TY - JOUR

T1 - Residues in the extracellular loop 4 are critical for maintaining the conformational equilibrium of the gamma-aminobutyric acid transporter-1.

AU - MacAulay, Nanna

AU - Meinild, Anne-Kristine

AU - Zeuthen, Thomas

AU - Gether, Ulrik

N1 - Keywords: Animals; Carrier Proteins; Electric Conductivity; Female; GABA Plasma Membrane Transport Proteins; Gene Expression; Hydrogen-Ion Concentration; Lithium; Membrane Potentials; Membrane Proteins; Membrane Transport Proteins; Methionine; Mutagenesis, Site-Directed; Oocytes; Organic Anion Transporters; Protein Conformation; Rats; Sodium; Structure-Activity Relationship; Threonine; Transfection; Tritium; Xenopus laevis; gamma-Aminobutyric Acid

PY - 2003

Y1 - 2003

N2 - We mutated residues Met345 and Thr349 in the rat gamma-aminobutyric acid transporter-1 (GAT-1) to histidines (M345H and T349H). These two residues are located four amino acids apart at the extracellular end of transmembrane segment 7 in a region of GAT-1 that we have previously suggested undergoes conformational changes critical for the transport process. The two single mutants and the double mutant (M345H/T349H) were expressed in Xenopus laevis oocytes, and their steady-state and presteady-state kinetics were examined and compared with wild type GAT-1 by using the two-electrode voltage clamp method. Oocytes expressing M345H showed a decrease in apparent GABA affinity, an increase in apparent affinity for Na+, a shift in the charge/voltage (Q/Vm) relationship to more positive membrane potentials, and an increased Li+-induced leak current. Oocytes expressing T349H showed an increase in apparent GABA affinity, a decrease in apparent Na+ affinity, a profound shift in the Q/Vm relationship to more negative potentials, and a decreased Li+-induced leak current. The data are consistent with a shift in the conformational equilibrium of the mutant transporters, with M345H stabilized in an outward-facing conformation and T349H in an inward-facing conformation. These data suggest that the extracellular end of transmembrane domain 7 not only undergoes conformational changes critical for the translocation process but also plays a role in regulating the conformational equilibrium between inward- and outward-facing conformations.

AB - We mutated residues Met345 and Thr349 in the rat gamma-aminobutyric acid transporter-1 (GAT-1) to histidines (M345H and T349H). These two residues are located four amino acids apart at the extracellular end of transmembrane segment 7 in a region of GAT-1 that we have previously suggested undergoes conformational changes critical for the transport process. The two single mutants and the double mutant (M345H/T349H) were expressed in Xenopus laevis oocytes, and their steady-state and presteady-state kinetics were examined and compared with wild type GAT-1 by using the two-electrode voltage clamp method. Oocytes expressing M345H showed a decrease in apparent GABA affinity, an increase in apparent affinity for Na+, a shift in the charge/voltage (Q/Vm) relationship to more positive membrane potentials, and an increased Li+-induced leak current. Oocytes expressing T349H showed an increase in apparent GABA affinity, a decrease in apparent Na+ affinity, a profound shift in the Q/Vm relationship to more negative potentials, and a decreased Li+-induced leak current. The data are consistent with a shift in the conformational equilibrium of the mutant transporters, with M345H stabilized in an outward-facing conformation and T349H in an inward-facing conformation. These data suggest that the extracellular end of transmembrane domain 7 not only undergoes conformational changes critical for the translocation process but also plays a role in regulating the conformational equilibrium between inward- and outward-facing conformations.

U2 - 10.1074/jbc.M213023200

DO - 10.1074/jbc.M213023200

M3 - Journal article

C2 - 12764157

VL - 278

SP - 28771

EP - 28777

JO - Journal of Biological Chemistry

JF - Journal of Biological Chemistry

SN - 0021-9258

IS - 31

ER -