Receptor Tyrosine Kinases Quiz

Learning Outcomes

• Session 1 learning outcome: Define kinases and phosphatases and their roles in signal transduction.
• Session 2 learning outcome: Explain the different mechanisms by which cells regulate and/or terminate responses to chemical signals.

Enzyme-Coupled Receptors

• Enzyme-coupled receptors are transmembrane proteins.
• These proteins display their ligand-binding domain on the outer surface of the plasma membrane.
• The cytoplasmic domain of the receptor acts as an enzyme or forms a complex with another protein that serves as an enzyme.

Receptor Tyrosine Kinases (RTKs)

• RTKs are the largest class of enzyme-coupled receptors.
• They consist of receptors with a cytoplasmic domain that functions as a tyrosine protein kinase.
• These kinases phosphorylate specific tyrosine residues on selected intracellular proteins.

Receptor Tyrosine Kinases (RTKs) - Regulation

• RTKs regulate cell proliferation and differentiation.
• Ligands for RTKs are soluble or membrane-bound peptide/protein hormones.
• Examples of these hormones include NGF, PDGF, FGF, EGF, and insulin.
• Ligand binding causes receptor dimerization; in some cases (e.g., insulin RTK), binding occurs to pre-existing dimers.

Signaling Proteins Acting Via Receptor Tyrosine Kinases

• Ligands include and examples of the respective Receptor proteins: EGF and EGF receptor, Insulin and Insulin receptor, IGF-1, IGF-2, and IGF receptor-1, NGF and TrkA, PDGF, PDGF receptors (a and β), M-CSF and M-CSF receptor, FGF, FGF receptor, VEGF and VEGF receptor, Ephrins and Eph receptors.
• Corresponding responses associated with the signaling are also listed.

Enzyme-Coupled Receptors - Further Details

• Activated RTKs recruit a complex of intracellular signaling proteins.
• Most RTKs activate the monomeric GTPase Ras.
• RTKs activate PI 3-kinase to produce lipid docking sites in the plasma membrane.
• Some receptors directly activate a fast track to the nucleus.
• Protein kinase networks integrate information to control complex cell behaviors.

RTK Signaling Pathways

• Nearly all RTKs signal via Ras/MAP kinase pathways, but other pathways exist.
• For example, insulin receptor uses Ras/MAP kinase to regulate gene expression and PI-3 kinase to regulate enzyme activity (e.g., glycogen synthase).

Ras Activation

• Ras is a small protein bound to a lipid tail in the cytoplasmic face of the plasma membrane.
• Ras-activating protein causes GDP to exchange to GTP activating Ras making it stimulate the next steps in the signaling pathway.
• Ras recruits and stimulates the Ras activating protein.

MAP Kinase Pathway

• MAP kinase is a protein kinase that performs a crucial step in relaying signals from cell-surface receptors to the nucleus, being the final step in a 3-kinase sequence (mitogen-activated protein kinase)
• MAP kinase phosphorylates and regulates downstream target proteins, which include transcription regulators.
• Change in gene expression and protein activity result in complex changes in cell behavior like proliferation and differentiation.

RTK Activity and Ras Activation

• EGF binding to receptor leads to dimerization and autophosphorylation on cytosolic tyrosines.
• Binding hormone causes receptor dimerization, kinase activation, and phosphorylation of cytosolic receptor tyrosine residues.

Adaptor Protein GRB2

• The adaptor protein GRB2 binds to receptor phosphotyrosine residues via its SH2 domain.
• Binding of GRB2 and Sos protein couples the receptor to inactive Ras.
• GRB2 contains SH3 domains, which allow the GEF protein (Sos) to bind the membrane complex and recruit Ras.

Ras-GTP Complex

• The activated Ras-GTP complex dissociates from Sos but remains tethered to the inner leaflet of the cytoplasmic membrane through a lipid anchor sequence.
• Activated Ras activates the MAP kinase portion of the signaling pathway.

Ras and Gα Subunits

• Activation of Ras and Gα subunits is triggered by hormone binding to a cell-surface receptor.
• Ras is a monomeric small GTPase switch protein.

Ras-GTP Hydrolysis

• Ras typically relies on GEFs for binding GTP and on GAPs for stimulation of GTP hydrolysis.

Other RTK Functions

• Once activated, Ras propagates signaling through a kinase cascade, resulting in the activation of MAP kinase family members.
• MAP kinase phosphorylates TFs regulating genes involved in the cell cycle and differentiation.
• RTK–Ras/MAP kinase signaling controls cell division, differentiation, and metabolism.

Receptor Internalization

• At high ligand concentrations, some cell surface receptors are internalized via endocytosis.
• Receptor internalization reduces the number of receptors on the cell surface and makes cells less sensitive to the ligand.

Mutant RTKs and Cancers

• Mutant RTKs or Ras/MAP kinase signaling proteins are commonly associated with cancers.
• Dominant Ras mutations that prevent GAP binding and lock Ras in the "on" state promote cancer.

MAP Kinase Cascade

• Ras activates MAP kinase via a phosphorylation cascade from Ras, Raf kinase, MEK kinase, and MAP kinase.
• Active MAP kinase translocates to the nucleus and activates many transcription factors.

Final Steps RTK-Ras MAP Kinase Signaling

• MAP kinase phosphorylates and activates p90RSK kinase in the cytoplasm.
• Both kinases enter the nucleus, phosphorylate transcription factors, and activate expression of genes that propel cells through the cell cycle.

Summary of RTK Signaling

• RTKs bind peptide hormones, behaving as dimers or dimerizing upon ligand binding.
• Ligand binding activates the receptor kinase, causing autophosphorylation of tyrosine residues.
• Ras, an intracellular GTPase, acts downstream from most RTKs and cycles between active (GTP-bound) and inactive (GDP-bound) forms.
• RTKs are coupled to Ras via GRB2 and Sos.
• Phosphorylated MAP kinase translocates into the nucleus and activates or inactivates transcription factors.

Summary of SH2 Domain in GRB2

• The SH2 domain in GRB2 is an adaptor protein that binds to specific phosphotyrosines in activated RTKs.
• Raf is recruited to the membrane by binding to Ras–GTP and activated, phosphorylating MEK, a dual specificity kinase that phosphorylates MAP kinase.
• Phosphorylated MAP kinase dimerizes and translocates to the nucleus, regulating gene expression.

PI-3-kinase-Akt Pathway

• RTKs can activate the PI-3-Akt signaling pathway.
• Extracellular survival signals (e.g., IGF) activate an RTK, which recruits and activates PI 3-kinase.
• This pathway promotes cell growth and survival.

PI3-Kinase Signaling

• PI3-kinase is an enzyme that phosphorylates inositol phospholipids in the plasma membrane.
• Phosphorylated lipids become docking sites for intracellular signaling proteins which move from cytosol to the membrane.

Activated Akt and Cell Survival

• Akt, a serine/threonine kinase, is one of the signaling proteins in the PI-3 kinase pathway.
• Akt promotes cell survival by inactivating Bad (a cytosolic protein) and thus inhibiting apoptosis.

PI3-Kinase-Akt and Tor

• Activated Akt activates Tor, a serine/threonine kinase.
• Tor stimulates protein synthesis and inhibits protein degradation, which promotes cell growth.

Inhibition of Receptor Tyrosine Kinases

• Excess mutant receptors can inhibit signaling through normal receptor tyrosine kinases.
• Mutant receptors lack kinase activity, preventing cross-phosphorylation of signaling proteins.

Mutant Proteins and Signaling

• Mutant proteins can help identify precise binding sites on intracellular signaling molecules.

Jamming the Pathway

• Recombinant DNA technology enables the creation of constitutively active forms of proteins like Ras.
• Constitutively active proteins mimic the effects of extracellular signals even without their presence.

Ras Mutations and Cancer

• Activating mutations in Ras (H-Ras, K-Ras, and N-Ras) are found in around 30% of human cancers.
• Mutations in Ras regulators like NF1 are also associated with certain cancers.

Enzyme-Linked Receptors and Nucleus

• Some enzyme-linked receptors activate a fast track to the nucleus.
• Cytokines bind to their receptors, activating JAK kinases that phosphorylate STATs, which enter the nucleus to control gene expression.

Cytokine Signaling Pathway

• The binding of cytokines causes a structural change to the receptor, leading to its dimerization.
• JAKs are brought into proximity and cross-phosphorylate each other, thus becoming activated.
• Activated JAKs phosphorylate tyrosine residues on the cytoplasmic tail of the receptor creating docking sites for STAT proteins.
• STAT proteins bind to the phosphorylated docking sites and are also activated by the JAKs.
• Activated STAT proteins dimerize and translocate to the nucleus.
• STAT dimers then bind to specific DNA sequences, thus regulating the transcription of target genes.

Summary of Pathways

Test your knowledge on receptor tyrosine kinases (RTKs) and their signaling pathways. This quiz covers the functions, structures, and mechanisms behind RTK activation and their role in cellular signaling. Challenge yourself with questions about ligands, phosphorylation, and downstream effects of RTK signaling.