Journal:Acta Cryst D:S205979832000772X

Structural evidence for mono- and di-carboxylates binding at pharmacologically relevant extracellular sites of a pentameric ligand gated ion channel

Zaineb Fourati, Ludovic Saugueta, and Marc Delarue ^[1]

Molecular Tour
Pentameric ligand-gated ion channels (pLGICs) are key receptors mediating electrochemical signaling in the nervous system. These receptors, that include the well-known GABAA and acetylcholine receptors, are activated by agonist/neurotransmitter binding to a specific orthosteric site in the extracellular domain, leading to channel opening and ion permeation inside the cell. They are organised as pentamers of identical or homologous subunits that respond in a concerted manner to agonist binding in the orthosteric site. Beyond the activation process, these receptors also respond to various chemical stimuli such as alcohol as well as some drugs that modulate the receptor function. Because these molecules bind and act at a distinct site from the orthosteric agonist site, they are classified as allosteric modulators. This allosteric modulation is a key feature of pLGIC pharmacological response. A comprehensive characterization of the allosteric modulation sites within pLGICs is thus a prerequisite toward the rational conception of efficient drugs targeting these receptors. In this study, we identify and characterize two modulation sites in the extracellular domain of GLIC, a bacterial pLGIC exhibiting similar pharmacological properties as its eukaryotic counterparts, especially cationic ones. In particular, we show that these pockets can accommodate different mono- and di- acetic acid derivatives, some of which are known to be allosteric inhibitors of GLIC. One of them, called the intersubunit site, partially overlaps the orthosteric (agonist) site; the other one is located behind the first one, is facing the interior of the vestibule and is called intrasubunit site.

The X-ray structure of GLIC in complex with succinate (PDB code 6hjz):

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• . The GLIC receptor is represented with white traces. Succinate binds 5 intrasubunit (cyan spacefill) and 5 intersubunit (hotpink spacefill) sites in the extracellular domain of GLIC.
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These extracellular sites are of great interest because they can accommodate drugs distinct from the common hydrophobic general anaesthetics that find their binding site through diffusion in the plasma membrane. Moreover, one of these sites, namely the intrasubunit one, is conserved in other cationic pLGICs such as the serotonin receptor, allowing a more specific targeting of cationic receptors. (PDB code 4pir). See also the static image below:

Succinate is shown as purple sticks

This latter site is likely to be a key pharmacological site and thus an important target for rational drug design against some neurological disorders involving human pLGICs.

PDB references: Xray structure of GLIC in complex with fumarate 6hj3; Xray structure of GLIC in complex with glutarate 6hja; Xray structure of GLIC in complex with malonate 6hjb; Xray structure of GLIC in complex with crotonate 6hji; Xray structure of GLIC in complex with succinate 6hjz; X-ray structure of GLIC in complex with propionate 6hpp.

References

1. ↑ Fourati Z, Sauguet L, Delarue M. Structural evidence for the binding of monocarboxylates and dicarboxylates at pharmacologically relevant extracellular sites of a pentameric ligand-gated ion channel. Acta Crystallogr D Struct Biol. 2020 Jul 1;76(Pt 7):668-675. PMID:32627739 doi:10.1107/S205979832000772X