G Proteins and GPCRs

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Questions and Answers

What is the characteristic that distinguishes trimeric G proteins activated by GPCRs from small monomeric G proteins like Rabs?

They are activated by GPCRs

They have GTPase activity

They are composed of alpha, beta, and gamma subunits (correct)

They are bound to the plasma membrane

How does cholera toxin affect α_s subunits?

It promotes G-protein activation

It covalently modifies α_s, blocking GTPase activity (correct)

It stimulates GTPase activity

It increases cAMP levels

What is the role of the βγ subunit in the G-protein cycle?

It regulates effectors independently of α-GTP

It dissociates from the α subunit upon G-protein activation (correct)

It inhibits Ca2+ channels

It stimulates adenylyl cyclase

What is the effect of α_i subunits on adenylyl cyclase?

They inhibit it Signup and view all the answers

Which of the following is NOT a function of the βγ subunit?

Inhibiting adenylyl cyclase Signup and view all the answers

What is the role of the γ phosphate in the G-protein cycle?

It dictates the active or inactive state of the G protein Signup and view all the answers

What is the result of GTP binding to the α subunit?

Dissociation of the G protein from the GPCR Signup and view all the answers

Which type of α subunit is involved in the parasympathetic response to ACh?

α_i Signup and view all the answers

What is the common structural feature among all GPCRs?

Extracellular N-terminal separated by 7 TMDs from cytosolic C-terminal Signup and view all the answers

What happens to the TMDs of GPCRs when they are activated?

A cleft opens between the cytosolic ends of TMDs 3, 5, 6, 7 Signup and view all the answers

What is the result of GPCR activation on G-proteins?

GDP is released from the G-protein Signup and view all the answers

What is the region into which the α-subunit of trimeric G proteins can insert?

A cleft between the cytosolic ends of TMDs 3, 5, 6, 7 Signup and view all the answers

What is the orientation of the N-terminal and C-terminal of GPCRs?

Extracellular N-terminal and cytosolic C-terminal Signup and view all the answers

What is the term coined by Sutherland to describe archetypal cAMP?

Second messenger Signup and view all the answers

What is the most important intracellular target for cAMP?

Protein kinase A Signup and view all the answers

What is the effect of caffeine and theophylline on phosphodiesterases (PDEs)?

They inhibit PDEs Signup and view all the answers

What is the function of adenylyl cyclase 1 (AC1)?

It acts as an AND gate Signup and view all the answers

What is the target of the drug Cilostazol, which is used to treat obstructed peripheral arteries?

Phosphodiesterase 3 Signup and view all the answers

What is the effect of PKA on phosphodiesterase 3 (PDE3)?

It stimulates PDE3 Signup and view all the answers

What is the function of the pseudo-substrate domain in each R subunit of PKA?

To block the active site of the C subunit Signup and view all the answers

What is the function of AKAPs in relation to PKA?

To assemble related proteins with PKA Signup and view all the answers

What is the effect of PKA phosphorylation on protein substrates?

Phosphorylation occurs on serine and threonine residues Signup and view all the answers

What is the role of phosphatases in relation to PKA activity?

Phosphatases reverse PKA-mediated phosphorylation Signup and view all the answers

What is the action of PDE5 on cGMP?

PDE5 degrades cGMP Signup and view all the answers

What is the effect of cAMP binding to the R subunit of PKA?

The R subunit dissociates from the C subunit Signup and view all the answers

What is the product of the reaction catalyzed by phospholipase C?

IP3 and DAG Signup and view all the answers

What is the effect of lithium on IP1?

It blocks IP1, preventing the conversion to inositol Signup and view all the answers

What is the result of IP3 binding to its receptors on the ER?

Release of Ca2+ from the ER Signup and view all the answers

What is the function of calmodulin in cellular signaling?

Ca2+ binding and regulation of downstream targets Signup and view all the answers

What is the precursor molecule for the synthesis of PIP2?

Phosphatidate Signup and view all the answers

What is the role of Ca2+ waves in intracellular signaling?

To increase frequency of Ca2+ spikes Signup and view all the answers

What is the significance of Ca2+ spikes in cellular signaling?

To protect cells from damaging effects of excessive Ca2+ increases Signup and view all the answers

What do Ca2+ indicators developed by Tsein reveal?

The spatial and temporal complexity of intracellular Ca2+ signals Signup and view all the answers

What is the advantage of spatially organized Ca2+ signals?

To allow Ca2+ from different sources to be delivered to different targets Signup and view all the answers

What is the role of IP3 receptors in the CICR mechanism?

To release Ca2+ from the ER Signup and view all the answers

Study Notes

G Proteins Activated by GPCRs

Trimeric G proteins are distinct from small monomeric G proteins like Rabs.
βγ subunits are inseparable and anchored to the plasma membrane.
α subunits are divided into 4 major families and bind and hydrolyze GTP.

α Subunit Structure and Function

GDP is bound to the inactive α-subunit in a deep pocket, restricting its escape.
Two conserved residues within two distinct domains of the α subunit are switches 1 and 2.
These switches make contact with the γ phosphate and pull in the switch regions, activating the G protein and breaking its contact with the βγ subunit.

G-Protein Activation and Dissociation

GPCRs promote G-protein activation, opening the deep pocket where GDP is tightly bound.
Binding of GTP to the α subunit causes dissociation from the βγ subunit.
The α-GTP and βγ subunits can now regulate their effectors.

Regulation of Activity

GTPase activity of α-GTP directly controls its activity and indirectly controls the activity of the βγ subunit.
The α-GTP subunit reassociates with the βγ subunit once it has been hydrolyzed to α-GDP.

α Subunit Families and Functions

The α_s subunit stimulates adenylyl cyclase.
The α_i subunit inhibits adenylyl cyclase and is involved in the parasympathetic response to ACh.
The α_q subunit stimulates phospholipase C.
The α_12 subunit regulates the cytoskeleton.

βγ Subunit Functions

The βγ subunit stimulates phospholipase C, inhibits Ca2+ channels, stimulates K+ channels, and stimulates P13K.

Toxins and Modifiers

Cholera toxin covalently modifies α_s, blocking GTPase activity and leading to fluid loss in the gut.
Pertussis toxin covalently modifies α_i, leading to uncoupling of the G-protein from GPCRs.

GPCR Structure

All GPCRs possess a similar structure, featuring an extracellular N-terminal region and a cytosolic C-terminal region separated by 7 transmembrane domains (TMDs).
The 7 TMDs are crucial for GPCR function, as they create a cleft when activated.

GPCR Activation

When a GPCR is activated, a cleft opens between the cytosolic ends of TMDs 3, 5, 6, and 7.
The cleft created by TMDs 3, 5, 6, and 7 serves as a binding site for the α-subunit of trimeric G proteins.
The GPCR causes the release of GDP from the G-protein following binding.

Allosteric Proteins and Signaling Cascades

Allosteric proteins transmit information from the plasma membrane (PM) via signaling cascades involving protein-protein and protein-small messenger interactions.

cAMP and Adenylyl Cyclases

cAMP is an intracellular messenger produced from ATP by adenylyl cyclases.
cAMP is inactivated by phosphodiesterases (PDEs).
Protein kinase A (PKA) is the primary target of cAMP.

Adenylyl Cyclase (AC) Forms and Regulation

There are 9 forms of adenylyl cyclase, all stimulated by α_s-GTP.
Each form of AC responds differently to other intracellular signals, such as Ca2+.
AC1 is stimulated by Ca2+ and responds optimally with Ca2+ and α_s-GTP, functioning as an AND gate.
Other forms of AC function as NOT gates, demonstrating integrative behaviors.

Phosphodiesterases (PDEs) and Regulation

PDEs are regulated and most are inhibited by caffeine and theophylline.
Inhibition of PDEs increases cAMP levels, contributing to the treatment of asthma.

Therapeutic Applications

Drugs that increase cAMP formation or reduce degradation can contribute to the treatment of asthma.
Ventolin, which activates receptors in the airway, increases cAMP and relieves asthma.
Cilostazol, which targets PDE3, is used to treat obstructed peripheral arteries.

Protein Kinase A (PKA)

Consists of 2 regulatory (R) subunits, each binding 2 cAMP molecules, and 2 catalytic (C) subunits, which phosphorylate substrates
R subunits have a pseudo-substrate domain that blocks the active site of the C subunit
Binding of cAMP to R subunits causes them to fall apart, unblocking the active site of the C subunit, allowing phosphorylation of substrates
Phosphorylation occurs on serine and threonine residues
Phosphorylation is reversed by phosphatases

A-Kinase Anchoring Proteins (AKAPs)

Scaffold proteins that associate with the R subunit of PKA
Anchor PKA at different places within the cell
Assemble related proteins with PKA, including targets of PKA

cGMP Signaling

Similar to cAMP signaling
Produced by guanylyl cyclases
Degraded by phosphodiesterases (PDEs)
Many actions mediated by protein kinase G

PDE5 and Viagra

PDE5 selectively degrades cGMP
Viagra prevents degradation of cGMP in blood vessels of the penis, causing them to dilate and accumulate blood

Phospholipase C (PLC) Signaling Pathway

PIP2 is converted to DAG and IP3 by phospholipase C (PLC)
DAG remains in the plasma membrane (PM) and activates protein kinase C (PKC)

DAG Metabolism

DAG is converted to phosphatidate, stimulated by phosphorylation

IP3 Metabolism and Function

IP3 is water-soluble and enters the cytosol, stimulating Ca2+ release from the endoplasmic reticulum (ER)
IP3 is converted to inositol via IP1, stimulated by dephosphorylation and inhibited by lithium

Lithium's Effect on IP3 Metabolism

Lithium blocks IP1, preventing the conversion to inositol, and is used to treat bipolar disorder by reducing signaling in over-active areas of the brain
Lithium prevents PIP2 reformation, reducing signaling in over-active areas of the brain

Phosphatidate and Inositol Conversion

Both phosphatidate and inositol are converted to PIP2

PLC Activation and Downstream Signaling

PLC can be activated by G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs)
Activated PLC produces DAGs, recruiting PKC to the PM and activating it, and IP3, releasing Ca2+ from the ER through IP3 receptors
Ca2+ released by IP3 receptors regulates many intracellular activities, often through the highly conserved Ca2+ binding protein, calmodulin

IP3 Receptor and Ca2+ Signals

IP3 receptor provides Ca2+ ions, which leads to the CICR (Calcium-Induced Calcium Release) mechanism.
Increased stimulus intensity leads to more frequent Ca2+ waves.
Ca2+ indicators developed by Tsein have revealed the spatial and temporal complexity of intracellular Ca2+ signals.

Characteristics of Ca2+ Signals

Many Ca2+ signals are delivered as Ca2+ spikes.
The frequency of Ca2+ spikes increases with stimulus intensity.
Ca2+ spikes may protect cells from damaging effects of excessive increases in cytosolic Ca2+ concentrations.
Ca2+ spikes allow for digital signalling, similar to action potentials.
Spatially organised Ca2+ signals enable Ca2+ from different sources to be delivered to different targets.

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Description

This quiz covers the structure and function of G proteins, including trimeric and monomeric forms, their activation by GPCRs, and the role of α and βγ subunits. It also explores the effects of cholera toxin and pertussis toxin on G protein activity.