Transmembrane Domain 8 of the γ-Aminobutyric Acid Transporter GAT-1 Lines a Cytoplasmic Accessibility Pathway into Its Binding Pocket

Abstract

GAT-1 is a sodium-and chloride-coupled ␥-aminobutyric acid (GABA) transporter, which fulfills an essential role in the synaptic transmission by this neurotransmitter. Cysteine-399 is the major site of inhibition of GAT-1 by membrane-permeant sulfhydryl reagents. This cysteine residue was previously thought to reside on a cytoplasmic loop connecting transmembrane domains (TMs) 8 and 9. However, the crystal structure of LeuT, a bacterial homologue of the mammalian neurotransmitter:sodium symporters, revealed that the residue corresponding to Cys-399 is in fact located in the middle of TM 8. This residue is located to the cytoplasmic side of Asp-395 and Ser-396, whose side chains are thought to ligand one of the two cotransported sodium ions. To determine how the sulfhydryl reagents approach cysteine-399, a cysteine scan of all 35 residues of TM 8 was performed. Sulfhydryl reagents inhibited transport when a cysteine residue was present at either of the positions 399, 402, 406, and 410. SKF-89976A and other non-transportable analogues, which are expected to lock the transporter in a conformation facing the extracellular medium, protected against the sulfhydryl modification at positions 399, 402, and 406. Such a protection was not seen by GABA itself, which actually modestly potentiated the modification at positions 399 and 402. Our results point to an ␣-helical stripe on TM8 lining an aqueous access pathway from the cytoplasm into the binding pocket, which gets occluded in the conformation of the transporter where the binding pocket is exposed to the extracellular medium.

SLC6 Neurotransmitter Transporters: Structure, Function, and Regulation

“…Hence, the tiagabine binding site overlaps with both the S1 and S2 sites, which might clarify the mixedtype mechanism of GABA uptake inhibition found in early pharmacological characterization of tiagabine (Braestrup et al, 1990). The binding site for the competitive GAT1 inhibitor SKF89976A was recently suggested to overlap the substrate binding site in GAT1, thereby stabilizing an outward-facing conformation of the transporter Dodd and Christie, 2007;Rosenberg and Kanner, 2008;Ben-Yona and Kanner, 2009), substantiating that the competitive inhibitor binding site in GATs overlaps the S1 substrate binding site as found for SLC6 monoamine transporters.…”

“…SCAM analysis of the GABA and glycine transporters has been carried out to a lesser extent, although several key residues for transporter function of GAT1 have been identified by mutagenesis studies (Kanner et al, 1994;Keshet et al, 1995;Mager et al, 1996;Bismuth et al, 1997;MacAulay et al, 2001;Zomot and Kanner, 2003;Zhou et al, 2004Zhou et al, , 2006Zomot et al, 2005;Rosenberg and Kanner, 2008;Ben-Yona and Kanner, 2009). For analysis of proximity relationships between specific residues, the primary approach has been to take advantage of the structural constraints required for construction of binding sites for divalent cations in proteins.…”

“…These include the use of voltage-clamp fluorometry to show movement of TM1 during transport (Meinild et al, 2009) and SCAM analysis to confirm that TM8 lines the intracellular pathway of GAT1 (Ben- Yona and Kanner, 2009). Furthermore, SCAM analysis of TM5 in SERT shows that the intracellular region of this domain becomes exposed to the intracellular side when SERT is locked in an inward-facing conformation; consistent with predictions that this region lines the intracellular pathway that becomes accessible in the predicted inward-facing state (Zhang and Rudnick, 2006).…”

Two Na+ Sites Control Conformational Change in a Neurotransmitter Transporter Homolog

“…Na ϩ , in the absence of substrate amino acids, shifts the distribution between these states in favor of outwardfacing (inward-closed) forms, an effect first observed with Tyt1 (1) and subsequently with LeuT (2, 3), GABA transporter (12), and SERT (13). Addition of leucine, a poor substrate for LeuT, stabilizes an intermediate conformation likely resembling crystal structures with substrate bound in an outward-occluded conformation (3,5), whereas a good substrate, such as alanine in LeuT or tyrosine in Tyt1, overcomes the conformational restriction imposed by Na ϩ , dramatically increasing the prevalence of inward-open conformations (1,4).…”

Amino Acid Residues Involved in the Substrate Specificity of TauT/SLC6A6 for Taurine and γ-Aminobutyric Acid

“…According to the previous reports, the polar side chain of Glu406 was suggested to make only a minor contribution to the substrate recognition of TauT since the polar side chain of Thr400 in hGAT1, corresponding to Glu406 of TauT, has been reported to play only a minor role in the function of hGAT1. 38,39) Thus, the higher affinity for GABA supports the involvement of Glu406 in determining the volume of the substrate pocket since it is thought that the Glu-to-Cys substitution at position 406 reduced the side chain bulkiness to improve the accessibility of GABA to the substrate pocket. These findings support the hypothesis that the lower affinity of E406C for taurine was caused by an increased volume of the substrate pocket, that weakened the interaction between taurine and the putative TM8 of TauT.…”