出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/02/06 23:27:10」(JST)
| Neurotransmitter-gated ion-channel transmembrane region | |||||||||
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Ligand-gated ion channel
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| Identifiers | |||||||||
| Symbol | Neur_chan_memb | ||||||||
| Pfam | PF02932 | ||||||||
| InterPro | IPR006029 | ||||||||
| PROSITE | PDOC00209 | ||||||||
| SCOP | 1cek | ||||||||
| SUPERFAMILY | 1cek | ||||||||
| TCDB | 1.A.9 | ||||||||
| OPM superfamily | 14 | ||||||||
| OPM protein | 2bg9 | ||||||||
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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),[1] such as a neurotransmitter.[2]
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 messengers), 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).[2][3]
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.[4] 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 none the less as a chemoreceptor.[5] This prokaryotic nAChR variant is know as the GLIC receptor, after the species in which it was identified; Gloeobacter Ligand-gated Ion Channel.
Vertebrate Anionic Cys-loop Receptors
| Type | Class | IUPHAR-recommended protein name[6] |
Gene | Previous names |
|---|---|---|---|---|
| GABAA | alpha | α1 α2 |
GABRA1 GABRA2 |
EJM, ECA4 |
| beta | β1 β2 |
GABRB1 GABRB2 |
|
|
| gamma | γ1 γ2 |
GABRG1 GABRG2 |
CAE2, ECA2, GEFSP3 | |
| delta | δ | GABRD | ||
| epsilon | ε | GABRE | ||
| pi | π | GABRP | ||
| theta | θ | GABRQ | ||
| rho | ρ1 ρ2 |
GABRR1 GABRR2 |
GABAC[7] | |
| Glycine (GlyR) |
alpha | α1 α2 |
GLRA1 GLRA2 |
STHE |
| beta | β | GLRB |
Vertebrate Cationic Cys-loop Receptors
| Type | Class | IUPHAR-recommended protein name [6] |
Gene | Previous names |
|---|---|---|---|---|
| Serotonin (5-HT) |
5-HT3 | 5-HT3A 5-HT3B |
HTR3A HTR3B |
5-HT3A 5-HT3B |
| Nicotinic acetylcholine (nAChR) |
alpha | α1 α2 |
CHRNA1 CHRNA2 |
ACHRA, ACHRD, CHRNA, CMS2A, FCCMS, SCCMS
|
| beta | β1 β2 |
CHRNB1 CHRNB2 |
CMS2A, SCCMS, ACHRB, CHRNB, CMS1D EFNL3, nAChRB2 |
|
| gamma | γ | CHRNG | ACHRG | |
| delta | δ | CHRND | ACHRD, CMS2A, FCCMS, SCCMS | |
| epsilon | ε | CHRNE | ACHRE, CMS1D, CMS1E, CMS2A, FCCMS, SCCMS | |
| Zinc-activated ion channel (ZAC) |
ZAC | ZACN | ZAC1, L2m LGICZ, LGICZ1 |
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.
| Type | Class | IUPHAR-recommended protein name [6] |
Gene | Previous names |
|---|---|---|---|---|
| AMPA | GluA | GluA1 GluA2 |
GRIA1 GRIA2 |
GLUA1, GluR1, GluRA, GluR-A, GluR-K1, HBGR1 GLUA2, GluR2, GluRB, GluR-B, GluR-K2, HBGR2 |
| Kainate | GluK | GluK1 GluK2 |
GRIK1 GRIK2 |
GLUK5, GluR5, GluR-5, EAA3 GLUK6, GluR6, GluR-6, EAA4 |
| NMDA | GluN | GluN1 NRL1A |
GRIN1 GRINL1A |
GLUN1, NMDA-R1, NR1, GluRξ1
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| GluN2A GluN2B |
GRIN2A GRIN2B |
GLUN2A, NMDA-R2A, NR2A, GluRε1 GLUN2B, NMDA-R2B, NR2B, hNR3, GluRε2 |
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| GluN3A GluN3B |
GRIN3A GRIN3B |
GLUN3A, NMDA-R3A, NMDAR-L, chi-1 GLU3B, NMDA-R3B |
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| ‘Orphan’ | (GluD) | GluD1 GluD2 |
GRID1 GRID2 |
GluRδ1 GluRδ2 |
ATP-gated channels open in response to binding the nucleotide ATP. They form trimers with two transmembrane helices per subunit and both the C and N termini on the intracellular side.
| Type | Class | IUPHAR-recommended protein name [6] |
Gene | Previous names |
|---|---|---|---|---|
| P2X | N/A | P2X1 P2X2 |
P2RX1 P2RX2 |
P2X1 P2X2 |
Phosphatidylinositol 4,5-bisphosphate (PIP2) binds to and directly agonizes Inward rectifying potassium channels(Kir).[8] 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.[9][10] In particular, the GABA and NMDA receptors are affected by anaesthetic agents at concentrations similar to those used in clinical anaesthesia.[11]
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"ligand-gated channel" at Dorland's Medical Dictionary
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| リンク元 | 「イオンチャネル型受容体」「イオンチャネル内蔵型受容体」 |
| 関連記事 | 「ionotropic」 |