Receptors and transduction

Question 1

What happens when a Gs protein is activated by a GPCR?

Answer 1

At rest Gs-protein binds GDP. When the receptor is activated, its affinity for Gs increases and it interacts with the Gs-protein. the alpha subunit catalyzes the exchange of GDP for GTP.
The GPCR, alpha subunit, and beta-gamma complex dissociate.
G alpha activates adenylyl cyclase. ATP is converted to cAMP and Pi.
G alpha catalyses GTP -> GDP. The GDP stays bound to the G protein, so the G protein reverts to the resting state. This switches off the G protein’s action. the three subunits of the G protein reassociate.

Question 2

What happens when Gq is activated by a GPCR?

Answer 2

When the GPCR is activated eg by adrenaline its affinity increases for the Gq protein. G alpha exchanges GDP for GTP, causing the beta-gamma complex to dissociate.
G alpha activates phospholipase C, which hydrolyses phosophotidylinositol biphosphate -> DAG and IP3.
DAG activates protein kinase C, while IP3 binds the endoplasmic reticulum causing release of calcium.

Question 3

What is prenylation?

Answer 3

Lipid modification of proteins. eg gamma subunit of a G protein is lipid modified allowing it to anchor the beta gamma complex to the plasma membrane.

Question 4

Which effectors can be controlled by G-proteins?

Answer 4

Adenylate cyclase - catalyses formation of cAMP, activating protein kinases to control cell function. This is activated by Gs pathways and inhibited by Gi pathways.
Phospholipase C - catalyses formation of IP3 and DAG, from membrane phospholipid. (increasing cytosolic Ca via IP3, and causing phosphorylation via DAG and protein kinase C)
Phospholipase A2 (forming arachidonic acid and eicosinaoids)
Ion channels such as K and Ca channels (Go signalling)
cGMP Phosphodiesterase (Gt signalling)

Question 5

How does desensitisation occur?

Answer 5

Receptor phosphorylation, via PKA, PKC, membrane bound GPCR kinases. PKA and PKC effects impair coupling between receptor and G protein (unselective)
Receptor internalisation/endocytosis - phosphorylated receptor residues become binding site for beta arrestins, which block interactions with G proteins and target the receptor for endocytosis.

Question 6

What are two important kinase-linked pathways?

Answer 6

Ras/Raf/mitogen-activated protein MAP kinase pathway important in cell division, growth, differentiation
Jak/Stat pathway activated by cytokines, controlling synthesis and release of inflammatory mediators

Question 7

How is calcium released inside a cell?

Answer 7

Inositol triphosphate activates its receptor IP3R, a ligand gated ion channel. This triggers opening of a calcium channel on the endoplasmic reticulum.

Question 8

What is a cys-loop receptor?

Answer 8

A group of LGIC receptor with characteristic loop of 13 amino acids between 2 cysteine residues joined by a disulfide bond. They are pentameric proteins and include nAChRs, 5HT3Rs and GABAaRs.

Question 9

What are some effects of G beta-gamma subunits?

Answer 9

They can activate potassium channels, inhibit voltage-gated calcium channels, activate GPCR kinases, activate mitogen-activated protein kinase cascade.
They are thought to require higher levels of GPCR activation.

Question 10

What is the Rho A/Rho A kinase system?

Answer 10

a system that controls activity of many signalling pathways controlling cell growth and proliferation, and smooth muscle contraction.

Question 11

How many nAChR subunits have been identified?

Answer 11

10 isoforms of α subunit
4 isoforms of β subunit
Gamma subunit, epsilon subunit and delta subunit

Question 12

Which part of the GPCR interacts with alpha of the G protein?

Answer 12

3rd intracellular loop

Question 13

What are the signalling effects of a Gt protein?

Answer 13

Activates cGMP phosphodiesterase in vertebrate rod photoreceptors
This can be activated by cholera

Question 14

What are the signalling effects of a G12/13 protein?

Answer 14

Regulates Rho family GTPase signaling

and cytoskeleton remodeling

Question 15

Which toxin can modify Gs signalling?

Answer 15

Cholera (activates)

Question 16

Which toxin can modify Gi signalling?

Answer 16

Pertussis (inhibits)

Question 17

Which G proteins are coupled with an M2 muscarinic receptor? (cholinergic)

Answer 17

Gi and Go

Question 18

What are the signalling effects of a Go protein?

Answer 18

Stimulates PLCβ
Activates K+ channels
inactivates Ca channels
(also inhibited by Pertussis)

Question 19

What is the G protein coupled with both alpha1 adrenoceptors and cholinergic M1 muscarinic receptors?

Answer 19

Gq

Question 20

How does an M2 muscarinic receptor affect the heart?

Answer 20

Via alpha of the Gi protein, adenylate cyclase is inhibited and K+ channels are opened, causing hyperpolarisation andcardiac inhibition.

Question 21

How do receptor tyrosine kinase proteins work?

Answer 21

They dimerise upon ligand binding leading to auto-phoshphorylation of tyrosine redisues - each RTK phosphorylates many tyrosines on the otherr RTK.
Phosphorylated tyrosine residues act as recognition sites for adapter proteins with SH2/3 domains.
Many different effectors may interact with the same receptor, causng amplification at each step.
Responses operate on the minutes/hours time scale.
Most RTKs are single membrane scanning proteins.

Question 22

How does the MAPK cascade work?

Answer 22

Ras is a small g protein attached to membrane. Inactivated it is bound to GDP. SH2- containing proteins joined to P’d tyrosine residues of RTKs cause Ras to bind GTP and become active. GTP-Ras activates the first serine-threonine kinase in the cascade. Each inase then activates the next MAP kinase by phosphorylating it. All three activate multiple substrates at once so the effect is amplified. The final enzyme phosphorylates transcription regulators, changing gene transcription. eg nerve growth factor, platelet derived growth factor.

Question 23

What are the different types of nuclear receptors?

Answer 23

• Steroid hormones present in the cytoplasm, and form homodimers. They migrate to the nucleus.
• those constitutively present in the nucleus and form heterodimers with retionoid x receptor eg thyroid hormone
• these bind hormone response elements in gene promoters and recruit coactivators or corepressor factors to initate changes in gene transcription

Question 24

What are the ligands of tyrosine kinase receptors and tyrosine kinase-linked receptors?

Answer 24

Insulin, epidermal growth factor

tyrosine kinase linked - growth hormone

Question 25

What is a ligand of serine/threonine kinase receptors?

Answer 25

Transforming growth factor

Question 26

What is a ligand of guanylate cyclase receptors?

Answer 26

Atrial natriuretic peptide