extracellular domain (ECD) of Cys-loop neurotransmitter-gated ion channels (also known as ligand-gated ion channel (LGIC)); This superfamily contains the extracellular domain (ECD) of Cys-loop neurotransmitter-gated ion channels, which include nicotinic acetylcholine receptor (nAChR), serotonin 5-hydroxytryptamine receptor (5-HT3), type-A gamma-aminobutyric acid receptor (GABAAR) and glycine receptor (GlyR). These ligand-gated ion channels (LGICs) are found across metazoans and have close homologs in bacteria. They are vital for communication throughout the nervous system. GABAAR and GlyR are anionic channels, both mediating fast inhibitory synaptic transmission. Cl- ions are selectively conducted through the GABAAR receptor pore, resulting in hyperpolarization of the neuron. nAChR is a non-selective cation channel that is permeable to Na+ and K+, and some subunit combinations are also permeable to Ca2+. Na+ enters and K+ exits to allow net flow of positively charged ions inward. 5-HT3, a cation-selective channel, binds serotonin and is permeable to Na+, K+, and Ca2+. It mediates neuronal depolarization and excitation within the central and peripheral nervous systems. These ligand-gated chloride channels are critical not only for maintaining appropriate neuronal activity, but have long been important therapeutic targets: benzodiazepines, barbiturates, some intravenous and volatile anaesthetics, alcohol, strychnine, picrotoxin, and ivermectin all derive their biological activity from acting on the inhibitory half of the Cys-loop receptor family. The ECD contains the ligand binding sites for these receptors.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|....*....
gi 767983205   1 MVLMTPTASMDDFTPSGEWDIVALPGRRTVNPQDPSYVDVTYDFIIKRK 49
Cdd:cd19027  130 MVLKTPTASMDDFTPSGEWDIVALPGRRTVNPLDPTYVDVTYDFIIKRK 178

transmembrane domain of nicotinic acetylcholine receptor (nAChR); This family contains transmembrane (TM) domain of the nicotinic acetylcholine receptor (nAChR). The transmembrane region consists of four transmembrane-spanning alpha-helical segments (M1-M4) that are linked by loops. The intracellular loop that links M1 and M2 determines the ion selectivity of the channel. nAChR is found in high concentrations at the nerve-muscle synapse, where it mediates fast chemical transmission of electrical signals in response to the endogenous neurotransmitter acetylcholine (ACh) released from the nerve terminal into the synaptic cleft. Thus far, seventeen nAChR subunits have been identified, including ten alpha subunits, four beta subunits and one gamma, delta, and epsilon subunit each, all found on the cell membrane that non-selectively conducts cations (Na+, K+, Ca++). These nAChR subunits combine in several different ways to form functional nAChR subtypes which are broadly categorized as either muscle subtype located at the neuromuscular junction or neuronal subtype that are found on neurons and on other cell types throughout the body. The muscle type of nAChRs are formed by the alpha1, beta1, gamma, delta, and epsilon subunits while the neuronal type are composed of nine alpha subunits and three beta subunits, which combine in various permutations and combinations to form functional receptors. Among various subtypes of neuronal nAChRs, the homomeric alpha7 and the heteromeric alpha4beta2 receptors are the main subtypes widely distributed in the brain and implicated in the pathophysiology of neurodevelopmental disorders such as schizophrenia and autism and neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease. Among subtypes of muscle nAChRs, the heteromeric subunits (alpha1)2, beta, gamma, and delta in fetal muscle, and the gamma subunit replaced by epsilon in adult muscle have been implicated in congenital myasthenic syndromes and multiple pterygium syndromes due to various mutations. This family also includes alpha- and beta-like nAChRs found in protostomia.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767983205  51 LFYTINLIIPCVLTTLLAILVFYLPSDCGEKMTLCISVLLALTFFLLLISKIVPPTSLDVPLIGKYLMFTMVLVTFSIVT 130
Cdd:cd19064    1 LFYTVNLIIPCVLISFLTVLVFYLPADSGEKITLSISVLLALTVFLLLIAEIIPPTSLVVPLIGKYLLFTMILVTLSIIV 80

                         90
                 ....*....|
gi 767983205 131 SVCVLNVHHR 140
Cdd:cd19064   81 TVIVLNVHHR 90

extracellular domain of nicotinic acetylcholine receptor subunit beta 4 (CHRNB4); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit beta 4 (beta4), encoded by the CHRNB4 gene and ubiquitously expressed on lung epithelial cells. The cluster of human neuronal nicotinic receptor gene CHRNA5-CHRNA3-CHRNB4 is related to drug-related behaviors and the development of lung cancer. One of the most broadly expressed subtype is the alpha-3 beta-4 nAChR, also known as the ganglion-type nicotinic receptor, located in the autonomic ganglia and adrenal medulla, where activation yields post- and/or pre-synaptic excitation, mainly by increased Na+ and K+ permeability. Beta4 forms heteromeric nAchRs to modulate receptor affinity for nicotine, but the exact pentameric stochiometry of alpha3beta4 receptor is not known; functional assemblies with varying subunit stoichiometries are possible.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|....*....
gi 767983205   1 MVLMTPTASMDDFTPSGEWDIVALPGRRTVNPQDPSYVDVTYDFIIKRK 49
Cdd:cd19027  130 MVLKTPTASMDDFTPSGEWDIVALPGRRTVNPLDPTYVDVTYDFIIKRK 178

transmembrane domain of nicotinic acetylcholine receptor (nAChR); This family contains transmembrane (TM) domain of the nicotinic acetylcholine receptor (nAChR). The transmembrane region consists of four transmembrane-spanning alpha-helical segments (M1-M4) that are linked by loops. The intracellular loop that links M1 and M2 determines the ion selectivity of the channel. nAChR is found in high concentrations at the nerve-muscle synapse, where it mediates fast chemical transmission of electrical signals in response to the endogenous neurotransmitter acetylcholine (ACh) released from the nerve terminal into the synaptic cleft. Thus far, seventeen nAChR subunits have been identified, including ten alpha subunits, four beta subunits and one gamma, delta, and epsilon subunit each, all found on the cell membrane that non-selectively conducts cations (Na+, K+, Ca++). These nAChR subunits combine in several different ways to form functional nAChR subtypes which are broadly categorized as either muscle subtype located at the neuromuscular junction or neuronal subtype that are found on neurons and on other cell types throughout the body. The muscle type of nAChRs are formed by the alpha1, beta1, gamma, delta, and epsilon subunits while the neuronal type are composed of nine alpha subunits and three beta subunits, which combine in various permutations and combinations to form functional receptors. Among various subtypes of neuronal nAChRs, the homomeric alpha7 and the heteromeric alpha4beta2 receptors are the main subtypes widely distributed in the brain and implicated in the pathophysiology of neurodevelopmental disorders such as schizophrenia and autism and neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease. Among subtypes of muscle nAChRs, the heteromeric subunits (alpha1)2, beta, gamma, and delta in fetal muscle, and the gamma subunit replaced by epsilon in adult muscle have been implicated in congenital myasthenic syndromes and multiple pterygium syndromes due to various mutations. This family also includes alpha- and beta-like nAChRs found in protostomia.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767983205  51 LFYTINLIIPCVLTTLLAILVFYLPSDCGEKMTLCISVLLALTFFLLLISKIVPPTSLDVPLIGKYLMFTMVLVTFSIVT 130
Cdd:cd19064    1 LFYTVNLIIPCVLISFLTVLVFYLPADSGEKITLSISVLLALTVFLLLIAEIIPPTSLVVPLIGKYLLFTMILVTLSIIV 80

                         90
                 ....*....|
gi 767983205 131 SVCVLNVHHR 140
Cdd:cd19064   81 TVIVLNVHHR 90

extracellular domain of nicotinic acetylcholine receptor subunit beta 4 (CHRNB4); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit beta 4 (beta4), encoded by the CHRNB4 gene and ubiquitously expressed on lung epithelial cells. The cluster of human neuronal nicotinic receptor gene CHRNA5-CHRNA3-CHRNB4 is related to drug-related behaviors and the development of lung cancer. One of the most broadly expressed subtype is the alpha-3 beta-4 nAChR, also known as the ganglion-type nicotinic receptor, located in the autonomic ganglia and adrenal medulla, where activation yields post- and/or pre-synaptic excitation, mainly by increased Na+ and K+ permeability. Beta4 forms heteromeric nAchRs to modulate receptor affinity for nicotine, but the exact pentameric stochiometry of alpha3beta4 receptor is not known; functional assemblies with varying subunit stoichiometries are possible.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|....*....
gi 767983205   1 MVLMTPTASMDDFTPSGEWDIVALPGRRTVNPQDPSYVDVTYDFIIKRK 49
Cdd:cd19027  130 MVLKTPTASMDDFTPSGEWDIVALPGRRTVNPLDPTYVDVTYDFIIKRK 178

extracellular domain of nicotinic acetylcholine receptor subunit beta 2 (CHRNB2); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit beta 2 (beta2), encoded by the CHRNB2 gene. The most abundant nicotinic subtype in the human brain is alpha4beta2 receptor which is known to assemble in two functional subunit stoichiometries, (alpha4)3(beta2)2 and (alpha4)2(beta2)3, the latter having a much higher affinity for both acetylcholine and nicotine. This subtype is implicated in the pathophysiology of neurodevelopmental disorders such as schizophrenia and autism, and neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. Thus, pharmacological ligands targeting this subtype have been researched and developed as a treatment approach implicated in these diseases. They include agonists such as varenicline and cytisine used as smoking cessation aids, as well as positive allosteric modulators (PAMs) such as desformylflustrabromine (dFBr), which are ligands that bind to nicotinic receptors at sites other than the orthosteric site where acetylcholine binds, and are not able to act as agonists on nAChR.

                         10        20        30        40        50
                 ....*....|....*....|....*....|....*....|....*....|
gi 767983205   1 MVLMTPTASMDDFTPSGEWDIVALPGRRTVNPQDPSYVDVTYDFIIKRKP 50
Cdd:cd19025  155 LVLRSDVASLDDFTPSGEWDIVALPGRRNENPNDSTYVDITYDFIIRRKP 204

transmembrane domain of 5-hydroxytryptamine 3 (5-HT3) receptor; This family contains transmembrane (TM) domain of the serotonin 5-HT3 receptors. The transmembrane region consists of four transmembrane-spanning alpha-helical segments (M1-M4) that are linked by loops. The intracellular loop that links M1 and M2 determines the ion selectivity of the channel. The 5-HT3 channel is cation-selective and mediates neuronal depolarization and excitation within the central and peripheral nervous systems. Like other ligand gated ion channels, the 5-HT3 receptor consists of five subunits arranged around a central ion conducting pore, which is permeable to Na+, K+, and Ca2+ ions. Binding of the neurotransmitter 5-hydroxytryptamine (serotonin) to the 5-HT3 receptor opens the channel, which then leads to an excitatory response in neurons, and the rapidly activating, desensitizing, inward current is predominantly carried by Na+ and K+ ions. This receptor is most closely related by homology to the nicotinic acetylcholine receptor (nAChR). Five subunits have been identified for this family: 5-HT3A, 5-HT3B, 5-HT3C, 5-HT3D, and 5-HT3E, encoded by HTR3A-E genes. Only 5-HT3A subunits are able to form functional homomeric receptors, whereas the 5-HT3B, C, D, and E subunits form heteromeric receptors with 5-HT3A. Different receptor subtypes are important mediators of nausea and vomiting during chemotherapy, pregnancy, and following surgery, while some contribute to neuro-gastroenterologic disorders such irritable bowel syndrome (IBS) and eating disorders as well as co-morbid psychiatric conditions. 5-HT3 receptor antagonists are established treatments for emesis and IBS, and are beneficial in the treatment of psychiatric diseases.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767983205  51 LFYTINLIIPCVLTTLLAILVFYLPSDCGEKMTLCISVLLALTFFLLLISKIVPPTSLDVPLIGKYLMFTMVLVTFSIVT 130
Cdd:cd19063    1 LLYVVNLLIPSIFLMLVDLASFYLPPNSGERLGFKVTLLLGYSVFLLILNDLLPATAIGTPLIGVYFVVCLALMVLSLLE 80

                 ....*....
gi 767983205 131 SVCVLNVHH 139
Cdd:cd19063   81 TILIVKLLH 89

extracellular domain of nicotinic acetylcholine receptor subunit beta 1 (CHRNB1); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit beta 1 (beta1), encoded by the CHRNB1 gene. It is a muscle type subunit found predominantly in the neuromuscular junction (NMJ), but also in other tissues and cell lines such as adrenal glands, carcinomas, brain, and lung. Simultaneous mRNA and protein expression of beta1 nAChR subunit is present in human placenta and skeletal muscle. The beta1 nAChR subunit forms a heteropentamer with either (alpha1)2, gamma and delta subunits in embryonic type or (alpha1)2, epsilon and delta subunits in adult type receptors. nAChRs containing beta1 subunits have been attributed to efficient clustering and anchoring of the receptors to the cytoskeleton which is important for formation of synapses in the NMJ. Mutations in the transmembrane domain region of this gene are associated with slow-channel congenital myasthenic syndrome (CMS).

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|
gi 767983205  11 DDFTPSGEWDIVALPGRRTVNPQDPSYVDVTYDFIIKRKP 50
Cdd:cd19024  174 NTFIENGQWSIEHKPSRKNQLPGDPLYEDITFYLIIRRKP 213

extracellular domain of nicotinic acetylcholine receptor subunit delta (CHRND); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit delta (delta), encoded by the CHRND gene and found in the muscle. Delta nAChR subunit forms a heteropentamer with either (alpha1)2, beta and gamma subunits in embryonic type or (alpha1)2, beta and epsilon subunits in adult type receptors. Defects in this gene are a cause of multiple pterygium syndrome lethal type (MUPSL), congenital myasthenic syndrome slow-channel type (SCCMS), and congenital myasthenic syndrome fast-channel type (FCCMS). The slow-channel congenital myasthenic syndromes (SCCMS) are caused by prolonged opening episodes of AChR due to dominant gain-of-function mutations in the transmembrane regions of the heteropentamer. These mutations produce an increase in the channel opening rate, a decrease in the channel closing rate, or an increase in the affinity of ACh for the AChR, resulting in the stabilization of the open state or the destabilization of the closed state of the AChR.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|..
gi 767983205  13 FTPSGEWDIVALPGRRTVN----PQDPSYVDVTYDFIIKRKP 50
Cdd:cd19028  180 FTENGEWEIVHKPAKKNIDpskpPESTKYQDITFYLIIKRKP 221

transmembrane domain of Cys-loop neurotransmitter-gated ion channels; This superfamily contains the transmembrane domain of Cys-loop neurotransmitter-gated ion channels, which include nicotinic acetylcholine receptor (nAChR), serotonin 5-hydroxytryptamine receptor (5-HT3), type-A gamma-aminobutyric acid receptor (GABAAR), and glycine receptor (GlyR). These ligand-gated ion channels (LGICs) are found across metazoans and have close homologs in bacteria. They are vital for communication throughout the nervous system where the sign of synaptic connections (excitatory or inhibitory) is determined by the charge of the ions that flow through these channels. In general, channels that conduct positive ions are excitatory, whereas channels that conduct negative ions are inhibitory. The transmembrane region consists of four transmembrane-spanning alpha-helical segments (M1-M4) that are linked by loops. The intracellular loop that links M1 and M2 determines the ion selectivity of the channel. GABAAR and GlyR are anionic channels, both mediating fast inhibitory synaptic transmission. Cl- ions are selectively conducted through the GABAAR receptor pore, resulting in hyperpolarization of the neuron. nAChR is a non-selective cation channel that is permeable to Na+ and K+, and some subunit combinations are also permeable to Ca2+. Na+ enters and K+ exits to allow net flow of positively charged ions inward. 5-HT3, a cation-selective channel, binds serotonin and is permeable to Na+, K+, and Ca2+. It mediates neuronal depolarization and excitation within the central and peripheral nervous systems. These ligand-gated chloride channels are critical not only for maintaining appropriate neuronal activity, but have long been important therapeutic targets: benzodiazepines, barbiturates, some intravenous and volatile anaesthetics, alcohol, strychnine, picrotoxin, and ivermectin all derive their biological activity from acting on the inhibitory half of the Cys-loop receptor family.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767983205  52 FYTINLIIPCVLTTLLAILVFYLPSDCGEKMTLCISVLLALTFFLLLISKIVPPTSLdVPLIGKYLMFTMVLVTFSIVTS 131
Cdd:cd03559    1 YYAVSLLLPSILIMVVSWVGFWLPPDSGERVSFKITLLLTYSVFLIIVSDTLPATPY-TPLIDVYFVVCMALLFIALLET 79

                         90
                 ....*....|.
gi 767983205 132 VCVLNVHHRSP 142
Cdd:cd03559   80 IFIVRLVHKQD 90

extracellular domain of neuronal acetylcholine receptor subunit alpha 7 (CHRNA7); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit alpha 7 (alpha7), encoded by the CHRNA7 gene. Alpha7 subunits form a homo-pentameric channel, displays marked permeability to calcium ions and is a major component of brain nicotinic receptors that are blocked by, and highly sensitive to, alpha-bungarotoxin. This protein is ubiquitously expressed in both the central nervous system and in the periphery, in several tissues, including adrenal, small intestine, testis, and stomach. CHRNA7 is located in a region identified as a major susceptibility locus for juvenile myoclonic epilepsy and a chromosomal location involved in the genetic transmission of schizophrenia. It is also genetically linked to other disorders with cognitive deficits, including bipolar disorder, ADHD, Alzheimer's disease, and Rett syndrome. An evolutionarily recent partial duplication of CHRNA7 on chromosome 15 forms a new gene, CHRFAM7A or FAM7A, which encodes the protein dup-alpha7. This protein assembles with alpha7 subunits, results in fewer binding sites and is a dominant negative regulator of alpha7 nAChR function.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|....*
gi 767983205   8 ASMDDFTPSGEWDIVALPGRRTVNPQD---PSYVDVTYDFIIKRK 49
Cdd:cd19020  136 ADISGYISNGEWDLVGVPGKRNEKFYEcckEPYPDVTFTVTMRRR 180

extracellular domain of nicotinic acetylcholine receptor subunit gamma (CHRNG); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit gamma (gamma), encoded by the CHRNG gene expressed during early fetal development, and replaced by the epsilon subunit in the adult. The gamma subunit forms a heteropentamer with (alpha1)2, beta, and delta and plays a role in neuromuscular organogenesis and ligand binding. Disruption of gamma subunit expression prevents the correct localization of the receptor in cell membranes. Mutations in CHRNG may cause the non-lethal Escobar variant (EVMPS) and lethal form (LMPS) of multiple pterygium syndrome (MPS), a condition characterized by prenatal growth failure with pterygium and akinesia leading to muscle weakness and severe congenital contractures, as well as scoliosis. Muscle-type acetylcholine receptor is the major antigen in the autoimmune disease myasthenia gravis.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|....
gi 767983205  11 DDFTPSGEWDIVALPGRRTVNPQ----DPSYVDVTYDFIIKRKP 50
Cdd:cd19029  150 EAFTENGEWAIRHRPAKKILNPQlpkdDLGYQEIVFYLIIQRKP 193

extracellular domain of nicotinic acetylcholine receptor subunit alpha 5 (CHRNA5); This subfamily contains the extracellular domain of nicotinic acetylcholine receptor subunit alpha 5 (alpha5), encoded by the CHRNA5 gene, which is part of the CHRNA5/A3/B4 gene cluster. Polymorphisms in this gene cluster have been identified as risk factors for nicotine dependence, lung cancer, chronic obstructive pulmonary disease, alcoholism, and peripheral arterial disease. A loss-of-function polymorphism in CHRNA5 is strongly linked to nicotine abuse and schizophrenia; the alpha5 nAChR subunit is strategically situated in the prefrontal cortex (PFC), where a loss-of-function in this subunit may contribute to cognitive disruptions in both disorders. Alpha5 forms heteropentamers with alpha3beta2 or alpha3beta4 nAChRs which increases the calcium permeability of the resulting receptors possibly playing significant roles in the initiation of ACh-induced signaling cascades under normal and pathological condition. Acetylcholine (ACh) release and signaling via alpha4/beta2 nAChR subunits plays a central role in the control of attention, but a subset of these oligomers also contains alpha5 subunit. A strong association is seen between a CHRNA5 polymorphism and the risk of lung cancer, especially in smokers.

                         10        20        30        40
                 ....*....|....*....|....*....|....*....|..
gi 767983205  12 DFTPSGEWDIVALPGRRTvNPQDPS--YVDVTYDFIIKRKPL 51
Cdd:cd19018  167 DFFDNGEWEIVSATGSKG-NRTDGCcwYPFITYSFIIRRLPL 207