CONTROL OF CYCLIN-B1 LOCALIZATION THROUGH REGULATED BINDING OF THE NUCLEAR EXPORT FACTOR CRM1

Activation of the Cyclin B/Cdc2 kinase complex triggers entry into mit osis in all eukaryotic cells. Cyclin B1 localization changes dramatica lly during the cell cycle, precipitously transiting from the cytoplasm to the nucleus at the beginning of mitosis. Presumably, this relocali zation promotes the phosphorylation of nuclear targets critical for ch romatin condensation and nuclear envelope breakdown. We show here that the previously characterized cytoplasmic retention sequence of Cyclin B1, responsible for its interphase cytoplasmic localization, is actua lly an autonomous nuclear export sequence, capable of directing nuclea r export of a heterologous protein, and able to bind specifically to t he recently identified export mediator, CRM1. We propose that the obse rved cytoplasmic localization of Cyclin Bl during interphase reflects the equilibrium between ongoing nuclear import and rapid CRM1-mediated export. In support of this hypothesis, we found that treatment of cel ls with leptomycin B, which disrupted Cyclin B1-CRM1 interactions, led to a marked nuclear accumulation of Cyclin B1. In mitosis, Cyclin B1 undergoes phosphorylation at several sites, a subset of which have bee n proposed to play a role in Cyclin B1 accumulation in the nucleus. Bo th CRM1 binding and the ability to direct nuclear export were affected by mutation of these phosphorylation sites; thus, we propose that Cyc lin B1 phosphorylation at the G(2)/M transition prevents its interacti on with CRM1, thereby reducing nuclear export and facilitating nuclear accumulation.