Functional Defects in the External and Internal Thin Gates of the γ-Aminobutyric Acid (GABA) Transporter GAT-1 Can Compensate Each Other

The GABA transporter GAT-1 belongs to the neurotransmitter:sodium:symporters which are crucial for synaptic transmission. GAT-1 mediates electrogenic transport of GABA together with sodium and chloride. Structure-function studies indicate that the bacterial homologue LeuT, which possess extra- and intracellular thin gates, is an excellent model for this class of neurotransmitter transporters. We recently showed that a conserved aspartate residue of GAT-1, Asp-451, whose LeuT equivalent participates in its thin extracellular gate, is functionally irreplaceable in GAT-1. Only the D451E mutant exhibited residual transport activity but with an elevated apparent sodium affinity as a consequence of an increased proportion of outward-facing transporters. Because during transport the opening and closing of external and internal gates should be tightly coupled, we have addressed the question of whether mutations of the intracellular thin gate residues Arg-44 and Asp-410 can compensate for the effects of their extracellular counterparts. Mutation of Asp-410 to glutamate resulted in impaired transport activity and a reduced apparent affinity for sodium. However, the transport activity of the double mutant D410E/D451E was increased by approximately 10-fold of that of each of the single mutants. Similar compensatory effects were also seen when other combinations of intra- and extracellular thin gate mutants were analyzed. Moreover, the introduction of D410E into the D451E background resulted in lower apparent sodium affinity than that of D451E alone. Our results indicate that a functional interaction of the external and internal gates of GAT-1 is essential for transport.