Functional characterization of the betaine/gamma-aminobutyric acid transporter BGT-1 expressed in Xenopus oocytes
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- Functional characterization of the betaine/gamma-aminobutyric acid transporter BGT-1 expressed in Xenopus oocytes
- Matskevitch, I; Wagner, CA; Stegen, C; Broer, S; Noll, B; Risler, T; Kwon, HM; Handler, JS; Waldegger, S; Busch, AE; Lang, F
- Issue Date
- AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
- JOURNAL OF BIOLOGICAL CHEMISTRY, v.274, no.24, pp.16709 - 16716
- Betaine is an osmolyte accumulated in cells during osmotic cell shrinkage. The canine transporter mediating cellular accumulation of the osmolyte betaine and the neurotransmitter γ-aminobutyric acid (BGT-1) was expressed in Xenopus oocytes and analyzed by two-electrode voltage clamp and tracer flux studies. Exposure of oocytes expressing BGT-1 to betaine or γ- aminobutyric acid (GABA) depolarized the cell membrane in the current clamp mode and induced an inward current under voltage clamp conditions. At 1 mM substrate the induced currents decreased in the following order: betaine = GABA > diaminobutyric acid = β-alanine > proline = quinidine > dimethylglycine > glycine > sarcosine. Both the V(max) and K(m) of GABA- and betaine-induced currents were voltage-dependent, and GABA- and betaine- induced currents and radioactive tracer uptake were strictly Na+-dependent but only partially dependent on the presence of Cl-. The apparent affinity of GABA decreased with decreasing Na+ concentrations. The K(m) of Na+ also depended on the GABA and Cl-concentration. A decrease of the Cl- concentration reduced the apparent affinity for Na+ and GABA, and a decrease of the Na+ concentration reduced the apparent affinity for Cl- and GABA. A comparison of 22Na+-, 36Cl-, and 14C-labeled GABA and 14C-labeled betaine fluxes and GABA- and betaine-induced currents yielded a coupling ratio of Na+/Cl-/organic substrate of 3:1:1 or 3:2:1. Based on the data, a transport model of ordered binding is proposed in which GABA binds first, Na+ second, and Cl- third. In conclusion, BGT-1 displays significant functional differences from the other members of the GABA transporter family.
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