Lecture 10: Intracellular signalling

Question 1

What is intracellular signalling?

Answer 1

Signals sent within the cell to stimulate a change in an activity of another or the same part of the cell

Question 2

What are examples of intracellular signalling molecules?

Answer 2

Ions, hydrophobic membrane-associated proteins, hydrophilic proteins in the cytosol, gases, and second messengers (eg. cAMP).

Question 3

What are transmembrane receptors?

Answer 3

A cell surface receptor that is either anchored in the membrane or passes through the membrane that binds to extracellular ligands.

Question 4

How are signalling molecules controlled ?

Answer 4

1. Post translational modification eg. Phosphorylation
2. Regulating whether a G protein has bound GDP or GTP
3. Provision of activators such as Ca2+ and cAMP

Question 5

How are proteins phosphorylated?

Answer 5

Protein kinases convert ATP to ADP and transfer phosphate group to protein.

Question 6

What are phosphatases?

Answer 6

An enzyme that removes a phosphate group from a protein by hydrolysis.

Question 7

What is hydrolysis?

Answer 7

The chemical breakdown of a compound due to reaction with water.

Question 8

How does receptor activation alter proteins?

Answer 8

Receptor activation causes a conformational change in the tertiary or quaternary structure that allows initiation of signalling.

Question 9

What is signal amplification?

Answer 9

Cascade of signalling molecules which can activate several different receptors

Question 10

What is signal integration?

Answer 10

Different signalling molecules need to be activated to come together and trigger next step in the signalling pathway

Question 11

What AA’s can be phosphorylated in a protein molecule?

Answer 11

Serine, threonine or tyrosine.

Question 12

Describe the GTP molecular switch mechanism?

Answer 12

Small GTPase uses hydrolysis to convert GTP to GDP, and exchange is promoted by guanine exchange factors (GEFs) . During the process water is split into OH- and H+.

Question 13

How does GTPase activity change in some cancers?

Answer 13

Ras is a small GTPase which can become mutated resulting in mutated GTPase activity leading to continuous activation.

Question 14

What are the 3 main categories of transmembrane receptors?

Answer 14

1. Receptors linked to ion channels
2. Receptors linked to G proteins
3. Receptors linked to enzymes

Question 15

What is the role of ion channels in intracellular signalling?

Answer 15

Ion channels transport ions along an electrochemical gradient to trigger a signalling cascade.

Question 16

How is ion channel opening regulated?

Answer 16

Ion channel opening is regulated and can be activated by either a change in membrane voltage or by a ligand.

Question 17

What are CAM kinases?

Answer 17

Calmodulin-dependent protein kinases

Question 18

What is Calcineurin?

Answer 18

Ca2+-dependent protein phosphatase

Question 19

What are the 3 subunits of G-proteins?

Answer 19

Alpha, beta and gamma

Question 20

What are the 3 alpha subunits of G-proteins?

Answer 20

Alpha-s, Alpha-q and Alpha-i

Question 21

What are G-proteins?

Answer 21

Heterotrimeric proteins anchored into the membrane.

Question 22

Describe the process of G-protein coupled receptor activation?

Answer 22

Ligand binds to the receptor and increases affinity for G-protein. G-protein undergoes a conformation change to its internal portion, releasing GDP from α-subunit which is replaced by GTP. α-subunit of G-protein can then initiate further signalling.

Question 23

Describe the β adrenergic receptor activation of Gαs proteins?

Answer 23

Adrenaline/salbutamol binds to receptors and triggers activation of G-protein and dissociate of GDP for GTP. Activated G-protein binds to and activates adenylate cyclase which converts ATP to cAMP. cAMP triggers a cascade event which activates PKA which effects gene transcription. Ultimately causes relaxation of blood vessels in skeletal muscle in terms of adrenaline and bronchodilation which elevates the symptoms of asthma in terms of salbutamol.

Question 24

Describe the α adrenergic receptor activation of Gαq proteins?

Answer 24

Signal molecule (acetylcholine) binds to receptors and triggers activation of alpha-q subunit of G-protein and dissociation of GDP for GTP. Activation of alpha-q subunit of G-protein triggers activation of phospholipase C which cleaves inositol phospholipids into IP3 (which triggers calcium release from ER) and DAG which ultimately activate PKC. Ultimately leads to contraction of smooth muscle in the vasculature of the gut and salivary secretion.

Question 25

What is the end result of Gas activation?

Answer 25

Activates PKA which effects gene transcription and ultimately causes relaxation of blood vessels in skeletal muscle and bronchodilation.

Question 26

What is the end result of Gaq activation?

Answer 26

Activate PKC which ultimately leads to contraction of smooth muscle in the vasculature of the gut and salivary secretion.

Question 27

Describe the activation of G-protein βγ subunits?

Answer 27

Binding of acetylcholine to receptor triggers activation of alpha-subunit of G-protein and dissociation of GDP to GTP. Activated G-protein binds to βγ complex and activates it. Activated βγ subunit binds to the K+ channels causing them to open resulting in potassium efflux out of the cell causing them to become hyperpolarized and the heart slows down.

Question 28

What is the end result of Gβγ activation?

Answer 28

Opening of K+ channels to cause cells to become hyperpolarized and slow the heart down

Question 29

What is cholera?

Answer 29

Cholerais an acute diarrhoeal infection caused by ingestion of food or water contaminated with the bacterium Vibriocholerae.

Question 30

How does cholera affect G-protein activation?

Answer 30

Cholera inhibits the GTPase activity of the subunit Gαs resulting in Cl- moving out of the cells lining the intestine.

Question 31

How does whooping cough affect G-protein inactivation?

Answer 31

Pertussis toxin from whooping cough renders Gαi inactive, so prevents the inhibition of adenylate cyclase. Signal stimulates coughing.

Question 32

What is the role of the G-protein alpha-I subunit?

Answer 32

Subunit inhibits adenylate cyclase activation.

Question 33

How does intracellular signalling work for receptors linked to enzymes?

Answer 33

Receptors are coupled to an enzymatic event.

Question 34

How do receptor tyrosine kinases work?

Answer 34

Ligand binding activates receptor dimerization and to trigger enzyme activity within the cytoplasmic domain. In the case of tyrosine kinase, tyrosine residues in the intracellular domains are auto-phosphorylated in response to the signal. Enzymatic activity riggers a downstream intracellular signalling cascade.

Question 35

What is receptor ‘cross-talk’?

Answer 35

Integration of signalling molecules. Signalling molecules can combine to boost signal, change signal or even stop signals.