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

1. Definition

G-protein-coupled receptors (GPCRs) are a large family of cell-surface receptors characterized by seven transmembrane α-helical domains that transduce extracellular signals into intracellular responses through heterotrimeric G proteins and associated signaling pathways.

GPCRs respond to diverse ligands including:

Hormones
Neurotransmitters
Chemokines
Lipids
Metabolites
Odorants
Light
Extracellular ions

They regulate:

Metabolism
Heart rate
Neurotransmission
Sensory perception
Smooth-muscle contraction
Secretion
Cell migration
Gene expression

2. Basic GPCR Architecture

A GPCR contains:

7 transmembrane helices
Extracellular N-terminus
Intracellular C-terminus
Three intracellular loops
Three extracellular loops

             EXTRACELLULAR
                  │
               Ligand
                  ↓
            ┌───────────┐
            │    N      │
            │    ↓      │
            │  ╱╲ ╱╲    │
            │ ╱  ╳  ╲   │
            │╱ ╲ ╱ ╲ ╲  │
            │  7 TM     │
            │           │
            │     ↓     │
            │     C     │
            └───────────┘
                  │
             INTRACELLULAR

The seven helices are conventionally numbered:

TM1 → TM7

3. The Heterotrimeric G Protein

The classical signaling partner of a GPCR is a heterotrimeric G protein consisting of:

       Gα
      /  \
     /    \
   Gβ────Gγ

Heterotrimeric G protein

The Gα subunit binds guanine nucleotides:

GDP → inactive state
GTP → active state

4. The Central GPCR Signaling Cycle

The core mechanism is:

Inactive GPCR
     ↓
Ligand binding
     ↓
Receptor conformational change
     ↓
G protein coupling
     ↓
GDP → GTP exchange on Gα
     ↓
Gα-GTP + Gβγ
     ↓
Effector activation
     ↓
Second messengers
     ↓
Protein kinases
     ↓
Cellular response

5. Inactive State

In the resting state:

GPCR
 │
 │
Gα-GDP
 │
Gβγ

The G protein exists as a heterotrimer.

The Gα subunit contains GDP.

This represents the predominantly inactive state.

6. Ligand Binding

When an agonist binds the GPCR:

Ligand
   ↓
GPCR
   ↓
Conformational change
   ↓
G protein interaction

The activated receptor acts as a guanine-nucleotide exchange factor (GEF) for Gα.

This is an important molecular concept.

7. GDP–GTP Exchange

The activated GPCR promotes:

GDP release → GTP binding

Gα-GDP
   ↓
GDP released
   ↓
GTP binds
   ↓
Gα-GTP

Because cellular GTP is abundant, GTP rapidly occupies the nucleotide-binding site.

8. G Protein Activation

Gα-GTP undergoes a conformational change and changes its interactions with Gβγ.

Gαβγ-GDP
     ↓
GPCR activation
     ↓
Gα-GTP + Gβγ

Both Gα-GTP and Gβγ can regulate downstream effectors.

This is an important refinement of the classical model.

9. Major Gα Families

The major functional families are:

Gs
Gi/o
Gq/11
G12/13

Each can regulate different downstream pathways.

10. Gs Pathway

Gs → stimulates adenylyl cyclase

Ligand
  ↓
GPCR
  ↓
Gs
  ↓
Adenylyl cyclase
  ↓
↑ cAMP
  ↓
PKA
  ↓
Cellular response

Key memory

Gs = stimulates cAMP

11. Adenylyl Cyclase

Adenylyl cyclase converts:

ATP → cAMP

cAMP is an important second messenger.

ATP
 ↓
Adenylyl cyclase
 ↓
cAMP
 ↓
PKA

12. cAMP–PKA Pathway

cAMP activates protein kinase A (PKA).

GPCR
 ↓
Gs
 ↓
Adenylyl cyclase
 ↓
↑ cAMP
 ↓
PKA
 ↓
Protein phosphorylation
 ↓
Cell response

PKA can phosphorylate:

Metabolic enzymes
Ion channels
Transcription factors
Other signaling proteins

13. CREB Signaling

One important target of PKA is:

CREB — cAMP response element-binding protein

Simplified pathway:

GPCR
 ↓
Gs
 ↓
↑ cAMP
 ↓
PKA
 ↓
CREB phosphorylation
 ↓
DNA regulatory regions
 ↓
Gene transcription

Thus a GPCR signal can alter gene expression.

14. Gi/o Pathway

Gi generally inhibits adenylyl cyclase.

Ligand
  ↓
GPCR
  ↓
Gi
  ↓
↓ Adenylyl cyclase
  ↓
↓ cAMP
  ↓
↓ PKA activity

Key memory

Gi = inhibits cAMP

15. Gβγ Signaling

The βγ complex is not merely a passive structural component.

Gβγ can regulate:

Ion channels
PI3K
Certain phospholipases
Other signaling proteins

GPCR
 ↓
Gα-GTP + Gβγ
       │
       ├──→ Effector 1
       ├──→ Effector 2
       └──→ Ion channel

16. Gq Pathway

Gq commonly activates:

Phospholipase C-β (PLCβ)

Ligand
 ↓
GPCR
 ↓
Gq
 ↓
PLCβ
 ↓
PIP2 cleavage
 ┌───────┴───────┐
 ↓               ↓
IP3              DAG
 ↓               ↓
Ca²⁺ release     PKC

This is one of the most important GPCR pathways.

17. PIP2 Cleavage

PLCβ cleaves the membrane phospholipid:

PIP2

into:

Thus:

PIP2 → IP3 + DAG

These act as second messengers.

18. IP3

IP3 diffuses through the cytoplasm and binds the:

IP3 receptor

located on the endoplasmic reticulum.

PLCβ
 ↓
PIP2
 ↓
IP3
 ↓
IP3 receptor
 ↓
ER Ca²⁺ release
 ↓
↑ Cytosolic Ca²⁺

19. DAG

DAG remains within the plasma membrane.

It contributes to activation of:

Protein kinase C — PKC

PIP2
 ↓
DAG
 ↓
PKC activation
 ↓
Protein phosphorylation
 ↓
Cellular response

Full PKC activation often involves Ca²⁺ depending on the PKC isoform.

20. Calcium as a Second Messenger

The increase in intracellular Ca²⁺ can regulate:

Calmodulin
CaMKs
PKC isoforms
Contractile proteins
Secretion
Metabolism
Gene transcription

GPCR
 ↓
Gq
 ↓
PLCβ
 ↓
IP3
 ↓
ER
 ↓
↑ Ca²⁺
 ↓
Calmodulin / CaMK / PKC
 ↓
Cellular response

21. G12/13 Pathway

G12/13 proteins are particularly associated with regulation of:

Rho GTPases
Cytoskeletal organization
Cell shape
Migration
Contractility

GPCR
 ↓
G12/13
 ↓
RhoGEFs
 ↓
RhoA
 ↓
Actin cytoskeleton
 ↓
Contractility / migration

22. Comparison of Major G Proteins

G protein	Major effector	Major effect
Gs	Adenylyl cyclase	↑ cAMP
Gi/o	Adenylyl cyclase	↓ cAMP
Gq/11	PLCβ	↑ IP3/DAG/Ca²⁺
G12/13	RhoGEFs	Rho activation/cytoskeleton

Memory trick

Gs → cAMP goes up

Gi → cAMP goes down

Gq → Ca²⁺

G12/13 → Rho

23. Second Messengers in GPCR Signaling

Important GPCR-associated second messengers include:

cAMP
IP3
DAG
Ca²⁺

These amplify the original extracellular signal.

1 ligand
   ↓
1 receptor
   ↓
Multiple G proteins
   ↓
Many second messengers
   ↓
Many protein targets

24. Signal Amplification

GPCRs are powerful signaling amplifiers.

A single ligand–receptor interaction can ultimately influence thousands of intracellular molecules.

Ligand
  │
  ↓
GPCR
  │
  ↓
G protein
  │
  ↓
Effector enzyme
  │
  ↓
Many second messengers
  │
  ↓
Many kinases
  │
  ↓
Many cellular targets

25. GTPase Timer

Gα has intrinsic GTPase activity.

It hydrolyzes:

GTP → GDP + Pi

Gα-GTP
   ↓
GTP hydrolysis
   ↓
Gα-GDP
   ↓
Inactive state

This provides an intrinsic mechanism for signal termination.

26. Regulators of G Protein Signaling

RGS proteins

Regulators of G-protein signaling

They accelerate GTP hydrolysis by functioning as GTPase-accelerating proteins (GAPs) for certain Gα subunits.

Gα-GTP
   ↓
RGS
   ↓
Faster GTP hydrolysis
   ↓
Gα-GDP

27. GPCR Desensitization

Continuous stimulation can reduce GPCR responsiveness.

One major mechanism involves:

GPCR kinases — GRKs

Persistent ligand
      ↓
GPCR activation
      ↓
GRK phosphorylation
      ↓
β-arrestin recruitment
      ↓
G-protein uncoupling

28. β-Arrestin

β-arrestins perform two major functions.

1. Desensitization

They prevent further efficient coupling of the receptor to G proteins.

2. Signaling

They can act as scaffolds for signaling pathways such as:

MAPK pathways
Other kinase cascades

Therefore:

β-arrestin is not simply an OFF switch.

29. GPCR Internalization

β-arrestin can facilitate receptor interaction with endocytic machinery.

GPCR activation
      ↓
GRK phosphorylation
      ↓
β-arrestin
      ↓
Clathrin-associated endocytosis
      ↓
Endosome

The receptor may then be:

Recycled
Degraded
Resensitized

30. GPCR Recycling

Some internalized receptors are returned to the plasma membrane.

GPCR
 ↓
Endocytosis
 ↓
Endosome
 ↓
Dephosphorylation / recycling
 ↓
Plasma membrane
 ↓
Resensitized receptor

This allows cells to restore responsiveness.

31. GPCR Downregulation

Persistent stimulation can cause receptor degradation.

Persistent stimulation
        ↓
Internalization
        ↓
Lysosomal trafficking
        ↓
Receptor degradation
        ↓
Reduced receptor number

This is called downregulation.

32. Desensitization vs Downregulation

Feature	Desensitization	Downregulation
Meaning	Reduced receptor responsiveness	Reduced receptor abundance
Time scale	Often relatively rapid	Often slower
Mechanisms	Phosphorylation, arrestin	Internalization, degradation
Receptor number	May remain initially	Decreases

33. Homologous Desensitization

When only the activated receptor becomes desensitized:

Homologous desensitization

GRKs are particularly important.

Activated GPCR
      ↓
GRK
      ↓
Specific receptor phosphorylation
      ↓
β-arrestin

34. Heterologous Desensitization

A signaling pathway activated by one receptor can cause reduced responsiveness of other receptors.

For example:

Receptor A
   ↓
Kinase activation
   ↓
Phosphorylation
   ↓
Receptor B
   ↓
Reduced responsiveness

This is called heterologous desensitization.

35. Biased Agonism

Different ligands can stabilize different GPCR conformations.

Ligand A
   ↓
GPCR conformation A
   ↓
G protein pathway

Ligand B
   ↓
GPCR conformation B
   ↓
β-arrestin pathway

This is called:

Biased agonism / functional selectivity

It is an important principle in modern pharmacology.

36. GPCR Allosteric Regulation

Ligands may bind:

Orthosteric site

The endogenous ligand-binding site.

Allosteric site

A distinct regulatory site.

          Ligand
             ↓
        Orthosteric
           site

       ┌─────────┐
       │  GPCR   │
       └─────────┘
             ↑
       Allosteric
         ligand

Allosteric ligands can modify:

Affinity
Efficacy
Receptor conformation
Signaling bias

37. GPCR Dimerization and Oligomerization

Some GPCRs can form:

Homodimers
Heterodimers
Higher-order complexes

This can influence:

Ligand recognition
Trafficking
Signaling
Pharmacological properties

The extent and physiological significance vary among receptors.

38. GPCR Crosstalk

GPCR signaling interacts with other receptor systems.

For example:

GPCR
 ↓
Src / metalloprotease activity
 ↓
Growth-factor receptor activation
 ↓
MAPK signaling

This type of interaction is called receptor crosstalk or transactivation, depending on mechanism.

39. GPCR and MAPK

GPCRs can activate MAPK pathways through several mechanisms.

GPCR
 ↓
G proteins / β-arrestin
 ↓
Ras
 ↓
Raf
 ↓
MEK
 ↓
ERK
 ↓
Nucleus
 ↓
Gene expression

This links GPCR signaling with proliferation and differentiation.

40. GPCR and PI3K–AKT

Some GPCRs activate PI3K–AKT signaling.

GPCR
 ↓
Gβγ / other signaling intermediates
 ↓
PI3K
 ↓
PIP3
 ↓
AKT
 ↓
Survival / metabolism / growth

41. GPCR and Rho GTPases

Particularly through G12/13:

GPCR
 ↓
G12/13
 ↓
RhoGEF
 ↓
RhoA
 ↓
ROCK
 ↓
Actomyosin
 ↓
Cell contraction / migration

This is particularly relevant to your previous topics on:

Actin cytoskeleton
Cell polarity
Mechanotransduction
ECM remodeling

42. GPCR Signaling and Calcium

GPCRs can regulate Ca²⁺ through several mechanisms.

Gq–PLC pathway

Gq
 ↓
PLCβ
 ↓
IP3
 ↓
ER
 ↓
Ca²⁺ release

Other mechanisms

Gβγ and other signaling pathways can regulate plasma-membrane calcium channels.

43. GPCRs in Sensory Biology

GPCRs are major sensory receptors.

Examples include:

Olfactory receptors

Detect odorants.

Rhodopsin

Detects light in photoreceptor cells.

Light
 ↓
Rhodopsin
 ↓
Transducin
 ↓
PDE
 ↓
↓ cGMP
 ↓
Ion-channel regulation
 ↓
Phototransduction

44. Rhodopsin — Special GPCR Signaling

Rhodopsin is a highly specialized GPCR.

In darkness:

↑ cGMP
 ↓
Cation channels open
 ↓
Photoreceptor depolarization

Light activates rhodopsin:

Light
 ↓
Rhodopsin
 ↓
Transducin
 ↓
PDE
 ↓
↓ cGMP
 ↓
Cation channels close
 ↓
Hyperpolarization

This demonstrates how GPCRs can control ion-channel activity.

45. GPCR Signaling and Metabolism

GPCRs regulate:

Glucose metabolism
Lipolysis
Appetite
Insulin secretion
Glucagon signaling

For example:

GPCR
 ↓
Gs
 ↓
cAMP
 ↓
PKA
 ↓
Metabolic enzyme regulation

46. GPCR Signaling and Gene Expression

Although GPCRs are membrane receptors, their effects can reach the nucleus.

Ligand
 ↓
GPCR
 ↓
G protein
 ↓
Second messenger
 ↓
Kinase
 ↓
Transcription factor
 ↓
DNA
 ↓
Gene expression

Examples include:

CREB
AP-1
NF-κB
Other context-dependent transcriptional regulators

47. Spatial Organization of GPCR Signaling

Modern cell biology recognizes that GPCR signaling is often spatially organized.

GPCRs can signal from:

Plasma membrane
Endosomes
Specialized membrane domains

Thus:

Where a receptor signals can influence what signal it produces.

48. Endosomal GPCR Signaling

Some internalized GPCRs remain signaling competent.

Plasma membrane
      ↓
GPCR activation
      ↓
Endocytosis
      ↓
Endosome
      ↓
Continued signaling

This challenges the older idea that:

internalization = complete signal termination

49. GPCR Signaling Is a Dynamic Cycle

The modern model:

        ACTIVATION
            ↓
         SIGNALING
            ↓
      PHOSPHORYLATION
            ↓
       DESENSITIZATION
            ↓
       INTERNALIZATION
            ↓
      ┌─────┴──────┐
      ↓            ↓
   RECYCLING     DEGRADATION
      ↓
  RESENSITIZATION

This cycle allows cells to precisely regulate signal intensity and duration.

50. Integrated GPCR Signaling

                         LIGAND
                            ↓
                          GPCR
                            ↓
                 G-protein activation
                            ↓
              ┌─────────────┼─────────────┐
              ↓             ↓             ↓
             Gs            Gi           Gq
              ↓             ↓             ↓
         Adenylyl       ↓ Adenylyl      PLCβ
          cyclase         cyclase          ↓
              ↓             ↓          PIP2 cleavage
            cAMP          ↓ cAMP       ┌────┴────┐
              ↓             ↓          ↓         ↓
             PKA           PKA         IP3       DAG
              ↓                         ↓         ↓
            CREB                      Ca²⁺       PKC
              │                         │         │
              └──────────┬──────────────┴─────────┘
                         ↓
                   CELL RESPONSE

51. GPCR Signaling and ECM

GPCR signaling can regulate ECM biology indirectly through:

Fibroblast activation
Rho/actomyosin signaling
MMP expression
Cell migration
Cytokine production

GPCR
 ↓
G protein
 ↓
Rho / MAPK / PI3K
 ↓
Cytoskeleton + gene expression
 ↓
MMP / ECM regulation
 ↓
ECM remodeling

This provides a direct connection to your preceding ECM remodeling and MMP topics.

52. High-Yield Comparison

G protein	Effector	Second messenger	Major downstream effect
Gs	Adenylyl cyclase ↑	cAMP ↑	PKA
Gi/o	Adenylyl cyclase ↓	cAMP ↓	Reduced PKA
Gq/11	PLCβ	IP3, DAG, Ca²⁺	PKC/Ca²⁺ signaling
G12/13	RhoGEFs	Rho signaling	Cytoskeletal remodeling

53. GPCR vs RTK

Feature	GPCR	RTK
Transmembrane domains	7	Usually 1
Major coupling	Heterotrimeric G proteins	Tyrosine kinase
Typical second messengers	cAMP, IP3/DAG, Ca²⁺	Often phosphotyrosine-dependent signaling
Major pathways	Gs/Gi/Gq/G12/13	RAS–MAPK, PI3K–AKT
Desensitization	GRKs/β-arrestin	Phosphatases/internalization etc.
Examples	β-adrenergic receptor	EGFR

54. Examination Answer

GPCR Signaling

G-protein-coupled receptors are seven-transmembrane cell-surface receptors that convert extracellular signals into intracellular responses through heterotrimeric G proteins. In the inactive state, the Gα subunit binds GDP and associates with Gβγ. Ligand binding induces a conformational change in the GPCR, allowing it to function as a guanine-nucleotide exchange factor and promote GDP–GTP exchange on Gα.

Gα-GTP and Gβγ can then regulate downstream effectors. Major Gα families include Gs, Gi/o, Gq/11 and G12/13. Gs stimulates adenylyl cyclase and increases cAMP, activating PKA. Gi/o generally inhibits adenylyl cyclase and decreases cAMP. Gq activates PLCβ, which cleaves PIP2 into IP3 and DAG, resulting in intracellular Ca²⁺ release and PKC activation. G12/13 activates Rho-associated signaling through RhoGEFs and regulates cytoskeletal organization.

Signal termination occurs through Gα GTP hydrolysis, often accelerated by RGS proteins. GPCRs can also undergo GRK-mediated phosphorylation followed by β-arrestin recruitment, producing desensitization and receptor internalization. Internalized receptors may be recycled or degraded.

Modern GPCR biology also includes β-arrestin-dependent signaling, biased agonism, allosteric regulation, receptor trafficking, endosomal signaling and receptor crosstalk. Thus, GPCR signaling is a dynamic and spatially regulated process rather than a simple ligand–receptor ON/OFF mechanism.

55. Viva Questions

Q1. What is a GPCR?
A seven-transmembrane receptor that transduces extracellular signals through heterotrimeric G proteins and other signaling mechanisms.

Q2. What are the three G-protein subunits?
Gα, Gβ and Gγ.

Q3. Which nucleotide is associated with inactive Gα?
GDP.

Q4. Which nucleotide is associated with active Gα?
GTP.

Q5. What is the role of GPCR in GDP–GTP exchange?
The activated receptor functions as a GEF for Gα.

Q6. What does Gs do?
Stimulates adenylyl cyclase and increases cAMP.

Q7. What does Gi do?
Generally inhibits adenylyl cyclase and decreases cAMP.

Q8. What does Gq activate?
PLCβ.

Q9. What are the products of PIP2 cleavage?
IP3 and DAG.

Q10. What does IP3 do?
Promotes Ca²⁺ release from the ER through IP3 receptors.

Q11. What is the function of DAG?
It contributes to activation of PKC.

Q12. What is G12/13 associated with?
Rho GTPase activation and cytoskeletal regulation.

Q13. What terminates Gα signaling?
GTP hydrolysis to GDP.

Q14. What are RGS proteins?
Regulators of G-protein signaling that accelerate GTP hydrolysis for certain Gα proteins.

Q15. What is β-arrestin?
A protein involved in GPCR desensitization, internalization and signaling.

Q16. What is biased agonism?
Preferential activation of particular signaling pathways by different ligands acting at the same receptor.

56. One-Minute Revision

                    GPCR
                      │
                   Ligand
                      ↓
             Conformational change
                      ↓
              GDP → GTP on Gα
                      ↓
          ┌───────────┼───────────┐
          ↓           ↓           ↓
         Gs          Gi          Gq
          ↓           ↓           ↓
        ↑cAMP       ↓cAMP        PLCβ
          ↓           ↓           ↓
         PKA         PKA      IP3 + DAG
          ↓                       ↓
        CREB                 Ca²⁺ + PKC
          │                       │
          └──────────┬────────────┘
                     ↓
                CELL RESPONSE

              G12/13 → Rho → Actin

Core memory line

Gs → ↑ cAMP → PKA

Gi → ↓ cAMP

Gq → PLC → IP3 + DAG → Ca²⁺/PKC

G12/13 → Rho → cytoskeleton

Gα-GTP → active

Gα-GTP → GDP → signal termination

GRK + β-arrestin → desensitization/internalization

Modern concept: GPCRs can signal through both G proteins and β-arrestin, from both plasma membrane and intracellular compartments.