Signal Transduction

Question 1

What are the two main types of kinases?

Answer 1

Tyrosine Kinase and serine/threonine kinase

Question 2

What are the two main types of GTP-binding proteins?

Answer 2

Trimeric G proteins and Monomeric GTPase

Question 3

How do G-protein-coupled receptors (GPCRs) work?

Answer 3

Ligand binds to receptor which activates a G-protein which in turn activates or inhibits another protein. Often this protein is an enzyme that generates a specific second messenger

Question 4

Describe the structure of a G-protein

Answer 4

They have 7 membrane spanning regions with their amino termini on the extracellular face and their carboxy termini on the cytoplasmic face.

Question 5

Why is signal transduction needed?

Answer 5

To mediate cell activity

Question 6

Most cell-surface receptors are coupled to what kind of G-protein and what does this mean?

Answer 6

To a trimeric G-protein, meaning it is compsed of three different subunits (alpha, beta and gamma)

Question 7

Describe the mechanism of GPCRs

Answer 7

The binding of the ligand causes a conformational change to the receptor causing it to bind to the G,alpha protein in a way that displaces GDP and GTP binds. This triggers G,beta,gamma dissociation activating downstream pathways. However this activation is short lived as GTP hydrolyses to GDP in seconds leading to the re-association of G,alpha with G,beta,gamma and the inactivation of adenylate cyclase

Question 8

The overall system of GPCRs require what?

Answer 8

A receptor, a transducer (G-protein) and an amplified (adenylate cyclase) which generates lots of second messenger

Question 9

Name three G,alpha subunits (different G proteins) and what they do

Answer 9

G(alpha)q or Gq - stimulates phospholipase C
Gs - Stimulates adenylate cyclase, increasing cAMP
Gi - Inhibits adenylate cyclase and therefore decreases cAMP

Question 10

What is the role of Phospholiapse C

Answer 10

To cleave PIP(2) into IP(3) and DAG. The IP(3) can open calcium channels in the ER releasing calcium. This causes PKC to translocate to the membrane where it is activated by DAG.

Question 11

How is PLC activated?

Answer 11

By Gq protein (G,alpha,q)

Question 12

What is calcium involved in?

Answer 12

Exocytosis, contraction, metabolism, gene transcription, fertilisation (so lots of cellular events)

Question 13

What causes the initial spark of fertilisation? and then what occurs after this?

Answer 13

PLC-zeta by triggering the opening of surface calcium channels. The wave of calcium then triggers the start of embryonic development and prevents other sperm from entering the cell.

Question 14

Where are the substrates for PKC?

Answer 14

Some in the cytoplasm but some isoforms can translocate to the nucleus and phosphorylate nuclear proteins. It can also act indirectly to alter gene transcription

Question 15

What is the receptor for adrenaline?

Answer 15

Beta (2) adrenergic receptor

Question 16

What happens when adrenaline binds to its receptor?

Answer 16

Receptor - Beta2 adrenergic receptor.

It causes the increase of intracellular concentrations of cAMP (Cyclic AMP) as the receptor couples to G(alpha)s.

Question 17

What is cAMP synthesised by and from?

Answer 17

From ATP by Adenylate cyclase

Question 18

What is cAMP degraded by?

Answer 18

cAMP phosphodiesterase

Question 19

How does cAMP activate the cAMP-dependant protein kinase (PKA)

Answer 19

cAMP binds to regulatory subunits of the PKA which causes the release of the catalytic subunits

Question 20

What does signal transduction result in?

Answer 20

The amplification of a signal meaning a small change at the top of the chain can have a large change at the bottom.

Question 21

What can PKA phosphorylate?

Answer 21

Substrates on serine or threonine residues

Question 22

What does adrenaline cuase?

Answer 22

The release of glucose and fatty acids from liver/fat and the increased contraction of cardiac muscle

Question 23

What can PKA do?

Answer 23

Phosphorylate a protein called CREB which can then bind to cAMP response element regions (CRE’s) and therefore switching on gene transcription

Question 24

Describe what cholera toxins do

Answer 24

It becomes cleaved and active and then enters the intestinal epithelium cells to activate Galpha(s). The overstimulation of cAMP production results in release of water and ions into the lumen of the small intestine leading to rapid fluid loss and dehydration.

Question 25

How does pertussis toxins work?

Answer 25

It inhibits Galpha(i) to increase cAMP production in lung epithelium.

Question 26

How do are receptor tyrosine kinases (RTKs) activated?

Answer 26

A signalling molecule in a form a dimer binds to the receptors causing two receptors to come together. They then phosphorylate each other. Intracellular proteins can then bind to these phosphates and mediate signal transduction

Question 27

What is an example of a small GTPase protein and how is it activated?

Answer 27

Ras and it is activated by receptor tyrosine kinase (RTK)

Question 28

Describe how Ras is activated?

Answer 28

Adaptor protein attached to RTK interacts with the Ras-activating protein. This protein then causes the GDP on the inactive Ras protein to switch to GTP, activating it.

Question 29

What small GTPases are direct targets for clostridial cytotoxins?

Answer 29

Rac/Rho subfamily

Question 30

Mutations in what small GTPase is found in aprox. 20% of cancers?

Answer 30

Ras

Question 31

What does Ras activate?

Answer 31

The MAPK (mitogen-activated protein kinase) pathway which results in changes in protein activity or changes in gene expression

Question 32

What is the EGFR (epidermal growth factor receptor) activated by and what odes it do?

Answer 32

It is a RTK which is activated by TGFἁ (transforming growth factor alpha), it then goes onto activate Ras via Grb2/SOS.

Question 33

Mutations in EGFR, Ras and Raf are associated with what?

Answer 33

Tumorigenesis