Question

I got wrong on this one. Any clarification?

Which of the following statements about the anaphase-promoting complex (APC) is true? Choose all that apply It promotes the degradation of proteins that regulate M-phase. It tags CDK with ubiquitin, so it gets sent to the proteasome Its activity is required for the cell to enter metaphase It is continuously active throughout the cell cycle It promotes the breakdown of cohesins It tags cyclin with ubiquitin, so it gets sent to the proteasome Its activity is stimulated by M-Cdk. It inhibits M-Cdk activity

Answer 1

Correct options are:

1.it promotes the degradation of protein that regulates M-Phase.

Explanation: APC (A ubiquitin ligase complex) complex is needed for the progression and exit of M- Phase. so it is necessary to degrade the proteins that regulate the M-phase via ubiquitination.

5. promotes the breakdown of cohesins.

Explanation: progression from metaphase to anaphase requires the separation of sister chromatids which are held together by cohesin. cohesins are degraded by separase which is normally inhibited by securin. APC targets securin for its degradation leading to activation of separase and subsequently separation of sister chromatids by the degradation of cohesins.

6. It targets cyclins with ubiquitin so it gets sent to the proteasome.

Explanation: APC is an E3 ubiquitin ligase complex and it regulates the cell cycle by ligating the cyclins (e.g cyclin B) for their degradation in 26s proteasome complex.

7. its activity is stimulated by M-Cdk.

Explanation: Cdk1 (a M-Cdk) activates the APC via phosphorylation.

8. It inhibits M-Cdk activity.

Explanation: In order to exit from metaphase and decondensation of chromosomes deactivation of M-Cdk is required. And it is done by the degradation of cyclins which can otherwise activate the M-Cdks. So yes it inhibits M-Cdk activity.

Incorrect answers:

2. It tags Cdk with ubiquitin so it gets sent to the proteasome.

Explanation: it is cyclins that are ubiquitylated by APC not Cdk.

3. Its activity is required for the cell to enter metaphase.

Explanation: As its name suggests it is anaphase-promoting complex, it must required for progression from metaphase to anaphase. Not for initiation of metaphase.

4. continuously active throughout the cell cycle.

Explanation: Its activation is regulated by different Phosphorylating complexes e.g cdc-Cdk1 and PKA. so it is not always active.