Question

Cyclin B1, a key cell cycle regulatory protein in vertebrates, is mostly cytosolic before mitosis. Early in mitosis, however, the protein is phosphorylated by certain protein kinases and consequently accumulates in the nucleus. How can phosphorylation bring about nuclear accumulation of this protein?

A. Phosphorylation within the nuclear localization signal inhibits the function of the signal.
B. Phosphorylation within the nuclear export signal enhances the function of the signal.
C. Phosphorylation within the nuclear export signal interferes with the function of the signal.
D. Phosphorylation elsewhere on the protein enhances binding to cytosolic proteins.

Answer 1

Phosphorylation within the nuclear export signal interferes with Cyclin B1's export function, leading to its accumulation in the nucleus. This is part of the cell cycle regulation by cyclins and cyclin-dependent kinases, which need to be phosphorylated to be active.

Step-by-step explanation:

The nuclear accumulation of Cyclin B1 in vertebrates during the early stages of mitosis is facilitated by phosphorylation. This phosphorylation occurs due to the action of protein kinases, which target specific locations on the Cyclin B1 protein. The correct response to how phosphorylation can lead to the nuclear accumulation of Cyclin B1 is: C. Phosphorylation within the nuclear export signal interferes with the function of the signal. This interference prevents Cyclin B1 from being exported to the cytosol and instead leads to its accumulation in the nucleus.

Cyclins and cyclin-dependent kinases (Cdks) are the main regulators of the cell cycle, with cyclin levels fluctuating to trigger the formation of Cdk/cyclin complexes at various checkpoints. For mitosis to proceed, these complexes must be phosphorylated to activate the proteins involved in cell cycle advancement. Without proper regulation, cells may proceed through the cycle without appropriate checkpoints, potentially contributing to the development of diseases like cancer.