Correct option is A
The question focuses on the regulation of PP2A-B55 during the M-phase of the cell cycle and its role in Cdk1 substrates.
Let's break down the statements:
A. PP2A-B55 activity is high during interphase but inhibited during early mitosis when M-Cdk activity rises.
- Correct. During interphase, PP2A-B55 is active and helps regulate Cdk1 substrates. However, during early mitosis, the activation of M-Cdk (M-phase Cdk) leads to the inhibition of PP2A-B55 to allow for the progression of the cell cycle.
B. M-Cdk turns off PP2A-B55 via the phosphorylation of an intermediary protein kinase called Greatwall.
- Correct. The activation of M-Cdk results in the phosphorylation of Greatwall, which, in turn, inhibits PP2A-B55 activity. This phosphorylation helps to ensure that the cell cycle continues through mitosis by blocking PP2A-B55's regulatory action on Cdk1 substrates.
C. M-Cdk turns on PP2A-B55 via the phosphorylation of an intermediary protein kinase called Greatwall.
- Incorrect. This statement contradicts the previous one. M-Cdk actually turns off PP2A-B55 via the phosphorylation of Greatwall, not activates it.
D. When anaphase is initiated and M-Cdk activity declines, PP2A-B55 promotes dephosphorylation of Cdk1 substrates.
- Correct. Once M-Cdk activity declines after anaphase, PP2A-B55 becomes active again, leading to the dephosphorylation of Cdk1 substrates, helping the cell cycle progress through the later stages of mitosis and exit mitosis.
Correct Combination:
- A, B, and D are correct statements.
- C is incorrect because it incorrectly states that M-Cdk activates PP2A-B55 through Greatwall phosphorylation, which is the opposite of what actually happens.
