Cell signalling 3

Question 1

Where are the photoreceptors in a rod cell?

Describe them

Answer 1

Outer segment
- opposite end to light + synaptic region

Discs of photoreceptor membrane containing Rhodopsin GPCR

Question 2

What does signalling amplification in rod cells allow?

Answer 2

Detection of low light levels for sensitivity

Question 3

What does adaptation in rod cells allow?

Answer 3

Detection of changes in light levels even in bright light

Question 4

How do rod cells detect changes in light levels even in bright light?

Answer 4

Many GPCRs are activated

• > closes many ion channels
• > causes cytoplasmic Ca2+ (and Na+) levels to fall

Ca2+ needed by enzymes involved in amplification

Question 5

Describe the pathway from light hitting the Rhodopsin GPCR to signals being sent to the brain

Answer 5

1. 1 Rhodopsin molecule absorbs 1 photon
2. Interacts with alpha-transducin -> changes its conformation -> subunit takes up GTP
3. Alph subunit activates cGMP phosphodiesterase
4. cGMP –> 5’GMP
5. Removes cGMP from Na+ ion channels
6. Decrease in Na+ inside cell
7. Changes charge across membrane
8. Interrupts the release of inhibitory neurotransmitters
9. Signal passed to brain

Question 6

How is the signal to the brain from the eye switched off?

i.e. how is the process reversed to resting state?

Answer 6

1. All-trans retinal needs to be replaced by a new 11-cis retinal molecule synthesised from VitA
2. GTPase converts GTP bound to alpha subunit into GDP
3. cGMP is synthesised from GTP by guanylyl cyclase
   - stimulated by low Ca2+ levels

Question 7

How does the absorption of a photon affect Rhodopsin GPCR?

Answer 7

Chromophore retinal is photoisomerised from 11-cis to all-trans retinal by photon absorption

Question 8

What is a mitogen?

What types of signalling are they?

Answer 8

A chemical substance, usually protein, that induces a cell to begin mitosis

Endocrine or paracrine

Question 9

Give 3 examples of mitogens

Answer 9

IGF = insulin growth factor
- cell survival + proliferation

PDGF = platelet-derived growth factor
- cell proliferation

NGF = nerve growth factor
- neuron survival

Question 10

How does receptor tyrosine kinase work?

Answer 10

1. Receptor dimerisation
   - signal holds 2 tyrosine kinase domains together
2. Autophosphorylation
   - cross-phosphorylate each other -> increases activity of TK domain
3. Signal protein binding
   - small change in conformation allows formation of docking domains for signals
   = activates intracellular signalling proteins
4. Downstream cascade activated

Question 11

How is receptor tyrosine kinase signalling reversed?

Answer 11

Phosphatase activity

Question 12

What is the Ras-GAP pathway important in?

Answer 12

Cell proliferation

Question 13

Describe the Ras protein pathway

Answer 13

1. activated RTK has an attached adaptor protein that binds to Ras-GEF
2. Activated Gas-GEF activates Ras protein (conformation change -> GDP is replaced by GTP)
3. Activated Ras protein interacts w/ next protein in cascade

Question 14

How is the Gas-GAP pathway reversed?

Answer 14

Promoted by interaction w/ Ras-GAP

GTPase activating protein

Question 15

What are MAP kinases?

What do they do?

Answer 15

Mitogen-activating kinases

Phosphorylate serine + threonine kinases

Question 16

Describe the cascade which Ras stimulates

Answer 16

1.Activated Gas protein activates MAP KKK (kinase x3)
2. Activated MAP KKK activates MAP KK using ATP
(2 Pi added)
3. MAP KK activates MAP K using ATP
(2 Pi added)
4. MAP K phosphorylates proteins or TFs using ATP -> ADP

Question 17

What type of gene are Ras genes?

Answer 17

Oncogenes
- mutated in 30% of cancers
(signals fire constantly -> cell proliferation constantly)

Question 18

How do RTKs stimulate cell survival?

Answer 18

1. Survival signal activates RTK
2. Activates PI 3-kinase
3. Phosphorylates inositol phospholipid
4. Activates protein kinase 1
5. Activates Akt via Phosphorylation
   (protein kinase 2 also helps to activate Akt)

Question 19

What does the phosphorylation of inositol phospholipids do?

Answer 19

Creates docking sites for target proteins

-including Akt + PK1

Question 20

What does AKT inactivating BAD do?

Answer 20

Prevents cell death

Question 21

Describe how AKT prevents cell death

Answer 21

Akt phosphorylates the BAD protein

• > releases active Bcl2
• > protists cell survival by inhibiting apoptosis

Question 22

What happens if AKT doesn’t phosphorylate BAD?

Answer 22

BAD would interact w/ Bcl2

–> cell death by apoptosis

Question 23

What does the activation of mTOR do?

Answer 23

Regulates cell growth

Question 24

Describe the pathway in which mTOR regulates cell growth

Answer 24

1. Growth factor activates RTK
2. activates PI 3-kinase
3. Activates AKT
4. Activates TOR
5. Inhibits protein degradation + stimulates protein synthesis

Question 25

What are the features of TOR?

Answer 25

Nutrient sensing
- if enough amino acids are around, this is relayed to TOR

(as if enough nutrients are mourned cell growth can occur)

Question 26

What is the importance of the Gas/MAP-kinase pathway?

Answer 26

> Outcome is often cell proliferation (depends on cell type)
Ras neutarlising antibodies can block cell proliferation
Ras gene is mutated in 30% of cancers
Ras + other members of pathway = oncogenes + tumour suppressors

Question 27

What are the 3 protein conformation ‘switches’ used in cell signalling pathways?

Answer 27

> signalling by protein phosphorylation
signalling by GTP-binding protein
protein-protein interaction (e.g. Ras: GTP -> MAP-KKK)

Question 28

What does the cross-take between signalling pathways allow?

Answer 28

The integration of diff info for the control of complex cell behaviours

Question 29

What do activated tyrosine RTKs frequently simulate?

Answer 29

The Ras/MAP-kinase pathway