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Mutation K448E in the external loop 5 of rat GABA transporter rGAT1 induces pH sensitivity and alters substrate interactions
Author(s) -
Forlani Greta,
Bossi Elena,
Ghirardelli Roberta,
Giovannardi Stefano,
Binda Francesca,
Bonadiman Lorena,
Ielmini Lorena,
Peres Antonio
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.0479c.xd
Subject(s) - chemistry , transporter , biophysics , gaba transporter , reversal potential , voltage clamp , membrane potential , protonation , steady state (chemistry) , xenopus , substrate (aquarium) , patch clamp , biochemistry , biology , ion , receptor , organic chemistry , gene , ecology
1 The effect of the mutation K448E in the rat GABA transporter rGAT1 was studied using heterologous expression in Xenopus oocytes and voltage clamp. 2 At neutral pH, the transport‐associated current vs. voltage ( I–V ) relationship of the mutated transporter was different from wild‐type, and the pre‐steady‐state currents were shifted towards more positive potentials. The mutated transporter showed an increased apparent affinity for Na + (e.g. 62 vs. 152 m m at −60 mV), while the opposite was true for GABA (e.g. 20 vs. 13 μ m at −60 mV). 3 In both isoforms changes in [Na + ] o shifted the voltage dependence of the pre‐steady‐state and of the transport‐associated currents by similar amounts. 4 In the K448E form, the moved charge and the relaxation time constant were shifted by increasing pH towards positive potentials. The transport‐associated current of the mutated transporter was strongly reduced by alkalinization, while acidification slightly decreased and distorted the shape of the I–V curve. Accordingly, uptake of [ 3 H]GABA was strongly reduced in K448E at pH 9.0. The GABA apparent affinity of the mutated transporter was reduced by alkalinization, while acidification had the opposite result. 5 These observations suggest that protonation of negatively charged residues may regulate the Na + concentration in the proximity of the transporter. Calculation of the unidirectional rate constants for charge movement shows that, in the K448E form, the inward rate constant is increased at alkaline pH, while the outward rate constant does not change, in agreement with an effect due to mass action law. 6 A possible explanation for the complex effect of pH on the transport‐associated current may be found by combining changes in local [Na + ] o with a direct action of pH on GABA concentration or affinity. Our results support the idea that the extracellular loop 5 may participate to form a vestibule to which sodium ions must have access before proceeding to the steps involving charge movement.