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Received June 17, 2002
Accepted after revision September 23, 2002
1 Laboratory of Cellular and Molecular Physiology, Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese and Department of Neurosciences, S. Raffaele Institute, Via Olgettina 58, 20132 Milano, Italy
2 Laboratory of Cellular and Molecular Physiology, Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese
3 Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese, Italy
* To whom correspondence should be addressed. E-mail: antonio.peres{at}uninsubria.it.
Most cotransporters characteristically display two main kinds of electrical activity: in the absence of organic substrate, transient presteady-state currents (Ipre) are generated by charge relocation during voltage steps; in the presence of substrate, sustained, transport-associated currents (Itr) are recorded. Quantitative comparison of these two currents, in Xenopus oocytes expressing the neural GABA cotransporter rGAT1, revealed several unforeseen consistencies between Ipre and Itr, in terms of magnitude and kinetic parameters. The decay rate constant (r) of Ipre and the quantity of charge displaced to an inner position in the transporter (Qin(0)) depended on voltage and ionic conditions. Saturating GABA concentrations, applied under the same conditions, suppressed Ipre (i.e. Qin() = 0) and produced a transport-associated current with amplitude Itr = Qin(0)r. At non-saturating levels of GABA, changes of Itr were compensated by corresponding variations in Qin, such that Ipre and Itr complemented each other, according to the relation: Itr = (Qin(0) - Qin) r. Complementarity of magnitude, superimposable kinetic properties and equal dependence on voltage and [Na+]o point to the uniqueness of the charge carrier for both processes, suggesting that transport and charge migration arise from the same molecular mechanism. The observed experimental relations were correctly predicted by a simple three-state kinetic model, in which GABA binding takes place after charge binding and inward migration have occurred. The model also predicts the observed voltage dependence of the apparent affinity of the transporter for GABA, and suggests a voltage-independent GABA binding rate with a value around 0.64 µM-1 s-1.
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