Cell Bio 1 Exam 2
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Be able to describe the major parts of GPCR signaling pathway
G protein–coupled receptors (GPCRs) are cell-surface receptors that respond to extracellular signaling molecules and activate intracellular signaling pathways.
The major components of a GPCR signaling pathway include:
Extracellular signaling molecule (ligand): A hormone or neurotransmitter that binds to a specific receptor and initiates signaling.
GPCR: Recognizes the extracellular ligand and undergoes a conformational change that activates the associated G protein.
Heterotrimeric G protein: Acts as an intracellular ON/OFF switch that transmits the signal from the activated receptor.
Effector protein: Usually an enzyme or ion channel that becomes activated or inhibited by the G protein.
Second messengers: Small intracellular signaling molecules, such as cAMP, IP3, DAG, and Ca²⁺, that help propagate and amplify the signal.
Protein kinases: Enzymes that phosphorylate specific target proteins, changing their activity and producing a cellular response.
General steps of the signaling pathway:
An extracellular ligand binds to its specific GPCR on the plasma membrane.
The activated receptor functions as a guanine nucleotide exchange factor (GEF), promoting the exchange of GDP for GTP on the Gα subunit.
The activated G protein interacts with an effector enzyme or ion channel.
The effector enzyme may generate multiple second messenger molecules.
Second messengers activate downstream proteins, including protein kinases.
Protein kinases phosphorylate target proteins, resulting in changes in cellular metabolism, movement, or gene expression.
Overall: GPCR signaling allows extracellular signals to be converted into specific intracellular responses and can amplify the original signal.
What are the 4 types of extracellular signaling?
Endocrine signaling
Signaling molecules are synthesized and secreted by signaling cells
These molecules, known as hormones, travel long distances through the circulatory system and affect distant target cells
Ex. Epinephrine and insulin
Paracrine Signaling
Signaling molecules are secreted by a cell and act on nearby target cells (acting locally on nearby cells)
Ex. Neurotransmitters and growth factors\
Autocrine signaling
A cell secretes a signaling molecule that binds to receptors on the same cell
This allows the cell to respond to signals that it produces itself
Important in regulating cell growth and other cellular activities
Ex. a cell responding to its own growth factors
Membrane protein signaling (contact-dependent signaling)
Signaling occurs when a membrane-bound signaling protein on one cell directly intercepts with a receptor on a neighboring cell
Remains attached to the plasma membrane
Requires direct contact
Important for cell communication and development
Describe the importance of protein kinases/phosphatases and GTP-binding proteins in regulation of protein activity
Protein kinases
Protein kinases are enzymes that regulate protein activity through phosphorylation & amplifies signal to change activity of target cells
They transfer the terminal phosphate group from ATP to specific amino acid residues on target proteins
These amino acids commonly include serine (Ser), threonine (Thr), and tyrosine (Tyr)
Protein phosphatases
Protein phosphatases regulate protein activity through dephosphorylation and allows target cells to return to normal activity
They hydrolyze and remove phosphate groups from phosphorylated proteins
GTP binding proteins (G proteins)
GTP binding proteins act as ON/OFF switches
When a G protein is bound to GTP, it is active (ON)
When it is bound to GDP, its inactive (OFF)
Guanine nucleotide exchange factors (GEFs) promote activation by stimulating the release of GDP and allowing GTP to bind.
GTPase activity causes GTP to be hydrolyzed into GDP + Pi, switching the protein OFF.
Accelerate GTP hydrolysis, helping terminate signaling.
Does GPCRs activate heterotrimeric or monomeric G proteins? What is the structure of the heterotrimeric G proteins?
GPCRs directly activate heterotrimeric G proteins, rather than monomeric G proteins
Heterotrimeric G protein consists of 3 subunits
Gα (alpha): Binds GDP or GTP + intrinsic GTPase activity.
Gβ (beta): Forms a complex with the gamma subunit + participates in intracellular signaling.
Gγ (gamma): Associates with the beta subunit + helps anchor G protein complex to membrane.
In the inactive state
The Gα subunit is bound to GDP.
The Gα, Gβ, and Gγ subunits remain as a heterotrimeric complex.
In the active state
A ligand binds to the GPCR, causing a conformational change
Activated GPCR acts as a GEF, promoting the exchange of GDP for GTP on Gα.
Gα-GTP separates from the Gβγ complex.
Both Gα-GTP and Gβγ can regulate effector proteins
Gα eventually hydrolyzes GTP into GDP, returning to its inactive state
Monomeric G proteins, such as Ras, consist of a single GTP-binding protein and are generally activated through other signaling proteins rather than directly by GPCRs.
What are second messengers and what role do they play in propagating the signal intracellularly?
Second messengers are small, nonprotein intracellular signaling molecules that transmit signals from activated cell-surface receptors to downstream target proteins.
The extracellular signaling molecule is considered the first messenger, while the intracellular molecules are the second messengers.
Second messengers are important because they help propagate and amplify the original signal within the cell.
Common second messengers include:
cAMP: Activates protein kinase A (PKA), which phosphorylates multiple target proteins.
IP3: Stimulates the release of Ca²⁺ from the endoplasmic reticulum.
DAG: Remains associated with the plasma membrane and helps activate protein kinase C (PKC).
Ca²⁺: Regulates numerous cellular responses, including muscle contraction
cGMP: Participates in signaling pathways involved in smooth muscle relaxation.
How second messengers propagate signals:
An extracellular ligand binds to receptor.
Receptor stimulates an intracellular signaling protein or enzyme.
Enzyme generates multiple second messenger molecules.
Second messengers interact with and activate target proteins.
Activated proteins produce cellular response.
List the steps of the epinephrine signaling pathway and the roles that each major molecule/protein/kinase plays in the process
Epinephrine is a hormone that activates GPCR signaling in liver and skeletal muscle cells.
Stimulates glycogen breakdown (glycogenolysis) & inhibits glycogen synthesis for energy
Steps of the epinephrine signaling pathway:
1. Epinephrine binds to the β-adrenergic receptor.
This causes a conformational change that activates the receptor.
2. The GPCR activates the heterotrimeric Gs protein.
The activated receptor functions as a GEF.
GDP is released from the Gαs subunit and replaced by GTP.
The activated Gαs-GTP separates from the Gβγ complex.
3. Gαs-GTP activates adenylyl cyclase.
Adenylyl cyclase is an effector enzyme located in the plasma membrane.
Its activation increases the production of cAMP.
4. Adenylyl cyclase converts ATP into cAMP.
cAMP acts as an intracellular second messenger.
Multiple cAMP molecules are generated, resulting in signal amplification.
5. cAMP activates protein kinase A (PKA).
cAMP binds to the regulatory subunits of inactive PKA.
This releases the catalytic subunits, allowing them to phosphorylate target proteins.
6. PKA activates phosphorylase kinase.
PKA phosphorylates and activates phosphorylase kinase.
Phosphorylase kinase then phosphorylates glycogen phosphorylase.
7. Glycogen phosphorylase becomes activated.
Activated glycogen phosphorylase stimulates glycogenolysis.
Glycogen is broken down to release glucose-1-phosphate, which can be converted into glucose-6-phosphate for energy metabolism.
8. PKA inhibits glycogen synthase.
PKA phosphorylates glycogen synthase, decreasing its activity.
This prevents the cell from simultaneously synthesizing glycogen while promoting glycogen breakdown.
9. The signal is terminated.
Gαs hydrolyzes GTP into GDP and becomes inactive.
Adenylyl cyclase activity decreases.
cAMP phosphodiesterase (PDE) converts cAMP into AMP.
Protein phosphatases remove phosphate groups from target proteins, helping restore their original activities.
Overall: Epinephrine activates the GPCR → Gαs → adenylyl cyclase → cAMP → PKA pathway, which stimulates glycogen breakdown and inhibits glycogen synthesis to increase glucose availability.
How are cAMP levels regulated?
cAMP is an intracellular second messenger whose concentration is regulated by its synthesis and degradation.
Adenylyl cyclase (AC):
Catalyzes the conversion of ATP into cAMP.
Activation of Gαs stimulates adenylyl cyclase, increasing intracellular cAMP concentrations.
Inhibitory G proteins (Gαi):
Certain GPCRs activate Gαi, which inhibits adenylyl cyclase.
This reduces cAMP production.
cAMP phosphodiesterase (PDE):
Catalyzes the hydrolysis of cAMP into AMP.
AMP does not function as a cAMP second messenger.
This decreases intracellular cAMP levels and helps terminate signaling.
Feedback regulation:
Signaling pathways can regulate adenylyl cyclase and PDE activity to control the strength and duration of the cAMP response.
GTP hydrolysis by Gαs also helps terminate stimulation of adenylyl cyclase.
Overall: cAMP levels increase when adenylyl cyclase is activated and decrease when cAMP is degraded by phosphodiesterase or when adenylyl cyclase activity is inhibited.
In unstimulated cells PKA exists in an inactive state. Explain how PKA switches from inactive to active
Protein kinase A (PKA) is a cAMP-dependent protein kinase that regulates multiple intracellular target proteins through phosphorylation.
In unstimulated cells, PKA exists as an inactive complex containing:
Two regulatory (R) subunits: Bind to and inhibit the catalytic subunits.
Two catalytic (C) subunits: Responsible for transferring phosphate groups from ATP to target proteins.
Activation of PKA:
An extracellular signal activates a GPCR and stimulates adenylyl cyclase.
Adenylyl cyclase increases intracellular cAMP concentrations.
cAMP binds to the regulatory subunits of PKA.
Binding of cAMP causes a conformational change in the regulatory subunits.
The inhibitory interaction between the regulatory and catalytic subunits is disrupted.
The catalytic subunits are released and become active.
Active catalytic subunits phosphorylate specific target proteins, changing their activities.
Deactivation of PKA:
When cAMP levels decrease, cAMP dissociates from the regulatory subunits.
The regulatory subunits can reassociate with and inhibit the catalytic subunits.
Protein phosphatases remove phosphate groups from downstream target proteins.
Overall: PKA switches from inactive to active when cAMP binds to its regulatory subunits and releases the active catalytic subunits.
Explain the general role of PKA activation/deactivation in glycogen metabolism
Glycogen metabolism involves two major processes:
Glycogenolysis: Breakdown of glycogen to provide glucose for energy metabolism.
Glycogenesis: Synthesis of glycogen for glucose storage.
These processes are regulated by two important enzymes:
Glycogen phosphorylase: Promotes glycogen breakdown.
Glycogen synthase: Promotes glycogen synthesis.
When PKA is activated:
Epinephrine or glucagon signaling increases cAMP concentrations, activating PKA.
PKA phosphorylates and activates phosphorylase kinase.
Phosphorylase kinase phosphorylates and activates glycogen phosphorylase, increasing glycogen breakdown.
PKA also phosphorylates and inhibits glycogen synthase, decreasing glycogen synthesis.
As a result, the cell promotes glycogenolysis while reducing glycogen storage.
This is useful when the body requires additional energy.
When PKA is deactivated:
cAMP concentrations decrease, reducing PKA activity.
Protein phosphatases dephosphorylate enzymes involved in glycogen metabolism.
Dephosphorylation inactivates phosphorylase kinase and glycogen phosphorylase, reducing glycogen breakdown.
Dephosphorylation activates glycogen synthase, allowing glycogen synthesis to increase.
This promotes glucose storage when the demand for immediate energy is reduced.
- Overall: PKA activation stimulates glycogen breakdown and inhibits glycogen synthesis, while PKA deactivation and protein dephosphorylation favor glycogen synthesis.
How is CREB activated and what role does it play in the cell?
CREB (cAMP response element-binding protein) is a transcription factor that links cAMP/PKA signaling to changes in gene expression.
CREB regulates the transcription of specific genes in response to extracellular signaling molecules.
Steps of CREB activation:
An extracellular ligand activates a GPCR.
The GPCR activates Gαs, which stimulates adenylyl cyclase.
Adenylyl cyclase increases intracellular cAMP concentrations.
cAMP activates PKA by releasing its catalytic subunits.
Active PKA catalytic subunits can translocate into the nucleus.
PKA phosphorylates CREB, activating its transcription-regulating function.
Phosphorylated CREB interacts with co-activator proteins, including CBP/p300.
The CREB complex binds to specific DNA regulatory sequences known as cAMP response elements (CREs).
This promotes the transcription of specific target genes.
Role of CREB:
Regulates gene expression in response to extracellular signals.
Helps produce longer-lasting cellular responses by influencing the synthesis of new proteins.
Participates in processes such as metabolism, cell survival, and neuronal function.
Overall: CREB allows a signal that begins at the plasma membrane to reach the nucleus and regulate gene transcription.
Describe the pathway leading to the generation of IP3 and DAG and how it affects calcium levels in the cell
Certain GPCRs activate the phospholipase C (PLC) signaling pathway, which produces two important second messengers: IP3 and DAG.
These second messengers are generated from phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid located in the plasma membrane.
Steps of the IP3/DAG signaling pathway:
An extracellular ligand binds to a specific GPCR.
The activated GPCR stimulates the heterotrimeric Gq protein.
GDP on the Gαq subunit is exchanged for GTP, activating Gαq.
Gαq-GTP activates phospholipase C-β (PLC-β).
PLC-β hydrolyzes PIP2 into two second messengers:
IP3 (inositol 1,4,5-trisphosphate)
DAG (diacylglycerol)
IP3 is soluble and diffuses through the cytosol to the endoplasmic reticulum (ER).
IP3 binds to IP3-gated Ca²⁺ channels on the ER membrane.
These channels open, allowing stored Ca²⁺ to move from the ER into the cytosol.
The increased cytosolic Ca²⁺ concentration activates calcium-dependent proteins and signaling pathways.
DAG remains associated with the plasma membrane and, together with Ca²⁺, helps activate protein kinase C (PKC).
Activated PKC phosphorylates target proteins, producing specific cellular responses.
Importance of Ca²⁺:
Cytosolic Ca²⁺ concentrations are normally maintained at low levels.
Even a small increase in Ca²⁺ can activate proteins and produce a significant cellular response.
Ca²⁺ participates in processes such as secretion, muscle contraction, and enzyme regulation.
Overall: GPCR activation of PLC produces IP3 and DAG. IP3 increases cytosolic Ca²⁺ by releasing calcium from the ER, while DAG and Ca²⁺ help activate PKC.
What is the connection between acetylcholine and smooth muscle relaxation?
Acetylcholine (ACh) is a neurotransmitter that can stimulate the relaxation of vascular smooth muscle through a GPCR-mediated signaling pathway.
This process involves endothelial cells, calcium ions, nitric oxide (NO), and cGMP.
Steps of the pathway:
Acetylcholine binds to a muscarinic GPCR on the surface of endothelial cells lining blood vessels.
The activated receptor stimulates Gq and phospholipase C.
PLC hydrolyzes PIP2 to produce IP3 and DAG.
IP3 stimulates the release of Ca²⁺ from the endoplasmic reticulum.
The increase in cytosolic Ca²⁺ activates calmodulin, which helps activate endothelial nitric oxide synthase (eNOS).
eNOS produces nitric oxide (NO).
NO diffuses from the endothelial cells into neighboring vascular smooth muscle cells.
NO activates soluble guanylyl cyclase, which converts GTP into cGMP.
cGMP activates protein kinase G (PKG).
PKG promotes a reduction in smooth muscle contraction, including by lowering the effective cytosolic Ca²⁺ signal.
This causes the vascular smooth muscle to relax, resulting in vasodilation.
- Overall: Acetylcholine stimulates endothelial cells to produce nitric oxide, which activates the NO → cGMP → PKG pathway in neighboring smooth muscle cells, causing relaxation.
Understand the clinical presentation of cholera and how it is connected to GPCRs
Cholera is an intestinal infection caused by the bacterium Vibrio cholerae.
The bacterium produces cholera toxin, which interferes with G protein signaling in intestinal epithelial cells.
Cholera is associated with severe watery diarrhea, electrolyte loss, and dehydration.
Connection to GPCR signaling:
Cholera toxin enters intestinal epithelial cells.
The toxin causes a covalent modification called ADP-ribosylation of the Gαs protein.
This modification inhibits the GTPase activity of Gαs, preventing the normal hydrolysis of GTP into GDP.
As a result, Gαs remains in its active GTP-bound state.
The continuously active Gαs stimulates adenylyl cyclase.
Adenylyl cyclase produces abnormally high levels of cAMP.
Increased cAMP causes prolonged activation of PKA.
PKA phosphorylates and activates the CFTR chloride channel in intestinal epithelial cells.
This increases the secretion of Cl⁻ into the intestinal lumen.
Sodium ions and water move into the intestinal lumen, resulting in excessive fluid loss.
Clinical presentation:
Profuse watery diarrhea, often described as rice-water stools.
Severe dehydration due to rapid loss of water and electrolytes.
Electrolyte imbalances that can cause muscle cramps and weakness.
Low blood pressure and potentially hypovolemic shock in severe cases.
Overall: Cholera toxin prevents Gαs from switching OFF, causing continuous cAMP production, excessive chloride and water secretion, and severe watery diarrhea.
Why are signaling responses involving modification of existing proteins generally faster than responses requiring changes in gene expression? Why is this biologically important?
Signaling responses involving the modification of existing proteins are generally faster because the proteins are already present within the cell and can be activated or inhibited immediately.
These responses often involve phosphorylation and dephosphorylation, which regulate protein activity without requiring the synthesis of new proteins.
Protein kinases and phosphatases can rapidly modify target proteins, allowing cells to respond to extracellular signals within seconds or minutes.
- Responses involving changes in gene expression:
Generally take longer because they require multiple steps before a cellular response can occur.
An extracellular signal must first activate an intracellular signaling pathway.
The signaling pathway activates specific transcription factors that regulate gene expression.
The DNA must be accessible, meaning the chromatin must be sufficiently open for transcription factors to bind.
The target gene must undergo transcription to produce mRNA.
The mRNA must then be translated to synthesize new proteins.
These processes generally require more time than modifying proteins that already exist.
- Biological importance:
Rapid protein modification allows cells to respond immediately to changes in their environment.
For example, epinephrine signaling activates existing enzymes involved in glycogen breakdown, allowing glucose to become available quickly.
Changes in gene expression allow cells to produce longer-lasting responses, including changes in growth, differentiation, and development.
Having both mechanisms allows cells to regulate immediate metabolic activities while also adapting to long-term changes.
- Overall: Modification of existing proteins produces faster cellular responses because it does not require new protein synthesis, while changes in gene expression are generally slower but allow longer-lasting changes in cellular function.
Compare and contrast TGF-B/Smads and JAK/STAT signaling pathways
TGF-β/Smad Signaling Pathway:
TGF-β (transforming growth factor-beta) is an extracellular signaling molecule that regulates cell growth, differentiation, and development.
TGF-β receptors are receptor serine/threonine kinases, meaning they phosphorylate serine and threonine residues on target proteins.
Steps of the pathway:
TGF-β binds to its specific receptor on the plasma membrane.
The type II receptor phosphorylates and activates the type I receptor.
The activated type I receptor phosphorylates Smad2 or Smad3.
Phosphorylated Smad2/3 associates with Smad4, forming a transcription-regulating complex.
The Smad complex translocates into the nucleus.
Smads interact with regulatory DNA sequences and cell-specific master transcription factors to regulate target gene expression.
This pathway is important in inhibiting cell proliferation and regulating growth and differentiation.
Negative feedback mechanisms, including inhibitory Smads and Ski proteins, help down-regulate signaling.
JAK/STAT Signaling Pathway:
The JAK/STAT pathway is activated by cytokine receptors and regulates gene expression related to immunity, cell division, and cell death.
Cytokine receptors do not contain intrinsic kinase activity but instead activate separate Janus kinases (JAKs).
Steps of the pathway:
A cytokine binds to its specific cell-surface receptor.
Receptor activation brings the associated JAK proteins close together.
JAK proteins become activated through phosphorylation.
Activated JAKs phosphorylate tyrosine residues on the receptor.
STAT (signal transducer and activator of transcription) proteins bind to the phosphorylated receptor.
JAKs phosphorylate the STAT proteins, causing them to dissociate and form dimers.
STAT dimers translocate into the nucleus.
STAT dimers bind to specific regulatory DNA sequences and activate transcription of target genes.
This pathway is important in regulating immune responses, cell proliferation, and cell survival.
Signaling is terminated by SHP1 phosphotyrosine phosphatase and SOCS proteins.
Similarities:
Both pathways begin when an extracellular signaling molecule binds to a cell-surface receptor.
Both involve phosphorylation to activate intracellular signaling proteins.
Both activate transcription factors that translocate into the nucleus.
Both regulate the transcription of specific target genes.
Both are involved in regulating cellular growth and other cellular functions.
Differences:
TGF-β/Smad uses receptor serine/threonine kinases, while JAK/STAT uses receptor-associated tyrosine kinases.
TGF-β/Smad activates Smad transcription factors, while JAK/STAT activates STAT transcription factors.
Smad2/3 associates with Smad4 to form a complex, while phosphorylated STAT proteins form dimers.
TGF-β/Smad mainly regulates cell growth, differentiation, and development, while JAK/STAT regulates immunity, cell division, and cell death.
TGF-β/Smad signaling is down-regulated by inhibitory Smads and Ski proteins, while JAK/STAT signaling is terminated by SHP1 and SOCS proteins.
What makes RTKs different from cytokine receptors?
Receptor tyrosine kinases (RTKs) and cytokine receptors are both cell-surface receptors that activate intracellular signaling pathways involving protein tyrosine phosphorylation.
The major difference is whether the receptor itself contains the kinase enzyme.
Receptor tyrosine kinases (RTKs):
RTKs contain an intrinsic tyrosine kinase domain within the receptor's own polypeptide chain.
This means the receptor itself has the enzymatic ability to phosphorylate tyrosine residues.
Ligand binding generally promotes receptor dimerization or rearrangement, leading to kinase activation.
Activated RTKs undergo autophosphorylation, producing phosphotyrosine sites that recruit intracellular signaling proteins.
RTKs regulate processes such as cell growth, proliferation, differentiation, and metabolism.
Examples include the epidermal growth factor receptor (EGFR/HER1) and insulin receptor.
Cytokine receptors:
Cytokine receptors do not have intrinsic tyrosine kinase activity.
Instead, they activate separate intracellular tyrosine kinases, commonly JAK proteins, that are associated with the receptor.
Ligand binding causes receptor subunits to come together or rearrange, allowing the associated JAKs to become activated.
Activated JAKs phosphorylate the receptor and downstream signaling proteins, including STAT transcription factors.
Cytokine receptors are especially important in immune responses, cell growth, and regulation of cell survival.
- Overall: RTKs contain their own tyrosine kinase enzyme, while cytokine receptors depend on separate receptor-associated kinases to phosphorylate target proteins.
Explain how the terms dimerization, autophosphorylation, and HER relate to RTKs
Receptor tyrosine kinases (RTKs) are activated through mechanisms involving receptor dimerization and phosphorylation.
The HER family is an important example of RTKs involved in cell growth and proliferation.
Dimerization:
Dimerization occurs when two receptor molecules associate to form a dimer.
For many RTKs, ligand binding promotes receptor dimerization.
This brings the intracellular kinase domains close enough to activate one another.
Dimerization can occur between:
Two identical receptors (homodimerization).
Two different receptors (heterodimerization).
This interaction is important because it allows receptor activation and initiation of downstream signaling.
Autophosphorylation:
Autophosphorylation is the process by which activated receptor tyrosine kinases phosphorylate tyrosine residues on the receptor complex.
After dimerization, the kinase domains become activated and phosphorylate specific tyrosine residues, commonly on the other receptor subunit.
These phosphorylated tyrosines act as binding sites for intracellular signaling proteins.
Adapter proteins and other signaling molecules recognize these sites and initiate downstream pathways, including the Ras/MAP kinase pathway.
This allows the extracellular signal to be transmitted into the cytosol.
HER (human epidermal growth factor receptor) family:
The HER family consists of four receptor tyrosine kinases:
HER1 (EGFR)
HER2
HER3
HER4
These receptors participate in signaling initiated by members of the epidermal growth factor family.
HER receptors can form different receptor dimers, allowing them to activate intracellular signaling pathways.
HER2 is especially important because it is a preferred dimerization partner for other HER receptors.
Increased expression of HER2 can cause cells to become more sensitive to growth factor signaling.
Clinical relevance of HER2:
Some breast cancers contain multiple copies of the HER2 gene, resulting in overproduction of HER2 receptors.
Increased HER2 expression promotes excessive receptor dimerization and signaling.
This can cause abnormal stimulation of cell growth and proliferation, contributing to tumor development.
Monoclonal antibodies that target HER2 can help block these signaling processes and are used in the treatment of certain HER2-positive cancers.
- Overall: Dimerization brings RTK molecules together, autophosphorylation creates binding sites for downstream signaling proteins, and HER receptors demonstrate how these processes regulate cell growth and can contribute to cancer when improperly regulated.
Describe in detail the steps of the Ras/MAP kinase pathway and the role it plays in cancer/ Include all the key players involved in the signaling cascade
The Ras/MAP kinase pathway is an intracellular signal transduction pathway that regulates cell proliferation, differentiation, and gene expression.
It is activated by many receptor tyrosine kinases and cytokine receptors.
The pathway involves the monomeric GTP-binding protein Ras, adapter proteins, and a cascade of protein kinases.
Ras belongs to the GTPase superfamily and functions as an intracellular ON/OFF switch.
Steps of the Ras/MAP kinase pathway:
1. A growth factor binds to a receptor tyrosine kinase (RTK).
An extracellular signaling molecule, such as epidermal growth factor (EGF), binds to its specific RTK.
This promotes receptor dimerization and activation of the receptor's tyrosine kinase activity.
2. The RTK undergoes autophosphorylation.
The activated receptor phosphorylates specific tyrosine residues on its cytosolic domain.
These phosphotyrosine residues create binding sites for intracellular signaling proteins.
3. The adapter protein GRB2 binds to the activated receptor.
GRB2 (growth factor receptor-bound protein 2) contains SH2 and SH3 domains.
Its SH2 domain recognizes and binds to specific phosphotyrosine sites on the activated receptor.
Its SH3 domains bind to proline-rich regions of the protein SOS.
4. GRB2 recruits SOS to the plasma membrane.
SOS (Son of Sevenless) functions as a guanine nucleotide exchange factor (GEF).
GRB2 recruits SOS to the membrane, where SOS can interact with Ras.
5. SOS activates Ras.
Ras is normally inactive when bound to GDP.
SOS promotes the release of GDP from Ras, allowing GTP to bind.
Ras becomes active in its GTP-bound state (Ras-GTP).
6. Activated Ras stimulates Raf.
Ras-GTP recruits and promotes activation of Raf, a MAP kinase kinase kinase (MAPKKK).
Raf is the first protein kinase in the three-kinase cascade.
7. Raf phosphorylates and activates MEK.
Activated Raf phosphorylates MEK, a MAP kinase kinase (MAPKK).
MEK is a dual-specificity kinase that can phosphorylate both threonine and tyrosine residues.
8. MEK phosphorylates and activates MAP kinase (ERK).
Activated MEK phosphorylates MAP kinase (MAPK/ERK).
Phosphorylation occurs on specific threonine and tyrosine residues within the activation loop.
This changes the conformation of MAPK, allowing its active site to interact with ATP and target proteins.
9. Activated MAP kinase phosphorylates downstream target proteins.
MAPK can phosphorylate cytosolic target proteins and transcription-regulating proteins.
Activated MAPK can also translocate into the nucleus.
10. MAP kinase regulates gene expression.
In the nucleus, MAPK phosphorylates transcription factors involved in regulating gene expression.
This stimulates the transcription of early response genes involved in cell growth, proliferation, and differentiation.
Role of the Ras/MAP kinase pathway in cancer:
The Ras/MAP kinase pathway normally regulates cell proliferation in response to extracellular growth factors.
However, mutations in Ras, RTKs, or other proteins in the MAP kinase cascade can cause the pathway to become constitutively active, meaning it remains active even without normal extracellular stimulation.
One important example involves mutations in the Ras protein.
Normally, Ras hydrolyzes GTP into GDP through its intrinsic GTPase activity, returning to its inactive state.
GTPase-activating proteins (GAPs) accelerate this process.
Certain oncogenic Ras mutations prevent Ras from efficiently hydrolyzing GTP.
For example, mutations affecting glycine at position 12 can interfere with GAP-mediated inactivation.
As a result, Ras remains in its active GTP-bound state and continuously stimulates the Raf → MEK → MAPK signaling cascade.
This can cause excessive transcription of genes involved in cell proliferation, contributing to uncontrolled cell growth and tumor formation.
Because abnormal activation of this pathway is common in cancer, proteins such as mutant Ras, Raf, and MEK are important targets for cancer therapeutics.
- Overall: The Ras/MAP kinase pathway transmits growth factor signals through RTK → GRB2 → SOS → Ras → Raf → MEK → MAPK, leading to changes in gene expression. Mutations that prevent the pathway from switching OFF can contribute to uncontrolled cell proliferation and cancer.
Describe the mechanism by which Wnt and NF-KB signaling are regulated by protein degradation
Both Wnt and NF-κB signaling pathways regulate gene expression through the controlled degradation of specific proteins.
These pathways involve ubiquitinylation (ubiquitination), which is the attachment of ubiquitin molecules to target proteins.
Polyubiquitinated proteins can be recognized and degraded by the proteasome, a protein complex responsible for breaking down selected cellular proteins.
Protein degradation is an irreversible process for the individual protein molecule, meaning the cell must synthesize a new protein to replace one that has been destroyed.
Wnt signaling pathway:
Wnt signaling regulates important cell fate decisions during animal development.
The pathway controls the stability of β-catenin, a protein that can function in the regulation of gene expression.
In the absence of Wnt:
When Wnt is absent, β-catenin is targeted by a destruction complex containing proteins such as APC, Axin, GSK3, and CK1.
The destruction complex promotes phosphorylation of β-catenin.
Phosphorylated β-catenin is recognized by the ubiquitinylation machinery.
β-catenin becomes ubiquitinated and is targeted for proteasomal degradation.
Because β-catenin is continuously degraded, it cannot accumulate in the cytosol or nucleus.
Wnt-responsive target genes remain largely inactive.
In the presence of Wnt:
Wnt binds to its cell-surface receptor complex, involving Frizzled and LRP5/6.
This activates intracellular signaling proteins, including Dishevelled (Dvl).
The signaling pathway inhibits the β-catenin destruction process.
β-catenin is no longer efficiently targeted for degradation and begins to accumulate in the cytosol.
Stabilized β-catenin translocates into the nucleus.
β-catenin interacts with TCF/LEF transcription factors and recruits transcriptional co-activators.
This activates Wnt-responsive genes involved in development, cell growth, and cell fate decisions.
- Clinical relevance: Abnormally active Wnt signaling can cause excessive β-catenin accumulation and inappropriate gene expression, contributing to the development and progression of certain cancers.
NF-κB signaling pathway:
NF-κB (nuclear factor kappa B) is a major transcriptional regulator involved in the mammalian immune system.
NF-κB controls the expression of genes associated with immune responses, inflammation, and cell survival.
Its activity is regulated by the degradation of an inhibitory protein called IκB.
In the absence of an activating signal:
NF-κB is located in the cytosol.
NF-κB is bound to the inhibitory protein IκB.
IκB prevents NF-κB from translocating into the nucleus and activating transcription.
As a result, NF-κB-dependent gene transcription remains inhibited.
In the presence of an activating signal:
An extracellular signal, such as an inflammatory cytokine, activates a cell-surface receptor.
The receptor activates an intracellular signaling pathway that stimulates the IκB kinase (IKK) complex.
Activated IKK phosphorylates IκB.
Phosphorylated IκB becomes ubiquitinated.
The ubiquitinated IκB is degraded by the proteasome.
Degradation of IκB releases NF-κB from its inhibitory complex.
Free NF-κB translocates into the nucleus.
NF-κB binds to specific regulatory DNA sequences and activates transcription of genes involved in immune and inflammatory responses.
Comparison of Wnt and NF-κB signaling:
Both pathways regulate transcription factors through ubiquitin-dependent protein degradation.
In the Wnt pathway, the absence of Wnt promotes degradation of β-catenin, while Wnt signaling prevents its degradation and allows it to regulate gene expression.
In the NF-κB pathway, an activating signal promotes degradation of the inhibitory protein IκB, allowing NF-κB to enter the nucleus.
Therefore, Wnt signaling works by preventing degradation of a transcriptional regulator, while NF-κB signaling works by promoting degradation of an inhibitory protein.
- Overall: Wnt and NF-κB signaling regulate gene expression by controlling protein stability through ubiquitination and proteasomal degradation, but they target different proteins to activate their respective pathways.
What is unique about Notch/Delta signaling?
Notch/Delta signaling is unique because it requires direct cell-to-cell contact between neighboring cells.
Unlike many other signaling pathways, the signaling molecule is not secreted and transported through extracellular fluid.
Instead, Delta is a membrane-bound signaling protein on one cell that binds to the Notch receptor on an adjacent cell.
This makes Notch/Delta an example of contact-dependent signaling.
Steps of the Notch/Delta signaling pathway:
The Delta ligand is expressed on the plasma membrane of a signaling cell.
Delta binds to the Notch receptor on the surface of a neighboring target cell.
This interaction promotes conformational changes and proteolytic cleavage of the Notch receptor.
Notch undergoes cleavage by proteases, including the γ-secretase complex.
Cleavage releases the Notch intracellular domain (NICD) from the membrane.
NICD translocates into the nucleus.
In the nucleus, NICD interacts with DNA-binding transcription-regulating proteins and co-activators.
This activates the transcription of specific target genes involved in cell differentiation and development.
What makes the pathway unique:
Direct contact is required: The signaling cell and target cell must be physically adjacent because Delta is membrane-bound.
Protein cleavage activates signaling: Unlike pathways that rely mainly on phosphorylation cascades, Notch signaling involves proteolytic cleavage of the receptor.
The receptor itself contributes to gene regulation: The released intracellular portion of Notch enters the nucleus and helps regulate transcription.
No conventional second messenger is required: The pathway does not depend on second messengers such as cAMP, IP3, or DAG to transmit the signal.
Important in development: Notch/Delta signaling regulates cell fate decisions and helps neighboring cells develop different functions.
- Overall: Notch/Delta signaling is unique because it requires direct contact between neighboring cells and uses proteolytic cleavage to release the Notch intracellular domain, which enters the nucleus and regulates gene expression.
What would happen if the Gαs subunit was unable to exchange GDP for GTP after epinephrine binds to its receptor?
Epinephrine would still bind to the β-adrenergic receptor on the plasma membrane.
However, the Gαs subunit would remain in its inactive GDP-bound state.
Since Gαs could not become activated, it would be unable to stimulate adenylyl cyclase.
Adenylyl cyclase would not produce the normal increase in cAMP.
Without increased cAMP, protein kinase A (PKA) would not become sufficiently activated.
This would reduce the activation of phosphorylase kinase and glycogen phosphorylase.
As a result, glycogenolysis would decrease, limiting the amount of glucose made available for energy metabolism.
Overall: The epinephrine signaling pathway would be disrupted, preventing the normal increase in glycogen breakdown.
What would happen if cAMP phosphodiesterase (PDE) was not functioning correctly?
cAMP phosphodiesterase normally hydrolyzes cAMP into AMP, helping terminate intracellular signaling.
If PDE was inactive, cAMP would not be degraded efficiently.
This would cause cAMP to remain elevated in the cytosol for a longer period.
Elevated cAMP would keep PKA activated longer than normal.
PKA would continue phosphorylating target proteins involved in cellular metabolism.
In cells responding to epinephrine, this could prolong activation of glycogen phosphorylase and inhibition of glycogen synthase.
As a result, glycogen breakdown could remain elevated while glycogen synthesis remains inhibited.
Overall: The cAMP/PKA pathway would remain active longer than normal, producing a prolonged cellular response.
What would happen if PKA could bind cAMP but its catalytic subunits were unable to phosphorylate target proteins?
cAMP would still bind to the regulatory subunits of PKA.
The catalytic subunits could still be released from the regulatory subunits.
However, the catalytic subunits would be unable to transfer phosphate groups from ATP to target proteins.
Phosphorylase kinase would not become properly activated through PKA phosphorylation.
Glycogen phosphorylase would receive less activation, reducing glycogen breakdown.
Glycogen synthase would not undergo normal PKA-dependent inhibition.
CREB would also receive less PKA-dependent phosphorylation, reducing activation of certain cAMP-responsive genes.
Overall: Even though cAMP could activate the PKA complex structurally, the pathway would fail to produce its normal phosphorylation-dependent cellular responses.
What would happen if phospholipase C-β (PLC-β) was unable to hydrolyze PIP2 into IP3 and DAG?
A ligand could still bind to a GPCR and activate the Gq protein.
However, activated Gαq would be unable to stimulate the normal production of IP3 and DAG through PLC-β.
Without IP3, the IP3-gated Ca²⁺ channels on the endoplasmic reticulum would not open in response to this pathway.
This would prevent the normal increase in cytosolic Ca²⁺ concentration.
Without DAG and the normal calcium increase, activation of protein kinase C (PKC) would decrease.
Calcium-dependent proteins and enzymes would also receive less stimulation.
This could affect cellular activities such as secretion, muscle contraction, and enzyme regulation.
Overall: The GPCR/Gq/PLC signaling pathway would be inhibited, reducing calcium-dependent cellular responses.
What would happen if endothelial cells could not produce nitric oxide (NO) after acetylcholine stimulation?
Acetylcholine could still bind to muscarinic GPCRs on endothelial cells.
The Gq/PLC pathway could still increase cytosolic Ca²⁺ concentrations.
However, if nitric oxide synthase (eNOS) was inactive, the endothelial cells would be unable to produce the normal amount of NO.
Without NO, soluble guanylyl cyclase in neighboring vascular smooth muscle cells would not receive normal stimulation.
This would reduce the production of cGMP from GTP.
Reduced cGMP would decrease activation of protein kinase G (PKG).
As a result, the smooth muscle cells would not undergo their normal NO-dependent relaxation.
Overall: Acetylcholine-induced vasodilation would be reduced, causing blood vessels to remain more constricted than they otherwise would.
What would happen if the TGF-β receptor was continuously active even without TGF-β binding?
Normally, TGF-β binds to its receptor and activates the receptor serine/threonine kinase pathway.
If the receptor was constitutively active, it could continuously phosphorylate downstream Smad proteins without requiring TGF-β.
Smad2/3 would become phosphorylated and associate with Smad4.
The Smad complex could continuously translocate into the nucleus and regulate target gene expression.
This could result in prolonged expression of genes involved in cell growth and differentiation.
In cells where TGF-β normally inhibits proliferation, continuous signaling could cause excessive suppression of cell division.
Overall: The TGF-β/Smad pathway would become overactive, potentially causing abnormal regulation of cell proliferation and differentiation.
What would happen if a mutation caused JAK proteins to remain continuously active without cytokine binding?
Normally, cytokine binding activates receptor-associated JAK proteins.
If JAK proteins were constitutively active, they could phosphorylate receptor tyrosine residues without normal cytokine stimulation.
STAT proteins could be recruited to the phosphorylated receptor and become phosphorylated.
Phosphorylated STAT proteins would form dimers and translocate into the nucleus.
STAT dimers could continuously activate transcription of target genes.
This could cause excessive expression of genes involved in immunity, cell division, and cell survival.
Prolonged activation could contribute to abnormal cell proliferation or inflammatory responses.
Overall: The JAK/STAT pathway would become overactive, causing gene expression to occur without normal extracellular regulation.
What would happen if GRB2 could no longer bind to phosphorylated tyrosine residues on an activated RTK?
The growth factor would still bind to the RTK, causing receptor dimerization and autophosphorylation.
However, GRB2 would be unable to bind to the receptor through its SH2 domain.
Without GRB2 recruitment, SOS would not be efficiently brought to the plasma membrane.
SOS would be unable to efficiently promote the exchange of GDP for GTP on Ras.
Ras would remain primarily in its inactive GDP-bound state.
This would decrease activation of the Raf → MEK → MAPK kinase cascade.
As a result, MAPK-dependent transcription factors would receive less stimulation.
Overall: The Ras/MAP kinase pathway would be inhibited, reducing growth factor-dependent gene expression and cell proliferation.
What would happen if the β-catenin destruction complex was unable to phosphorylate β-catenin in the absence of Wnt?
Normally, the β-catenin destruction complex phosphorylates β-catenin when Wnt is absent.
Phosphorylation allows β-catenin to become ubiquitinated and targeted for proteasomal degradation.
If the destruction complex could not phosphorylate β-catenin, β-catenin would not be efficiently recognized for degradation.
As a result, β-catenin would accumulate in the cytosol even without Wnt stimulation.
Accumulated β-catenin could translocate into the nucleus and interact with TCF/LEF transcription factors.
This could cause continuous activation of Wnt-responsive genes involved in cell growth and cell fate decisions.
Overall: The Wnt signaling pathway could become constitutively active, potentially contributing to abnormal cell proliferation and cancer.
What would happen if the Notch intracellular domain was released normally but could not bind to transcription-regulating proteins in the nucleus?
Delta would still bind to the Notch receptor on an adjacent cell.
The Notch receptor would still undergo proteolytic cleavage, releasing its intracellular domain.
The Notch intracellular domain could still translocate into the nucleus.
However, it would be unable to form a functional transcription-regulating complex with its nuclear binding partners.
As a result, Notch-responsive target genes would not be properly activated.
This would interfere with the regulation of genes involved in cell fate determination.
Overall: Notch/Delta signaling would be disrupted at the level of gene expression, potentially affecting normal cell differentiation and development.