Cyclin B1, a Key Cell Cycle Regulatory Protein in Vertebrates

Multiple Choice

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.