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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.

Genetic Classification[edit]

Ion channel genes[edit]

(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[edit]






Cyclic nucleotide-gated

Human channels with two TM helices in each subunit, as in bacteria[edit]


G coupled protien receptors[edit]

Rhodopsin-like GCPRs[edit]

Subfamily A1[edit]
Subfamily A2[edit]
Subfamily A3[edit]
Subfamily A4[edit]
Subfamily A5[edit]
Subfamily A6[edit]
Subfamily A7[edit]
Subfamily A8[edit]
Subfamily A9[edit]
Subfamily A10[edit]
Subfamily A11[edit]
Subfamily A12[edit]
Subfamily A13[edit]
Subfamily A14[edit]
Subfamily A15[edit]
Subfamily A16[edit]
Subfamily A17[edit]
Subfamily A18[edit]
Subfamily A19[edit]

Secretin receptor family[edit]

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

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

The receptors in this family have 7 transmembrane helices,[3] 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.[4]

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[edit]

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

Subfamily B2[edit]

Subfamily B2 contains receptors with long extracellular N-termini, such as the leukocyte cell-surface antigen CD97; calcium-independent receptors for latrotoxin (such as O94910, and brain-specific angiogenesis inhibitor receptors (such as O14514) amongst others.

Subfamily B3[edit]

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.

Unclassified subfamilies[edit]
Unclassified members[edit]

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

Metabotropic Glutamate/Pheromone receptors[edit]

Eight different types of mGluRs, labeled mGluR1 to mGluR8 (GRM1 to GRM8), are divided into groups I, II, and III.[5][6][7][8] Receptor types are grouped based on receptor structure and physiological activity.[9] The mGluRs are further divided into subtypes, such as mGluR7a and mGluR7b.

Overview of glutamate receptors
Family Receptors [10][11] Gene Mechanism[10] Function Agonists & Activators Antagonists Synapse site
Group I mGluR1 GRM1 Gq, ↑Na+,[7]K+,[7]glutamate[8] mainly postsynaptic[14]
mGluR5 GRM5 Gq, ↑Na+,[7]K+,[7]glutamate[8]
Group II mGluR2 GRM2 Gi/G0 mainly presynaptic[14]
mGluR3 GRM3 Gi/G0
Group III mGluR4 GRM4 Gi/G0 mainly presynaptic[14]
mGluR6 GRM6 Gi/G0
mGluR7 GRM7 Gi/G0
mGluR8 GRM8 Gi/G0

Fungal mating pheromone receptors[edit]

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 STE2 GPCR
PDB 1pjd EBI.jpg
Structure of a Peptide Segment of the 6th Transmembrane Domain of the Saccharomyces cerevisiae alpha-Factor Receptor.[16]
Symbol STE2
Pfam PF02116
InterPro IPR000366
SCOP 1pjd
OPM superfamily 6
OPM protein 2k9p

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.[17][18][19] 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[edit]

Slime mold cyclic AMP receptor
Symbol Dicty_CAR
Pfam PF05462
Pfam clan CL0192
InterPro IPR000848

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.[20][21][22] 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.

See also[edit]


Smoothened, frizzled class receptor
Available structures
PDB Ortholog search: PDBe, RCSB
Symbols SMO ; FZD11; Gx; SMOH
External IDs OMIM601500 MGI108075 HomoloGene4115 IUPHAR: 239 ChEMBL: 5971 GeneCards: SMO Gene
RNA expression pattern
PBB GE SMO 218629 at tn.png
More reference expression data
Species Human Mouse
Entrez 6608 319757
Ensembl ENSG00000128602 ENSMUSG00000001761
UniProt Q99835 P56726
RefSeq (mRNA) NM_005631 NM_176996
RefSeq (protein) NP_005622 NP_795970
Location (UCSC) Chr 7:
129.19 – 129.21 Mb
Chr 6:
29.74 – 29.76 Mb
PubMed search [1] [2]

Smoothened is a G protein-coupled receptor[23] protein encoded by the SMO gene of the hedgehog signaling pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine.[24]

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.[25] 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.[26]

SMO can function as an oncogene. Activating SMO mutations can lead to unregulated activation of the hedgehog pathway and cancer.[27]

Overview of signal transduction pathways involved in apoptosis.


Frizzled/Smoothened family membrane region
Crystal structure of the cysteine-rich domain of mouse frizzled 8 (mfz8)[28]
Symbol Frizzled
Pfam PF01534
Pfam clan GPCR_A
InterPro IPR000539
TCDB 9.A.14
OPM superfamily 6
OPM protein 4jkv

Frizzled is a family of G protein-coupled receptor proteins[29] 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[edit]

Frizzled proteins and the genes that encode them have been identified in an array of animals, from sponges to humans.[30]


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.[30] 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.[31][32] 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.[32]

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.[32] This signature is usually found downstream of the Fz domain (InterPro: IPR000024)

=Cysteine-rich domain[edit]

Frizzled proteins include cysteine-rich domain that is conserved in diverse proteins, including several receptor tyrosine kinases.[33][34][35] 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[edit]

The following is a list of the ten known human frizzled receptors:

Overview of signal transduction pathways involved in apoptosis.

Kinase/cell surface receptors[edit]

receptor protein-tyrosine kinase
VEGF receptors.png
EC number
IntEnz IntEnz view
ExPASy NiceZyme view
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO
Symbol ?
Pfam PF07714
OPM superfamily 207
OPM protein 2k1k

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.[36] 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.[37] 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.[38]

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[edit]

Approximately 20 different RTK classes have been identified.[39]

  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[edit]

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.[40]

Receptor Bind to [40] Function[40]
pattern recognition receptors (PRRs)
(e.g. TLRs, NLRs)
pathogen-associated molecular patterns (PAMP) Mediate cytokine production --> inflammation --> destroying pathogen
killer activated and killer inhibitor receptors (KARs and KIRs) Avails NK cells to identify abnormal host cells (KAR) or inhibit inappropriate host cell destruction (KIR)
complement receptors complement proteins on e.g. microbes Allow phagocytic and B cells to recognize microbes and immune complexes
Fc receptors epitope-antibody complexes Stimulate phagocytosis
B cell receptors epitopes B cell differentiation into plasma cells and proliferation
T cell receptors linear epitopes bound to MHC Activate T cells
Cytokine receptors cytokines regulation and co-ordination of immune responses

[[Nuclear receptors]][edit]

The following is a list of the 48 known human nuclear receptors[41] categorized according to sequence homology.[42][43]

Subfamily Group Member
NRNC Symbol[42] Abbreviation Name Gene Ligand(s)
1 Thyroid Hormone Receptor-like A Thyroid hormone receptor NR1A1 TRα Thyroid hormone receptor-α THRA thyroid hormone
NR1A2 TRβ Thyroid hormone receptor-β THRB
B Retinoic acid receptor NR1B1 RARα Retinoic acid receptor-α RARA vitamin A and related compounds
NR1B2 RARβ Retinoic acid receptor-β RARB
NR1B3 RARγ Retinoic acid receptor-γ RARG
C Peroxisome proliferator-activated receptor NR1C1 PPARα Peroxisome proliferator-activated receptor-α PPARA fatty acids, prostaglandins
NR1C2 PPAR-β/δ Peroxisome proliferator-activated receptor-β/δ PPARD
NR1C3 PPARγ Peroxisome proliferator-activated receptor-γ PPARG
D Rev-ErbA NR1D1 Rev-ErbAα Rev-ErbAα NR1D1 heme
NR1D2 Rev-ErbAβ Rev-ErbAα NR1D2
F RAR-related orphan receptor NR1F1 RORα RAR-related orphan receptor-α RORA cholesterol, ATRA
NR1F2 RORβ RAR-related orphan receptor-β RORB
NR1F3 RORγ RAR-related orphan receptor-γ RORC
H Liver X receptor-like NR1H3 LXRα Liver X receptor-α NR1H3 oxysterols
NR1H2 LXRβ Liver X receptor-β NR1H2
NR1H4 FXR Farnesoid X receptor NR1H4
I Vitamin D receptor-like NR1I1 VDR Vitamin D receptor VDR vitamin D
NR1I2 PXR Pregnane X receptor NR1I2 xenobiotics
NR1I3 CAR Constitutive androstane receptor NR1I3 androstane
X NRs with two DNA binding domains[44][45] NR1X1 2DBD-NRα
2 Retinoid X Receptor-like A Hepatocyte nuclear factor-4 NR2A1 HNF4α Hepatocyte nuclear factor-4-α HNF4A fatty acids
NR2A2 HNF4γ Hepatocyte nuclear factor-4-γ HNF4G
B Retinoid X receptor NR2B1 RXRα Retinoid X receptor-α RXRA retinoids
NR2B2 RXRβ Retinoid X receptor-β RXRB
NR2B3 RXRγ Retinoid X receptor-γ RXRG
C Testicular receptor NR2C1 TR2 Testicular receptor 2 NR2C1
NR2C2 TR4 Testicular receptor 4 NR2C2
E TLX/PNR NR2E1 TLX Homologue of the Drosophila tailless gene NR2E1
NR2E3 PNR Photoreceptor cell-specific nuclear receptor NR2E3
F COUP/EAR NR2F1 COUP-TFI Chicken ovalbumin upstream promoter-transcription factor I NR2F1
NR2F2 COUP-TFII Chicken ovalbumin upstream promoter-transcription factor II NR2F2
NR2F6 EAR-2 V-erbA-related NR2F6
3 Estrogen Receptor-like A Estrogen receptor NR3A1 ERα Estrogen receptor-α ESR1 estrogens
NR3A2 ERβ Estrogen receptor-β ESR2
B Estrogen related receptor NR3B1 ERRα Estrogen-related receptor-α ESRRA
NR3B2 ERRβ Estrogen-related receptor-β ESRRB
NR3B3 ERRγ Estrogen-related receptor-γ ESRRG
C 3-Ketosteroid receptors NR3C1 GR Glucocorticoid receptor NR3C1 cortisol
NR3C2 MR Mineralocorticoid receptor NR3C2 aldosterone
NR3C3 PR Progesterone receptor PGR progesterone
NR3C4 AR Androgen receptor AR testosterone
4 Nerve Growth Factor IB-like A NGFIB/NURR1/NOR1 NR4A1 NGFIB Nerve Growth factor IB NR4A1
NR4A2 NURR1 Nuclear receptor related 1 NR4A2
NR4A3 NOR1 Neuron-derived orphan receptor 1 NR4A3
5 Steroidogenic Factor-like A SF1/LRH1 NR5A1 SF1 Steroidogenic factor 1 NR5A1 phosphatidylinositols
NR5A2 LRH-1 Liver receptor homolog-1 NR5A2 phosphatidylinositols
6 Germ Cell Nuclear Factor-like A GCNF NR6A1 GCNF Germ cell nuclear factor NR6A1
0 Miscellaneous B DAX/SHP NR0B1 DAX1 Dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1 NR0B1
NR0B2 SHP Small heterodimer partner NR0B2

Typical Classifications of receptor families[edit]

(gleanings from Receptor_(biochemistry)#Structure)

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

(ionotropic receptors)[edit]

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

Vertebrate Anionic Cys-loop Receptors

Type Class IUPHAR-recommended
protein name[46]
Gene Previous names
GABAA alpha α1
beta β1

gamma γ1
delta δ GABRD
epsilon ε GABRE
pi π GABRP
theta θ GABRQ
rho ρ1
alpha α1

beta β GLRB

Vertebrate Cationic Cys-loop Receptors

Type Class IUPHAR-recommended
protein name [46]
Gene Previous names
5-HT3 5-HT3A
Nicotinic acetylcholine
alpha α1

beta β1

Zinc-activated ion channel

Other ionotropic receptors:

in other (nonhuman) organisms

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)[edit]

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).[48]
  9. Involved in growth and metastasis of some types of tumors.[49]

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.[50] These receptors are coupled to different intracellular effector systems via G-proteins.

kinase linked and related receptors[edit]

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[edit]

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

Nuclear receptors[edit]

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.[51]

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