3. Signalling mechanisms in growth and division Flashcards
What sort of enzymes are Cdks?
Serine-threonine kinases
What things are required for Cdk activation?
Specific cyclin
Phosphorylation
Summarise the life cycle of a cyclin
Expressed at apporpriate point in cell cycle
Once it has bound to a Cdk to activate it = degraded
What is M-phase Promoting Factor made up of?
Cdk1-Cyclin B
Recall the phosphorylation reactions involved in Cdk activation
- Phosphorylation by CAK
2. Removal of inhibitory phosphate by Wee1
How is Wee1 phosphate removal activated?
CDC25 phosphatase
Describe the positive feedback loop involved in Cdk control
CDC25P –> increased MPF –> drives increased CDC25P activation
What is the function of MPF?
Phosphorylates nuclear lamins to initiate nuclear envelope breakdown
How is MPF inactivated?
Kinetochore attachment to spindle releases signal to degrade cyclin B
Describe the mechanisms controlling entry into G1 phase
- Mitogenic Growth Factor binds RPTK
- RPTK activates Ras
- Ras –> MAPK/ERK cascade
- MAPK/ERK cascade –> c-Myc expression
- Cdk4/6-Cyclin D complex required
Recall the kinases specific to the MAPK/ERK pathway
Raf/MEK/ERK
What is the role of Grb2?
Binds to TRPK to inactivate Ras
Describe the control of Ras
Activated when bound to GTP, inactive when bound to GDP
- Exchange factor Sos activates to Ras-GTP
- GAP hydrolyses to Ras-GDP to inactivate
What is the role of the Cdk4/6-Cyclin D complex?
- Activate start kinase (Cdk2-Cyclin E)
2. Phosphorylate retinoblastoma
Recall the role and control of retinoblastoma protein (pRb)
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
What sort of protein is retinoblastoma?
Tumour suppressor gene
Recall 3 targets of E2F TFs
- Increase cyclin E expression, Cyclin A at higher concentrations
- Regulate proto-oncogenes eg Myc
- Control S phase proteins eg thymidine kinase
Recall the feedback loop that E2F is involved in
E2F stimu;ates Cdk2-Cyclin E which in turn stimulates pRb: pRb causes more E2F to be released = positive feedback
Recall the families of CKIs, their members and their function
- INK4 (P15/16/18/19) = displaces cyclin D from Cdk4/6
2. CIP/KIP (P21/27/57) = inhibits all Cdk-Cyclin complexes
Which transcription factor is required for entry into the cell cycle?
c-Myc
How do growth factors activate RPTKs? (3 steps)
- Monomeric GF brings 2 receptors togetehr to form active dimer
- 2 elements of dimer cross-phosphorylate tyrosine residues in the intracellular domain
- Adaptor proteins use phosphorylated tyrosine residues as docking sites
How does Herceptin target the cell cycle?
Inhibits Her2 Receptor tyrosine kinase
What is the key adaptor protein involved in cell cycle control?
Grb2
Recall the sites of interaction of Grb2
- SH2 domain interactions = binds Grb to phosphorylated tyrosine resides of RPTKs
- SH3 domain interactions = binds Grb to proline-rich regions of Sos
What does GAP stand for with relation to cell cycle control?
GTPase Activating Proteins
Recall 2 common Ras mutations and their effect
- 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
