The RanGTP-importin β gradient (red) promotes microtubule nucleation (blue) and stabilization (green).

CAUDRON/AAAS

One RanGTP gradient builds on another to spatially guide mitotic spindle assembly, say Maïwen Caudron, Eric Karsenti (European Molecular Biology Laboratory, Heidelberg, Germany), and colleagues.

Chromatin was known to promote spindle assembly via its effects on the Ran GTPase. RanGTP is abundant near chromosomes due to activation by a Ran–guanosine nucleotide exchange factor localized on chromosomes, but the extent of RanGTP localization and its function remained unknown. “Researchers suspected that long-range spindle effects were being mediated by chromosomes, but they couldn't prove this for certain,” says Karsenti.

Combining mathematical modeling and fluorescent microscopy, Karsenti's group now shows that RanGTP diffusion away from chromosomes creates an interrelated series of short- and long-range protein gradients. The gradient of free RanGTP is very steep, but built upon it are shallower gradients of RanGTP–importin β and...

The Rockefeller University Press
2005
The Rockefeller University Press
2005
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