Receptor Types and Signaling Mechanisms

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Questions and Answers

What is the main function of guanylyl cyclase in its receptor form?

  • Dimerize with other receptors
  • Convert ATP to cAMP
  • Convert GTP to cGMP (correct)
  • Phosphorylate serine and threonine residues

What causes the activation of receptor guanylyl cyclase?

  • Binding of TGFß
  • Binding of ANP (correct)
  • Binding of growth factors
  • Binding of insulin

What do receptor serine/threonine kinases primarily phosphorylate?

  • GTP to cGMP
  • Serine and threonine residues on target proteins (correct)
  • cAMP to AMP
  • Tyrosine residues on receptor proteins

What initiates the MAP kinase signalling pathway?

<p>RAS activation (A)</p> Signup and view all the answers

Which receptors are involved in insulin-mediated glucose uptake?

<p>Receptor tyrosine kinases (D)</p> Signup and view all the answers

What is the role of phosphotyrosine motifs created by receptor tyrosine kinases?

<p>They recruit intracellular signalling proteins (A)</p> Signup and view all the answers

What is the effect of atrial natriuretic peptide (ANP) when signaling through guanylyl cyclase?

<p>Vasodilation of blood vessels (A)</p> Signup and view all the answers

Which type of receptor is formed by the dimerization of two molecules of insulin?

<p>Receptor tyrosine kinase (D)</p> Signup and view all the answers

What initiates the mechanism of signalling for receptor tyrosine phosphatase?

<p>Ligand binding to the receptor (D)</p> Signup and view all the answers

Which of the following best describes the role of tyrosine phosphatase activity in receptor signalling?

<p>Dephosphorylates target proteins to modulate their activity (B)</p> Signup and view all the answers

What is a distinctive feature of receptor tyrosine kinases compared to G protein-coupled receptors?

<p>Catalytic activity that triggers phosphorylation (C)</p> Signup and view all the answers

In the context of signalling pathways, what is a primary function of CD45?

<p>Induces the maturation of lymphocytes (A)</p> Signup and view all the answers

How do GPCRs primarily relay their signals?

<p>Via secondary messengers like cAMP (D)</p> Signup and view all the answers

What is the main difference in signal duration between GPCRs and RTKs?

<p>RTKs activate for hours while GPCRs for seconds (B)</p> Signup and view all the answers

What common outcome do both GPCRs and RTKs achieve in their signalling pathways?

<p>Phosphorylation of target proteins (A)</p> Signup and view all the answers

What type of receptors do not require dimerization for their activation?

<p>GPCRs (D)</p> Signup and view all the answers

What is the main effect of cholera toxin on the Ga subunit?

<p>It causes ADP-ribosylation, preventing GTP hydrolysis. (B)</p> Signup and view all the answers

Which of the following statements about the action of G proteins is incorrect?

<p>Gi subunits stimulate adenylyl cyclase. (A)</p> Signup and view all the answers

What is the main physiological effect of increased levels of cAMP and PKA due to pertussis toxin acting on G proteins?

<p>Induction of whooping cough (C)</p> Signup and view all the answers

What is the function of adenylyl cyclase in the G protein signaling pathway?

<p>Convert ATP into cAMP. (D)</p> Signup and view all the answers

Which subtypes of muscarinic receptors are classified as Gq-coupled stimulatory receptors?

<p>M1, M3, and M5 (C)</p> Signup and view all the answers

What is the consequence of activating protein kinase A (PKA) after cAMP binding?

<p>PKA phosphorylates target proteins. (B)</p> Signup and view all the answers

Which receptors are known to couple with Gs subunits?

<p>Beta-adrenergic receptors. (A), A2A/B adenosine receptors. (D)</p> Signup and view all the answers

What is the primary role of phospholipase C (PLC) in signal transduction?

<p>Cleavage of PIP2 into IP3 and DAG (B)</p> Signup and view all the answers

How does pertussis toxin affect the Gi subunit?

<p>It locks the subunit into an inactive configuration. (C)</p> Signup and view all the answers

Which response is mediated by G proteins containing the alphaq11 subunit in acetylcholine signaling?

<p>Mediated salivary secretion (B)</p> Signup and view all the answers

What happens to IP3 after it functions in signaling within the cytosol?

<p>It is converted into IP2, halting the signal (D)</p> Signup and view all the answers

Which of the following describes the role of G proteins in cellular signaling?

<p>They mediate the exchange of GDP for GTP. (C)</p> Signup and view all the answers

What is the role of cAMP in the signaling pathway activated by GPCR?

<p>It serves as a second messenger that activates PKA. (C)</p> Signup and view all the answers

What autonomic effects are primarily associated with muscarinic receptors?

<p>Salivary secretion and bronchial smooth muscle contraction (B)</p> Signup and view all the answers

What is the role of DAG in the signaling process following the action of phospholipase C?

<p>It recruits protein kinase C (PKC) in the membrane (D)</p> Signup and view all the answers

How does activation of M3 receptors by alphaq11-containing G proteins affect respiratory function?

<p>It causes bronchospasm (B)</p> Signup and view all the answers

What initiates the activation of phospholipase Cß?

<p>Binding of a hormone to a G protein-coupled receptor (C)</p> Signup and view all the answers

What is the primary function of calmodulin in cellular signaling?

<p>Binding calcium ions to stimulate other proteins (C)</p> Signup and view all the answers

Why does diacylglycerol (DAG) remain in the plasma membrane?

<p>It is hydrophobic and interacts with the lipid bilayer (A)</p> Signup and view all the answers

What intracellular responses do calmodulin kinases (CaMKs) mediate?

<p>Phosphorylation of serine and threonine residues (B)</p> Signup and view all the answers

What effect does activated protein kinase C (PKC) have on cells?

<p>Regulation of cellular transcription factors (D)</p> Signup and view all the answers

How does IP3 facilitate the release of calcium ions into the cytosol?

<p>By interacting with receptors on the endoplasmic reticulum (D)</p> Signup and view all the answers

What is the mechanism through which alpha1 adrenergic receptors induce vasoconstriction?

<p>Increased intracellular calcium levels (D)</p> Signup and view all the answers

Which component serves as the most critical calcium-binding protein mediating intracellular responses?

<p>Calmodulin (B)</p> Signup and view all the answers

What is the primary function of Ras-GEF in the activation of Ras?

<p>It exchanges the GDP bound to Ras for GTP. (A)</p> Signup and view all the answers

Which statement correctly describes the role of adaptor proteins in RAS activation?

<p>They dock on phosphotyrosine residues and recruit RAS-GEF. (B)</p> Signup and view all the answers

In the MAPK signaling cascade, what is the primary role of MAPKKK?

<p>It acts as the first kinase and phosphorylates MAPKK. (D)</p> Signup and view all the answers

What occurs once the Ras protein is activated?

<p>It becomes covalently attached to a lipid and anchors to the plasma membrane. (A)</p> Signup and view all the answers

How are tyrosine kinases activated in tyrosine kinase-associated receptors?

<p>Via dimerization of the receptor which activates associated kinases. (D)</p> Signup and view all the answers

What do activated tyrosine kinases do in this signaling mechanism?

<p>They phosphorylate themselves and the receptor to form phosphotyrosine motifs. (A)</p> Signup and view all the answers

Which function explains the role of phosphotyrosine motifs in signaling by tyrosine kinase-associated receptors?

<p>They provide a docking site for intracellular signaling molecules. (D)</p> Signup and view all the answers

Which kinase is typically referred to as the third kinase in the MAPK signaling cascade?

<p>ERK (C)</p> Signup and view all the answers

Flashcards

What triggers the activation of a G protein?

The exchange of GDP for GTP on the alpha subunit of the G protein, leading to its activation.

What is a G-protein coupled receptor (GPCR)?

A type of receptor that uses a G protein to relay signals within the cell.

What is a second messenger?

A molecule that acts as a messenger inside the cell, relaying signals from the GPCR.

What does the Ga-GTP subunit do?

A subunit of the G protein that activates adenylyl cyclase, the enzyme that converts ATP into cAMP.

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What is adenylyl cyclase?

An enzyme that converts ATP into cAMP, a second messenger involved in various cellular processes.

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How does cholera toxin (CTx) affect the G protein?

This toxin binds to the alpha subunit of the G protein and prevents the hydrolysis of GTP, leading to persistent activation of the G protein.

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What is Gi?

A family of G proteins that inhibit adenylyl cyclase.

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How does pertussis toxin affect the Gi subunit?

This toxin locks the alphai subunit into an inactive configuration, preventing its function.

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G protein containing alphaq11 subunits

A type of G protein that activates phospholipase C (PLC) upon binding to its receptor.

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Muscarinic receptors

A family of metabotropic receptors activated by acetylcholine, playing a crucial role in the autonomic nervous system.

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Phospholipase C (PLC)

A key enzyme that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG), both important secondary messengers.

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Inositol trisphosphate (IP3)

A water-soluble second messenger that travels to the endoplasmic reticulum (ER) and triggers the release of calcium.

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Diacylglycerol (DAG)

A lipid-based second messenger that remains in the cell membrane, where it activates protein kinase C (PKC).

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Protein kinase C (PKC)

A calcium-dependent protein kinase that plays a role in various cellular processes, including cell growth and differentiation.

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Gq signaling pathway

The process of activating the phospholipase C pathway, which in turn leads to the release of calcium and activation of protein kinase C.

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Calcium signaling

The result of the activation of G proteins containing alphaq11 subunits, leading to the release of intracellular calcium and activation of other signaling pathways.

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What are receptor guanylyl cyclases?

These receptors contain two guanylyl cyclase domains which convert GTP to cyclic GMP (cGMP). cGMP then activates downstream kinases to trigger cellular responses.

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Explain the signaling mechanism of receptor guanylyl cyclases.

Binding of a ligand, like atrial natriuretic peptide (ANP), induces a conformational change in the receptor, causing it to dimerize and activate. This activates the guanylyl cyclase activity, generating cGMP. The increased cGMP then activates other signaling molecules, determining the cellular response.

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What is generated by the guanylyl cyclase activity of the receptor?

cGMP is generated.

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What is the effect of ANP signaling via guanylyl cyclase?

ANP signaling via guanylyl cyclase leads to vasodilation, relaxing vascular smooth muscle and dilating blood vessels.

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Describe receptor serine/threonine kinases.

These receptors contain serine-threonine kinase domains that phosphorylate target proteins, similar to protein kinase A (PKA).

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Explain the signaling mechanism of receptor serine/threonine kinases.

First messenger binds to receptor type II, initiating a cascade. Receptor type I then binds to form a ternary complex with type II and the first messenger. Type II receptor phosphorylates type I, activating its serine-threonine kinase activity. The activated type I then phosphorylates target proteins, often SMAD proteins. This pathway plays a role in TGF-β mediated cell proliferation.

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Describe receptor tyrosine kinases (RTKs).

These receptors contain tyrosine kinase domains that phosphorylate themselves and other proteins.

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Explain the mechanism of signaling for receptor tyrosine kinases.

Binding of two insulin molecules causes the receptor to dimerize. The receptors then use their cytoplasmic tyrosine kinase activity to phosphorylate each other at multiple tyrosine residues, creating 'phosphotyrosine motifs'.

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What is IP3 and what does it do?

A second messenger that promotes calcium release from the endoplasmic reticulum (ER) and increases intracellular free Ca2+, ultimately leading to cellular responses.

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What allows the release of Ca2+ into the cytosol?

IP3 binds to its receptor, IP3R (IP3 receptor), located on the membrane of the ER. This binding triggers the release of Ca2+ into the cytosol.

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What transports Ca2+ back into the ER?

The SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) pump is responsible for transporting Ca2+ back into the ER. It uses ATP as its energy source.

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What is the most important calcium-binding protein involved in intracellular responses?

Calmodulin (CaM) is a calcium-binding protein that plays a central role in mediating various intracellular responses triggered by Ca2+. It binds to four Ca2+ ions, forming the Ca2+-CaM complex.

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What does the Ca2+-CaM complex activate?

The complex of Ca2+ bound to Calmodulin (Ca2+-CaM) activates various downstream proteins, including PDE (phosphodiesterase) and CaMKs (calmodulin kinases).

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What do calmodulin kinases (CaMKs) do?

CaMKs phosphorylate serine and threonine residues on specific substrate proteins, playing a role in various cellular processes, such as smooth muscle contraction.

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What are the effects of DAG and how does it work?

DAG (diacylglycerol) is a second messenger that activates specific protein kinases, primarily protein kinase C (PKC). It plays a role in various cellular processes, including cell proliferation, transcription factor activity, and receptor desensitization.

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How does PKC interact with IP3 signaling?

PKCs can amplify the effects of IP3 signaling, enhancing downstream signaling pathways triggered by IP3 and Ca2+. It is also involved in regulating cell shape and transcription factor activity.

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Cytokine Signalling Pathway

A signalling pathway mediated by tyrosine kinase-associated receptors often involving the release of cytokines, like IL-6, in response to inflammation or infection.

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Receptor Tyrosine Phosphatases (RTPs)

Receptors that contain tyrosine phosphatase domains. These enzymes remove phosphate groups from tyrosine residues on target proteins, effectively 'turning off' their activity.

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RTP Signalling Mechanism

Signal Transduction via RTPs involves a conformational change in the receptor upon ligand binding, activating its tyrosine phosphatase activity. This dephosphorylates target proteins and can trigger phosphorylation of downstream proteins.

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Function of RTP Activity

The tyrosine phosphatase activity of the receptor removes phosphate groups from tyrosine residues on target proteins, altering their cellular function and activity.

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RTPs and Downstream Phosphorylation

Dephosphorylation of target proteins by RTPs can trigger phosphorylation of downstream proteins, often involved in signalling cascades and cellular processes.

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Example: CD45 and Lymphocyte Maturation

CD45, a receptor tyrosine phosphatase, plays a key role in lymphocyte maturation by binding to its receptor and initiating a signalling cascade.

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GPCRs vs. RTKs: Structure

GPCRs: 7 transmembrane helices, RTKs: 1 transmembrane helix.

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GPCRs vs. RTKs: Enzymatic Activity

GPCRs: No enzymatic activity, activate G proteins. RTKs: Have enzymatic activity, directly phosphorylate target proteins.

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Adaptor Protein Role in Ras Activation

An adaptor protein binds to a phosphotyrosine residue on an activated receptor tyrosine kinase (RTK), facilitating the recruitment of RAS-GEF, a crucial intermediary in activating Ras.

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How does RAS-GEF Activate Ras?

RAS-GEF, a guanine nucleotide exchange factor, stimulates Ras to release its bound GDP and bind to GTP, activating Ras.

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Activated Ras: Function and Localization

The activated Ras protein, now bound to GTP, anchors itself to the plasma membrane via a lipid group and initiates the transmission of the signal to downstream pathways.

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MAPKKK: The Initiator

The first kinase in the MAP kinase cascade (e.g., RAF) is activated by Ras-GTP. It then phosphorylates MAPKK, transferring a phosphate from ATP to ADP.

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MAPKK: The Middleman

The second kinase in the MAP kinase cascade, activated by MAPKKK. It phosphorylates MAPK.

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MAPK: The Messenger

The third kinase in the MAP kinase cascade, activated by MAPKK. It translocates to the nucleus to phosphorylate transcription factors, thereby altering gene expression.

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Tyrosine Kinase-Associated Receptors

These receptors lack intrinsic kinase domains but are associated with cytoplasmic tyrosine kinases, which are non-covalently bound.

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Signaling Mechanism of Tyrosine Kinase-Associated Receptors

Binding of a signaling molecule to the receptor causes a conformational change, leading to dimerization. This dimerization activates the associated tyrosine kinases, such as JAK2. These kinases then phosphorylate tyrosine residues on both themselves and the receptor, creating 'phosphotyrosine motifs.' These motifs recruit intracellular signaling molecules, such as STAT proteins, initiating the cellular response.

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Study Notes

Receptor Types and Signaling Mechanisms

  • Four main receptor classes: ligand-gated ion channels, G-protein coupled receptors (GPCRs), enzyme-linked receptors, and nuclear receptors.
  • G-protein activation: GPCR activation triggers GDP-GTP exchange on the G protein alpha subunit. The alpha-GTP subunit dissociates from the beta-gamma complex, activating downstream effectors.
  • Adenylyl cyclase (AC) activation: Activated G protein (Gas) stimulates AC, converting ATP to cAMP (a second messenger).
  • Protein Kinase A (PKA) activation: cAMP binds to PKA's regulatory subunits, releasing the catalytic subunits, which become active.
  • Cholera toxin (CTx) effect: CTx ADP-ribosylates the Gas subunit, preventing GTP hydrolysis; leading to sustained cAMP production and diarrhea.
  • Pertussis toxin (PTx) effect: PTx ADP-ribosylates Gai, preventing its inhibitory action on AC, leading to increased cAMP and whooping cough symptoms.
  • Gq/11 protein activation: Gq/11 activates phospholipase C (PLC), cleaving PIP2 into IP3 and DAG.
  • IP3 function: IP3 releases calcium from the endoplasmic reticulum (ER) into the cytoplasm.
  • DAG function: DAG activates protein kinase C (PKC) in the membrane.
  • Calcium signaling: Calcium acts as a second messenger; CaM (calmodulin) binds to calcium, activating CaMKs (calmodulin kinases).
  • Calmodulin kinases (CaMKs): CaMKs phosphorylate target proteins, influencing various cellular responses, including smooth muscle contraction.
  • Alpha1-adrenergic receptor function: Gq-coupled, increasing intracellular calcium leading to vasoconstriction.
  • Beta2-adrenergic receptor function: Gs-coupled, increasing cAMP, leading to vasodilation.

Enzyme-Linked Receptors

  • Receptor guanylyl cyclases: Convert GTP to cGMP, activating downstream kinases. ANP (atrial natriuretic peptide) is an example.
  • Receptor serine/threonine kinases: Phosphorylate target proteins (e.g., SMAD proteins), mediating responses like cell proliferation.
  • Receptor tyrosine kinases (RTKs): Phosphorylate themselves and other proteins, initiating phosphorylation cascades. Examples include insulin signaling.
  • Tyrosine kinase-associated receptors: Lack intrinsic kinase activity but associate with cytoplasmic kinases (e.g., JAK2), triggering similar phosphorylation cascades. Examples include signaling by cytokines.
  • Receptor tyrosine phosphatases: Dephosphorylate target proteins, modulating cellular activity by reversing phosphorylation events initiated by other receptors. Examples include lymphocytes maturation (activation of CD45)

GPCRs vs. RTKs

  • GPCRs: 7 transmembrane helices; no intrinsic enzymatic activity; signal relayed via secondary messengers (e.g., cAMP, IP3/DAG); shorter duration of signaling.
  • RTKs: Single transmembrane helix; intrinsic enzymatic activity; signal relayed via phosphorylation cascades; longer signaling duration.

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