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Volume 16, Number 17,
Issue of September 1, 1996
pp. 5405-5414
Copyright ©1996 Society for Neuroscience
Ion Binding and Permeation at the GABA Transporter GAT1
Received April 26, 1996; revised June 14, 1996; accepted June 18, 1996.
Sela Mager1,
Nurit Kleinberger-Doron2,
Gilmor I. Keshet2,
Norman Davidson1,
Baruch I. Kanner2, and
Henry A. Lester1
1 Division of Biology, California Institute of
Technology, Pasadena, California 91125, and 2 Department of
Biochemistry, Hadassah Medical School, The Hebrew University, Jerusalem
91120, Israel
This study addresses the binding of ions and the permeation of
substrates during function of the GABA transporter GAT1. GAT1 was
expressed in Xenopus oocytes and studied
electrophysiologically as well as with [3H]GABA flux;
GAT1 was also expressed in mammalian cells and studied with
[3H]GABA and [3H]tiagabine binding. Voltage
jumps, Na+ and Cl concentration jumps, and
exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all
produce capacitive charge movements. Occlusive interactions among these
three types of perturbations show that they all measure the same
population of charges. The concentration dependences of the charge
movements reveal (1) that two Na+ ions interact with the
transporter even in the absence of GABA, and (2) that Cl
facilitates the binding of Na+. Comparison between the
charge movements and the transport-associated current shows that this
initial Na+-transporter interaction limits the overall
transport rate when [GABA] is saturating. However, two classes of
manipulation treatment with high-affinity uptake blockers and the W68L
mutation ``lock'' Na+ onto the transporter by slowing or
preventing the subsequent events that release the substrates to the
intracellular medium. The Na+ substitutes Li+
and Cs+ do not support charge movements, but they can
permeate the transporter in an uncoupled manner. Our results (1)
support the hypothesis that efficient removal of synaptic transmitter
by the GABA transporter GAT1 depends on the previous binding of
Na+ and Cl , and (2) indicate the important
role of the conserved putative transmembrane domain 1 in interactions
with the permeant substrates.
Key words:
sodium;
chloride;
GAT1;
GABA;
transporter;
Xenopus oocyte
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