- Receptor tyrosine kinase
Receptor tyrosine kinases (RTK)s are the high affinity
cell surface receptors for many polypeptide growth factors, cytokines and hormones. Of the ninety unique tyrosine kinase genes idenitified in the human genome, 58 encode receptor tyrosine kinase proteins.cite journal | author =Robinson DR, Wu YM, Lin SF. | title = The protein tyrosine kinase family of the human genome | journal = Oncogene | year=2000 | volume=19 | issue=49 | pages=5548–5557 | pmid = 11114734 | doi = 10.1038/sj.onc.1203957] Receptor tyrosine kinases have been shown to be not only key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer.cite journal | author =Zwick, E. Bange, J. Ullrich, A. | title = Receptor tyrosine kinase signalling as a target for cancer intervention strategies | journal = Endocr. Relat. Cancer | year=2001 | volume=8 | issue=3 | pages=161–173 | pmid = 11566607 | doi = 10.1677/erc.0.0080161]
Receptor tyrosine kinase classes
Approximately 20 different RTK classes have been identified. [ www.genome.ad.jp [http://www.genome.ad.jp/kegg-bin/get_htext?htext=hsa04000.keg&filedir=%2fkegg%2fbrite%2fhsa&highlight=3791&option=-a&extend=C162&close=C162#C162] Retrieved on
#RTK class I (EGF receptor family)
#RTK class II (
#RTK class III (PDGF receptor family)
#RTK class IV (FGF receptor family)
#RTK class V (
#RTK class VI (HGF receptor family)
#RTK class VII (
#RTK class IX (
#RTK class X (
#RTK class XI ( family)
#RTK class XII (
#RTK class XIII (
#RTK class XV (
#RTK class XVI (RYK receptor family)
#RTK class XVII (MuSK receptor family)
Most RTKs are single subunit receptors but some e.g. the
insulin receptorexist as multimeric complexes. Each monomerhas a single transmembranespanning domain composed of 25-38 amino acids, an extracellular N-terminalregion and an intracellular C-terminalregion. The extracellular N-terminal region is composed of a very large protein domainwhich binds to extracellular ligands e.g. a particular growth factoror hormone. The intracellular C-terminal region comprises domains responsible for the kinaseactivity of these receptors.
biochemistry, a "kinase" is a type of enzymethat transfers phosphategroups (see below) from high-energy donor molecules, such as ATP (see below) to specific target molecules (substrates); the process is termed " phosphorylation". The opposite, an enzyme that removes phosphate groups from targets, is known as a phosphatase. Kinase enzymes that specifically phosphorylate tyrosine amino acids are termed tyrosine kinases.
When a growth factor binds to the extracellular domain of an RTK, its
dimerizationis triggered with other adjacent RTKs. Dimerizationleads to a rapid activation of the protein's cytoplasmic kinase domains, the first substrate for these domains being the receptor itself. The activated receptor as a result then becomes autophosphorylated on multiple specific intracellular tyrosineresidues.
The phosphorylation of specific
tyrosineresidues within the activated receptor creates binding sites for Src homology 2 (SH2) and phosphotyrosinebinding (PTB) domain containing proteins.cite journal | author =Pawson, T. | title = Protein modules and signalling networks | journal = Nature | year=1995 | volume=373 | issue=6515 | pages=573–580 | pmid = 7531822 | doi = 10.1038/373573a0] Specific proteins containing these domains include Src and phospholipase Cγ, the phosphorylation and activation of these two proteins on receptor binding leading to the initiation of signal transductionpathways. Other proteins that interact with the activated receptor act as adaptor proteins and have no intrinsic enzymatic activity of their own. These adaptor proteins link RTK activation to downstream signal transductionpathways, such as the MAP kinase signalling cascade.
Fibroblast growth factor receptor (FGFR) family
fibroblast growth factors are the largest family of growth factor ligands comprising of 23 members.cite journal | author =Ornitz DM. and Itoh, N. | title = Fibroblast growth factors | journal = Genome Biol. | year=2001 | volume=2 | issue=3 | pages=REVIEWS 3005 | pmid = 11276432 | doi = 10.1186/gb-2001-2-3-reviews3005] The natural alternate splicing of four fibroblast growth factor receptor (FRFR) genes results in the production of over 48 different isoforms of FGFR.cite journal | author =Duchesne L, Tissot B. "et al" | title = N-glycosylation of fibroblast growth factor receptor 1 regulates ligand and heparan sulfate co-receptor binding | journal = J. Biol. Chem. | year=2006 | volume=281 | issue=37 | pages=27178–27189 | pmid = 16829530 | doi = 10.1074/jbc.M601248200] These isoforms vary in their ligand binding properties and kinase domains, however all share a common extracellular region composed of three immunoglobulin(Ig) like domains (D1-D3), and thus belong to the immunoglobulin superfamily.cite journal | author =Coutts JC, and Gallagher JT. | title = Receptors for fibroblast growth factors | journal = Immunol. Cell. Biol. | year=1995 | volume=73 | issue=6 | pages=584–589 | pmid = 8713482 | doi = 10.1038/icb.1995.92] Interactions with FGFs occur via FGFR domains D2 and D3. Each receptor can be activated by several FGFs. In many cases the FGFs themselves can also activate more than one receptor, this is not the case with FGF-7 however which can only activate FGFR2b. A gene for a fifth FGFR protein, FGFR5, has also been identified. In contrast to FGFRs 1-4 it lacks a cytoplasmic tyrosine kinase domain and one isoform, FGFR5γ, only contains the extracellular domains D1 and D2.cite journal | author = Sleeman M, Fraser J. "et al" | title = Identification of a new fibroblast growth factor receptor, FGFR5 | journal = Gene | year=2001 | volume=271 | issue=2 | pages=171–182 | pmid = 11418238 | doi = 10.1016/S0378-1119(01)00518-2]
Vascular endothelial growth factor receptor (VEGFR) family
Vascular endothelial growth factor(VEGF) is one of the main inducers of endothelial cellproliferation and permeability of blood vessels. Two RTKs bind to VEGF at the cell surface, VEGFR-1 ( Flt-1) and VEGFR-2 ( KDR/Flk-1).cite journal | author = Robinson, CJ and Stringer, SE. | title = The splice variants of vascular endothelial growth factor (VEGF) and their receptors | journal = J. Cell. Sci. | year=2001 | volume=114 | issue=5 | pages=853–865 | pmid = 11181169]
The VEGF receptors have an extracellular portion consisting of seven Ig-like domains so, like FGFRs, belong to the immunoglobulin superfamily. They also possess a single transmembrane spanning region and an intracellular portion containing a split
tyrosine-kinasedomain. VEGF-A binds to VEGFR-1 ( Flt-1) and VEGFR-2 ( KDR/Flk-1). VEGFR-2 appears to mediate almost all of the known cellular responses to VEGF. The function of VEGFR-1 is less well defined, although it is thought to modulate VEGFR-2 signaling. Another function of VEGFR-1 may be to act as a dummy/decoy receptor, sequestering VEGF from VEGFR-2 binding (this appears to be particularly important during vasculogenesis in the embryo). A third receptor has been discovered (VEGFR-3), however, VEGF-A is not a ligand for this receptor. VEGFR-3 mediates lymphangiogenesisin response to VEGF-C and VEGF-D.
RET receptor family
The natural alternate splicing of the "RET"
generesults in the production of 3 different isoforms of the protein RET. RET51, RET43 and RET9 contain 51, 43 and 9 amino acids in their C-terminaltail respectively.cite journal | author = Myers SM, Eng C. "et al" | title = Characterization of RET proto-oncogene 3' splicing variants and polyadenylation sites: a novel C-terminus for RET | journal =Oncogene | year=1995 | volume=11 | issue=10 | pages=2039–2045 | pmid = 7478523] The biological roles of isoforms RET51 and RET9 are the most well studied " in-vivo" as these are the most common isoforms in which RET occurs.
RET is the receptor for members of the
glial cell line-derived neurotrophic factor(GDNF) family of extracellular signalling molecules or ligands (GFLs).cite journal | author = Baloh RH, Enomoto H. "et al" | title = The GDNF family ligands and receptors - implications for neural development | journal = Curr. Opin. Neurobiol. | year=2000 | volume=10 | issue=1 | pages=103–110 | pmid = 10679429 | doi = 10.1016/S0959-4388(99)00048-3]
In order to activate RET GFLs first need to form a complex with a
glycosylphosphatidylinositol(GPI)-anchored co-receptor. The co-receptors themselves are classified as members of the GDNF receptor-α (GFRα) protein family. Different members of the GFRα family (GFRα1-GFRα4) exhibit a specific binding activity for a specific GFLs.cite journal | author = Airaksinen MS, Titievsky A, Saarma M. | title = GDNF family neurotrophic factor signaling: four masters, one servant? | journal = Mol. Cell Neurosci. | year=1999 | volume=13 | issue=5 | pages=313–325 | pmid = 10356294 | doi = 10.1006/mcne.1999.0754] Upon GFL-GFRα complex formation, the complex then brings together two molecules of RET, triggering trans-autophosphorylation of specific tyrosineresidues within the tyrosine kinasedomain of each RET molecule. Phosphorylationof these tyrosines then initiates intracellular signal transductionprocesses.cite journal | author = Arighi E, Borrello MG, Sariola H. | title = RET tyrosine kinase signaling in development and cancer | journal = Cytokine Growth Factor Rev. | year=2005 | volume=16 | issue=4-5 | pages=441–467 | pmid = 15982921 | doi = 10.1016/j.cytogfr.2005.05.010]
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