THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: Catalytic receptors

The Concise Guide to PHARMACOLOGY 2019/20 is the fourth in this series of biennial publications. The Concise Guide provides concise overviews of the key properties of nearly 1800 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (http://www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide represents approximately 400 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point‐in‐time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.14751. Catalytic receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein‐coupled receptors, ion channels, nuclear hormone receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid‐2019, and supersedes data presented in the 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC‐IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

receptor. A third group is the extrinsic protein tyrosine kinase receptors, where the catalytic activity resides in a separate protein from the binding site. Examples of this group include the GDNF and ErbB receptor families, where one, catalytically silent, member of the heterodimer is activated upon binding the ligand, causing the second member of the heterodimer, lacking ligand binding capacity, to initiate signaling through tyrosine phosphorylation. A fourth group, the receptor threonine/serine kinase (RTSK) family, exemplified by TGF-β and BMP receptors, has intrinsic serine/threonine protein kinase activity in the heterodimeric functional unit. A fifth group is the receptor tyrosine phosphatases (RTP), which appear to lack cognate ligands, but may be triggered by events such as cell:cell contact and have identified roles in the skeletal, hematopoietic and immune systems.
A further group of catalytic receptors for the Guide is the integrins, which have roles in cell:cell communication, often associated with signaling in the blood.

Cytokine receptor family
Catalytic receptors → Cytokine receptor family Overview: Cytokines are not a clearly defined group of agents, other than having an impact on immune signalling pathways, although many cytokines have effects on other systems, such as in development. A feature of some cytokines, which allows them to be distinguished from hormones, is that they may be produced by "non-secretory" cells, for example, endothelial cells. Within the cytokine receptor family, some subfamilies may be identified, which are described elsewhere in the Guide to PHARMA-COLOGY, receptors for the TNF family, the TGF-β family and the chemokines. Within this group of records are described Type I cytokine receptors, typified by interleukin receptors, and Type II cytokine receptors, exemplified by interferon receptors. These receptors possess a conserved extracellular region, known as the cytokine receptor homology domain (CHD), along with a range of other structural modules, including extracellular immunoglobulin (Ig)-like and fibronectin type III (FBNIII)-like domains, a transmembrane domain, and intracellular homology domains. An unusual feature of this group of agents is the existence of soluble and decoy receptors. These bind cytokines without allowing signalling to occur. A further attribute is the produc-tion of endogenous antagonist molecules, which bind to the receptors selectively and prevent signalling. A commonality of these families of receptors is the ligand-induced homo-or heterooligomerisation, which results in the recruitment of intracellular protein partners to evoke cellular responses, particularly in inflammatory or haematopoietic signalling. Although not an exclusive signalling pathway, a common feature of the majority of cytokine receptors is activation of the JAK/STAT pathway. This cascade is based around the protein tyrosine kinase activity of the Janus kinases (JAK), which phosphorylate the receptor and thereby facilitate the recruitment of signal transducers and activators of transcription (STATs). The activated homo-or heterodimeric STATs function principally as transcription factors in the nucleus. Type I cytokine receptors are characterized by two pairs of conserved cysteines linked via disulfide bonds and a C-terminal WSXWS motif within their CHD. Type I receptors are commonly classified into five groups, based on sequence and structual homology of the receptor and its cytokine ligand, which is potentially more reflective of evolutionary relationships than an earlier scheme based on the use of common signal transducing chains within a receptor complex.
Type II cytokine receptors also have two pairs of conserved cysteines but with a different arrangement to Type I and also lack the WSXWS motif.

IL-3 receptor family
Catalytic receptors → Cytokine receptor family → IL-3 receptor family Overview: The IL-3 receptor family signal through a receptor complex comprising of a ligand-specific α subunit and a common β chain (CSF2RB, P32927), which is associated with Jak2 and signals primarily through Stat5.

IL-6 receptor family
Catalytic receptors → Cytokine receptor family → IL-6 receptor family Overview: The IL-6 receptor family signal through a ternary receptor complex consisting of the cognate receptor and either the IL-6 signal transducer gp130 (IL6ST, P40189) or the oncostatin M-specific receptor, β subunit (OSMR, Q99650), which then activates the JAK/STAT, Ras/Raf/MAPK and PI 3-kinase/PKB signalling modules. Unusually amongst the cytokine receptors, the CNTF receptor is a glycerophosphatidylinositol-linked protein.

Nomenclature
Interleukin

IL-12 receptor family
Catalytic receptors → Cytokine receptor family → IL-12 receptor family Overview: IL-12 receptors are a subfamily of the IL-6 receptor family. IL12RB1 is shared between receptors for IL-12 and IL-23; the functional agonist at IL-12 receptors is a heterodimer of IL-12A/IL-12B, while that for IL-23 receptors is a heterodimer of IL-12B/IL-23A.

Nomenclature
Interleukin

Immunoglobulin-like family of IL-1 receptors
Catalytic receptors → Cytokine receptor family → Immunoglobulin-like family of IL-1 receptors Overview: The immunoglobulin-like family of IL-1 receptors are heterodimeric receptors made up of a cognate receptor subunit and an IL-1 receptor accessory protein, IL1RAP (Q9NPH3, also known as C3orf13, IL-1RAcP, IL1R3). They are characterised by extracellular immunoglobulin-like domains and an intracellular Toll/Interleukin-1R (TIR) domain.

Nomenclature
Interleukin

Comments:
Inhibitors of other receptor tyrosine kinases, such as semaxanib, which inhibits VEGF receptor function, may also inhibit Ret function [157]. Mutations of RET and GDNF genes may be involved in Hirschsprung's disease, which is characterized by the absence of intramural ganglion cells in the hindgut, often resulting in intestinal obstruction.

Catalytic receptors → Integrins
Overview: Integrins are unusual signalling proteins that function to signal both from the extracellular environment into the cell, but also from the cytoplasm to the external of the cell. The intracellular signalling cascades associated with integrin activation focus on protein kinase activities, such as focal adhesion kinase and Src. Based on this association between extracellular signals and intracellular protein kinase activity, we have chosen to include integrins in the 'Catalytic receptors' section of the database until more stringent criteria from NC-IUPHAR allows precise definition of their classification.
Integrins are heterodimeric entities, composed of α and β subunits, each 1TM proteins, which bind components of the extracellular matrix or counter-receptors expressed on other cells. One class of integrin contains an inserted domain (I) in its α subunit, and if present (in α1, α2, α10, α11, αD, αE, αL, αM and αX), this I domain contains the ligand binding site. All β subunits possess a similar I-like domain, which has the capacity to bind ligand, often recognising the RGD motif. The presence of an α subunit I domain precludes ligand binding through the β subunit. Integrins provide a link between ligand and the actin cytoskeleton (through typically short intracellular domains). Integrins bind several divalent cations, including a Mg 2+ ion in the I or I-like domain that is essential for ligand binding. Other cation binding sites may regulate integrin activity or stabilise the 3D structure. Integrins regulate the activity of particular protein kinases, including focal adhesion kinase and integrin-linked kinase. Cellular activation regulates integrin ligand affinity via inside-out signalling and ligand binding to integrins can regulate cellular activity via outsidein signalling.

Comments: Integrin ligands
Collagen is the most abundant protein in metazoa, rich in glycine and proline residues, made up of cross-linked triple helical structures, generated primarily by fibroblasts. Extensive post-translational processing is conducted by prolyl and lysyl hydroxylases, as well as transglutaminases. Over 40 genes for collagen-α subunits have been identified in the human genome. The collagen-binding integrins α1β1, α2β1, α10β1 and α11β1 recognise a range of triple-helical peptide motifs including GFOGER (O = hydroxyproline), a synthetic peptide derived from the primary sequence of collagen I (COL1A1 (COL1A1, P02452)) and collagen II (COL2A1 (COL2A1, P02458)). Laminin is an extracellular glycoprotein composed of α, β and γ chains, for which five, four and three genes, respectively, are identified in the human genome. It binds to α1β1, α2β1, α3,β1, α7β1 and α6β4 integrins10. fibrinogen (FGA FGB FGG, P02671 P02675 P02679) is a glycosylated hexamer composed of two α (FGA, P02671), two β (FGB, P02675) and two γ (FGG, P02679,) subunits, linked by disulphide bridges. It is found in plasma and alpha granules of platelets. It forms cross-links between activated platelets mediating aggregation by binding αIIbβ3; proteolysis by thrombin cleaves short peptides termed fibrinopeptides to generate fibrin, which polymerises as part of the blood coagulation cascade. fibronectin (FN1, P02751) is a disulphide-linked homodimer found as two major forms; a soluble dimeric form found in the plasma and a tissue version that is polymeric, which is secreted into the extracellular matrix by fibroblasts. Splice variation of the gene product (FN1, P02751) generates multiple isoforms. vitronectin (VTN, P04004) is a serum glycoprotein and extracellular matrix protein which is found either as a monomer or, following proteolysis, a disulphide -linked dimer. osteopontin (SPP1, P10451) forms an integral part of the mineralized matrix in bone, where it undergoes extensive posttranslation processing, including proteolysis and phosphorylation. von Willebrand factor (VWF, P04275) is a glycoprotein synthesised in vascular endothelial cells as a disulphide-linked homodimer, but multimerises further in plasma and is deposited on vessel wall collagen as a high molecular weight multimer. It is responsible for capturing platelets under arterial shear flow (via GPIb) and in thrombus propagation (via integrin αIIbβ3).

Catalytic receptors → Pattern recognition receptors
Overview: Pattern Recognition Receptors (PRRs, [208]) (nomenclature as agreed by NC-IUPHAR sub-committee on Pattern Recognition Receptors, [22]) participate in the innate immune response to microbial agents, the stimulation of which leads to activation of intracellular enzymes and regulation of gene transcription. PRRs express multiple leucine-rich regions to bind a range of microbially-derived ligands, termed PAMPs or pathogen-associated molecular patterns or endogenous ligands, termed DAMPS or damage-associated molecular patterns. These include peptides, carbohydrates, peptidoglycans, lipoproteins, lipopolysaccharides, and nucleic acids. PRRs include both cell-surface and intracellular proteins. PRRs may be divided into signalling-associated members, identified here, and endocytic members, the function of which appears to be to recognise particular microbial motifs for subsequent cell attachment, internalisation and destruction. Some are involved in inflammasome formation, and modulation of IL-1β cleavage and secretion, and others in the initiation of the type I interferon response.

Toll-like receptor family
Catalytic receptors → Pattern recognition receptors → Toll-like receptor family Overview: Members of the toll-like family of receptors (nomenclature recommended by the NC-IUPHAR subcommittee on pattern recognition receptors, [22]) share significant homology with the interleukin-1 receptor family and appear to require dimerization either as homo-or heterodimers for functional activity.
Eritoran (E5564) is a lipid A analogue, which has been described as a TLR4 antagonist [99]. TLR4 signals in conjunction with the co-factor MD-2 (LY96).
Involved in the detection of bacterial flagellin; pro-inflammatory.

NOD-like receptor family
Catalytic receptors → Pattern recognition receptors → NOD-like receptor family Overview: The nucleotide-binding oligomerization domain, leucine-rich repeat (NLR) family of receptors (nomenclature recommended by the NC-IUPHAR subcommittee on pattern recognition receptors [22]) share a common domain organisation. This consists of an N-terminal effector domain, a central nucleotidebinding and oligomerization domain (NOD; also referred to as a NACHT domain), and C-terminal leucine-rich repeats (LRR) which have regulatory and ligand recognition functions. The type of effector domain has resulted in the division of NLR family members into two major sub-families, NLRC and NLRP, along with three smaller sub-families NLRA, NLRB and NLRX [212]. NLRC members express an N-terminal caspase recruitment domain (CARD) and NLRP members an N-terminal Pyrin domain (PYD). Upon activation the NLRC family members NOD1 (NLRC1) and NOD2 (NLRC2) recruit a serine/threonine kinase RIPK2 (receptor interacting serine/threonine kinase 2, O43353, also known as CARD3, CARDIAK, RICK, RIP2) leading to signalling through NFκB and MAP kinase. Activation of NLRC4 (previously known as IPAF) and members of the NLRP3 family, including NLRP1 and NLRP3, leads to formation of a large multiprotein complex known as the inflammasome. In addition to NLR proteins other key members of the inflammasome include the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD, also known as PYCARD, CARD5, TMS1, Q9ULZ3) and inflammatory caspases. The inflammasome activates the pro-inflammatory cytokines IL-1β (IL1B, P01584) and IL-18 (IL18, Q14116) [22,41].
Nomenclature nucleotide binding oligomerization domain containing 1 nucleotide binding oligomerization domain containing 2 Comments -NOD2 has also been reported to be activated by ssRNA [187] although this has not been widely reproduced.
-NLRC4 forms an inflammasome with the NAIP proteins following recognition of bacterial flagellin and type III secretion system rod proteins by the NAIPs.
Loss-of-function mutations of NLRP3 are associated with cold autoinflammatory and Muckle-Wells syndromes. This family also includes NLR family, apoptosis inhibitory protein (NAIP, Q13075) which can be found in the 'Inhibitors of apoptosis (IAP) protein family' in the Other protein targets section of the Guide.  Overview: There are three human RIG-I-like receptors (RLRs) which are cytoplasmic pattern recognition receptors (PRRs) of the innate immune system. They detect non-self cytosolic double-stranded RNA species and and 5'-triphosphate single-stranded RNA from various sources and are essential for inducing production of type I interferons, such as IFNβ, type III interferons, and other anti-pathogenic effectors [21,22]. They function as RNA helicases (EC 3.6.4.13) using the energy from ATP hydrolysis to unwind RNA. NPRs, but with a truncated intracellular domain which appears to couple, via the G i/o family of G proteins, to activation of phospholipase C, inwardly-rectifying potassium channels and inhibition of adenylyl cyclase activity [161].

Receptor tyrosine kinases (RTKs)
Catalytic receptors → Receptor kinases → TK: Tyrosine kinase → Receptor tyrosine kinases (RTKs) Overview: Receptor tyrosine kinases (RTKs), a family of cellsurface receptors, which transduce signals to polypeptide and protein hormones, cytokines and growth factors are key regulators of critical cellular processes, such as proliferation and differentiation, cell survival and metabolism, cell migration and cell cycle control [14,67,219]. In the human genome, 58 RTKs have been identified, which fall into 20 families [125]. All RTKs display an extracellular ligand binding domain, a single transmembrane helix, a cytoplasmic region containing the protein tyrosine kinase activity (occasionally split into two domains by an insertion, termed the kinase insertion), with juxtamembrane and C-terminal regulatory regions. Agonist binding to the extracellular domain evokes dimerization, and sometimes oligomerization, of RTKs (a small subset of RTKs forms multimers even in the absence of activating ligand). This leads to autophosphorylation in the tyrosine kinase domain in a trans orientation, serving as a site of assembly of protein complexes and stimulation of multiple signal transduction pathways, including phospholipase C-γ, mitogen-activated protein kinases and phosphatidylinositol 3-kinase [219].
RTKs are of widespread interest not only through physiological functions, but also as drug targets in many types of cancer and other disease states. Many diseases result from genetic changes or abnormalities that either alter the activity, abundance, cellular distribution and/or regulation of RTKs. Therefore, drugs that modify the dysregulated functions of these RTKs have been developed which fall into two categories. One group is often described as 'biologicals', which block the activation of RTKs directly or by chelating the cognate ligands, while the second are small molecules designed to inhibit the tyrosine kinase activity directly.  [73]. ERBB2 (also known as HER-2 or NEU) appears to act as an essential partner for the other members of the family without itself being activated by a cognate ligand [74]. Ligands of the ErbB family of receptors are peptides, many of which are generated by proteolytic cleavage of cell-surface proteins. HER/ErbB is the viral counterpart to the receptor tyrosine kinase EGFR. All family members heterodimerize with each other to activate downstream signalling pathways and are aberrantly expressed in many cancers, particularly forms of breast cancer and lung cancer. Mutations in the EGFR are responsible for acquired resistance to tyrosine kinase inhibitor chemotherapeutics.

Nomenclature
epidermal growth factor receptor erb-b2 receptor tyrosine kinase 2 erb-b2 receptor tyrosine kinase 3 erb-b2 receptor tyrosine kinase 4  [73], to evoke cellular responses, mediated through multiple intracellular adaptor proteins. Exceptionally amongst the catalytic receptors, the functional receptor in the insulin receptor family is derived from a single gene product, cleaved post-translationally into two peptides, which then cross-link via disulphide bridges to form a heterotetramer. Intriguingly, the endogenous peptide ligands are formed in a parallel fashion with post-translational processing producing a heterodimer linked by disulphide bridges. Signalling through the receptors is mediated through a rapid autophosphorylation event at intracellular tyrosine residues, followed by recruitment of multiple adaptor proteins, notably IRS1 (P35568), IRS2 (Q9Y4H2), SHC1 (P29353), GRB2 (P62993) and SOS1 (Q07889).
Serum levels of free IGFs are kept low by the action of IGF binding proteins (IGFBP1-5, P08833, P18065, P17936, P22692, P24593), which sequester the IGFs; overexpression of IGFBPs may induce apoptosis, while IGFBP levels are also altered in some cancers.

Nomenclature
Insulin receptor Insulin-like growth factor I receptor Insulin receptor-related receptor -Comments: There is evidence for low potency binding and activation of insulin receptors by IGF1. IGF2 also binds and activates the cation-independent mannose 6-phosphate receptor (also known as the insulin-like growth factor 2 receptor; IGF2R; P11717), which lacks classical signalling capacity and appears to subserve a trafficking role [139]. INSRR, which has a much more discrete localization, being predominant in the kidney [117], currently lacks a cognate ligand or evidence for functional impact. Antibodies targetting IGF1, IGF2 and the extracellular portion of the IGF1 receptor are in clinical trials.
PQ401 inhibits the insulin-like growth factor receptor [5], while BMS-536924 inhibits both the insulin receptor and the insulinlike growth factor receptor [234]. Comments Upregulation of CSF1R expression is associated with migroglial activation and immune pathology in Alzhermer's disease (AD) [71,77]. Pharmacological inhibition of CSF1R with GW-2580 reduces microglial proliferation and prevents disease progression in a mouse model of AD, but this does not correlate with amyloid-β plaque numbers [171].

Comments:
The VEGFR, as well as VEGF ligands, have been targeted by antibodies and tyrosine kinase inhibitors. DMH4 [57], Ki8751 [116] and ZM323881, a novel inhibitor of vascular endothelial growth factor-receptor-2 tyrosine kinase activity [231] are described as VEGFR2-selective tyrosine kinase inhibitors. Bevacizumab is a monoclonal antibody directed against VEGF-A, used clinically for the treatment of certain metastatic cancers; an antibody fragment has been used for wet age-related macular degeneration.  Agonists -palifermin --Comments: Splice variation of the receptors can influence agonist responses. FGFRL1 (Q8N441) is a truncated kinase-null analogue. Various antibodies and tyrosine kinase inhibitors have been developed against FGF receptors [129,252]. PD161570 is an FGFR tyrosine kinase inhibitor [9], while PD173074 has been described to inhibit FGFR1 and FGFR3 [200].

Type VI RTKs: PTK7/CCK4
Catalytic receptors → Receptor kinases → TK: Tyrosine kinase → Receptor tyrosine kinases (RTKs) → Type VI RTKs: PTK7/CCK4 Overview: The PTK7 receptor is associated with polarization of epithelial cells and the development of neural structures. Sequence analysis suggests that the gene product is catalytically inactive as a protein kinase, although there is evidence for a role in Wnt signalling [180]. The neurotrophin receptor family of RTKs include trkA, trkB and trkC (tropomyosin-related kinase) receptors, which respond to NGF, BDNF and neurotrophin-3, respectively. They are associated primarily with proliferative and migration effects in neural systems.
Various isoforms of neurotrophin receptors exist, including truncated forms of trkB and trkC, which lack catalytic domains. p75 (TNFRSF16, also known as nerve growth factor receptor), which has homologies with tumour necrosis factor receptors, lacks a tyrosine kinase domain, but can signal via ceramide release and nuclear factor κB (NF-κB) activation. Both trkA and trkB contain two leucine-rich regions and can exist in monomeric or dimeric forms.

Nomenclature
neurotrophic receptor tyrosine kinase 1 neurotrophic receptor tyrosine kinase 2 neurotrophic receptor tyrosine kinase 3 have been used to label the trkA and trkB receptor, respectively. p75 influences the binding of NGF (NGF, P01138) and neurotrophin-3 (NTF3, P20783) to trkA. The ligand selectivity of p75 appears to be dependent on the cell type; for example, in sympathetic neurones, it binds neurotrophin-3 (NTF3, P20783) with comparable affinity to trkC [44]. Small molecule agonists of trkB have been described, including LM22A4 [148], while ANA12 has been described as a noncompetitive antagonist of BDNF binding to trkB [27]. GNF5837 is a family-selective tyrosine kinase inhibitor [2], while the tyrosine kinase activity of the trkA receptor can be inhibited by GW441756 Overview: Members of the ROR family appear to be activated by ligands complexing with other cell-surface proteins. Thus, ROR1 and ROR2 appear to be activated by Wnt-5a (WNT5A, P41221) binding to a Frizzled receptor thereby forming a cell-surface multiprotein complex [75].

Nomenclature
receptor tyrosine kinase like orphan receptor 1 receptor tyrosine kinase like orphan receptor 2 Type X RTKs: HGF (hepatocyte growth factor) receptor family Catalytic receptors → Receptor kinases → TK: Tyrosine kinase → Receptor tyrosine kinases (RTKs) → Type X RTKs: HGF (hepatocyte growth factor) receptor family Overview: HGF receptors regulate maturation of the liver in the embryo, as well as having roles in the adult, for example, in the innate immune system. HGF is synthesized as a single gene product, which is post-translationally processed to yield a heterodimer linked by a disulphide bridge. The maturation of HGF is enhanced by a serine protease, HGF activating complex, and inhibited by HGF-inhibitor 1 (SPINT1, O43278), a serine protease inhibitor. MST1, the ligand of RON, is two disulphide-linked peptide chains generated by proteolysis of a single gene product.

Comments:
The tyrosine kinase inhibitors of DDR, imatinib and nilotinib, were identified from proteomic analysis [43]. Other collagen receptors include glycoprotein VI (Q9HCN6), leukocyte-associated immunoglobulin-like receptor 1 (Q6GTX8), leukocyte-associated immunoglobulin-like receptor 2 (Q6ISS4) and osteoclast-associated immunoglobulin-like receptor (Q8IYS5). Overview: The LMR kinases are unusual amongst the RTKs in possessing a short extracellular domain and extended intracellular domain (hence the 'Lemur' name reflecting the long tail). A precise function for these receptors has yet to be defined, although LMR1 was identified as a potential marker of apoptosis [63], giving rise to the name AATYK (Apoptosis-associated tyrosine kinase); while over-expression induces differentiation in neuroblastoma cells [183].
Nomenclature apoptosis associated tyrosine kinase lemur tyrosine kinase 2 lemur tyrosine kinase 3 Overview: The LTK family appear to lack endogenous ligands. LTK is subject to tissue-specific splice variation, which appears to generate products in distinct subcellular locations. ALK fusions created by gene translocations and rearrangements are associated with many types of cancer, including large cell lymphomas, inflammatory myofibrilastic tumours and non-small cell lung cancer [144]. Selective inhibitors -- Comments -Crizotinib appears to be a selective ALK inhibitor acting on the tyrosine kinase activity [68] Type XX RTKs: STYK1 Catalytic receptors → Receptor kinases → TK: Tyrosine kinase → Receptor tyrosine kinases (RTKs) → Type XX RTKs: STYK1 Overview: Similar to the LMR RTK family, STYK1 has a truncated extracellular domain, but also displays a relatively short intracellular tail beyond the split kinase domain. STYK1 (also known as Novel Oncogene with Kinase-domain, NOK) has been suggested to co-localize with activated EGF receptor [48].

Overview:
Receptor serine/threonine kinases (RTSK), EC 2.7.11.30, respond to particular cytokines, the transforming growth factor β (TGFβ) and bone morphogenetic protein (BMP) families, and may be divided into two subfamilies on the basis of structural similarities. Agonist binding initiates formation of a cell-surface complex of type I and type II RSTK, possibly heterotetrameric, where where both subunits express serine/threonine kinase activity. The type I receptor serine/threonine kinases are also known as activin receptors or activin receptor-like kinases, ALKs, for which a systematic nomenclature has been proposed (ALK1-7). The type II protein phosphorylates the kinase domain of the type I partner (sometimes referred to as the signal propagat-ing subunit), causing displacement of the protein partners, such as the FKBP12 FK506-binding protein FKBP1A (P62942) and allowing the binding and phosphorylation of particular members of the Smad family. These migrate to the nucleus and act as complexes to regulate gene transcription. Type III receptors, sometimes called co-receptors or accessory proteins, regulate the signalling of the receptor complex, in either enhancing (for example, presenting the ligand to the receptor) or inhibitory manners. TGFβ family ligand signalling may be inhibited by endogenous proteins, such as follistatin (FST, P19883), which binds and neutralizes activins to prevent activation of the target receptors. Endogenous agonists, approximately 30 in man, are often de-scribed as paracrine messengers acting close to the source of production. They are characterized by six conserved cysteine residues and are divided into two subfamilies on the basis of sequence comparison and signalling pathways activated, the TGFβ/activin/nodal subfamily and the BMP/GDF (growth/ differentiation factor)/MIS (Müllerian inhibiting substance) subfamily. Ligands active at RSTKs appear to be generated as large precursors which undergo complex maturation processes [128]. Some are known to form disulphide-linked homo-and/or heterodimeric complexes. Thus, inhibins are α subunits linked to a variety of β chains, while activins are combinations of β subunits.