Ligand-gated ion channel
|Neurotransmitter-gated ion-channel transmembrane region|
Ligand-gated ion channel
Ligand-gated ion channels (LGICs) are a group of transmembrane ion channel proteins which open to allow ions such as Na+, K+, Ca2+, or Cl− to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.
These proteins are typically composed of at least two different domains: a transmembrane domain which includes the ion pore, and an extracellular domain which includes the ligand binding location (an allosteric binding site). This modularity has enabled a 'divide and conquer' approach to finding the structure of the proteins (crystallising each domain separately). The function of such receptors located at synapses is to convert the chemical signal of presynaptically released neurotransmitter directly and very quickly into a postsynaptic electrical signal. Many LGICs are additionally modulated by allosteric ligands, by channel blockers, ions, or the membrane potential. LGICs are classified into three superfamilies which lack evolutionary relationship: Cys-loop receptors, Ionotropic glutamate receptors and ATP-gated channels.
LGICs can be contrasted with metabotropic receptors (which use second messenger activated ion channels), voltage-gated ion channels (which open and close depending on membrane potential), and stretch-activated ion channels (which open and close depending on mechanical deformation of the cell membrane).
The cys-loop receptors are named after a characteristic loop formed by a disulfide bond between two cysteine residues in the N terminal extracellular domain. They are subdivided with respect to the type of ion that they conduct (anionic or cationic) and further into families defined by the endogenous ligand. They are usually pentameric with each subunit containing 4 transmembrane helices constituting the transmembrane domain, and a beta sheet sandwich type, extracellular, N terminal, ligand binding domain. Some also contain an intracellular domain like shown in the image.
The prototypic ligand-gated ion channel is the nicotinic acetylcholine receptor. It consists of a pentamer of protein subunits (typically ααβγδ), with two binding sites for acetylcholine (one at the interface of each alpha subunit). When the acetylcholine binds it alters the receptor's configuration (twists the T2 helices which moves the leucine residues, which block the pore, out of the channel pathway) and causes the constriction in the pore of approximately 3 angstroms to widen to approximately 8 angstroms so that ions can pass through. This pore allows Na+ ions to flow down their electrochemical gradient into the cell. With a sufficient number of channels opening at once, the inward flow of positive charges carried by Na+ ions depolarizes the postsynaptic membrane sufficiently to initiate an action potential.
While single-cell organisms like bacteria would have little apparent need for the transmission of an action potential, a bacterial homologue to an LGIC has been identified, hypothesized to act nonetheless as a chemoreceptor. This prokaryotic nAChR variant is known as the GLIC receptor, after the species in which it was identified; Gloeobacter Ligand-gated Ion Channel.
Vertebrate Anionic Cys-loop Receptors
|CAE2, ECA2, GEFSP3|
Vertebrate Cationic Cys-loop Receptors
protein name 
|ACHRA, ACHRD, CHRNA, CMS2A, FCCMS, SCCMS
|CMS2A, SCCMS, ACHRB, CHRNB, CMS1D
|delta||δ||CHRND||ACHRD, CMS2A, FCCMS, SCCMS|
|epsilon||ε||CHRNE||ACHRE, CMS1D, CMS1E, CMS2A, FCCMS, SCCMS|
|Zinc-activated ion channel
|ZAC||ZACN||ZAC1, L2m LGICZ, LGICZ1|
Ionotropic glutamate receptors (iGluR)
The ionotropic glutamate receptors bind the neurotransmitter glutamate. They form tetramers with each subunit consisting of an extracellular amino terminal domain (ATD, which is involved tetramer assembly), an extracellular ligand binding domain (LBD, which binds glutamate), and a transmembrane domain (TMD, which forms the ion channel). The transmembrane domain of each subunit contains three transmembrane helices as well as a half membrane helix with a reentrant loop. The structure of the protein starts with the ATD at the N terminus followed by the first half of the LBD which is interrupted by helix 1,2 and 3 of the TMD before continuing with the final half of the LBD and then finishing with helix 4 of the TMD at the C terminus. This means there are three links between the TMD and the extracellular domains. Each subunit of the tetramer has a binding site for glutamate formed by the two LBD sections forming a clamshell like shape. Only two of these sites in the tetramer need to be occupied to open the ion channel. The pore is mainly formed by the half helix 2 in a way which resembles an inverted potassium channel.
protein name 
|GLUA1, GluR1, GluRA, GluR-A, GluR-K1, HBGR1
GLUA2, GluR2, GluRB, GluR-B, GluR-K2, HBGR2
GLUA3, GluR3, GluRC, GluR-C, GluR-K3
GLUA4, GluR4, GluRD, GluR-D
|GLUK5, GluR5, GluR-5, EAA3
GLUK6, GluR6, GluR-6, EAA4
GLUK7, GluR7, GluR-7, EAA5
GLUK1, KA1, KA-1, EAA1
GLUK2, KA2, KA-2, EAA2
|GLUN1, NMDA-R1, NR1, GluRξ1
|GLUN2A, NMDA-R2A, NR2A, GluRε1
GLUN2B, NMDA-R2B, NR2B, hNR3, GluRε2
GLUN2C, NMDA-R2C, NR2C, GluRε3
GLUN2D, NMDA-R2D, NR2D, GluRε4
|GLUN3A, NMDA-R3A, NMDAR-L, chi-1
protein name 
Phosphatidylinositol 4,5-bisphosphate (PIP2) binds to and directly agonizes Inward rectifying potassium channels(Kir). PIP2 is a plasma membrane lipid and its definitive role in gating ion channels was only recently demonstrated by X-ray crystallography.
Ligand-gated ion channels are likely to be the major site at which anaesthetic agents and ethanol have their effects, although unequivocal evidence of this is yet to be established. In particular, the GABA and NMDA receptors are affected by anaesthetic agents at concentrations similar to those used in clinical anaesthesia.
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- Ligand-Gated Ion Channel database at European Bioinformatics Institute. Verified availability April 11, 2007.
- "Revised Recommendations for Nomenclature of Ligand-Gated Ion Channels". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.