Translocation of cyclin B1 to the nucleus at prophase requires a phosphorylation-dependent nuclear import signal

Background: At M phase, cyclin B1 is phosphorylated in the cytoplasmic rete ntion sequence (CRS), which is required for nuclear export. During interpha se, cyclin B1 shuttles between the nucleus and the cytoplasm because consti tutive nuclear import is counteracted by rapid nuclear export. In M phase, cyclin B moves rapidly into the nucleus coincident with its phosphorylation , an overall movement that might be caused simply by a decrease in its nucl ear export. However, the questions of whether CRS phosphorylation is requir ed for cyclin B1 translocation in mitosis and whether a reduction in nuclea r export is sufficient to explain its rapid relocalisation have not been ad dressed. Results: We have used two forms of green fluorescent protein to analyse sim ultaneously the translocation of wild-type cyclin B1 and a phosphorylation mutant of cyclin B1 in mitosis, and correlated this with an in vitro nuclea r import assay. We show that cyclin B1 rapidly translocates into the nucleu s approximately 10 minutes before breakdown of the nuclear envelope, and th at this movement requires the CRS phosphorylation sites. A cyclin B1 mutant that cannot be phosphorylated enters the nucleus after the wild-type prote in. Phosphorylation of the CRS creates a nuclear import signal that enhance s cyclin B1 import in vitro and in vivo, in a manner distinct from the prev iously described import of cyclin B1 mediated by importin beta. Conclusions: We show that phosphorylation of human cyclin B1 is required fo r its rapid translocation to the nucleus towards the end of prophase. Phosp horylation enhances cyclin B1 nuclear import by creating a nuclear import s ignal. The phosphorylation of the CRS is therefore a critical step in the c ontrol of mitosis.