User:RotogenRay/Receptors

Receptors and their associated ligands, agonists, antagonists, second messengers, etc. play an important role in facilitating all organisms' ability to respond to the environment. Likewise, categorization and contectualization of receptor families is tantamount to elucidate understanding of their function in the organisim. However, the challenge with categorization and current efforts to that end is determining a common denominator, or criteria for classification. With such a wide variety of known receptors and associated genetics, and many still unknown and currently researched cellular sensory and signal transduction mechanisms, and given the fact that different parts of the body tend to use certain receptor systems in different ways, there is a disconnect between the observation of receptor structure, location-specific function, and the larger effect on the whole organism to the presence of metabolic response modifiers, an excess or defficiet of ligands, receptor agonists, antagonists, cofactors and second messengers, etc.

Ion channel genes
(from Ion_channel_family

Ion channels and their associated receptors are most useful when fast neurotransmission or signal transduction is necessary. However many ion channels serve regulation or passive functions.

Human channels with six transmembrane helices in each subunit
Cation
 * MCOLN1; MCOLN2; MCOLN3;
 * PKD1L3;
 * TRPA1
 * TPCN1; TPCN2
 * TRPC1: TRPC3; TRPC4; TRPC5; TRPC6; TRPC7
 * TRPM1; TRPM2; TRPM3; TRPM4; TRPM5; TRPM6; TRPM7; TRPM8
 * TRPV1; TRPV2; TRPV3; TRPV4; TRPV5; TRPV6

Calcium
 * CACNA1A; CACNA1B; CACNA1C; CACNA1D; CACNA1E; CACNA1F; CACNA1G; CACNA1H; CACNA1I; CACNA1S
 * CATSPER1; CATSPER2; CATSPER3; CATSPER4
 * RYR1; RYR2; RYR3

Potassium
 * KCNA1; KCNA2; KCNA3; KCNA4; KCNA5; KCNA6; KCNA7; KCNA10
 * KCNB1; KCNB2
 * KCNC1; KCNC2; KCNC3; KCNC4
 * KCND1; KCND2; KCND3
 * KCNF1
 * KCNG1; KCNG2; KCNG3; KCNG4
 * KCNH1; KCNH2; KCNH3; KCNH4; KCNH5; KCNH6; KCNH7; KCNH8
 * KCNMA1
 * KCNQ1; KCNQ2; KCNQ3; KCNQ4; KCNQ5
 * KCNS1; KCNS2; KCNS3
 * KCNV1; KCNV2

Sodium
 * NALCN
 * SCN1A; SCN2A; SCN2A2; SCN3A; SCN4A; SCN5A; SCN7A; SCN8A; SCN9A; SCN10A; SCN11A
 * SLC9A10; SLC9A11

Proton
 * HVCN1

Cyclic nucleotide-gated
 * CNGA1; CNGA2; CNGA3; CNGA4
 * CNGB1; CNGB3
 * HCN1; HCN2; HCN3; HCN4
 * ITPR1; ITPR2; ITPR3

Human channels with two TM helices in each subunit, as in bacteria
Potassium
 * KCNK1; KCNK2; KCNK3; KCNK4; KCNK5; KCNK6; KCNK7; KCNK9; KCNK10; KCNK12; KCNK13; KCNK15; KCNK16; KCNK17; KCNK18
 * KCNN1; KCNN2; KCNN3; KCNN4;
 * KCNT1; KCNT2

Subfamily A1

 * Chemokine receptor
 * Chemokine (C-C motif) receptor 1 (, CKR1)
 * Chemokine (C-C motif) receptor 2 (, CKR2)
 * Chemokine (C-C motif) receptor 3 (, CKR3)
 * Chemokine (C-C motif) receptor 4 (, CKR4)
 * Chemokine (C-C motif) receptor 5 (, CKR5)
 * Chemokine (C-C motif) receptor 8 (, CKR8)
 * Chemokine (C-C motif) receptor-like 2 (, CKRX)
 * chemokine (C motif) receptor 1 (, CXC1)
 * chemokine (C-X3-C motif) receptor 1 (, C3X1)
 * GPR137B (, TM7SF1)

Subfamily A2

 * Chemokine receptor
 * Chemokine (C-C motif) receptor-like 1 (, CCR11)
 * Chemokine (C-C motif) receptor 6 (, CKR6)
 * Chemokine (C-C motif) receptor 7 (, CKR7)
 * Chemokine (C-C motif) receptor 9 (, CKR9)
 * Chemokine (C-C motif) receptor 10 (, CKRA)
 * CXC chemokine receptors
 * Chemokine (C-X-C motif) receptor 3
 * Chemokine (C-X-C motif) receptor 4 (, Fusin)
 * Chemokine (C-X-C motif) receptor 5
 * Chemokine (C-X-C motif) receptor 6 (, BONZO)
 * Chemokine (C-X-C motif) receptor 7 (, RDC1)
 * Interleukin-8 (IL8R)
 * IL8R-α (, CXCR1)
 * IL8R-β (, CXCR2)
 * Adrenomedullin receptor
 * Duffy blood group, chemokine receptor (, DUFF)
 * G Protein-coupled Receptor 30 (, CML2, GPCR estrogen receptor)

Subfamily A3

 * Angiotensin II receptor
 * Angiotensin II receptor, type 1 (, AG2S)
 * Angiotensin II receptor, type 2 (, AG22)
 * Apelin receptor (, APJ)
 * Bradykinin receptor
 * Bradykinin receptor B1 (, BRB1)
 * Bradykinin receptor B2 (, BRB2)
 * GPR15 (, GPRF)
 * GPR25

Subfamily A4

 * Opioid receptor
 * delta Opioid receptor (, OPRD)
 * kappa Opioid receptor (, OPRK)
 * mu Opioid receptor (, OPRM)
 * Nociceptin receptor (, OPRX)
 * Somatostatin receptor
 * Somatostatin receptor 1 (, SSR1)
 * Somatostatin receptor 2 (, SSR2)
 * Somatostatin receptor 3 (, SSR3)
 * Somatostatin receptor 4 (, SSR4)
 * Somatostatin receptor 5 (, SSR5)
 * GPCR neuropeptide receptor
 * Neuropeptides B/W receptor 1 (, GPR7)
 * Neuropeptides B/W receptor 2 (, GPR8)
 * GPR1 orphan receptor

Subfamily A5

 * Galanin receptor
 * Galanin receptor 1 (, GALR)
 * Galanin receptor 2 (, GALS)
 * Galanin receptor 3 (, GALT)
 * Cysteinyl leukotriene receptor
 * Cysteinyl leukotriene receptor 1
 * Cysteinyl leukotriene receptor 2
 * Leukotriene B4 receptor
 * Leukotriene B4 receptor (, P2Y7)
 * Leukotriene B4 receptor 2
 * Relaxin receptor
 * Relaxin/insulin-like family peptide receptor 1 (, LGR7)
 * Relaxin/insulin-like family peptide receptor 2 (, GPR106)
 * Relaxin/insulin-like family peptide receptor 3 (, SALPR)
 * Relaxin/insulin-like family peptide receptor 4 (, GPR100/GPR142)
 * KiSS1-derived peptide receptor (GPR54)
 * Melanin-concentrating hormone receptor 1 (, GPRO)
 * Urotensin-II receptor (, UR2R)

Subfamily A6

 * Cholecystokinin receptor
 * Cholecystokinin A receptor (, CCKR)
 * Cholecystokinin B receptor (, GASR)
 * Neuropeptide FF receptor
 * Neuropeptide FF receptor 1 (, FF1R)
 * Neuropeptide FF receptor 2 (, FF2R)
 * Orexin receptor
 * Hypocretin (orexin) receptor 1 (, OX1R)
 * Hypocretin (orexin) receptor 2 (, OX2R)
 * Vasopressin receptor
 * Arginine vasopressin receptor 1A (, V1AR)
 * Arginine vasopressin receptor 1B (, V1BR)
 * Arginine vasopressin receptor 2 (, V2R)
 * Gonadotrophin releasing hormone receptor (, GRHR)
 * Pyroglutamylated RFamide peptide receptor (, GPR103)
 * GPR22 (, GPRM)
 * GPR176 (, GPR)

Subfamily A7

 * Bombesin receptor
 * Bombesin-like receptor 3
 * Neuromedin B receptor
 * Gastrin-releasing peptide receptor
 * Endothelin receptor
 * Endothelin receptor type A (, ET1R)
 * Endothelin receptor type B (, ETBR)
 * GPR37 (, ETBR-LP2)
 * Neuromedin U receptor
 * Neuromedin U receptor 1
 * Neuromedin U receptor 2
 * Neurotensin receptor
 * Neurotensin receptor 1 (, NTR1)
 * Neurotensin receptor 2 (, NTR2)
 * Thyrotropin-releasing hormone receptor (, TRFR)
 * Growth hormone secretagogue receptor
 * GPR39
 * Motilin receptor (, GPR38)

Subfamily A8

 * Anaphylatoxin receptors
 * C3a receptor (, C3AR)
 * C5a receptor (, C5AR)
 * Chemokine-like receptor 1 (, CML1)
 * Formyl peptide receptor
 * Formyl peptide receptor 1 (, FMLR)
 * Formyl peptide receptor-like 1 (, FML2)
 * Formyl peptide receptor-like 2 (, FML1)
 * MAS1 oncogene
 * MAS1 (, MAS)
 * MAS1L (, MRG)
 * GPR1
 * GPR32 (, GPRW)
 * GPR44
 * GPR77 (, C5L2)

Subfamily A9

 * Melatonin receptor
 * Melatonin receptor 1A (, ML1A)
 * Melatonin receptor 1B (, ML1B)
 * Neurokinin receptor
 * Tachykinin receptor 1 (, NK1R)
 * Tachykinin receptor 2 (, NK2R)
 * Tachykinin receptor 3 (, NK3R)
 * Neuropeptide Y receptor
 * Neuropeptide Y receptor Y1 (, NY1R)
 * Neuropeptide Y receptor Y2 (, NY2R)
 * Pancreatic polypeptide receptor 1 (, NY4R)
 * Neuropeptide Y receptor Y5 (, NY5R)
 * Prolactin-releasing peptide receptor (PRLHR, GPRA)
 * Prokineticin receptor 1 (, GPR73)
 * Prokineticin receptor 2 (, PKR2)
 * GPR19 (, GPRJ)
 * GPR50 (, ML1X)
 * GPR75
 * GPR83 (, GPR72)

Subfamily A10

 * Glycoprotein hormone receptor
 * FSH-receptor
 * Luteinizing hormone/choriogonadotropin receptor (, LSHR)
 * Thyrotropin receptor
 * Leucine-rich repeat-containing G protein-coupled receptor 4 (, GPR48)
 * Leucine-rich repeat-containing G protein-coupled receptor 5 (, GPR49)
 * Leucine-rich repeat-containing G protein-coupled receptor 6

Subfamily A11

 * GPR40-related receptor
 * Free fatty acid receptor 1 (, GPR40)
 * Free fatty acid receptor 2 (, GPR43)
 * Free fatty acid receptor 3 (, GPR41)
 * GPR42 (, FFAR1L)
 * P2 purinoceptor
 * Purinergic receptor P2Y1
 * Purinergic receptor P2Y2
 * Purinergic receptor P2Y4
 * Purinergic receptor P2Y6
 * Purinergic receptor P2Y8
 * Purinergic receptor P2Y11
 * Hydroxycarboxylic acid receptor 1 (, GPR81)
 * Hydroxycarboxylic acid receptor 2, Niacin receptor 1 (, GPR109A)
 * Hydroxycarboxylic acid receptor 3, Niacin receptor 2 (, GPR109B, HM74)
 * GPR31 (, GPRV)
 * GPR82
 * Oxoglutarate (alpha-ketoglutarate) receptor 1 (, GPR80)
 * Succinate receptor 1 (, GPR91)

Subfamily A12

 * P2 purinoceptor
 * Purinergic receptor P2Y12
 * Purinergic receptor P2Y13 (, GPR86)
 * Purinergic receptor P2Y14 (, UDP-glucose receptor, KI01)
 * GPR34
 * GPR87
 * GPR171 (, H963)
 * Platelet-activating factor receptor (, PAFR)

Subfamily A13

 * Cannabinoid receptor
 * Cannabinoid receptor 1 (brain) (, CB1R)
 * Cannabinoid receptor 2 (macrophage) (, CB2R)
 * Lysophosphatidic acid receptor
 * Lysophosphatidic acid receptor 1
 * Lysophosphatidic acid receptor 2
 * Lysophosphatidic acid receptor 3
 * Sphingosine 1-phosphate receptor
 * Sphingosine 1-phosphate receptor 1
 * Sphingosine 1-phosphate receptor 2
 * Sphingosine 1-phosphate receptor 3
 * Sphingosine 1-phosphate receptor 4
 * Sphingosine 1-phosphate receptor 5
 * Melanocortin/ACTH receptor
 * Melanocortin 1 receptor (, MSHR)
 * Melanocortin 3 receptor
 * Melanocortin 4 receptor
 * Melanocortin 5 receptor
 * ACTH receptor, ACTR)
 * GPR3
 * GPR6
 * GPR12 (, GPRC)

Subfamily A14

 * Eicosanoid receptor
 * Prostaglandin D2 receptor (, PD2R)
 * Prostaglandin E1 receptor (, PE21)
 * Prostaglandin E2 receptor (, PE22)
 * Prostaglandin E3 receptor (, PE23)
 * Prostaglandin E4 receptor (, PE24)
 * Prostaglandin F receptor (, PF2R)
 * Prostaglandin I2 (prostacyclin) receptor (, PI2R)
 * Thromboxane A2 receptor (, TA2R)

Subfamily A15

 * Lysophosphatidic acid receptor
 * Lysophosphatidic acid receptor 4
 * Lysophosphatidic acid receptor 5
 * Lysophosphatidic acid receptor 6
 * P2 purinoceptor
 * Purinergic receptor P2Y10 (, P2Y10)
 * Protease-activated receptor
 * Coagulation factor II (thrombin) receptor-like 1 (, PAR2)
 * Coagulation factor II (thrombin) receptor-like 2 (, PAR3)
 * Coagulation factor II (thrombin) receptor-like 3 (, PAR4)
 * Epstein-Barr virus induced gene 2 (lymphocyte-specific G protein-coupled receptor)
 * Proton-sensing G protein-coupled receptors
 * GPR4
 * GPR65
 * GPR68
 * GPR132 (, G2A)
 * GPR17 (, GPRH)
 * GPR18 (, GPRI)
 * GPR20 (, GPRK)
 * GPR35
 * GPR55
 * Coagulation factor II receptor (, THRR)

Subfamily A16

 * Opsins
 * Rhodopsin (, OPSD)
 * Opsin 1 (cone pigments), short-wave-sensitive (color blindness, tritan) (, OPSB) (blue-sensitive opsin)
 * Opsin 1 (cone pigments), medium-wave-sensitive (color blindness, deutan) (, OPSG) (green-sensitive opsin)
 * Opsin 1 (cone pigments), long-wave-sensitive (color blindness, protan) (, OPSR) (red-sensitive opsin)
 * Opsin 3, Panopsin
 * Opsin 4, Melanopsin
 * Opsin 5 (, GPR136)
 * Retinal G protein coupled receptor
 * Retinal pigment epithelium-derived rhodopsin homolog (, OPSX) (visual pigment-like receptor opsin)

Subfamily A17

 * 5-Hydroxytryptamine (5-HT) receptor
 * 5-HT2A (, 5H2A)
 * 5-HT2B (, 5H2B)
 * 5-HT2C (, 5H2C)
 * 5-HT6 (, 5H6)
 * Adrenergic receptor
 * Alpha1A (, A1AA)
 * Alpha1B (, A1AB)
 * Alpha1D (, A1AD)
 * Alpha2A (, A2AA)
 * Alpha2B (, A2AB)
 * Alpha2C (, A2AC)
 * Beta1 (, B1AR)
 * Beta2 (, B2AR)
 * Beta3 (, B3AR)
 * Dopamine receptor
 * D1 (, DADR)
 * D2 (, D2DR)
 * D3 (, D3DR)
 * D4 (, D4DR)
 * D5 (, DBDR)
 * Trace amine receptor
 * TAAR1 (, TAR1)
 * TAAR2 (, GPR58)
 * TAAR3 (, GPR57)
 * TAAR5 (, PNR)
 * TAAR6 (, TAR4)
 * TAAR8 (, GPR102)
 * TAAR9 (, TAR3)
 * Histamine H2 receptor (, HH2R)

Subfamily A18

 * Histamine H1 receptor (, HH1R)
 * Histamine H3 receptor
 * Histamine H4 receptor
 * Adenosine receptor
 * A1 (, AA1R)
 * A2a (, AA2A)
 * A2b (, AA2B)
 * A3 (, AA3R)
 * Muscarinic acetylcholine receptor
 * M1 (, ACM1)
 * M2 (, ACM2)
 * M3 (, ACM3)
 * M4 (, ACM4)
 * M5 (, ACM5)
 * GPR21 (, GPRL)
 * GPR27
 * GPR45 (, PSP24)
 * GPR52
 * GPR61
 * GPR62
 * GPR63
 * GPR78
 * GPR84
 * GPR85
 * GPR88
 * GPR101
 * GPR161 (, RE2)
 * GPR173 (, SREB3)

Subfamily A19

 * 5-Hydroxytryptamine (5-HT) receptor
 * 5-HT1A (, 5H1A)
 * 5-HT1B (, 5H1B)
 * 5-HT1D (, 5H1D)
 * 5-HT1E (, 5H1E)
 * 5-HT1F (, 5H1F)
 * 5-HT4
 * 5-HT5A (, 5H5A)
 * 5-HT7 (, 5H7)

Unclassified

 * Olfactory receptor
 * Vomeronasal receptor
 * VN1R1
 * VN1R2
 * VN1R3
 * VN1R4
 * VN1R5

Secretin receptor family
Secretin family of 7 transmembrane receptors This family is known as Family B, the secretin-receptor family or family 2 of the G-protein-coupled receptors (GPCR). Many secretin receptors are regulated by peptide hormones from the glucagon hormone family.

The secretin-receptor family GPCRs include


 * vasoactive intestinal peptide receptors
 * secretin,
 * calcitonin and
 * parathyroid hormone/parathyroid hormone-related peptides.

These receptors activate adenylyl cyclase and the phosphatidyl-inositol-calcium pathway.

The receptors in this family have 7 transmembrane helices, like rhodopsin-like GPCRs. However, there is no significant sequence identity between these two GPCR families and the secretin-receptor family has its own characteristic 7TM signature.

The secretin-receptor family GPCRs exist in many animal species and have not been identified in plants, fungi or prokaryotes. Three distinct sub-families (B1-B3) are recognized.

Subfamily B1
Subfamily B1 contains classical hormone receptors, such as receptors for secretin and glucagon, that are all involved in cAMP-mediated signalling pathways.


 * Pituitary adenylate cyclase-activating polypeptide type 1 receptor
 * PACAPR (ADCYAP1R1)
 * Calcitonin receptor
 * CALCR
 * Corticotropin-releasing hormone receptor
 * CRHR1; CRHR2
 * Glucose-dependent insulinotropic polypeptide receptor/Gastric inhibitory polypeptide receptor
 * GIPR
 * Glucagon receptor
 * GCGR
 * Glucagon receptor-related
 * GLP1R; GLP2R;
 * Growth hormone releasing hormone receptor
 * GHRHR
 * Parathyroid hormone receptor
 * PTHR1; PTHR2
 * Secretin receptor
 * SCTR
 * Vasoactive intestinal peptide receptor
 * VIPR1; VIPR2

Subfamily B2
Subfamily B2 contains receptors with long extracellular N-termini, such as the leukocyte cell-surface antigen CD97; calcium-independent receptors for latrotoxin (such as, and brain-specific angiogenesis inhibitor receptors (such as ) amongst others.


 * Brain-specific angiogenesis inhibitor
 * BAI1; BAI2; BAI3
 * CD97 antigen
 * CD97
 * EMR hormone receptor
 * CELSR1; CELSR2; CELSR3; EMR1; EMR2; EMR3; EMR4
 * GPR56 orphan receptor
 * GPR56; GPR64; GPR97; GPR110; GPR111; GPR112; GPR113; GPR114; GPR115; GPR123; GPR125; GPR126; GPR128; GPR133; GPR144; GPR157
 * Latrophilin receptor
 * ELTD1; LPHN1; LPHN2; LPHN3

Subfamily B3
Subfamily B3 includes Methuselah and other Drosophila proteins. Other than the typical seven-transmembrane region, characteristic structural features include an amino-terminal extracellular domain involved in ligand binding, and an intracellular loop (IC3) required for specific G-protein coupling.


 * Diuretic hormone receptor

Unclassified subfamilies

 * Ig-hepta receptor
 * GPR116

Unclassified members
HCTR-5; HCTR-6; KPG 006; KPG 008

Metabotropic Glutamate/Pheromone receptors
Eight different types of mGluRs, labeled mGluR1 to mGluR8 ( to ), are divided into groups I, II, and III. Receptor types are grouped based on receptor structure and physiological activity. The mGluRs are further divided into subtypes, such as mGluR7a and mGluR7b.

Fungal mating pheromone receptors
This receptor resembles many 7 transmembrane G-protiens found in humans, though it is know known at this time if human genetics code for this type of receptor

Fungal pheromone mating factor receptors form a distinct family of G-protein-coupled receptors.

Mating factor receptors STE2 and STE3 are integral membrane proteins that may be involved in the response to mating factors on the cell membrane. The amino acid sequences of both receptors contain high proportions of hydrophobic residues grouped into 7 domains, in a manner reminiscent of the rhodopsins and other receptors believed to interact with G-proteins.

Cyclic AMP receptors
Cyclic AMP receptors  from slime molds are a distinct family of G-protein coupled receptors. These receptors control development in Dictyostelium discoideum.

The cyclic AMP receptors coordinate aggregation of individual cells into a multicellular organism, and regulate the expression of a large number of developmentally-regulated genes. The amino acid sequences of the receptors contain high proportions of hydrophobic residues grouped into 7 domains, in a manner reminiscent of the rhodopsins and other receptors believed to interact with G-proteins. However, while a similar 3D framework has been proposed to account for this, there is no significant sequence similarity between these families: the cAMP receptors thus bear their own unique '7TM' signature.

Smoothened
Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog signaling pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine.

Cellular localization plays an essential role in the function of SMO. Stimulation of the patched receptor by the sonic hedgehog ligand leads to translocation of SMO to the primary cilium. Furthermore, SMO that is mutated in the domain required for ciliary localisation cannot contribute to pathway activation. SMO has also been shown to bind the kinesin motor protein Costal-2 and play a role in the localization of the Ci (Cubitus interruptus transcription factor) complex.

SMO can function as an oncogene. Activating SMO mutations can lead to unregulated activation of the hedgehog pathway and cancer.

Agonists

 * Smoothened agonist (SAG)
 * Purmorphamine

Antagonists

 * Cyclopamine
 * Itraconazole
 * Vismodegib (Erivedge), a smoothened receptor inhibitor for the treatment of basal-cell carcinoma, being investigated for the treatment of other types of cancer

Frizzled
Frizzled is a family of G protein-coupled receptor proteins that serves as receptors in the Wnt signaling pathway and other signaling pathways. When activated, Frizzled leads to activation of Dishevelled in the cytosol.

Species distribution
Frizzled proteins and the genes that encode them have been identified in an array of animals, from sponges to humans.

Function
Frizzled proteins also play key roles in governing cell polarity, embryonic development, formation of neural synapses, cell proliferation, and many other processes in developing and adult organisms. These processes occur as a result of one of three signaling pathways. These include the canonical Wnt/β-catenin pathway, Wnt/calcium pathway, and planar cell polarity (PCP) pathway. Mutations in the human frizzled-4 receptor have been linked to familial exudative vitreoretinopathy, a rare disease affecting the retina at the back of the eye, and the vitreous, the clear fluid inside the eye.

The frizzled (fz) locus of Drosophila coordinates the cytoskeletons of epidermal cells, producing a parallel array of cuticular hairs and bristles. In fz mutants, the orientation of individual hairs with respect both to their neighbours and to the organism as a whole is altered. In the wild-type wing, all hairs point towards the distal tip.

In the developing wing, Fz has 2 functions: it is required for the proximal-distal transmission of an intracellular polarity signal; and it is required for cells to respond to the polarity signal. Fz produces an mRNA that encodes an integral membrane protein with 7 putative transmembrane (TM) domains. This protein should contain both extracellular and cytoplasmic domains, which could function in the transmission and interpretation of polarity information. This signature is usually found downstream of the Fz domain

Cysteine-rich domain
Frizzled proteins include cysteine-rich domain that is conserved in diverse proteins, including several receptor tyrosine kinases. In Drosophila melanogaster, members of the Frizzled family of tissue-polarity genes encode proteins that appear to function as cell-surface receptors for Wnts. The Frizzled genes belong to the seven transmembrane class of receptors (7TMR) and have in their extracellular region a cysteine-rich domain that has been implicated as the Wnt binding domain. Sequence similarity between the cysteine-rich domain of Frizzled and several receptor tyrosine kinases, which have roles in development, include the muscle-specific receptor tyrosine kinase (MuSK), the neuronal-specific kinase (NSK2), and ROR1 and ROR2. The structure of this domain is known and is composed mainly of alpha helices. This domain contains ten conserved cysteines that form five disulphide bridges.

Group members
The following is a list of the ten known human frizzled receptors:
 * Frizzled-1
 * Frizzled-2
 * Frizzled-3
 * Frizzled-4
 * Frizzled-5
 * Frizzled-6
 * Frizzled-7
 * Frizzled-8
 * Frizzled-9
 * Frizzled-10

Kinase/cell surface receptors
Receptor tyrosine kinases (RTK)s are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins. Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer. Receptor tyrosine kinases are part of the larger family of protein tyrosine kinases, encompassing the receptor tyrosine kinase proteins which contain a transmembrane domain, as well as the nonreceptor tyrosine kinases which do not possess transmembrane domains.

Receptor tyrosine kinase (RTK) in this article is also known as tyrosine receptor kinase (TRK) or tyrosine kinase receptor (TKR) depending on the permutation. However, Trk receptor is the name in the context of neurobiology.

Receptor tyrosine kinase classes
Approximately 20 different RTK classes have been identified.


 * 1) RTK class I (EGF receptor family) (ErbB family)
 * 2) RTK class II (Insulin receptor family)
 * 3) RTK class III (PDGF receptor family)
 * 4) RTK class IV (FGF receptor family)
 * 5) RTK class V (VEGF receptors family)
 * 6) RTK class VI (HGF receptor family)
 * 7) RTK class VII (Trk receptor family)
 * 8) RTK class VIII (Eph receptor family)
 * 9) RTK class IX (AXL receptor family)
 * 10) RTK class X (LTK receptor family)
 * 11) RTK class XI (TIE receptor family)
 * 12) RTK class XII (ROR receptor family)
 * 13) RTK class XIII (DDR receptor family)
 * 14) RTK class XIV (RET receptor family)
 * 15) RTK class XV (KLG receptor family)
 * 16) RTK class XVI (RYK receptor family)
 * 17) RTK class XVII (MuSK receptor family)

Immune receptors
The main receptors in the immune system are pattern recognition receptors (PRRs), Toll-like receptors (TLRs), killer activated and killer inhibitor receptors (KARs and KIRs), complement receptors, Fc receptors, B cell receptors and T cell receptors.

Nuclear receptors]]
The following is a list of the 48 known human nuclear receptors categorized according to sequence homology.

Typical Classifications of receptor families
(gleanings from Receptor_(biochemistry))

The structures of receptors are very diverse and can broadly be classified into the following categories:

(ionotropic receptors)
These associated receptors are typical targets of fast neurotransmission or used for rapid signal transduction

Vertebrate Anionic Cys-loop Receptors

Vertebrate Cationic Cys-loop Receptors

Other ionotropic receptors:


 * AMPA receptor
 * Glycine receptor
 * GRIN1
 * GRIN3A
 * GRIN3B
 * GRINL1A
 * GRINL1B
 * HTR3A
 * HTR3B
 * HTR3C
 * HTR3D
 * HTR3E
 * Kainate receptor
 * NMDA receptor
 * P2X purinoreceptor

in other (nonhuman) organisms
 * GLIC

Activation of these receptor results in changes in ion movement across the membrane. They have a hetero structure. Each subunit consists of the extracellular ligand-binding domain and a transmembrane domain where the transmembrane domain in turn includes four transmembrane alpha helixes. The ligand binding cavities are located at the interface between the subunits.

G protein-coupled receptors (metabotropic)
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:


 * 1) The visual sense: The opsins use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. Rhodopsin, for example, uses the conversion of 11-cis-retinal to all-trans-retinal for this purpose
 * 2) The gustatory sense (taste): GPCRs in taste cells mediate release of gustducin in response to bitter- and sweet-tasting substances.
 * 3) The sense of smell: Receptors of the olfactory epithelium bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
 * 4) Behavioral and mood regulation: Receptors in the mammalian brain bind several different neurotransmitters, including serotonin, dopamine, GABA, and glutamate
 * 5) Regulation of immune system activity and inflammation: Chemokine receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as histamine receptors bind inflammatory mediators and engage target cell types in the inflammatory response
 * 6) Autonomic nervous system transmission: Both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
 * 7) Cell density sensing: A novel GPCR role in regulating cell density sensing.
 * 8) Homeostasis modulation (e.g., water balance).
 * 9) Involved in growth and metastasis of some types of tumors.

They are the largest family of receptors including the receptors for several hormones and slow transmitters e.g. dopamine, metabotropic glutamate, olfactory sense, etc. Composed of seven transmembrane alpha helices. The loops connecting the alpha helices form extracellular and intracellular domains. The binding site for larger peptidic ligands is usually located in the extracellular domain whereas the binding site for smaller non-peptidic ligands is often located between the seven alpha helices and one extracellular loop. These receptors are coupled to different intracellular effector systems via G-proteins.

kinase linked and related receptors
These receptors are composed of an extracellular domain containing the ligand binding site and an intracellular domain, often with enzymatic function, linked by a single transmembrane alpha helix. e.g. the insulin receptor.

Immune receptors
Immune receptors function to recognize pathogens and induce or inhibit certain immune responses

Nuclear receptors
While they are called Nuclear receptors, these are actually located in the cytosol and migrate to the nucleus after binding with their ligands. They are composed of a C-terminal ligand binding region, a core DNA-binding domain (DBD) and an N-terminal domain that contains the AF1(activation function 1) region. The core region has two zinc fingers that are responsible for recognising the DNA sequences specific to this receptor. The N-terminal interacts with other cellular transcription factors in a ligand independent manner and depending on these interactions it can modify the binding/activity of the receptor. Steroid and thyroid hormone receptors are examples of such receptors.