Receptor Tyrosine Kinases (RTKs): The Mitogen Activated Protein Kinase (MAPK) Pathway

Receptor tyrosine kinases (RTKs)

enzyme-coupled receptors that regulate critical cellular processes including proliferation, differentiation, metabolism, survival, migration, and development. They function by converting extracellular ligand binding into intracellular phosphorylation-based signaling cascades.

• Large family of single-pass transmembrane receptors

• Possess intrinsic tyrosine kinase activity in their cytosolic domain

• Activated by growth factors, hormones, and developmental cues

• e.g. insulin receptor, vascular Endothelial Growth Factor Receptor (VEGFR), Platelet-Derived Growth Factor Receptor (PDGFR), Epidermal Growth Factor Receptor (EGFR/HER1)

Insulin Receptor

• Regulates glucose uptake and glycogen synthesis

• Controls metabolic pathways

Vascular Endothelial Growth Factor Receptor (VEGFR)

• Stimulates angiogenesis (new blood vessel formation)

• Important in wound healing and cancer vascularization

Platelet-Derived Growth Factor Receptor (PDGFR)

• Embryonic development

• Cell proliferation and migration

• Tissue repair

Epidermal Growth Factor Receptor (EGFR/HER1)

• Cell growth

• Proliferation

• Differentiation

• Frequently dysregulated in cancers

Why RTKs Matter

• Coordinate long-term cellular responses through gene expression changes

• Central to development and tissue homeostasis

• Mutations/overactivation linked to cancer and developmental disorders

General RTK Structure

• Extracellular ligand-binding domain

• Single transmembrane α-helix

• Cytoplasmic tyrosine kinase domain

• Tyrosine-rich C-terminal tail

Stepwise Activation of EGFR (Representative RTK)

1. resting state

2. ligand binding

3. Receptor Dimerization

4. Trans-Autophosphorylation / Cross-Phosphorylation

5. Full Kinase Activation

Resting state

• Receptor monomers exist largely unphosphorylated

• Kinase domain minimally active

• Dimerization interface hidden/inactive

Ligand binding

• EGF binds extracellular ligand-binding domain

• Causes conformational change in receptor

receptor dimerisation

• Two ligand-bound receptors dimerize

• Dimerization brings kinase domains together

Trans-Autophosphorylation / Cross-Phosphorylation

• Each kinase phosphorylates tyrosines on the partner receptor

• Includes phosphorylation of activation loop/activation lip

Full Kinase Activation

• Additional tyrosine phosphorylation increases catalytic activity

• Creates phosphotyrosine docking sites for signaling proteins

Asymmetric Kinase Domain Dimer

• One kinase domain acts as “activator/donor”

• Other acts as “receiver/acceptor”

• Conformational changes remove activation loop blockade from active site

Ras

• Small monomeric GTP-binding protein (small G-protein)

• Anchored to inner plasma membrane

• Functions as a binary molecular switch

Ras States

• Inactive State - Bound to GDP

• Active State - Bound to GTP

Ras Regulatory Proteins: GEFs (Guanine Nucleotide Exchange Factors)

• Promote release of GDP

• Allow GTP binding

• Example: Sos

Ras Regulatory Proteins: GAPs (GTPase Activating Proteins)

• Accelerate GTP hydrolysis

• Convert Ras-GTP → Ras-GDP

• Turn off signaling

Why Ras Is a “Timer”

• Intrinsic GTPase activity slowly hydrolyzes GTP

• GAPs speed this process to terminate signal

Ras Clinical Significance

• Mutations that reduce GTPase activity lock Ras in active GTP-bound form

• Common in cancers (e.g., KRAS mutations in colorectal cancer)

Molecular Mechanisms of Ras–Raf–MAPK Signalling

1. Recruitment of Adaptor Proteins

2. Activation of Ras

3. Activation of Raf (MAPKKK)

4. Activation of MEK (MAPKK)

5. Activation of ERK/MAPK

6. Nuclear Signaling

7. Gene Expression Changes

Recruitment of Adaptor Proteins

• Phosphotyrosines on activated RTK recruit GRB2 via its SH2 domain

• GRB2 binds Sos through its SH3 domains

Activation of Ras

• Sos acts as a GEF for Ras

• Promotes GDP release from Ras

• GTP binds Ras due to high cytosolic GTP concentration

• Ras becomes active (Ras-GTP)

Activation of Raf (MAPKKK)

• Ras-GTP recruits Raf to membrane

• Ras binds Raf regulatory domain

• Raf undergoes:

  • Partial activation by conformational change/dephosphorylation

  • Dimerization

  • Phosphorylation of activation loop serine/threonines

• Fully active Raf formed

Activation of MEK (MAPKK)

• Raf phosphorylates MEK

• MEK is a dual-specificity kinase

• Phosphorylates both Tyr and Thr residues

Activation of ERK/MAPK

• MEK phosphorylates ERK/MAPK on:

  • Threonine-183

  • Tyrosine-185

• Dual phosphorylation fully activates ERK

Nuclear Signaling

• Active ERK dimerizes

• ERK translocates to nucleus with p90RSK/p90 kinase

• Phosphorylates transcription factors:

  • TCF (Ternary Complex Factor)

  • SRF (Serum Response Factor)

Gene Expression Changes

• TCF/SRF bind Serum Response Elements (SREs)

• Activate immediate early genes such as c-fos

• Promote proliferation/differentiation programs

Enzyme cascade

A signaling mechanism where one activated enzyme activates multiple downstream enzymes in sequence.

Cascade in MAPK Pathway

RTK → Ras → Raf → MEK → ERK → Transcription Factors → Gene Expression

Advantages of Enzyme Cascades

• Signal Amplification

• Signal Integration

• Specificity

• Regulation/Control

• Temporal Control

Signal Amplification

• One activated receptor can activate many Ras molecules

• One Raf can activate many MEK molecules

• One MEK can activate many ERK molecules

• Small extracellular signal produces large intracellular response

Signal Integration

• Multiple upstream pathways can converge on same cascade

• Allows coordination of diverse signals

Specificity

Scaffold proteins/localization ensure correct pathway activation

Regulation/Control

• Multiple checkpoints for modulation

• Phosphatases can reverse phosphorylation at many levels

Temporal Control

• Duration/intensity of signaling influences outcome

• Transient vs sustained ERK activation can produce different cell fates

Comparison: RTKs vs GPCRs

RTKs

• Intrinsic enzymatic activity (tyrosine kinase)

• Often mediate long-term effects

• Commonly alter gene expression, proliferation, development

GPCRs

• No intrinsic enzyme activity

• Signal through heterotrimeric G proteins

• Often mediate short-term changes in:

  • Metabolism

  • Movement

  • Secretion