Cell Communication: GPCRs Overview

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Questions and Answers

What is the primary function of GTPase activating proteins (GAPs) in relation to Galpha subunits?

To activate second messengers produced by Galpha subunits

To stimulate the intrinsic GTPase activity of Galpha subunits (correct)

To bind with downstream signaling proteins

To inhibit Galpha subunits from hydrolyzing GTP

Which role does the Gβγ subunit play in regulating cardiac function?

Inhibits the dissociation of Gα and Gβγ

Directly hydrolyzes GTP to GDP

Stimulates cAMP production to increase heart rate

Activates K+ channels to decrease heart rate (correct)

What characterizes second messengers in signaling pathways mediated by G-proteins?

They act directly on the G-protein subunits

They are produced before the first messenger activates the G-proteins

They are typically larger proteins that bind to receptors

They are synthesized at the membrane and diffuse into the cytosol (correct)

Which of the following is NOT a characteristic of the Galpha subunit when bound to GTP?

It activates downstream signaling proteins directly Signup and view all the answers

What happens to the Galpha subunit when it transitions from GTP-bound to GDP-bound state?

It dissociates from Gβγ and inactivates signaling Signup and view all the answers

What is the primary GTP-binding protein in heterotrimeric G-proteins?

G-alpha Signup and view all the answers

In which conformation do G-alpha, G-beta, and G-gamma subunits associate together?

Inactive conformation Signup and view all the answers

What triggers GDP release from the G-alpha subunit?

Binding of a ligand to GPCR Signup and view all the answers

What happens to the G-alpha subunit upon GTP binding?

It dissociates from G-beta and G-gamma Signup and view all the answers

Which subunits of heterotrimeric G-proteins are involved in downstream signaling?

G-alpha and G-beta-gamma Signup and view all the answers

Which Galpha subunit is responsible for inhibiting adenylyl cyclase?

G-alpha-i Signup and view all the answers

How do G-alpha and G-gamma subunits interact with cellular membranes?

They are covalently attached to membrane lipids Signup and view all the answers

What is the effect of GPCR activation on the G-alpha subunit?

It leads to G-alpha dissociation from G-beta-gamma Signup and view all the answers

What is the primary role of G-proteins in signal transduction?

To cycle between active and inactive states depending on bound nucleotides Signup and view all the answers

Which amino acid residues are primarily targeted for phosphorylation by kinases?

Serine, threonine, and tyrosine Signup and view all the answers

What accelerates the hydrolysis of GTP to GDP in G proteins?

GTPase Activating Proteins (GAPs) Signup and view all the answers

What effect do GEFs have on G proteins?

They induce the release of GDP and bind to GTP Signup and view all the answers

How do the distinct GTP and GDP states influence G proteins?

They drive allosteric changes that regulate binding properties Signup and view all the answers

What is the primary function of G-protein coupled receptors (GPCRs)?

Transmitting signals inside the cell upon ligand binding Signup and view all the answers

What is a primary function of signal transduction pathways?

Integrating multiple signals to yield distinct responses Signup and view all the answers

What is the structural characteristic of GPCRs?

Seven transmembrane α-helices Signup and view all the answers

Which statement about feedback regulation in signal transduction pathways is true?

It regulates an upstream component of the pathway Signup and view all the answers

Which of the following describes the role of antagonists in relation to GPCRs?

They prevent agonists from binding and keep the GPCR inactive Signup and view all the answers

What distinguishes small GTPases from heterotrimeric GTPases?

Heterotrimeric GTPases contain a specific helical domain Signup and view all the answers

How do small molecules interact with GPCRs?

They can act as either agonists or antagonists Signup and view all the answers

Why are GPCRs significant in pharmacology?

They are major targets for drug development Signup and view all the answers

What type of response is initiated by the beta-adrenergic receptor when activated?

Fight or flight response Signup and view all the answers

Which of the following statements about GPCR signaling is correct?

Conformational changes are transmitted to the cytosolic side Signup and view all the answers

What role does naloxone play in relation to opioid receptors?

It competes with opioids and keeps the receptor inactive Signup and view all the answers

What happens if a cell is deprived of survival signals?

The cell will activate programmed cell death. Signup and view all the answers

How do different target cells respond to acetylcholine despite having the same receptor type?

The physiological response varies in different target cells. Signup and view all the answers

What characterizes fast signaling pathways?

They alter the activities of existing proteins quickly. Signup and view all the answers

Which type of signaling occurs when two cells are in direct physical contact?

Contact-dependent signaling Signup and view all the answers

Which signaling method communicates over long distances via the bloodstream?

Endocrine signaling Signup and view all the answers

What defines autocrine signaling?

A cell secretes a signal that binds to its own receptors. Signup and view all the answers

Which signaling mechanism has effects that occur over short distances, typically within a few cell diameters?

Paracrine signaling Signup and view all the answers

In which scenario would a signal exert its effect the slowest?

Changes in gene expression and protein synthesis. Signup and view all the answers

What does the diverse responses to a single signaling molecule typically depend on?

The specific receptor type expressed by the target cell. Signup and view all the answers

Which of the following statements is true regarding signals acting on different time scales?

Different signals can exert rapid or slow effects based on their mechanisms. Signup and view all the answers

What is the role of cAMP in cellular signaling?

It acts as a second messenger to activate protein kinase A (PKA). Signup and view all the answers

How does caffeine affect cAMP levels in neurons?

It inhibits phosphodiesterase, thus keeping cAMP around longer. Signup and view all the answers

What initiates the signaling process that leads to glycogen breakdown in muscles?

Binding of epinephrine to GPCR. Signup and view all the answers

What is the outcome of cAMP activation of PKA in gene expression?

Phosphorylation of transcription factors in the nucleus. Signup and view all the answers

Which of the following describes the role of phospholipase C (PLC)?

It cleaves inositol phospholipid to produce diacylglycerol and IP3. Signup and view all the answers

What does diacylglycerol do in the signaling cascade?

It recruits protein kinase C (PKC) to the membrane. Signup and view all the answers

What is the significance of the rise in Ca+2 concentration in cells?

It regulates gene expression by activating specific transcription factors. Signup and view all the answers

What is the function of phosphorylase kinase in glycogen metabolism?

It phosphorylates glycogen phosphorylase to promote glycogen breakdown. Signup and view all the answers

Which statement is true regarding cAMP signaling pathways?

cAMP can mediate both fast and slow signaling effects. Signup and view all the answers

What is the role of inositol 1,4,5-trisphosphate (IP3) in cellular signaling?

It binds to Ca+2 ion channels in the ER, releasing Ca+2 into the cytosol. Signup and view all the answers

What types of processes does the rise in cAMP influence?

Hormone production and long-term memory formation. Signup and view all the answers

How does the signaling via cAMP and PKA compare to other signaling pathways?

It is rapid and does not require new gene expression for an immediate effect. Signup and view all the answers

What is the initial step when GPCR is activated by a ligand?

It activates heterotrimeric G-proteins. Signup and view all the answers

What effect does calcium binding to PKC have?

It activates PKC to phosphorylate downstream targets. Signup and view all the answers

Study Notes

Cell Communication 1: GPCRs

This lecture investigates cellular signaling, focusing on G-protein Coupled Receptors (GPCRs), their activators, G-proteins, and downstream effectors.
Signaling pathways are divided into three components:
- Cells detect a signal via an appropriate receptor.
- The signal is relayed within the cell via a series of proteins.
- The cell responds by changing cellular protein activity, gene expression, or both.
Biological signals are diverse, ranging from light, small molecules (e.g., dopamine, morphine), proteins (e.g., Wnt signaling), to mechanical signals.
A cell phone analogy for signal transduction: receiving radio signals, converting these into sounds, and vice-versa demonstrates how cells receive signals from various sources and convert them to intracellular responses.
Input from the environment (or other cells) elicits specific responses. Cells convert these external signals into intracellular changes.
Cells can respond only to signals they can detect.

Signaling Pathways

Eukaryotic cells depend on multiple extracellular signals.
Signals can be combined or act alone; this determines cellular responses.
Cells only respond to signals they detect and cells can respond differently to the same signal based on cell type and internal factors.
Cells use multiple signaling pathways and receptors to generate responses.
Cells can respond to signals over varying distances, ranging from direct contact (contact-dependent) to signaling over distances like local signaling (paracrine), or long distance signaling (endocrine).
Autocrine signaling is when a cell signals to itself.

GPCRs

GPCRs are the largest family of cell surface receptors in animals (over 700 in humans).
GPCRs detect various signals, including odorants, light, and other molecules.
GPCRs have a 7 transmembrane helix structure. They have extracellular N-terminus and extracellular loops.
Ligand binding to the GPCR's outer surface triggers conformational changes that impact the intracellular components of the receptor allowing for intracellular signaling.
GPCRs are major drug targets for many medications.
Different GPCRs can be activated by various agonists (molecules that activate the receptors) but also inhibited by antagonists (molecules that bind and prevent activation).

GTP-Binding Proteins (G-Proteins)

G-proteins cycle between active and inactive conformations based on whether they bind GTP or GDP.
These proteins are central to intracellular communication.
G-proteins exist in active (GTP bound) or inactive (GDP bound) states; they regulate downstream effector activities.
G proteins are usually regulated by accessory proteins (e.g. GEFs and GAPs) which promote either the exchange GDP for GTP or stimulate the hydrolysis of GTP to GDP.
GPCR activation drives GTP exchange on Galpha.
G proteins dissociate into two signaling complexes when activated.

Signal Transduction Pathways

Signal transduction pathways convert a signal from one form to another, allowing the signal to be transmitted and amplified within the cell.
Receptors detect input, relay signals to target proteins, and then activate processes within the cell.
Pathways can create molecules which amplify signals initiating a variety of possible downstream events within the cell
Signals can have rapid/on-off effects (e.g., milliseconds to minutes) or sustained/long-term effects (e.g., minutes to hours), depending on the nature of the signal and pathways/effectors involved.

Effectors

G-proteins can regulate ion channels allowing changes in cell membrane permeability.
G-proteins activate membrane-bound enzymes (e.g., adenylyl cyclase and phospholipase C) that make second messengers (e.g., cAMP and IP3).
Second messengers trigger downstream actions and effects in the cell to change its state.

Calcium

Low intracellular calcium levels are maintained by calcium pumps on the cell membrane and within organelles like the ER.
Calcium signaling is an important intracellular signal controlling multiple cellular processes and is not limited to activating proteins like PKC.

PKA

PKA phosphorylates other proteins, ultimately leading to changes in gene expression; involved in signaling pathways that have both fast and immediate effects and long-term regulatory effects on cell function.
PKA is involved in glycogen utilization by the skeletal muscles when activated by cAMP.
PKA phosphorylates glycogen phosphorylase, leading to glycogen breakdown and releasing glucose for energy.
PKA signaling can also involve changes in long term gene expression.

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Description

This quiz explores the fascinating world of cellular signaling through G-protein Coupled Receptors (GPCRs). It covers the components of signaling pathways, types of biological signals, and how cells process and respond to these signals. Gain a deeper understanding of how cellular communication shapes biological responses in various contexts.