Assessing structure, function and druggability of major inhibitory neurotransmitter gamma-aminobutyrate symporter subtypes.
Ambient level of gamma -aminobutyric acid (GABA), the major inhibitory neurotransmitter of the brain is mediated by neuronal and glial GABA transporters (GATs), members of the sodium and chloride ion-dependent solute carrier family. The neuronal GABA transporter subtype (GAT-1) has already been proven to be the target for the antiepileptic drug Tiagabine. However, druggability of glial GAT-2 and GAT-3 is yet to be established. Recent advances in structure elucidation of a bacterial orthologue leucine transporter in complex with different substrates substantiate homology modeling of human GATs (hGATs). These modeling studies can provide mechanistic clues for structure-based prediction of the potential of medicinal chemistry campaigns. A recently identified characteristic structural feature of the occluded conformation of hGATs is that similar extra- and intracellular gates are formed by middle-broken transmembrane helices TM1 and TM6. Binding crevice formed by unwound segments of broken helices facilitates symport of GABA with Na+ ion via fitting of GABA to TM1-bound Na+ closely inside. Favored accommodation of substrate inhibitors with high docking score predicts efficient inhibition of the neuronal hGAT-1 if the TM1-TM8 binding prerequisite for GABA was used. Docking, molecular dynamics and transport data indicate, that amino acids participating in substrate binding of the neuronal hGAT-1 and the glial hGAT-2 and hGAT-3 subtypes are different. By contrast, substrate binding crevices of hGAT-2 and hGAT-3 cannot be distinguished, avoiding sensible prediction of efficient selective substrate inhibitors. Glial subtypes might be specifically distinguished by interfering Zn2+ binding in the second extracellular loop of hGAT-3. Formation of the unique ring-like Na+-GABA complex in the occluded binding crevices anticipates family member symporters exploring chemiosmotic energy via reversible chemical coupling of Na+ ion.
Kardos
J
J
Department of Neurochemistry, Institute of Biomolecular Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri út 59-67, Budapest H-1025, Hungary.
Palló
A
A
Bencsura
A
A
Simon
A
A
D001426:Bacterial Proteins / Q000737:chemistry / Q000145:classification / Q000378:metabolism; D050485:GABA Plasma Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism; D058805:GABA Uptake Inhibitors; D006801:Humans; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D040681:Structural Homology, Protein; D013379:Substrate Specificity; D015032:Zinc / Q000737:chemistry
D001426:Bacterial Proteins; D050485:GABA Plasma Membrane Transport Proteins; D058805:GABA Uptake Inhibitors; D015032:Zinc