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Questions and Answers

What is the primary role of kinases in the signal transduction pathway described?

  • To add phosphates to proteins (correct)
  • To catalyze the breakdown of ATP
  • To activate receptor molecules directly
  • To remove phosphates from proteins

Which process is responsible for turning off the signal in a signaling cascade?

  • Dephosphorylation (correct)
  • Phosphorylation
  • Signal amplification
  • Transcriptional activation

In the context of signal transduction, what is the term for events that occur closer to the response rather than the receptor?

  • Receptor-mediated events
  • Cascading events
  • Downstream events (correct)
  • Upstream events

What is the effect of signal transduction pathways on the cellular response?

<p>They can either enhance or diminish cellular activities. (C)</p> Signup and view all the answers

What is the specific role of phosphatases in the signaling pathways?

<p>To remove phosphates and deactivate target proteins. (A)</p> Signup and view all the answers

What is a common feature of both G protein signaling pathways and the MAP-kinase pathway?

<p>Both involve a series of phosphorylation events. (D)</p> Signup and view all the answers

Which type of protein actively regulates G protein activity, often modulating the effect of receptors?

<p>G-protein–receptor kinases (GRKs) (C)</p> Signup and view all the answers

What is a characteristic of an inactive protein kinase in the signaling cascade?

<p>It cannot catalyze the phosphorylation of substrates. (C)</p> Signup and view all the answers

What is the role of phosducin in G protein signaling pathways?

<p>It modifies the βγ subunits of G proteins. (C)</p> Signup and view all the answers

How do receptors with inherent tyrosine kinase activity generally function?

<p>They undergo autophosphorylation upon ligand binding. (B)</p> Signup and view all the answers

What is the specific role of the SH2 domain in the Ras-MAP kinase pathway?

<p>It forms binding sites for proteins with phosphotyrosine residues. (B)</p> Signup and view all the answers

Which factors act as upstream regulators in the MAP kinase pathway?

<p>Grb2 and SOS proteins. (A)</p> Signup and view all the answers

What is the significance of receptor autophosphorylation in signaling?

<p>It creates binding sites for signal transducer proteins. (A)</p> Signup and view all the answers

Which of the following describes the relationship between Ras and Raf in the MAP kinase pathway?

<p>Ras activates Raf through a conformational change. (A)</p> Signup and view all the answers

Which process is primarily modulated by G-protein-receptor kinases (GRKs)?

<p>Receptor desensitization. (D)</p> Signup and view all the answers

What is the primary function of SOS in G protein signaling?

<p>It functions as a GEF for Ras. (C)</p> Signup and view all the answers

What physiological effects arise from the generation of free βγ subunits in G protein signaling?

<p>They can phosphorylate ligand-occupied receptors. (A)</p> Signup and view all the answers

How do βγ subunits influence the mitogen-activated protein kinase (MAP-kinase) pathway?

<p>They activate the MAP-kinase pathway at an early step. (C)</p> Signup and view all the answers

What is the primary role of phosducin in G protein signaling?

<p>To sequester βγ subunits and prevent their reassociation. (D)</p> Signup and view all the answers

What role does the GoLoco protein play in G protein signaling?

<p>It releases free βγ dimers without activating receptors. (C)</p> Signup and view all the answers

What is one way that βγ subunits can regulate receptors?

<p>By directing GRKs to phosphorylate ligand-occupied receptors. (C)</p> Signup and view all the answers

Which cellular process is influenced by the βγ subunit complex due to its membrane tethering?

<p>Membrane trafficking processes. (C)</p> Signup and view all the answers

Which statement correctly describes the action of free βγ subunits in G protein signaling?

<p>They can activate several effector proteins in addition to influencing the MAP-kinase pathway. (C)</p> Signup and view all the answers

What effect does the sequestration of βγ subunits by phosducin have on G protein activity?

<p>It eventually inhibits G protein activity by curtailing βγ activity. (A)</p> Signup and view all the answers

Flashcards

Signal Transduction Pathway

A series of steps in which a signal from outside the cell is converted into a response within the cell.

Upstream Events

Events in a signal transduction pathway closer to the receptor.

Downstream Events

Events in a signal transduction pathway closer to the response.

Protein Phosphorylation

Adding phosphate groups to proteins, often activating them.

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Protein Kinases

Enzymes that add phosphate groups to proteins (phosphorylation).

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Protein Phosphatases

Enzymes that remove phosphate groups from proteins (dephosphorylation).

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Molecular Switch

Phosphorylation/dephosphorylation system that turns activities on and off.

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Signal Transduction Effects

Two major downstream impacts of signal transduction pathways affecting plasma membrane receptors on cells.

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Receptors with inherent Tyrosine Kinase activity (RTKs)

Membrane-bound receptors that possess intrinsic tyrosine kinase activity, directly phosphorylating tyrosine residues upon activation.

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Receptor Tyrosine Kinases (RTKs) activation

Activated by growth factors, binding to RTKs triggers dimerization. Then, autophosphorylation on specific tyrosine residues occur.

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Autophosphorylation

The process where a receptor tyrosine kinase phosphorylates itself on tyrosine residues after dimerization.

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SH2 domain

A protein domain that recognizes and binds to phosphotyrosine residues on activated receptor tyrosine kinases.

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Adaptor protein Grb2

A protein with an SH2 domain that binds to phosphotyrosine residues on growth factor receptors, activating a downstream signaling pathway upon binding.

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Ras protein

A small monomeric G protein activated by SOS (a guanine nucleotide exchange factor), triggering the MAP kinase pathway.

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MAP kinase pathway

A signaling cascade initiated by RTK activation, involving Ras and subsequent kinase activations, ultimately resulting in cellular responses.

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Monomeric G protein

A type of G protein that exists as a single polypeptide chain, rather than a complex of multiple subunits.

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G protein activation

The process where a receptor and G protein interact, leading to the release of free α and βγ subunits, triggering various cellular responses.

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βγ subunits

A dimer (two-unit structure) of proteins that detach from the G protein complex upon activation, and can bind to and activate other proteins, leading a variety of effects.

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GRK

A protein that phosphorylates (adds a phosphate group to) the activated receptor, leading to receptor inactivation or other downstream effects.

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GoLoco protein

A protein that can cause the release of βγ subunits from the G protein-receptor complex without prior receptor activation.

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Phosducin

A protein that binds to βγ subunits, preventing re-association with α subunit, and affecting G protein activity.

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ERK pathway

The Extracellular-regulated kinase pathway which is part of the mitogen-activated protein kinase pathway.

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G protein heterotrimer

A protein complex composed of an α subunit and a βγ dimer, usually involved in transmitting signals from activated receptors.

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Study Notes

Cell Signaling Overview

  • Cell signaling, or cell-to-cell communication, is vital for multicellular organisms.
  • Intercellular signaling mechanisms allow one cell to send a message changing another cell's function.
  • Cell communication occurs in multiple ways.

Types of Cell Signaling

  • Juxtacrine/Contact-Dependent: Direct contact between cells, via gap junctions or cell surface proteins.
  • Intercellular Signaling (Contact-Independent): Signaling molecules secreted by one cell and received by others. This includes several forms:
    • Endocrine: Hormones travel long distances via blood, affecting target cells at distant locations.
    • Paracrine: Secreted molecules affect nearby target cells locally, without the involvement of the bloodstream.
    • Synaptic/Neuronal: Nerve cells release neurotransmitters to target cells.
    • Autocrine: A cell secretes signaling molecules that bind to receptors on itself (direct feedback loop).

Intracellular Signal Transduction

  • Key components of a signal cascade, such as:

    • Receptors
    • Signal transduction, includes a sequence of reactions that mediate the response to a signal.
    • Response.

  • Signals relay in multistep pathways, amplifying the initial signal.

Signal Termination

  • Termination of signals within the cell.
    • The first process is messenger degradation/removal.
    • Specific steps within signal transduction pathways include receptor desensitization or inactivation of the second messenger.
    • Processes that terminate signals are carefully controlled.

Chemical Messengers

  • Chemical messengers that act as signals include:
    • Neurotransmitters released in the nervous system.
    • Hormones released by glands and transported in the circulatory system.

Types of Receptors

  • Intracellular receptors: Bind to small, hydrophobic molecules (e.g., steroid hormones). These messengers readily diffuse across plasma membrane.
  • Plasma membrane receptors: Bind to large, hydrophilic molecules, unable to cross membrane. These include:
    • Ion channel receptors
    • Receptors that are enzymes (kinases) or that bind and activate enzymes.
    • G-protein-coupled receptors

G-Protein Coupled Receptors (GPCRs)

  • Contain 7 transmembrane (7TM) domains (alpha-helices).
  • Ligand binding alters receptor conformation, leading to G protein activation.
  • G protein activation subsequently influences effector proteins (e.g., enzymes, ion channels).
  • The G protein cycle typically involves GTP binding and hydrolysis. The hydrolysis returns the G protein back to its inactive state.
  • G proteins are divided into families, each with specific functions (e.g., Gs, Gi, Gq).

Small G Proteins

  • Small GTPases, associated with cellular functions like membrane trafficking, gene activation, and cell growth.
  • Examples include Ras, Rho, Rac, etc.
  • Small G proteins control processes, especially by activating a cascade of kinases.

Receptors that are Kinases

  • Receptors with internal tyrosine kinase activity.
  • Receptors that recruit intracellular tyrosine kinases.
  • Involved in signal transduction pathways.
  • Example pathways include the Ras-MAP kinase pathway.
  • These pathways work through signal protein assemblies, which have three dimensional shapes (e.g., SH2 and SH3 domains).

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