Functional characterization of the betaine/gamma-aminobutyric acid transporter BGT-1 expressed in Xenopus oocytes

Betaine is an osmolyte accumulated in cells during osmotic cell shrinkage. The canine transporter mediating cellular accumulation of the osmolyte beta ine and the neurotransmitter gamma-aminobutyric acid (BGT-1) was expressed in Xenopus oocytes and analyzed by two-electrode voltage clamp and tracer f lux studies. Exposure of oocytes expressing BGT-1 to betaine or gamma-amino butyric acid (GABA) depolarized the cell membrane in the current clamp mode and induced an inward current under voltage clamp conditions. At 1 mM subs trate the induced currents decreased in the following order: betaine GABA > diaminobutyric acid = beta-alanine > proline quinidine > dimethylglycine > glycine > sarcosine. Both the V-max and K-m of GABA- and betaine-induced c urrents were voltage-dependent, and GABA- and betaine-induced currents and radioactive tracer uptake were strictly Na+-dependent but only partially de pendent on the presence of Cl-. The apparent affinity of GABA decreased wit h decreasing Naf concentrations. The K-m of Na+ also depended on the GABA a nd Cl- concentration. A decrease of the Cl- concentration reduced the appar ent affinity for Na+ and GABA, and a decrease of the Na+ concentration redu ced the apparent affinity for Cl- and GABA. A comparison of Na-22(+)-, Cl-3 6(-)-, and C-14-labeled GABA and C-14-labeled betaine fluxes and GABA- and betaine-induced currents yielded a coupling ratio of Na+/Cl-/organic substr ate of 3:1:1 or 3:2:1, Based on the data, a transport model of ordered bind ing is proposed in which GABA binds first, Na+ second, and Cl- third. In co nclusion, BGT-1 displays significant functional differences from the other members of the GABA transporter family.