3. Signalling mechanisms in growth and division

Question 1

What sort of enzymes are Cdks?

Answer 1

Serine-threonine kinases

Question 2

What things are required for Cdk activation?

Answer 2

Specific cyclin

Phosphorylation

Question 3

Summarise the life cycle of a cyclin

Answer 3

Expressed at apporpriate point in cell cycle

Once it has bound to a Cdk to activate it = degraded

Question 4

What is M-phase Promoting Factor made up of?

Answer 4

Cdk1-Cyclin B

Question 5

Recall the phosphorylation reactions involved in Cdk activation

Answer 5

1. Phosphorylation by CAK

2. Removal of inhibitory phosphate by Wee1

Question 6

How is Wee1 phosphate removal activated?

Answer 6

CDC25 phosphatase

Question 7

Describe the positive feedback loop involved in Cdk control

Answer 7

CDC25P –> increased MPF –> drives increased CDC25P activation

Question 8

What is the function of MPF?

Answer 8

Phosphorylates nuclear lamins to initiate nuclear envelope breakdown

Question 9

How is MPF inactivated?

Answer 9

Kinetochore attachment to spindle releases signal to degrade cyclin B

Question 10

Describe the mechanisms controlling entry into G1 phase

Answer 10

1. Mitogenic Growth Factor binds RPTK
2. RPTK activates Ras
3. Ras –> MAPK/ERK cascade
4. MAPK/ERK cascade –> c-Myc expression
5. Cdk4/6-Cyclin D complex required

Question 11

Recall the kinases specific to the MAPK/ERK pathway

Answer 11

Raf/MEK/ERK

Question 12

What is the role of Grb2?

Answer 12

Binds to TRPK to inactivate Ras

Question 13

Describe the control of Ras

Answer 13

Activated when bound to GTP, inactive when bound to GDP

1. Exchange factor Sos activates to Ras-GTP
2. GAP hydrolyses to Ras-GDP to inactivate

Question 14

What is the role of the Cdk4/6-Cyclin D complex?

Answer 14

1. Activate start kinase (Cdk2-Cyclin E)

2. Phosphorylate retinoblastoma

Question 15

Recall the role and control of retinoblastoma protein (pRb)

Answer 15

Unphosphorylated in G0 as bound to E2F transcription factors in cytoplasm
When phosphorylated by SK, E2F released
E2F TFs bind to promoters on cell-cycle progression genes including cyclin E

Question 16

What sort of protein is retinoblastoma?

Answer 16

Tumour suppressor gene

Question 17

Recall 3 targets of E2F TFs

Answer 17

1. Increase cyclin E expression, Cyclin A at higher concentrations
2. Regulate proto-oncogenes eg Myc
3. Control S phase proteins eg thymidine kinase

Question 18

Recall the feedback loop that E2F is involved in

Answer 18

E2F stimu;ates Cdk2-Cyclin E which in turn stimulates pRb: pRb causes more E2F to be released = positive feedback

Question 19

Recall the families of CKIs, their members and their function

Answer 19

1. INK4 (P15/16/18/19) = displaces cyclin D from Cdk4/6

2. CIP/KIP (P21/27/57) = inhibits all Cdk-Cyclin complexes

Question 20

Which transcription factor is required for entry into the cell cycle?

Answer 20

c-Myc

Question 21

How do growth factors activate RPTKs? (3 steps)

Answer 21

1. Monomeric GF brings 2 receptors togetehr to form active dimer
2. 2 elements of dimer cross-phosphorylate tyrosine residues in the intracellular domain
3. Adaptor proteins use phosphorylated tyrosine residues as docking sites

Question 22

How does Herceptin target the cell cycle?

Answer 22

Inhibits Her2 Receptor tyrosine kinase

Question 23

What is the key adaptor protein involved in cell cycle control?

Answer 23

Grb2

Question 24

Recall the sites of interaction of Grb2

Answer 24

1. SH2 domain interactions = binds Grb to phosphorylated tyrosine resides of RPTKs
2. SH3 domain interactions = binds Grb to proline-rich regions of Sos

Question 25

What does GAP stand for with relation to cell cycle control?

Answer 25

GTPase Activating Proteins

Question 26

Recall 2 common Ras mutations and their effect

Answer 26

1. V12Ras = Gly–> Val at Pos 12: prevent GAP binding

2. L61Ras = Glu –> Leu at Pos 61: inhibits GAP so Ras is constantly bound to GTP