autophosphorylation
   HOME

TheInfoList



OR:

Autophosphorylation is a type of
post-translational modification Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribosome ...
of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s. It is generally defined as the
phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
of the
kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
by itself. In
eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, this process occurs by the addition of a
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...
group to
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
,
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COOâ ...
or
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
residues within protein kinases, normally to regulate the catalytic activity.Petsko, GA and Ringe, D 2009, 'Protein Structure and Function', Oxford University Press Inc., New York, U.S.A Autophosphorylation may occur when a kinases' own
active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...
catalyzes the phosphorylation reaction (cis autophosphorylation), or when another kinase of the same type provides the active site that carries out the chemistry (trans autophosphorylation). The latter often occurs when kinase molecules dimerize. In general, the phosphate groups introduced are gamma phosphates from
nucleoside triphosphate A nucleoside triphosphate is a nucleoside containing a nitrogenous base bound to a 5-carbon sugar (either ribose or deoxyribose), with three phosphate groups bound to the sugar. They are the molecular precursors of both DNA and RNA, which are cha ...
s, most commonly ATP.


Function

Protein kinases, many of which are regulated by autophosphorylation, are vital in controlling the cellular proliferation, differentiation, metabolism, migration and survival.
Mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
s in the
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s encoding them or their potential activators or repressors can affect any number of functions within an organism. Phosphorylation is easily reversed by
phosphatase In biochemistry, a phosphatase is an enzyme that uses water to cleave a phosphoric acid Ester, monoester into a phosphate ion and an Alcohol (chemistry), alcohol. Because a phosphatase enzyme catalysis, catalyzes the hydrolysis of its Substrate ...
s. Therefore, it is an effective method of turning 'on' and 'off' kinase activity. Because of this it is recognized as an essential process in cell signaling. Addition of a negatively charged phosphate group brings about a change in the microenvironment that may lead to attraction or repulsion of other residues or molecules. The result may be a conformational change to expose or hide catalytic or allosteric seats from the surface. If the phosphorylated residue resides within the catalytic seat itself, it may facilitate or prevent substrate binding by means of charge-interaction, or by providing or preventing complementary shapes necessary for molecular recognition. In addition, the phosphate group yields several potential areas for
hydrogen-bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
or establishment of salt-bridges, of which the latter generally involves an
arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the am ...
residue. Binding of effector molecules may be affected in a similar manner if the phosphorylated residue makes part of the
allosteric site In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...
. Autophosphorylation has also been reported to have an effect on the cell's ability for endocytosis and
proteolysis Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...
.


Process and structure

Kinases are either phosphorylated on
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
and/or
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COOâ ...
residues, or solely on tyrosine residues. This serves as a means to classify them as either Ser/Thr- or Tyr-kinases. Several residues within the
primary structure Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthes ...
may be autophosphorylated simultaneously. The phosphoacceptors often reside within loops in the protein structure suitably termed '
activation loop In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other proteins or RNA. IDPs rang ...
s'. The structures of some autophosphorylation complexes are known from crystals of protein kinases in which the phosphorylation site (Ser, Thr, or Tyr) of one monomer in the crystal is sitting in the active site of another monomer of the crystal in a manner similar to known peptide-substrate/kinase structures. The known structures include: * Tyr phosphorylation sites in juxtamembrane regions: ** human cKIT, Tyr568 (PDB: 1PKG) ** human CSF1R, Tyr561 (PDB: 3LCD, homologous to cKIT site) ** human EPHA2, Tyr594 (PDB: 4PDO, two residues after the cKIT and CSF1R sites) * Tyr phosphorylation sites in kinase insert regions: ** human FGFR1, Tyr583 (PDB: 3GQI) ** human FGFR3, Tyr577 (PDB: 4K33, homologous to the FGFR1 site, domain interface identical to FGFR1 structure) * Tyr phosphorylation sites in activation loops: ** human IGF1R, Tyr1165 (PDB: 3D94) ** human IGF1R, Tyr1166 (PDB: 3LVP) ** human LCK, Tyr394 (PDB: 2PL0, homologous to the IGF1R Tyr1165 site) * Ser/Thr
phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
sites in activation loops: ** human PAK1, Thr423 (PDB: 3Q4Z, 4O0R, 4O0T, 4P90, 4ZLO, 4ZY4, 4ZY5, 4ZY6, 5DEY; the 4ZY4 and 4ZY5 structures provide complete coordinates for the substrate activation loop) ** human IRAK4, Thr345 (PDB: 4U97, 4U9A) * N or C terminal tails Ser/Thr phosphorylation sites: ** C. elegans CaMKII, C-terminal tail, Thr284 (PDB: 3KK8, 3KK9) ** human CaMKII, C-terminal tail, Thr287 (PDB: 2WEL, homologous to the C. elegans site) ** human CLK2, N-terminal tail, Ser142 (PDB: 3NR9) In general, the structures of the phosphorylation of internal loops involve important domain-domain contacts that have been confirmed by site-directed mutagenesis, while the phosphorylation of positions in the N or C terminal tails more than 10 amino acids away from the kinase domain do not involve important domain-domain contacts away from the substrate binding site.


Signaling pathways and trans-autophosphorylation

Among a number of various molecules,
Receptor Tyrosine Kinase Receptor tyrosine kinases (RTKs) 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 ...
s (RTKs) play a critical role in transducing signals through a range of
signaling pathway In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
s. All RTKs consists of an extracellular
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
binding region, a single transmembrane helix and a cytoplasmic region (the tyrosine kinase domain). Prior to ligand stimulation most RTKs present as a monomer on the surface of cells. Ligand binding to the extracellular domain induces dimerization. Dimerization of RTKs leads to autophosphorylation of tyrosine in the catalytic core of the dimer, and finally stimulation of the tyrosine kinase activity and cell signaling. It is thus an example of a trans-autophosphorylation reaction, where one receptor subunit of the dimer phosphorylates the other subunit.


Examples of RTKs which undergo autophosphorylation


Epidermal growth factor receptor

An example of RTKs that undergo autophosphorylation is the
Epidermal Growth Factor Epidermal growth factor (EGF) is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR. Human EGF is 6-k Da and has 53 amino acid residues and three intramolecular disulfide bonds. EGF was originally descr ...
receptor (EGFR). EGFR was the first discovered example of RTKs. Following ligand binding, a conformational change occurs in the EGFR monomers. This leads to EGFR dimerization. Dimerization brings the two receptors into close proximity. This stimulates the kinase activity of EGFR, which leads to transautophosphorylation on multiple tyrosine residues in C-terminal end of the molecule. The phosphorylated tyrosine residue can then serve as a docking site for downstream signaling proteins. (Fig. 1).


Insulin receptors

Another example is the binding of
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the ''INS'' gene. It is considered to be the main anabolic hormone of the body. It regulates the metabolism o ...
to
insulin receptor The insulin receptor (IR) is a transmembrane receptor that is activated by insulin, IGF-I, IGF-II and belongs to the large class of receptor tyrosine kinase. Metabolically, the insulin receptor plays a key role in the regulation of glucose ho ...
s. Once released into the bloodstream insulin can bind to receptors on the surface of cells in muscle or other tissues. This receptor is a protein with an (αβ)2
quaternary structure Protein quaternary structure is the fourth (and highest) classification level of protein structure. Protein quaternary structure refers to the structure of proteins which are themselves composed of two or more smaller protein chains (also refe ...
. The two large α-subunits are extracellular, while the smaller β-subunits have a transmembrane domain as well as extra-and intracellular domains. In the absence of insulin, the two intracellular domains of the β subunits are relatively distant. Binding with insulin triggers a conformational change in the receptor that brings them closer together. Each β subunit intracellular domain is a tyrosine kinase that phosphorylates its partner in the receptor.


Cancer


Src kinases

The Src-family kinases are examples of proteins that utilize autophosphorylation to sustain their activated states. Src kinases are involved in intracellular signaling pathways that influence cell growth and cell adhesion strength. The latter contributes to the control of cell migration. In this way, src-kinase deregulation can enhance tumor growth and invasive potential of cancer cells. The activity of src kinases is regulated by both phosphorylation and intramolecular interactions involving the SH2 and SH3 domains. The probable activation mechanism of src kinase in cancer is as follows: * 1. The src kinase is kept in an inactive form through the binding of SH2 to a phosphotyrosine * 2. Dephosphorylation of tyr-527 releases SH2 as well as SH3 domain. * 3. Subsequent autophosphorylation of tyr-416 activates the kinase. * 4. The constitutive activation of src kinase observed in cancer can be due to deletion of tyr-527, displacement of SH3 and SH2-mediated interactions by high affinity ligands with constantly autophosphorylated tyr-416.(Fig. 2).


Ataxia telangiectasia mutated kinase (ATM kinase)

ATM kinase, a member of the PI3-like family of serine/threonine kinases plays a critical role in maintaining the stability of the genome, which is of fundamental importance to the survival of all organisms. It exerts its effect by phosphorylating target proteins such as
P53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
, MDM2 and chk2. Activation of ATM is facilitated by autophosphorylation. The inactive ATM exists as dimer, where the kinase domain of one monomer is bound to the internal domain of the other monomer, containing ser-1981. It will therefore be inaccessible to cellular substrates. In response to DNA damage, the kinase domain of one monomer phosphorylates ser-1981 of the other interacting ATM, resulting in subunit dissociation and ATM activation. The activated ATM triggers a sequence of events including cell cycle arrest which allows time for the repair of the damaged DNA. If damaged DNA is left unrepaired, it can lead to cell death or genomic instability, cancer and other pathologies.


See also

*
Phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
*
Kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...


References

{{reflist Cell biology Cell signaling Post-translational modification Phosphorus