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Receptor Tyrosine Kinase Signaling

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  • Last Updated : 13 Feb, 2023
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Cell signalling is a cell’s capacity to accept, process, and transmit messages to its surroundings and to itself. Cell signalling is a basic characteristic of all prokaryotic and eukaryotic cellular life. Extracellular signals (or signals that originate outside of a cell) can be physical agents such as mechanical pressure, electricity, temperature, light, or chemical signals (e.g., small molecules, peptides, or gas).

RTK Signaling

A tyrosine kinase is an enzyme that transfers a phosphate group from ATP to the tyrosine residues of particular proteins within the cell. Many cellular functions use it as an “on” or “off” switch. Tyrosine kinases are members of the protein kinase family, which also attaches phosphates to other amino acids including serine and threonine. Protein phosphorylation by kinases is a key method for signal transmission and controlling cellular function, such as cell division.

Protein kinases can get altered and become trapped in the “on” state, causing uncontrolled cell proliferation, an essential stage in the formation of cancer. As a result, kinase inhibitors such as imatinib and osimertinib are frequently useful in cancer treatment.

Receptor Tyrosine Kinase Signaling


RTK Receptor

Protein kinases are a class of enzymes that have a catalytic component that transfers the gamma (terminal) phosphate from nucleoside triphosphates (typically ATP) to one or more amino acid residues in the side-chain of a protein substrate, causing a conformational change that affects protein activity. The enzymes are divided into two groups based on substrate specificity: serine/threonine-specific and tyrosine-specific.


Protein kinase domains are found in protein tyrosine kinase proteins and consist of an N-terminal lobe with 5 beta-sheet strands and an alpha helix termed the C-helix, as well as a C-terminal domain with 6 alpha helices (helices D, E, F, G, H, and I). Catalysis is controlled by two loops in the kinase domain’s core. The HRD motif can be found in the catalytic loop (usually with sequence His-Arg-Asp). During catalysis, the aspartic acid of this motif creates a hydrogen bond with the substrate OH group on Tyr. The activation loop, whose location and conformation determine whether the kinase is active or inactive, is the other loop. The DFG motif starts the activation loop (usually with the sequence Asp-Phe-Gly). The Protein Data Bank contains about 1500 3D structures of tyrosine kinases. PDB: 1IRK, the crystal structure of the human insulin receptor’s tyrosine kinase domain, is one example.


A serine/threonine protein kinase enzyme phosphorylates the OH group of serine or threonine amino-acid residues with similar side chains. Serin/threonine kinases account for at least 350 of the 500 or so human protein kinases (STK). A transferase enzyme that transfers phosphates to the oxygen atom of a protein’s serine or threonine side chain is known as a serine/threonine protein kinase. This process is known as phosphorylation. Protein phosphorylation, in particular, is a critical posttranslational alteration that is involved in a variety of biological functions.

RTK ( Receptor Tyrosine Kinases)

Many polypeptide growth factors, cytokines, and hormones have high-affinity cell surface receptors called receptor tyrosine kinases (RTKs). 58 of the 90 distinct tyrosine kinase genes found in the human genome encode proteins known as receptor tyrosine kinases. It has been demonstrated that receptor tyrosine kinases play a crucial role in the initiation and development of many different forms of cancer in addition to being important regulators of normal cellular activities. Mutations in receptor tyrosine kinases trigger a number of signalling cascades that have diverse impacts on the expression of proteins. Receptor tyrosine kinases are a subset of the wider family of protein tyrosine kinases that includes both non-receptor tyrosine kinases and receptor tyrosine kinase proteins that feature transmembrane domains.

There were 58 receptor tyrosine kinases (RTKs) identified in 2004, which were divided into 20 subfamilies. They are essential for many different cellular processes, such as differentiation, adhesion, motility, growth (via signalling neurotrophins), and death. RTKs are made up of an intracellular catalytic domain that can bind certain substrates and phosphorylate them, a transmembrane domain, and an extracellular domain that can bind a particular ligand. Numerous RTKs are implicated in the development of cancer, whether by chromosomal translocation, gene mutation, or simple overexpression. Every time, the outcome is a hyperactive kinase that provides the cancer cells with an abnormal, ligand-independent, uncontrolled growth stimulation.

 Mechanism of RTK Signaling

  • Extracellular ligand binding frequently initiates or maintains receptor dimerization by a range of mechanisms.
  • As a result, each receptor monomer’s cytoplasmic tyrosine can be trans-phosphorylated by its paired receptor, causing a signal to go across the plasma membrane.
  • Src homology 2 (SH2) and phosphotyrosine binding (PTB) domain-containing proteins are able to bind to the active receptor through the phosphorylation of certain tyrosine residues.
  • Src and phospholipase C are two examples of certain proteins that include these domains.
  • Signal transduction pathways start when these two proteins are phosphorylated and activated upon receptor contact.
  • Without their own intrinsic enzymatic activity, other proteins that connect with the active receptor serve as adapter proteins.
  • These adapter proteins connect RTK activation to later signalling cascades, such as the MAP kinase signalling cascade.
  • The tyrosine kinase receptor, c-met, is an illustration of a crucial signal transduction pathway that is necessary for the survival and proliferation of migratory myoblasts during myogenesis.
  • Lack of c-met impairs secondary myogenesis and, like LBX1, hinders the development of limb musculature.
  • The local signalling that occurs when FGFs (Fibroblast Growth Factors) interact with their RTK receptors is referred to as paracrine signalling.
  • RTK receptors can activate multiple signal transduction pathways since they phosphorylate several tyrosine residues.

Activation of Receptor Tyrosine Kinase

RTK Activation


Receptor tyrosine kinases (RTKs) and non-receptor tyrosine kinases are further categories of tyrosine kinases (NRTKs). Of the 90 tyrosine kinases that have been identified, 58 belong to 20 subfamilies of RTKs, whereas 32 belong to 10 subfamilies of NRTKs. An amino-terminal extracellular domain with a ligand binding site, a single transmembrane -helix, an intracellular tyrosine kinase domain, a tyrosine-rich carboxy-(C) terminal, and juxtamembrane regions make up the core structural elements shared by all RTKs.

  • RTKs are activated when a ligand binds to its receptors, causing the receptor to dimerize.
  • There have been four general modes suggested.
    • Ligand-mediated dimerization is one of them.
    • Dimerization of receptor connections caused by ligands
    • Dimerization of receptors by ligands with connections to accessory molecules
    • Dimerization is mediated by receptors.
  • Once ligand-induced dimerization takes place, the transmembrane (TM) domain of the intracellular tyrosine kinase domain (TKD) is activated.
  • With regard to the location of the catalytic domains, the RTK TM dimer interface is quite precise and includes crucial structural data.
  • It’s interesting to note that studies have shown that, if the catalytic domains are orientated correctly, swapping the TM domains across various receptors can still cause constitutive activation.

 Tyrosine Kinase Pathway

  • Cells have a variety of RTKs that bind to a wide range of extracellular signalling molecules, many of which are generated locally and are at low levels in the body.
  • These small-scale cell-to-cell interactions are vital for the formation and upkeep of tissues’ spatial orientation, which is essential for higher-order functioning.
  • Two kinds of signalling molecules that bind to RTKs with particular significance are growth factors and hormones.
  • RTKs can also be bound to and activated by extracellular matrix proteins and specific surface proteins on adjacent cells.
  • Cell development and differentiation are hampered by malfunctioning RTKs.
  • RTKs are the targets of several medicines used in cancer treatment as a result.

Because it comprises three serine-threonine kinases, the mitogen-activated protein (MAP) kinase cascade is one of the most frequent intracellular signalling pathways induced by RTKs. Ras, a little G protein attached to the plasma membrane, is activated to begin the process. Ras is connected to GDP when it is not functioning. However, they induce Ras to bind GTP instead of GDP and become active when SH2-containing proteins team up with activated RTKs. The first serine-threonine kinase in the MAP kinase cascade is then activated by the GTP-bound Ras, which is not a kinase in and of itself. The subsequent kinase in this cascade is phosphorylated by each of the three kinases before being activated. Each step in this pathway amplifies the initial signal since all three of the kinases phosphorylate several substrates. The pathway’s last enzyme then changes the transcription of genes by phosphorylating transcription regulators. This pathway is used by several growth factors, such as platelet-derived growth factors and nerve growth factors.

Not all RTKs interact with the nucleus via the MAP kinase cascade. For instance, the protein kinase cascade (different from the MAP kinase cascade) that transmits the signal to the nucleus is activated when insulin-like growth factor receptors phosphorylate inositol phospholipids in the cell membrane. Other RTKs approach the nucleus more directly. Signal transducers and activators of transcription (STAT) proteins, which are transcriptional regulators, bind to phosphorylated tyrosines in cytokine and certain hormone receptors. When STAT proteins are active, they enter the nucleus directly and alter transcription.

Functions of Tyrosine Kinase

  1. Tyrosine kinases are proteins that catalyse the phosphorylation of tyrosine residues.
  2. Phosphorylation of tyrosine residues regulates several aspects of proteins, including enzyme activity, subcellular localization, and molecular interaction. Furthermore, tyrosine kinases participate in a variety of signal transduction cascades in which extracellular signals are transferred across the cell membrane to the cytoplasm and, in certain cases, to the nucleus, where gene expression is altered.
  3. Tyrosine kinases play important roles in a range of processes, routes, and activities in the body.
  4. Tyrosine kinase activity in the nucleus is associated with cell-cycle control and transcription factor characteristics.

Mechanisms of RTK

The majority of the time, receptor-specific ligands activate RTKs. Growth factor ligands bind to RTK extracellular regions, and ligand-induced receptor dimerization and/or oligomerization activates the receptor. Trans-autophosphorylation of each TKD and the release of the cis-autoinhibition are made possible for the majority of RTKs by the ensuing conformational alterations. The TKD can now adopt an active conformation thanks to this conformational shift. Additionally, a large number of downstream signalling proteins with Src homology-2 (SH2) or phosphotyrosine-binding (PTB) domains are recruited and activated by the autophosphorylation of RTKs. These domains interact with downstream mediators to spread crucial cellular signalling pathways by binding to certain phosphotyrosine residues within the receptor.

Mechanism of downstream signalling activation

Numerous downstream signalling proteins are drawn in as a result of the activation and subsequent autophosphorylation of RTKs. Direct recruitment of SH2 domain-containing proteins to the receptor is possible, as is indirect recruitment through docking proteins that bind to RTKs via their PTB domains. Activated RTKs have the capacity to bind to and control a broad variety of signalling pathways, including RAS/MAPK, PI-3 K/AKT, and JAK2/STAT signalling, as a result of the presence of multiple phosphotyrosine and the participation of various docking proteins. In order to activate transcriptional pathways involved in controlling a variety of cellular functions, RTKs work as a node that transmits complex information about cell growth and migration from the extracellular environment to the cell nucleus.

Receptor Tyrosine Kinase Activation in Cancer Cells

The aberrant activation of RTKs is a complex process that not only involves the RTKs but also their partner molecules and environment. The mechanisms of oncogenic RTK activation are further complicated by their connection with several subgroups of cellular components. The following four primary pathways have been put up as causes of abnormal activation:

  • Increased RTK expression
  • Mutations that increase function
  • Chromosomal translocations
  • Autocrine activation.

Along with these fundamental pathways, other factors that can affect oncogenic RTK activation include kinase domain duplications, microRNAs, alterations in the tumour microenvironment, protein tyrosine phosphatases, altered endocytic/trafficking genes, and geographic dysregulation of RTKs.

Clinical Significance of RTK

Tyrosine kinases are particularly significant today due to their potential use in cancer therapy. Imatinib is a medication that inhibits the activity of certain tyrosine kinases by binding to their catalytic cleft. During infection, a variety of viruses target tyrosine kinase function. Furthermore, tyrosine kinase can sometimes act improperly, leading to non-small cell lung cancer. Non-small cell lung cancer is a frequent and widespread cancer that kills more people than breast, colorectal, and prostate cancer combined.

FAQs on RTK Signaling

Question 1: What is the Tyrosine Kinase Mechanism?


Tyrosine kinases are enzymes that preferentially phosphorylate tyrosine residue in a wide range of substrates. Receptor tyrosine kinase is triggered when a ligand attaches to a receptor’s extracellular domain. Receptor dimerization is brought on by ligands, which are extracellular signal molecules, such as EGF and PDGF (except Insulin receptors).

Question 2: How is the Tyrosine Kinase Formed?


A fusion gene is produced when pieces of chromosomes 9 and 22 separate and switch places. The ABL gene from chromosome 9 and the BCR gene from chromosome 22 combine to form the BCR-ABL fusion gene. Tyrosine kinase activity is necessary for the transformation of BCR-ABL.

Question 3: Tyrosine kinase is what Sort of Inhibitor?


Targeted therapies include tyrosine kinase inhibitors (TKIs). TKIs are administered orally as tablets. A targeted treatment lessens the harm done to healthy cells by identifying and attacking particular cancer cell types.

Question 4: What Exactly is the Tyrosine Kinase Receptor Signalling Pathway?


A subclass of tyrosine kinases known as receptor tyrosine kinases (RTKs) is responsible for mediating cell-to-cell communication and regulating a variety of intricate biological processes, such as cell proliferation, motility, differentiation, and metabolism.

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