Review3D structure and allosteric modulation of the transmembrane domain of pentameric ligand-gated ion channels
Introduction
Pentameric ligand-gated ion channels (pLGICs) are transmembrane protein complexes that mediate chemical communication between cells in both the central and peripheral nervous systems (Taly et al., 2009; Miller and Smart, 2010). These transmembrane receptors are grouped into distinct families, named according to neurotransmitter pharmacology. In vertebrates, nicotinic acetylcholine (ACh), serotonin (5HT3R), and zinc (ZAC) receptors are linked to cation-selective channels, while γ‑aminobutyric acid (GABAA and GABAA-ρ) and Glycine receptors are linked to an anionic chloride ion-selective channel. In invertebrates, γ-aminobutyric acid, serotonin, glutamate, histamine and proton-gated channels have also been identified. More than 40 distinct pLGIC subunits exist in humans, including numerous subunit types within each of the neurotransmitter receptor families. Within a family, these subunits combine to form a variety of both homomeric and heteromeric complexes, each with different functional and pharmacological profiles.
The primary role of most pLGICs is to convert the binding of a neurotransmitter into the opening of a transmembrane ion channel leading either to intracellular excitation or inhibition, depending on the ionic selectivity of the channel. These two primary functions, ligand-binding and ion channel conductance, are located in distinct structural domains; an extracellular domain (ECD), which carries two to five neurotransmitter binding sites per receptor, and a transmembrane domain (TMD), which carries a single ion channel along the symmetry axis of the protein (Corringer et al., 2000, Smit et al., 2001, Brejc et al., 2001, Miyazawa et al., 2003, Unwin, 2005). X-ray structural data for homologs of the nicotinic ACh receptor ECD have been collected for nearly a decade (Kalamida et al., 2007, Rucktooa et al., 2009) thus providing atomic-level insight into the nature of pLGIC-ligand interactions. In contrast, X-ray structural data for the TMD has been lacking until recently, thus precluding atomic-level investigation of the structural features responsible for both channel conductance/gating and the mechanisms by which pLGICs interact with surrounding lipids, such as cholesterol, and other hydrophobic allosteric effectors, notably general anesthetics, alcohols, and neurosteroids. This short review presents our current knowledge of the structural architecture of the TMD taking into account recent advances derived from the discovery (Tasneem et al., 2005, Bocquet et al., 2007) and subsequent X-ray structure determination (Hilf and Dutzler, 2008, Hilf and Dutzler, 2009, Bocquet et al., 2009) of bacterial members of the pLGICs super-family. In light of this new structural insight, we review and discuss the role of the TMD in the allosteric modulation of pLGICs, focusing on two examples; protein–lipid interactions at the nicotinic ACh receptor and anesthetic action at anion-selective pLGICs.
Section snippets
General structure of pentameric ligand-gated ion channels
To date, structural data have been collected for 1) the Torpedo nicotinic ACh receptor, which is easily purified in large quantities thus allowing detailed analysis using electron microscopy and chemical labeling approaches (Unwin, 2005), 2) the acetylcholine binding proteins (AChBPs), which are water-soluble pentamers homologous to the ECD of the nicotinic ACh receptor. Structures of AChBPs have been solved by X-ray crystallography in the presence and absence of a series of nicotinic agonists
Structure of the transmembrane domain
Accumulated biochemical and structural data provide a consistent picture of the TMD structure with the ion channel located along the central symmetry axis of the pentamer bordered by five M2 segments, one from each of the five subunits. In each case, M2 adopts an α-helical conformation, as shown by affinity labeling (Giraudat et al., 1986), the substituted cystein accessibility method (SCAM) (Akabas et al., 1994), electron microscopy (Unwin, 2005), and X-ray crystallography (Hilf and Dutzler,
Lipids as allosteric modulators of nicotinic ACh receptor function
Nicotinic ACh receptors are found throughout the central and peripheral nervous systems, as well as abundantly in the electric ray, Torpedo. In addition to their primordial role in neuromuscular and motor autonomous transmission, nicotinic ACh receptors are involved in central neurological functions, including control of voluntary motion, memory and attention, sleep and wakefulness, reward (nicotine) and pain. Seventeen genes encoding for nicotinic ACh receptor subunits have been identified in
Allosteric modulation of anionic pLGICs
GABA, the major inhibitory neurotransmitter in the brain, mediates both phasic (synaptic) and tonic (extra-synaptic) inhibition via GABAA receptors (Olsen and Sieghart, 2009), while Glycine receptors are expressed predominantly at the periphery (Breitinger and Becker, 2002, Betz and Laube, 2006). Due to their widespread expression in the brain, GABAA receptors play a major role in virtually all brain physiological functions and are the targets of numerous classes of drugs. Nineteen genes coding
Conclusions
The above discussion highlights the complex and diverse effects of allosteric modulators that act within the TMD of pLGICs. Despite the diversity of effects and sites of action, the data also reveal two distinct, but related themes, with regards to the sites of allosteric modulation within the TMD:
First, many of hydrophobic/amphipathic allosteric regulators that act on the TMD bind to pockets located at interfaces between subunits. Some act at interfaces located between the subunits within the
Acknowledgements
This work was supported by the Commission of the European Communities (Neurocypres project), the Louis D. Foundation from the Institut de France, by the Canadian Institute of Health Research, and by a travel grant from the French Government to JEB. We thank Amanda Huynh for assistance with the figures.
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