Tyrosine phosphorylation

Tyrosine phosphorylation is the addition of a phosphate (PO43-) group to the amino acid tyrosine on the protein. This transfer of phosphate group from ATP to the amino acid tyrosine on the protein is made possible through the enzyme tyrosine kinase. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity.

History
In the summer of 1979 during studies of polyomavirus middle T and v-Src associated kinase activities led to the discovery of tyrosine phosphorylation as a new type of protein modification. Following the 1979 discovery that Src is a tyrosine kinase, the number of distinct tyrosine kinases grew rapidly, accelerated by the advent of rapid DNA sequencing technology and PCR. About one year later was discovered an important role for tyrosine phosphorylation in growth factor signaling and proliferation, and by extension in oncogenesis through hijacking of growth factor tyrosine phosphorylation signaling pathways. The most important role played the year 1990 in which was detected Receptor tyrosine kinases (RTK)s initiate intracellular signaling. Phosphotyrosine (P.Tyr) residues on activated RTKs are recognized by a phosphodependent-binding domain, the SH2 domain. The recruitment of SH2 domain proteins to autophosphorylated RTKs at the plasma membrane is essential for initiating and propagating downstream signaling. SH2 domain proteins may have a variety of functions, including adaptor proteins to recruit other signaling proteins, enzymes that act on membrane molecules, such as phospholipases, cytoplasmic tyrosine kinases that relay signals, E3 ubiquitin ligases, and transcription factors. In the year 1995 were found proteins containing a second type of P.Tyr-binding domain, PTB in the RTK signaling. Gradually grow the number of identified tyrosine kinases and receptor tyrosine kinases. In the year 2002 of the 90 human tyrosine kinases 58 are RTKs and opposing the action of the tyrosine kinases are 108 protein phosphatases that can remove phosphate from P.Tyr in proteins.

Signal transduction
Ushiro and Cohen (1980) were the first to establish the important role of phosphorylation of tyrosine as a regulator of intracellular processes and to reveal changes of tyrosine kinase activity of proteins in mammalian cells Subsequently, the change of the protein tyrosine kinase activity was shown to underlie the Ras-MAPK signaling pathway regulated by Mitogen-Activated Protein (MAP) kinases. The classical scheme of transmission of the proliferative signals through the pathway mediated by growth factors (Ras-MAPK pathway) includes:


 * 1) association of growth factor with receptor
 * 2) dimerization of receptor and autophosphorylation of receptor tyrosine kinase (RTK)
 * 3) module coupling of RTK with adaptor SH2−domain proteins; activation of Ras
 * 4) phosphorylation and activation of MAP kinases
 * 5) transmission of signal into genome.

Another pathway of transmission of proliferative signals into genome, with participation of growth factors and tyrosine kinases, is the monocascade STAT pathway activated by receptors of growth factors and cytokines. The essence of this transmission consists in direct activation by tyrosine kinases of the STAT (signal transducer and activator of transcription) proteins located in the cytoplasm. This transmission is also provided by the SH2−domain contacts responsible for coupling of phosphotyrosine-containing proteins.

PTK
In tyrosine phosphorylation are important two classes of tyrosine kinase, receptor tyrosine kinase and nonreceptor ryrosine kinase. Receptor tyrosine kinases are type I transmembrane proteins possessing an N-terminal extracellular domain, which can bind activating ligands, a single transmembrane domain, and a C-terminal cytoplasmic domain that includes the catalytic domain. Nonreceptor tyrosine kinases lack a transmembrane domain, most are soluble intracellular proteins, but a subset associate with membranes via a membrane-targeting posttranslational modification, such as an N-terminal myristoyl group, and can act as the catalytic subunit for receptors that lack their own catalytic domain.

Reaction
Protein tyrosine kinases (PTK) catalyze the transfer of the γ-phosphate group from ATP to the hydroxyl group of tyrosine residues, whereas protein tyrosine phosphatases (PTP) remove the phosphate group from phosphotyrosine.

Growth factor signaling
Tyrosine phosphorylation of certain target proteins is required for ligand stimulation of their enzymatic activity. In response to EGF, PDGF, or FGF receptor activation, the SH2 domains of PLCγ bind to specific phosphotyrosines in the C-terminal tails of these receptors. Binding of PLCγ to the activated receptor facilitates its efficient tyrosine phosphorylation by the RTK. PDGF-induced activation of phospholipase C activity is abrogated in cells expressing PLCγ mutated in the tyrosine phosphorylation sites.

Cell adhesion, spreading, migration and shape
Phosphorylation on tyrosine residues, which are localized on membrane proteins, stimulates a cascade of signaling pathways that control cell proliferation, migration, and adhesion. These tyrosine residues are phosphorylated very nearly. For example p140Cap (Cas-associated protein) are phosphorylated within 15 min of cell adhesion to integrin ligands.

Cell differentiation in development
Tyrosine phosphorylation mediates in signal transduction pathways during germ cell development and to determine their association with the differentiation of a functional gamete. Until testicular germ cells differentiate into spermatozoa, cAMP-induced tyrosine phosphorylation is not detectable. Entry of these cells into the epididymis is accompanied by sudden activation of the tyrosine phosphorylation pathway, initially in the principal piece of the cell and subsequently in the midpiece.

Cell cycle control
Going through the phases of cell cycle is also dependent on tyrosine phosphorylation. In late G2 phase, it is present as an inactive complex of tyrosine-phosphorylated p34cdc2 and unphosphorylated cyclin Bcdc13. In M phase, its activation as an active MPF displaying histone H1 kinase (H1K) originates from the concomitant tyrosine dephosphorylation of the p34cdc2 subunit and the phosphorylation of the cylin Bcdc13 subunit. As cells leave the S phase and enter the G2 phase, a massive tyrosine phosphorylation of p34cdc2 occurs.

Gene regulation and transcription
Regulation with tyrosine phosphorylation plays a very important role in gene regulation. Tyrosine phosphorylation can influence the formation of different transcription factors and the subsequent development of their product. One of these cases is tyrosine phosphorylation of caveolin 2 (Cav-2) that negatively regulates the anti-proliferative function of transforming growth factor beta (TGF-beta) in endothelial cells. Only tyrosine phosphorylation is essential for the negative regulation of anti-proliferative function and signaling of TGF-β in ECs.

Endocytosis and exocytosis
In these two very important processes play phosphorylation on tyrosine residues an important role. Ligand-dependent endocytosis, which is not coupled to secretion, is known to be regulated via tyrosine phosphorylation. The effect of tyrosine phosphorylation is specific to rapid endocytosis. Dynamin is tyrosine phosphorylated in rapid endocytosis as well as in ligand dependent endocytosis.

Insulin stimulation of glucose uptake
Insulin binds to the insulin receptor at the cell surface and activates its tyrosine kinase activity, leading to autophosphorylation and phosphorylation of several receptor substrates. Phosphorylation of selected tyrosine sites on receptor substrates is known to activate different pathways leading to increased glucose uptake, lipogenesis, and glycogen and protein synthesis, as well as to stimulation of cell growth. In addition to activation of these pathways by tyrosine phosphorylation, several mechanisms of downregulating the response to insulin stimulation have also been identified.

Angiogenesis (formation of new blood vessels)
Protein tyrosine phosphorylation of capillary endothelial cells plays an important role in their proliferation. This phosphorylation can form new blood vessels.

Regulation of ion channels in nerve transmission
A lot of studies demonstrating high levels of protein-tyrosine kinases and phosphatases in the central nervous system have suggested that tyrosine phosphorylation is also involved in the regulation of neuronal processes. High levels of protein-tyrosine kinases and phosphatases and their substrates at synapses, both presynaptically and postsynaptically, suggest that tyrosine phosphorylation may regulate synaptic transmission. The role of tyrosine phosphorylation in the regulation of ligand-gated ion channels in the central nervous system has been less clear. The major excitatory neurotransmitter receptors in the central nervous system are the glutamate receptors. These receptors can be divided into three major classes: AMPA, kainate, and NMDA receptors, based on their selective agonists and on their physiological properties. Recent studies have provided evidence that NMDA receptors are regulated by tyrosine phosphorylation.

Tyrosine kinase and diseases
Tyrosine kinases are critical mediators of intracellular signaling and of intracellular responses to extracellular signaling. Changes in tyrosine kinase activity are implicated in numerous human diseases, including cancers, diabetes, and pathogen infectivity. One of diseases, which are very big problem in our society, is Acquired Immune Deficiency Syndrome also known as AIDS. Understanding the mechanism of CD4-mediated negative signaling is of particular interest in view of the progressive depletion of the CD4+ subset of T lymphocytes by the human immunodeficiency virus (HIV), which causes AIDS. T-cells from HIV-infected individuals also display activation defects, and undergo spontaneous apoptosis in culture. Similarities between the inhibitory effects of anti-CD4 antibodies and HIV-derived gp 120 immune complexes on T-cells suggest that sequestration of this and/or other putative similarities between the inhibitory effects of anti-CD4 antibodies and HIV-derived gp 120 immune complexes on T-cells suggest that sequestration of this and/or other putative p56lck substrate(s) by gp 120-mediated CD4 ligation in HIV-infected individuals may play a role in the loss of CD4+ cells and the inhibition of their activation.