The aromatic and charge pairs of the thin extracellular gate of the γ-aminobutyric acid transporter GAT-1 are differently impacted by mutation

Molecular basis of human GABA transporter 3 inhibition

γ-Aminobutyric acid (GABA) transporters (GATs) are sodium- and chloride-dependent transporters that mediate the reuptake of the inhibitory neurotransmitter GABA after its release from synaptic vesicles. GAT3 transports GABA from the synaptic cleft into astrocytes and modulates synaptic signaling. GAT3 has been implicated in various neurological disorders and neurodegenerative diseases, rendering it a therapeutically important drug target. To understand the mechanism of transport and inhibition, here we determine cryo-electron microscopy structures of human GAT3 in its apo form and in complex with the selective inhibitor SNAP-5114. Unexpectedly, we have discovered that SNAP-5114 acts as a noncompetitive inhibitor at GAT3. SNAP-5114 binds at the orthosteric substrate binding pocket of GAT3 in its inward-open conformation, in agreement with its noncompetitive inhibition of GABA transport. In the apo state, GAT3 also adopts an inward-open conformation with the orthosteric substrate binding pocket exposed to cytoplasm, while an extensive network of interactions closes the extracellular gate. The structures, complemented with mutagenesis and radioligand uptake assays, show that the increased orthosteric substrate binding pocket volume and bulky moieties of SNAP-5114, drive the selective inhibition of GAT3 over GAT1. Our structural and functional studies reveal the mechanism of selective inhibition of GAT3 and provide a framework for GAT3-targeted rational drug design.

Cryo-EM structure of GABA transporter 1 reveals substrate recognition and transport mechanism

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) is cleared from the synaptic cleft by the sodium- and chloride-coupled GABA transporter GAT1. Inhibition of GAT1 prolongs the GABAergic signaling at the synapse and is a strategy to treat certain forms of epilepsy. In this study, we present the cryo-electron microscopy structure of Rattus norvegicus GABA transporter 1 (rGAT1) at a resolution of 3.1 Å. The structure elucidation was facilitated by epitope transfer of a fragment-antigen binding (Fab) interaction site from the Drosophila dopamine transporter (dDAT) to rGAT1. The structure reveals rGAT1 in a cytosol-facing conformation, with a linear density in the primary binding site that accommodates a molecule of GABA, a displaced ion density proximal to Na site 1 and a bound chloride ion. A unique insertion in TM10 aids the formation of a compact, closed extracellular gate. Besides yielding mechanistic insights into ion and substrate recognition, our study will enable the rational design of specific antiepileptics.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

Substrate-induced unlocking of the inner gate determines the catalytic efficiency of a neurotransmitter:sodium symporter

Neurotransmitter:sodium symporters (NSSs) mediate reuptake of neurotransmitters from the synaptic cleft and are targets for several therapeutics and psychostimulants. The prokaryotic NSS homologue, LeuT, represents a principal structural model for Na(+)-coupled transport catalyzed by these proteins. Here, we used site-directed fluorescence quenching spectroscopy to identify in LeuT a substrate-induced conformational rearrangement at the inner gate conceivably leading to formation of a structural intermediate preceding transition to the inward-open conformation. The substrate-induced, Na(+)-dependent change required an intact primary substrate-binding site and involved increased water exposure of the cytoplasmic end of transmembrane segment 5. The findings were supported by simulations predicting disruption of an intracellular interaction network leading to a discrete rotation of transmembrane segment 5 and the adjacent intracellular loop 2. The magnitude of the spectroscopic response correlated inversely with the transport rate for different substrates, suggesting that stability of the intermediate represents an unrecognized rate-limiting barrier in the NSS transport mechanism.