Role of importin-β in the control of nuclear envelope assembly by Ran

Compartmentalization of the genetic material into a nucleus bounded by a nuclear envelope (NE) is the hallmark of a eukaryotic cell. The control of NE assembly is poorly understood, but in a cell-free system made from Xenopus eggs, NE assembly involves the small GTPase Ran [1, 2]. In this system, Sepharose beads coated with Ran induce the formation of functional NEs in the absence of chromatin [2]. Here, we show that importin-β, an effector of Ran involved in nucleocytoplasmic transport and mitotic spindle assembly, is required for NE assembly induced by Ran. Concentration of importin-β on beads is sufficient to induce NE assembly in Xenopus egg extracts. The function of importin-β in NE assembly is disrupted by a mutation that decreases affinity for nucleoporins containing FxFG repeats. By contrast, a truncated protein that cannot interact with importin-α is functional. Thus, importin-β functions in NE assembly by recruiting FxFG nucleoporins rather than by interaction through importin-α with karyophilic proteins carrying classical nuclear localization signals. Importin-β links NE assembly, mitotic spindle assembly, and nucleocytoplasmic transport to regulation by Ran and may coordinate these processes during cell division.