Signalling mechanisms of growth and division Flashcards

Question

Describe the **positive feedback** effect of MPF.

Answer 1

MPF = Cdk1 + cyclin B * Once MPF is activated, Cdk1 phosphorylates more **Cdc25** enzymes, activating them * This leads to even more MPF molecules being dephosphorylated and activated * *i.e. Having the inactivating phosphate removed* * This further drives mitosis

Answer 2

* The active MPF *(cdk1/cycB complex)* phosphorylates various substrates to initiate mitosis * *e.g. Phosphorylates proteins to trigger chromatin condensation* * **BUT, phosphorylation of certain substrates have an **inhibitory** effect * *i.e. Prevents those substrates from carrying out their function* * So essentially at the spindle assembly checkpoint *(end of metaphase)*, the active MPF puts mitosis **on hold** by phosphorylation *(i.e. inhibition)* of some key substrates * Kinetochores signal that they are fully attached *(→ = leads to)*: * → Cyclin B is degraded * → Cdk1 is inactivated * → Key substrates **dephosphorylated** * → Mitosis progresses *(onto anaphase)* SUMMARY: * MPF in the G₂ checkpoint: * Once activated, MPF phosphorylates key substrates * This activates the substrates required for prophase - metaphase * This inhibits the substrates required for after metaphase * MPF in the spindle assembly (M) checkpoint: * MPF degraded due to degradation of cyclin B * The post-metaphase substrates are dephosphorylated and become active again

Answer 3

**No** - different cyclins **and** different Cdks required at different stages of cell cycle IMPORTANT (about cyclins): * **Different cyclins** can bind to the same Cdk at different stages of the cell cycle and give the Cdk a **different subtrate specificity** * EXAMPLE: * Cdk2 activated by both cyclin E and cyclin A but at different stages of the cell cycle * This is important because substrate accessibility changes throughout the cell cycle * *i.e. There are different molecules that need to be phosphoryated (substrates) at different stages of the cell cycle*

Answer 4

Promotes G₀ → G₁ transition * Recap: * Growth factor → RPTK → Grb2 → Sos → Ras → ERK cascade * ERK *(kinase III in the cascade)* stimulates increased expression of **immediate early gene** transcription factors * EXAMPLES: c-jun, c-Fos, **c-Myc** * ****These stimulate the transcription of other genes * EXAMPLE: **cyclin D** * *Stimulated by the transcription factor examples given above* NOTE: * Immediate early gene = genes which are activated (trasncribed) transiently and rapidly in response to a stimulus * These genes mainly code for transcription factors which then go on to activate other genes * Other genes - i.e. the ones which are more relevant to the context *(e.g. cell cycle specific)*

Answer 5

* Cyclin D activates **Cdk4** or **Cdk6** * This leads to the stimulation of **cyclin E** synthesis * *Cyclin E required from G₁ → S, which means the cell has committed to divide* IMPORTANT: * Cyclin D is an **oncogene** * *Overexpressed in 50% of breast cancers*

Answer 6

Cdks become **sequentially active** and stimulate synthesis of genes required for next phase *(i.e. synthesis of the next cyclin)* * e.g. Cyclin D-Cdk4/6 stimulates expression of cyclin E * This gives direction and timing to cycle * *Direction - keeps the cell cycle going in **one direction*** The activation of **cyclins** are **cyclical** * This is because they are susceptible to degradation * *Cyclical = goes up and then down*

Answer 7

pRb = retinoblastoma protein * pRb acts as a 'brake' on the cell cycle. * Cdks phosphorylate (at multiple sites) & progressively inactivate pRb * Rb is a **'tumour suppressor'** Mechanism of action: * In **G₀**, pRb protein is bound to **E2F** *(transcription factor)*, rendering E2F **inactive** * Cyclin D-Cdk4/6 phosphorylates pRb * This results in a conformational change of pRb it releases E2F * E2F is now **active** - free to bind to gene promoters to drive transcription * e.g. E2F promotes transcription of **cyclin E** *(G₁ → S)* REMEMBER: * c-Myc stimulates cyclin D synthesis * Cyclin D: G₀ → G₁

Answer 8

* c-Myc stimulates cyclin D transcription → cyclin D-Cdk4/6 * Cyclin D-Cdk4/6 phosphorylates pRb → E2F released * ***Some** E2F released* * *This is because there are different isoforms (conformations) of mono-phosphorylated pRb as there are multiple phosphorylation sites on the protein* * *So some isoforms release E2F while others don't* * E2F stimulates cyclin E transcription → cyclin E-Cdk2 * Cycln E-Cdk2 **further** phosphorylates pRb → **more** E2F released * *More phosphorylated isoforms (conformations) release E2F* * The increase in E2F concentration means that it can now bind to targets with **lower affinity** * *The cyclin A gene transcription is not activated until the E2F concentration is high enough* * **Higher** E2F concentration stimulates cyclin A transcription → cyclin A-cdk2 * Cyclin A-cdk2 further phosphorylates pRb → more E2F * More E2F stimulates phosphorylation of another transcription factor (TF) * *pRb binds differently to different transcription factors* * *So you may need different levels of phosphorylation (i.e. different conformations) to release different transcription factors from pRb* * *i.e. to **inactivate** pRb* * TF stimulates cyclin B transcription → cyclin B-cdk1 EXPLANATION: * Progressive phosphorylation leads to change in conformation * Phosphorylating pRb more releases more E2F * Because the more phosphorylated isoforms of pRb will have a conformation which causes it to release E2F * pRb binds differently to different transcription factors * So you may need different levels of phosphorylation to release different transcription factors from pRb * *i.e. to **inactivate** pRb*

Answer 9

CKI = Cdk inhibitor * They bind to Cdks or the Cdk/cyclin complex * Therefore, they prevent the Cdks from carrying out their function

Answer 10

These are all proteins **INK4 family** * p15^INK4b * p16^INK4a * p18^INK4c * p19^INK4d These are **G₁ phase** CKIs * They inhibit **Cdk4/6** by displacing cyclin D **CIP/KIP family** * p21^CIP1/WAF1 * p27^KIP1 * p57^KIP2 These are **S phase** CKIs * They inhibit **all Cdks** by binding to the Cdk/cyclin complex * *Essentially inhibits all the CKIs involved in the S phase* * *Cyclin E-Cdk2* * *Cyclin A-Cdk2* **CKIs must be degraded to allow cell cycle progression** Link to cancer: * p27^KIP1is a **tumour suppressor** * Reduced expression correlates with poor prognosis in many malignancies * *Technically they could all be tumour suppressors as CKIs inhibit cell cycle progression* * *This helps prevent uncontrollable proliferation, which is essentially what cancer is* * *But this particularly CKI is known as a tumour suppressor as reduced expression has been linked to cancer*