Mitotic functions of the Ran GTPase network: the importance of being in the right place at the right time.

The Ran GTPase has important roles in nucleocytoplasmic transport, cell cycle progression, nuclear organization and nuclear envelope (NE) assembly. In this review, we discuss emerging evidence that implicate the Ran GTPase system in mitotic control in mammalian cells. Recent work indicates that members of the Ran network control two fundamental aspects of the mammalian mitotic apparatus: (i) centrosome and spindle pole function, and (ii) kinetochore function. It is also emerging that, after NE breakdown, specific Ran network components assemble in local combinations at crucial sites of the mitotic apparatus. In the light of these findings, the original notion that nucleotide-bound forms of the Ran GTPase are distributed along a unique "gradient" in mitotic cells should be re-examined. Available data also suggest that the Ran system is deregulated in certain cellular contexts: this may represent a favoring condition for the onset and propagation of mitotic errors that can predispose cells to become genetically unstable and facilitate neoplastic growth.