Ran-independent nuclear import of cyclin B1–Cdc2 by importin β

Mitosis is triggered in vertebrate cells by the cyclin B1–Cdc2 complex. The activation of this complex at the end of G(2) phase is accompanied by its translocation from the cytoplasm to the nucleus. We used digitonin-permeabilized human cells to analyze the mechanism by which cyclin B1–Cdc2 is imported into the nucleus. Cyclin B1–Cdc2 import was not blocked by inhibitors of the importin α-dependent import pathway or by dominant negative versions of the GTPase Ran or importin β. However, the rate of cyclin B1 import was decreased by immunodepletion of importin β from cytosol. Purified importin β promoted cyclin B1 import in the absence of cytosol or Ran and in the presence of the dominant negative Ran mutant. We conclude that cyclin B1 import is mediated by an unusual importin β-dependent mechanism that does not require Ran.