltage at half-maximal charge (V0.5) was + 25 mV, and the effective valence of the moveable charge (zδ) was 1.6. In contrast to the ON transients, which relaxed with a time constant of ≤30 msec, the OFF transients had a time constant of 1.1 sec. Reduction in external Na+ ([Na+]o) and Cl- ([Cl-]o) concentrations shifted the Q-V relationship to negative membrane potentials. At zero [Na+]o (106 mM Cl-), no mGAT3-mediated transients were observed, and at zero [Cl-]o (100 mM Na+), the charge movements decreased to ≈30% of the maximal charge (Qmax). GABA led to the elimination of charge movements. The half-maximal concentrations for Na+ activation, Cl- enhancement, and GABA elimination of the charge movements were 48 mM, 19 mM, and 5 μM, respectively. Qmax and Imax obtained in the same cells yielded the mGAT3 turnover rate, 1.7 sec-1 at -50 mV. The low turnover rate of mGAT3 may be due to the slow return of the empty transporter from the internal to the external membrane surface.

Original languageEnglish
Pages (from-to)57-73
Number of pages17
JournalJournal of Membrane Biology
Issue number1
StatePublished - 1 Nov 2002


FundersFunder number
National Institute of General Medical SciencesS06GM053933


    • Charge movement
    • Chloride dependence
    • GABA
    • Mouse GAT3
    • Neurotransmitter transporter


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