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RTKs and
MAPK Signaling

Explore how receptor tyrosine kinases activate the MAPK/ERK cascade to control cell growth, differentiation, and survival.

Biochemistry

What Are Receptor Tyrosine Kinases?

Receptor tyrosine kinases (RTKs) are transmembrane proteins that span the plasma membrane. They have an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular kinase domain that phosphorylates tyrosine residues.

Anatomy of an RTK Monomer

Plasma Membrane Ligand- Binding Domain Kinase Domain Tyr residues (unphosphorylated) Extracellular Intracellular Ligand

Dimerization and Autophosphorylation

Binding Kinase Binding Kinase Ligand P P P P then Grb2 SOS Ras GTP-bound Trans-autophosphorylation

The MAPK/ERK Cascade

The mitogen-activated protein kinase (MAPK) cascade is a three-tier phosphorylation relay. Once Ras is activated, it triggers a sequential kinase chain: Raf \( \rightarrow \) MEK \( \rightarrow \) ERK. Each kinase phosphorylates and activates the next.

\( \text{Ras-GTP} \)
Raf (MAPKKK) phosphorylates MEK (MAPKK) phosphorylates ERK (MAPK) Gene expression, proliferation, survival Signal Amplification

Why Three Tiers?

Amplification
Each activated kinase phosphorylates many copies of the next, exponentially increasing signal strength at every tier.
Regulation
Multiple tiers offer multiple control points. Phosphatases can shut down the cascade at any level.
Specificity
Scaffold proteins bind all three kinases, channeling signals to the correct downstream targets and preventing crosstalk.

RTKs vs GPCRs

RTKs GPCRs
Transmembrane passes 1 (single helix) 7 (serpentine)
Catalytic activity Intrinsic kinase None: uses G-proteins
Activation mechanism Dimerization + autophosphorylation Conformational change activates G-protein
Primary downstream Ras/MAPK, PI3K/Akt cAMP, IP3/DAG, ion channels
Typical ligands Growth factors (EGF, PDGF) Hormones, neurotransmitters

Grb2, SOS, and Ras Activation

1
Grb2 docks on p-Tyr. The adaptor protein Grb2 binds phosphotyrosine residues on the activated RTK via its SH2 domain.
2
Grb2 recruits SOS. Grb2's SH3 domains bind the guanine nucleotide exchange factor (GEF) called SOS, bringing it to the membrane.
3
SOS activates Ras. SOS promotes the exchange of \( \text{GDP} \) for \( \text{GTP} \) on Ras, switching Ras to its active (GTP-bound) state.
4
\( \text{Ras-GTP} \) recruits Raf. Active Ras binds the Raf kinase (MAPKKK) and activates it at the membrane, initiating the MAPK cascade.

Ras: The Molecular Switch

\( \text{Ras} \) \( \text{GDP-bound} \)
INACTIVE SOS (GEF)
\( \text{Ras} \) \( \text{GTP-bound} \)
ACTIVE GAP + intrinsic GTPase Raf (MAPKKK)

How to Trace the Pathway

1
Identify the signal. A growth factor (e.g., EGF) binds the extracellular domain of the RTK.
2
Dimerization. Two RTK monomers form a dimer. Their kinase domains cross-phosphorylate each other.
3
Adaptor recruitment. Grb2 (SH2 domain) docks on p-Tyr and recruits SOS (GEF) to the membrane.
4
Ras activation. SOS catalyzes \( \text{GDP} \rightarrow \text{GTP} \) exchange on Ras.
5
MAPK cascade. \( \text{Ras-GTP} \) activates Raf \( \rightarrow \) MEK \( \rightarrow \) ERK in a phosphorylation relay.
6
Nuclear response. ERK translocates to the nucleus, phosphorylates transcription factors, and drives gene expression for growth/proliferation.

Example: EGF Drives Cell Proliferation

Scenario: Epidermal Growth Factor (EGF) binds EGFR (an RTK) on an epithelial cell. Trace the signal from receptor to gene expression.

1
EGF binds EGFR's extracellular domain.
2
EGFR dimerizes; kinase domains trans-phosphorylate tyrosines.
3
Grb2 binds p-Tyr \( \rightarrow \) recruits SOS \( \rightarrow \) activates Ras.
4
\( \text{Ras-GTP} \) activates Raf \( \rightarrow \) MEK \( \rightarrow \) ERK.
5
ERK enters the nucleus, phosphorylates transcription factors (e.g., Elk-1, c-Myc), upregulating genes for cell division.

Example: Oncogenic Ras Mutation

Scenario: A point mutation in the KRAS gene produces a Ras protein that cannot hydrolyze GTP. What happens?

Constitutive activation
The MAPK cascade fires nonstop, driving uncontrolled proliferation.
Clinical impact
KRAS mutations are found in ~25% of all human cancers, especially pancreatic and colorectal.

Example: Targeting the Pathway with Drugs

Scenario: A patient has a tumor driven by overactive EGFR signaling. Which component of the pathway could a drug target?

Receptor level
Cetuximab: monoclonal antibody that blocks EGF binding to EGFR.
Kinase level
Erlotinib: small-molecule inhibitor that blocks EGFR's kinase domain (competes with ATP).
Downstream level
MEK inhibitors (trametinib): block MEK activity in the MAPK cascade even if upstream signals persist.

What Does ERK Do?

Once activated, ERK phosphorylates a wide array of substrates both in the cytoplasm and nucleus:

Transcription factors
Elk-1, c-Fos, c-Myc: drive genes for proliferation and survival.
Cytoplasmic targets
RSK kinases, cytoskeletal proteins: regulate cell motility and protein synthesis.
Negative feedback
ERK phosphorylates SOS, reducing Ras activation: a built-in brake on the cascade.

Scaffold Proteins Ensure Specificity

Scaffold (KSR) Raf MEK ERK Prevents crosstalk Increases efficiency Enhances specificity ✕ Wrong substrate

Common Mistakes

✕ Confusing GEF and GAP
GEFs (SOS) activate Ras by loading GTP. GAPs inactivate Ras by stimulating GTP hydrolysis. They have opposite effects.
✕ RTKs "use G-proteins"
RTKs use Ras (a small GTPase), not heterotrimeric G-proteins. G-proteins are specific to GPCRs.
✕ Forgetting dimerization
A single RTK monomer cannot autophosphorylate. Two monomers must dimerize for trans-phosphorylation.

Pathway Components at a Glance

Component Role Key Detail
RTK Receptor with kinase activity Dimerizes, trans-autophosphorylates
Grb2 Adaptor protein SH2 domain binds p-Tyr
SOS GEF for Ras Catalyzes \( \text{GDP} \rightarrow \text{GTP} \) exchange
Ras Molecular switch (GTPase) Active when GTP-bound
Raf MAPKKK First kinase in the cascade
MEK MAPKK Dual-specificity kinase (Thr + Tyr)
ERK MAPK Phosphorylates TFs in nucleus

Question 1 of 5

What event directly follows ligand binding to an RTK?

Question 2 of 5

Which protein acts as a guanine nucleotide exchange factor (GEF) for Ras in the RTK/MAPK pathway?

Question 3 of 5

A mutation prevents Ras from hydrolyzing GTP. What is the most likely consequence?

Question 4 of 5

What is the correct order of kinase activation in the MAPK cascade?

Question 5 of 5

How do scaffold proteins contribute to MAPK signaling?