The Ran GTPase regulates nucleocytoplasmic transport, mitotic spindle formation and nuclear envelope assembly. During interphase, Ran-GTP dissociates nuclear import complexes, releasing cargo molecules from their importin carriers. Studies in oocyte extracts indicate that it might function similarly at mitosis: it appears to release aster-promoting factors such as TPX2 from inhibitory importin complexes and thereby stimulate spindle assembly. Patrizia Lavia and co-workers have examined its role during spindle assembly in mammalian somatic cells, focusing on the involvement of centrosomes – an important difference between somatic cells and meiotic extracts. They have found that overexpression of the Ran-binding protein RanBP1, which reduces Ran-GTP levels, induces formation of multipolar spindles. They show that this is because the mother and daughter centrioles separate at spindle poles and go on to anchor distinct, functional microtubule arrays (see ). The authors use inhibitors to demonstrate that this `centrosome splitting'depends on microtubule integrity and Eg5 – a kinesin motor that controls centrosome separation at the onset of mitosis. Moreover, they demonstrate that a fraction of endogenous RanBP1 is stably associated with centrosomes. Their findings thus indicate that the Ran network has a novel role in centrosome organization in which microtubule dynamics and/or transport mechanisms play a part.
