Master’s-Level Cell Biology & Advanced Molecular Biology Notes

1. Definition

Receptor tyrosine kinases (RTKs) are a major family of cell-surface transmembrane receptors that convert extracellular signals into intracellular responses by activating protein tyrosine kinase activity.

RTKs regulate:

Cell proliferation
Cell survival
Differentiation
Cell migration
Metabolism
Growth
Development
Angiogenesis
Tissue repair

Important RTKs include receptors for:

Epidermal growth factor (EGF)
Fibroblast growth factors (FGFs)
Platelet-derived growth factor (PDGF)
Vascular endothelial growth factor (VEGF)
Insulin
Neurotrophins

2. Basic RTK Structure

A typical RTK contains three major regions:

              EXTRACELLULAR
                    │
                Ligand-binding
                    domain
                    │
              ┌───────────┐
              │           │
              │    RTK    │
              │           │
              └─────┬─────┘
                    │
             Single TM helix
                    │
              ┌─────┴─────┐
              │           │
              │ Tyrosine  │
              │  kinase   │
              │  domain   │
              └─────┬─────┘
                    │
             Cytoplasmic tail
                    │
              INTRACELLULAR

Major components

Extracellular ligand-binding domain
Single transmembrane α-helix
Intracellular tyrosine kinase domain
Cytoplasmic regulatory regions

3. Important Difference from GPCRs

RTKs differ fundamentally from GPCRs.

Feature	GPCR	RTK
Transmembrane segments	7	Usually 1
Main signaling partner	Heterotrimeric G protein	Protein/adaptor signaling complexes
Intrinsic kinase activity	No	Usually yes
Major phosphorylation	Various downstream proteins	Tyrosine residues
Major pathways	cAMP, IP3/DAG	RAS–MAPK, PI3K–AKT, PLCγ
Typical function	Broad physiological signaling	Growth, survival, differentiation, metabolism

4. General RTK Signaling Mechanism

The canonical sequence is:

Ligand
  ↓
RTK binding
  ↓
Receptor dimerization / rearrangement
  ↓
Kinase activation
  ↓
Tyrosine phosphorylation
  ↓
Recruitment of signaling proteins
  ↓
Intracellular signaling cascades
  ↓
Cellular response

5. Step 1 — Ligand Binding

An extracellular ligand binds the receptor.

For example:

EGF → EGFR

          EGF
           ↓
      ┌─────────┐
      │  RTK    │
      └────┬────┘
           │
          TM
           │
       Kinase

Ligand binding induces structural rearrangements that promote receptor activation.

6. Step 2 — Receptor Dimerization

Many RTKs become activated by formation of receptor dimers.

Before ligand

 RTK       RTK
  │         │


After ligand

   Ligand
      ↓
 RTK ═══ RTK
      ↓
    Dimer

Dimerization brings the intracellular kinase domains into proximity.

Important qualification

Not every RTK follows exactly the same activation mechanism. Some receptors can exist as preformed complexes and become activated through ligand-induced conformational rearrangement.

7. Step 3 — Kinase Activation

The intracellular kinase domains become activated.

The receptor then phosphorylates tyrosine residues on the receptor itself.

This is called:

Autophosphorylation

More precisely, in many RTKs it is trans-autophosphorylation, because one receptor kinase phosphorylates the other receptor molecule in the dimer.

RTK-A kinase ───→ Tyr on RTK-B
RTK-B kinase ───→ Tyr on RTK-A

8. ATP and Tyrosine Phosphorylation

The kinase uses ATP as the phosphate donor.

Conceptually:

ATP + protein-Tyr → ADP + protein-Tyr-P

The phosphorylated tyrosines become docking sites for intracellular signaling proteins.

9. Phosphotyrosine Docking Sites

Activated RTKs function as signaling platforms.

Activated RTK
     │
 ─── Tyr-P
 ─── Tyr-P
 ─── Tyr-P
     │
     ├──→ Adaptor protein
     ├──→ Enzyme
     ├──→ Scaffold
     └──→ Signaling complex

Proteins recognize phosphorylated tyrosines through specialized domains.

10. SH2 and PTB Domains

Two important phosphotyrosine-recognition modules are:

SH2 domains

Src homology 2 domains

They recognize specific phosphotyrosine-containing sequences.

PTB domains

Phosphotyrosine-binding domains

They can recognize phosphotyrosine-containing motifs in specific sequence contexts.

These interactions provide molecular specificity.

11. Major RTK Signaling Pathways

The three high-yield pathways are:

                    RTK
                     │
       ┌─────────────┼──────────────┐
       ↓             ↓              ↓
   RAS–MAPK       PI3K–AKT       PLCγ
       ↓             ↓              ↓
 Proliferation    Survival       Ca²⁺/PKC
 Differentiation  Metabolism

12. RAS–MAPK Pathway

This is one of the most important RTK pathways.

RTK
 ↓
GRB2
 ↓
SOS
 ↓
RAS-GTP
 ↓
RAF
 ↓
MEK
 ↓
ERK
 ↓
Nucleus
 ↓
Gene expression

This pathway is strongly associated with:

Cell proliferation
Differentiation
Development

13. GRB2

GRB2 = Growth factor receptor-bound protein 2

GRB2 is an adaptor protein.

It contains:

SH2 domain
SH3 domains

The SH2 domain binds phosphorylated receptor tyrosines, while SH3 domains interact with proline-rich regions of other proteins such as SOS.

14. SOS and RAS Activation

SOS = Son of Sevenless

SOS functions as a guanine nucleotide exchange factor (GEF) for RAS.

RTK-P
  ↓
GRB2
  ↓
SOS
  ↓
RAS-GDP → RAS-GTP

RAS is a small GTPase.

RAS-GTP = active

RAS-GDP = inactive

15. RAF–MEK–ERK Cascade

Active RAS recruits and activates RAF.

RAS-GTP
   ↓
RAF
   ↓
MEK
   ↓
ERK
   ↓
Nucleus
   ↓
Transcription factors
   ↓
Gene expression

This is called the:

MAPK cascade

16. Why Is It Called a Kinase Cascade?

Each kinase activates downstream components through phosphorylation.

RAF
 ↓
MEK
 ↓
ERK
 ↓
Target proteins

This produces:

Signal amplification
Signal integration
Temporal regulation

17. ERK and Gene Expression

Activated ERK can influence nuclear transcription factors.

RTK
 ↓
RAS
 ↓
RAF
 ↓
MEK
 ↓
ERK
 ↓
Nucleus
 ↓
Transcription factors
 ↓
Gene expression

The resulting genes can regulate:

Cell-cycle progression
Differentiation
Growth
Survival

18. PI3K–AKT Pathway

Another major RTK pathway is:

RTK
 ↓
PI3K
 ↓
PIP3
 ↓
AKT
 ↓
mTOR and other targets
 ↓
Growth / survival / metabolism

19. PI3K

Phosphoinositide 3-kinase (PI3K) phosphorylates membrane phosphoinositides.

A major reaction is:

PIP2 → PIP3

PIP3 acts as a membrane-associated signaling platform.

20. AKT Recruitment

PIP3 recruits proteins containing PH domains, including:

AKT
PDK1

This brings signaling proteins into proximity at the plasma membrane.

PI3K
 ↓
PIP3
 ↓
AKT recruitment
 ↓
AKT activation

21. AKT Functions

AKT regulates:

Cell survival
Protein synthesis
Metabolism
Cell growth
Apoptosis
Cell-cycle regulation

One major downstream pathway involves:

mTOR

RTK
 ↓
PI3K
 ↓
AKT
 ↓
mTOR
 ↓
Protein synthesis / growth

22. PTEN — Important Negative Regulator

PTEN is a major antagonist of PI3K signaling.

It converts PIP3 toward PIP2.

Simplified:

PI3K
PIP2 ─────→ PIP3
             ↑
             │
           PTEN
             │
             ↓
            PIP2

Thus:

PI3K promotes AKT signaling

PTEN restrains PI3K–AKT signaling

23. PLCγ Pathway

Some RTKs activate:

Phospholipase C-γ (PLCγ)

RTK
 ↓
PLCγ
 ↓
PIP2
 ↓
IP3 + DAG
 ↓
Ca²⁺ + PKC
 ↓
Cellular response

This resembles the Gq–PLCβ pathway but uses a different phospholipase:

GPCR/Gq → PLCβ
RTK → PLCγ

24. IP3 and Calcium

IP3 stimulates Ca²⁺ release from the ER.

PLCγ
 ↓
IP3
 ↓
IP3 receptor on ER
 ↓
Ca²⁺ release
 ↓
Ca²⁺-dependent signaling

25. DAG and PKC

DAG remains associated with the membrane.

Together with appropriate cofactors, it promotes activation of protein kinase C isoforms.

PIP2
 ↓
DAG
 ↓
PKC
 ↓
Protein phosphorylation

26. RTK Signaling as a Network

RTKs do not activate a single linear pathway.

A single activated receptor can simultaneously activate several pathways.

                    RTK
                     │
        ┌────────────┼────────────┐
        ↓            ↓            ↓
       RAS          PI3K         PLCγ
        ↓            ↓            ↓
      MAPK          AKT       IP3 + DAG
        ↓            ↓            ↓
   Proliferation   Survival    Ca²⁺/PKC

Therefore RTKs function as signaling hubs.

27. EGFR

EGFR = Epidermal Growth Factor Receptor

Also called:

ERBB1 / HER1

EGFR is one of the best-studied RTKs.

Ligands include:

EGF
TGF-α
Other EGFR-family ligands

EGF
 ↓
EGFR
 ↓
Dimerization
 ↓
Tyrosine phosphorylation
 ↓
RAS–MAPK
PI3K–AKT
PLCγ

28. ERBB Family

The ERBB/HER family includes:

EGFR / HER1 / ERBB1
HER2 / ERBB2
HER3 / ERBB3
HER4 / ERBB4

These receptors can form different receptor complexes.

Important point

HER2 has unusual ligand-binding characteristics and is a powerful signaling partner in ERBB receptor complexes.

HER3 has impaired intrinsic kinase activity and can signal effectively through partnership with other ERBB receptors.

29. Insulin Receptor

The insulin receptor is an RTK with a distinctive architecture.

It exists as a disulfide-linked receptor complex.

Insulin
  ↓
Insulin receptor
  ↓
Tyrosine phosphorylation
  ↓
IRS proteins
  ↓
PI3K–AKT
  ↓
Metabolic effects

The insulin receptor illustrates how RTKs regulate metabolism in addition to growth.

30. IRS Proteins

IRS = Insulin receptor substrate

After receptor activation, IRS proteins become phosphorylated and serve as signaling platforms.

Major downstream pathway:

Insulin receptor
 ↓
IRS
 ↓
PI3K
 ↓
AKT
 ↓
Metabolic responses

31. VEGF Receptors

VEGF receptors are important in:

Angiogenesis

VEGF
 ↓
VEGFR
 ↓
RTK activation
 ↓
PI3K / MAPK / other pathways
 ↓
Endothelial-cell responses
 ↓
Angiogenesis

32. FGFR

Fibroblast growth factor receptors regulate:

Development
Cell proliferation
Differentiation
Tissue repair

FGF
 ↓
FGFR
 ↓
RTK activation
 ↓
MAPK / PI3K pathways
 ↓
Cellular response

33. PDGFR

Platelet-derived growth factor receptors regulate:

Fibroblast proliferation
Migration
Vascular biology
Tissue repair

They are particularly important in wound healing and mesenchymal-cell signaling.

34. Neurotrophin Receptors

Some neurotrophin receptors are RTKs.

Examples:

TrkA
TrkB
TrkC

They regulate:

Neuronal survival
Differentiation
Axonal growth
Synaptic functions

35. RTKs and Cell Cycle

RTK signaling can promote cell-cycle entry.

Growth factor
 ↓
RTK
 ↓
RAS–MAPK
 ↓
Transcriptional changes
 ↓
Cyclin expression
 ↓
CDK activation
 ↓
G1 → S progression

This provides a major link between extracellular growth factors and cell-cycle regulation.

36. RTKs and Cell Survival

RTKs can promote survival through PI3K–AKT.

RTK
 ↓
PI3K
 ↓
AKT
 ↓
Anti-apoptotic signaling
 ↓
Cell survival

AKT can influence multiple components of the apoptotic machinery.

37. RTKs and Cell Migration

RTK signaling can regulate:

Actin polymerization
Rho GTPases
Focal adhesions
Cell polarity
Cytoskeletal remodeling

RTK
 ↓
PI3K / Rho / MAPK
 ↓
Actin + focal adhesion remodeling
 ↓
Cell migration

This connects RTK signaling with your earlier topics on integrins, actin cytoskeleton and mechanotransduction.

38. RTK–Integrin Crosstalk

RTKs and integrins frequently cooperate.

Growth factor
     ↓
    RTK
     ↘
       Signaling network
     ↗
  Integrin
     ↓
     ECM

This allows cells to integrate:

Chemical signals
Adhesion signals
Mechanical signals

39. RTK Transactivation

One receptor system can influence another.

For example:

GPCR
 ↓
Signaling intermediates
 ↓
EGFR activation
 ↓
MAPK

This is an example of RTK transactivation.

40. Negative Regulation of RTKs

RTK signaling must be tightly controlled.

Major mechanisms include:

Protein tyrosine phosphatases
Receptor internalization
Ubiquitination
Endosomal sorting
Lysosomal degradation
Negative feedback pathways

41. Protein Tyrosine Phosphatases

Protein tyrosine phosphatases remove phosphate groups from tyrosine residues.

RTK-Tyr-P
    ↓
Tyrosine phosphatase
    ↓
RTK-Tyr

This can reduce signaling.

42. Receptor Internalization

Activated RTKs can be internalized.

RTK activation
      ↓
Endocytosis
      ↓
Endosome
      ↓
 ┌────┴─────┐
 ↓          ↓
Recycle    Degrade

The balance between recycling and degradation affects signal duration.

43. Ubiquitination

Some RTKs are tagged with ubiquitin following activation.

A major example is EGFR.

RTK
 ↓
Ubiquitination
 ↓
Endosomal sorting
 ↓
Lysosome
 ↓
Degradation

Ubiquitination therefore contributes to receptor downregulation.

44. Cbl

Cbl is an important E3 ubiquitin ligase involved in regulation of activated RTKs, particularly EGFR-family signaling.

Activated RTK
 ↓
Cbl recruitment
 ↓
Ubiquitination
 ↓
Endocytosis / degradation

45. Signal Duration

RTK signaling is determined not simply by whether the receptor is activated, but by:

Strength of activation
Duration
Subcellular location
Receptor trafficking
Feedback
Phosphatase activity
Downstream pathway state

Signal
 │
 │─────── Strong + transient
 │
 │        vs
 │
 │──────── Weak + prolonged
 └──────────────────── Time

Different temporal patterns can produce different cellular outcomes.

46. RTK Mutations and Disease

Abnormal RTK signaling can contribute to:

Cancer
Developmental disorders
Metabolic disease
Fibrotic processes

Mechanisms include:

Activating mutations
Gene amplification
Receptor overexpression
Gene rearrangements
Excess ligand production
Failure of negative regulation

47. Oncogenic RTK Signaling

A simplified cancer model:

RTK mutation / amplification
          ↓
Persistent receptor activity
          ↓
RAS–MAPK + PI3K–AKT
          ↓
↑ proliferation
↑ survival
          ↓
Tumor progression

48. Constitutive Activation

A mutant RTK may signal even without ligand.

Normal:

Ligand → RTK → Signal


Mutant:

No ligand
   ↓
RTK*
   ↓
Continuous signaling

This is called:

Constitutive activation

49. RTK Inhibitors

Because abnormal RTK signaling occurs in several diseases, RTKs are important therapeutic targets.

Two broad approaches are:

Monoclonal antibodies

Can target extracellular receptor regions or ligands.

Small-molecule kinase inhibitors

Can inhibit intracellular kinase activity.

Mechanistically:

RTK
 ↓
Kinase inhibition
 ↓
↓ phosphorylation
 ↓
↓ downstream signaling

50. RTK Signaling and Systems Biology

A modern view treats RTK signaling as a network rather than a linear pathway.

                       RTK
                        │
          ┌─────────────┼─────────────┐
          ↓             ↓             ↓
        RAS             PI3K         PLCγ
          ↓             ↓             ↓
        RAF             AKT        IP3/DAG
          ↓             ↓             ↓
        MEK            mTOR          Ca²⁺
          ↓             │             │
        ERK             └─────┬───────┘
          │                   │
          └─────────┬─────────┘
                    ↓
             Cellular response

Crosstalk and feedback make RTK signaling highly dynamic.

51. Master-Level Concept: RTKs as Signaling Hubs

An activated RTK can be viewed as a phosphotyrosine signaling platform.

             RTK
              │
       Tyr-P Tyr-P Tyr-P
        │      │      │
        ↓      ↓      ↓
      GRB2    PI3K    PLCγ
        │      │      │
       RAS    AKT    Ca²⁺
        │      │      │
       MAPK   mTOR   PKC
        │      │      │
        └──────┼──────┘
               ↓
         Cellular response

This is more accurate than viewing an RTK as a simple linear switch.

52. RTK Signaling and Cellular Outcomes

Pathway	Major cellular functions
RAS–RAF–MEK–ERK	Proliferation, differentiation
PI3K–AKT	Survival, growth, metabolism
mTOR	Protein synthesis, growth, metabolism
PLCγ–IP3/DAG	Ca²⁺ signaling, PKC
Rho GTPases	Cytoskeleton, migration
STAT-associated pathways	Context-dependent transcription

53. High-Yield RTK Signaling Diagram

                         GROWTH FACTOR
                              ↓
                         ┌─────────┐
                         │   RTK   │
                         └────┬────┘
                              ↓
                         DIMERIZATION
                              ↓
                       TYROSINE-P
                              ↓
            ┌─────────────────┼─────────────────┐
            ↓                 ↓                 ↓
           GRB2              PI3K              PLCγ
            ↓                 ↓                 ↓
           SOS               PIP3              PIP2
            ↓                 ↓              ┌──┴──┐
         RAS-GTP              AKT             IP3  DAG
            ↓                 ↓                ↓    ↓
           RAF               mTOR             Ca²⁺  PKC
            ↓                 ↓                │    │
           MEK            Growth/survival      └─┬──┘
            ↓                                     ↓
           ERK                              Cellular response
            ↓
       Gene expression
            ↓
      Proliferation /
      differentiation

54. RTK vs GPCR: Examination Comparison

Characteristic	GPCR	RTK
Receptor architecture	7 TM	Single TM
Intracellular mechanism	G proteins	Tyrosine kinase
Receptor phosphorylation	Regulatory, not intrinsic kinase activity	Central activation mechanism
Main second messengers	cAMP, IP3, DAG, Ca²⁺	Often phosphotyrosine-dependent networks
Major pathways	Gs/Gi/Gq/G12/13	MAPK, PI3K–AKT, PLCγ
Common roles	Hormonal/neural/sensory	Growth/development/metabolism
Examples	β-adrenergic receptor	EGFR, insulin receptor

55. Examination Answer

Receptor Tyrosine Kinases

Receptor tyrosine kinases are single-pass transmembrane receptors that possess intrinsic intracellular tyrosine kinase activity. They play major roles in cell proliferation, differentiation, survival, metabolism, migration and development.

Binding of an extracellular ligand generally induces receptor dimerization or a conformational rearrangement that activates the intracellular kinase domains. The receptors undergo trans-autophosphorylation on tyrosine residues using ATP. These phosphotyrosine residues serve as docking sites for intracellular proteins containing SH2 or PTB domains.

Major signaling pathways include the GRB2–SOS–RAS–RAF–MEK–ERK pathway, which regulates proliferation and differentiation; the PI3K–PIP3–AKT–mTOR pathway, which promotes survival, growth and metabolism; and the PLCγ–IP3/DAG pathway, which regulates Ca²⁺ and PKC signaling.

RTK signaling is terminated or attenuated through protein tyrosine phosphatases, receptor internalization, ubiquitination, endosomal sorting and degradation. Dysregulated RTK signaling caused by receptor overexpression, amplification or activating mutations can contribute to cancer and other diseases.

56. Viva Questions

Q1. What is an RTK?
A transmembrane receptor with intrinsic or closely associated tyrosine kinase activity that converts extracellular signals into intracellular responses.

Q2. How many transmembrane domains does a typical RTK have?
One.

Q3. What happens after ligand binding?
The receptor undergoes dimerization or conformational rearrangement, kinase activation and tyrosine phosphorylation.

Q4. What is trans-autophosphorylation?
One receptor kinase phosphorylates tyrosine residues on the other receptor molecule in an activated receptor complex.

Q5. Why are phosphotyrosines important?
They serve as docking sites for signaling proteins containing SH2 or PTB domains.

Q6. What is the role of GRB2?
It acts as an adaptor linking activated RTKs to SOS and RAS.

Q7. What is SOS?
A guanine-nucleotide exchange factor that promotes RAS-GDP → RAS-GTP.

Q8. Name the MAPK cascade.
RAS → RAF → MEK → ERK.

Q9. What does PI3K produce?
PIP3 from PIP2.

Q10. What is the major downstream kinase activated by the PI3K pathway?
AKT.

Q11. What does PTEN do?
It antagonizes PI3K signaling by converting PIP3 toward PIP2.

Q12. Which phospholipase is activated downstream of many RTKs?
PLCγ.

Q13. What are the products of PLCγ-mediated PIP2 cleavage?
IP3 and DAG.

Q14. Name three important RTKs.
EGFR, insulin receptor and VEGFR.

Q15. How can RTKs contribute to cancer?
Through amplification, overexpression, activating mutations or failure of receptor downregulation, resulting in persistent signaling.

57. One-Minute Revision

                  LIGAND
                     ↓
                    RTK
                     ↓
            DIMERIZATION /
          CONFORMATIONAL CHANGE
                     ↓
          TYROSINE PHOSPHORYLATION
                     ↓
             DOCKING PROTEINS
                     ↓
       ┌─────────────┼──────────────┐
       ↓             ↓              ↓
      GRB2           PI3K           PLCγ
       ↓             ↓              ↓
      SOS           PIP3           PIP2
       ↓             ↓           ┌──┴──┐
     RAS-GTP         AKT          IP3  DAG
       ↓             ↓             ↓    ↓
      RAF           mTOR          Ca²⁺  PKC
       ↓             │             │    │
      MEK            └──────┬──────┴────┘
       ↓                    ↓
      ERK             CELLULAR RESPONSE
       ↓
 GENE EXPRESSION
       ↓
PROLIFERATION /
DIFFERENTIATION

Core memory line

Ligand → RTK dimerization/rearrangement → Tyr phosphorylation → docking proteins → MAPK + PI3K/AKT + PLCγ → cellular response

GRB2 → SOS → RAS → RAF → MEK → ERK

PI3K → PIP3 → AKT → mTOR

PLCγ → IP3 + DAG → Ca²⁺ + PKC

PTEN opposes PI3K

RTKs are signaling hubs, not simple ON/OFF switches.

Receptor Tyrosine Kinases (RTKs)