Receptor tyrosine kinase
- RTKs are transmembrane glycoproteins that function as enzyme linked receptors.
- RTKs are directly linked to intracellular enzymes and have tyrosine kinase activity.
- RTK signaling pathways have a wide spectrum of functions including regulation of cell proliferation and differentiation, promotion of cell survival, and modulation of cellular metabolism.
(Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme).
What does it mean to say that these receptors have tyrosine kinase activity?
This means that RTKs have a region that can carry out the phosphorylation of tyrosine residues on a protein.
Since they catalyze the ATP-dependent phosphorylation of their target proteins at specific Tyr residues, the receptors got the name receptor tyrosine kinase.
(A kinase is an enzyme that phosphorylates its substrate (here it is tyrosine aminoacid)– it catalyzes a reaction between its substrate and an energy-carrier molecule like ATP, resulting in the ATP losing a phosphate group to react with the substrate to form a product that is highly energized.
Structure of receptor tyrosine kinase
The human genome encodes 59 receptor protein-tyrosine kinases, including the receptors for EGF, NGF, PDGF, insulin, and many other growth factors.
All these receptors share a common structural organization:
- N – Terminal extracellular domain (Containing ligand-binding site)
- Single hydrophobic transmembrane α helix
- Cytosolic C – terminal domain (Includes a region with protein tyrosine kinase activity)
The structures of three distinct subfamilies of receptor protein-tyrosine kinases are shown in figure 1.
- EGF receptor
- Insulin receptor
- PDGF receptor
The EGF receptor and insulin receptor both have cysteine-rich extracellular domains, whereas the PDGF receptor has immunoglobulin (Ig)-like domains.
- Most of the receptor protein-tyrosine kinases consist of single polypeptides. That means most RTKs are monomeric. Eg: EGF receptor.
- (Ligand binding to the extracellular domain of these receptors induces formation of receptor dimers).
Some of the RTKs are dimeric. Eg: Insulin receptor
- The insulin receptor is a disulfide linked dimer of two pairs of polypeptide chains (In the absence of hormone).
- Binding of ligand to this type of RTK alters its conformation in such a way that the receptor becomes activated.
Regardless of the mechanism by which ligand binds and locks an RTK into a functional dimeric state, the next step is universal.
Overall scheme of signal transduction and response involving receptor tyrosine kinases
1) Arrival of signal at the cell surface
Many protein growth factors variously control the differentiation, proliferation, migration, metabolic state, and survival of their target cells by binding to their cognate receptor tyrosine kinases (RTKs).
What signals bind to receptor tyrosine kinases?
The ligands for RTKs are soluble or membrane-bound peptide growth factors or peptide hormones
Peptide growth factors: Nerve growth factor (NGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF)
Peptide hormone: Insulin.
Monomeric ligands for RTKs: FGF, EGF
Some monomeric ligands, including FGF, bind tightly to heparan sulfate, a negatively charged polysaccharide component of the extracellular matrix. This association enhances ligand binding to the monomeric receptor and formation of a dimeric receptor-ligand complex
The ligands for some RTKs are dimeric.
Eg: NGF, PDGF (They consists of two identical polypeptide chains)
These growth factors directly induce receptor dimerization by simultaneously binding to two different receptor molecules.
2) Signal recognition and binding by the extracellular domains of receptor tyrosine kinases
RTKs are activated by the binding of a cognate protein growth factor to their ectodomains.
Each RTK have specific ligands (Example: EGF receptor binds only to epidermal growth factor).
Different ligands employ different strategies by which they achieve the stable dimeric conformation.
- One ligand may bind with two receptor molecules (1 ligand: 2 receptor complex)
(Example: growth hormone and growth hormone receptor)
- Two ligands binds simultaneously to two receptors (2 ligand: 2 receptor complex)
(Example: VEGF and VEGFR)
3) Ligand induced dimerization and autophosphorylation of receptors
Since the enzyme-linked receptors are having a single transmembrane helix, they have a different strategy from GPCR for transducing the extracellular signal.
- The binding of a signal molecule to the extracellular domain of a receptor tyrosine kinase causes two receptor molecules to associate into a dimer.
- This brings the cytoplasmic tails of the receptors close to each other and thereby activating their tyrosine kinase activity.
(Tyrosine kinases are enzymes that selectively phosphorylates tyrosine residue in different substrates)
- The activated tails of the dimerized receptor then cross phosphorylate each other on several tyrosine residues. This is called autophosphorylation.
- Phosphorylation first occurs at a regulatory site known as the activation lip. Phosphorylation of the lip causes conformational changes that allow the kinase domain to phosphorylate other tyrosine residues in the receptor and in signal transduction proteins.
- Phosphorylation takes place both at catalytic and non-catalytic sites of the enzyme tyrosine kinase. Such autophosphorylation plays two key roles in signaling from these receptors.
1) Phosphorylation of tyrosine residues within the catalytic domain increases protein kinase activity.
2) Phosphorylation of tyrosine residues outside of the catalytic domain creates specific binding sites for additional proteins that transmit intracellular signals downstream of the activated receptors.
Exceptional case (dimerization):
For RTKs that are permanent dimers, such as the insulin receptor, the tyrosine kinase domain is thought to be activated by a ligand induced structural change that is transmitted across the membrane.
4) Assembly of signaling complex on the cytoplasmic tail of receptors
- The phosphorylation of tyrosines on the receptor tails triggers the assembly of an intracellular signaling complex on the tails.
- Phosphotyrosine residues in activated RTKs serve as docking sites for proteins involved in downstream signal transduction (as many as 10 or 20 different molecules— which themselves can become activated upon binding).
- Many phosphotyrosine residues in activated RTKs interact with adapter proteins (small proteins that contain SH2, PTB, or SH3 domains but have no intrinsic enzymatic or signaling activities)
- These proteins couple activated RTKs to other components of signal-transduction pathways and helps them to relay the signal to the cell’s interior.
Some of these proteins function solely as physical adaptors (They help build a large signaling aggregate by coupling the receptor to other proteins, which in turn may bind to and activate yet other proteins that pass the message along).
An important effector protein that is subsequently activated by the signaling complexes on the receptor tyrosine kinases is called Ras.
- Nearly all RTKs signal via Ras/MAP kinase pathways. They also may signal via other pathways.
- For example, the insulin receptor uses the Ras/MAP kinase pathway to regulate gene expression and the PI-3 kinase pathway to regulate enzyme activity (e.g., glycogen synthase).
Step 5: Activation of Ras protein
- GEFs facilitate Ras activation by promoting GDP exchange to GTP
Example: Sos (GEF)
Step 6: Stimulation of MAP kinase cascade
Active Ras.GTP stimulate MAP Kinase cascade by activating Raf
Step 7: MAP kinase phosphorylates target enzymes and transcriptional activators
MAP kinases phosphorylate TFs that regulate genes involved in the cell cycle and in differentiation.
Step 8: Changes in protein activities and gene expression
RTK-Ras/MAP kinase signaling controls cell division, differentiation, and metabolism.
For more details (step 6, step 7 and step 8) refer the following lecture note