Ran GTPase: a master regulator of nuclear structure and function during the eukaryotic cell division cycle?

Ran is an abundant GTPase that is highly conserved in eukaryotic cells and has been implicated in many aspects of nuclear structure and function, espe cially determining the directionality of nucleocytoplasmic transport during interphase. However, cell-free systems have recently shown that Ran plays distinct roles in mitotic spindle assembly and nuclear envelope (NE) format ion in vitro. During spindle assembly, Ran controls the formation of comple xes with importins, the same effectors that control nucleocytoplasmic trans port. Here,we review these advances and discuss a general model for Ran in the coordination of nuclear processes throughout the cell division cycle vi a common biochemical mechanisms.