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Petr Kalab, Ph.D.

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RanGTP gradient in interphase and mitotic cells
RanGTP gradient in interphase and mitotic cells
The guanine nucleotide exchange factor for Ran called RCC1 binds to DNA and is imported to nuclei, while RanGAP is mostly cytoplasmic. Majority of Ran is highly mobile throughout the cell.
As a result, a steep concentration gradient of RanGTP is formed across nuclear envelope (NE). After the NE breakdown in cells undergoing open mitosis, a diffusion-limited gradient of RanGTP surrounds the condensed chromosomes.
The RanGTP-regulated gradients of importin cargos
The RanGTP-regulated gradients of importin cargos
The binding of RanGTP to importins dissociates their complexes with nuclear localization signal (NLS) -containing cargos. In cells with nuclei, this mechanism underlies the RanGTP-regulated nuclear import. In open mitosis, the highly dynamic Ran-regulated importin - NLS interactions continue but the absence of the NE prevents local accumulation of the NLS -containing cargos. Instead, the RanGTP gradient induces the formation of a downstream gradient of cargos released from importins. The mitotic free cargo gradient overlaps the RanGTP gradient and is less steep.
Some spindle assembly factors (SAFs) contain NLS and are inhibited when bound to importins. The importins currently known to be involved in such regulation are importin beta acting either alone or in complex with importin alpha-1, and importin 7. The RanGTP-induced gradient of importin-free, active SAFs which surrounds the mitotic chromosomes thus helps to direct the mitotic spindle assembly to correct location within the mitotic cell.
The detection of RanGTP-regulated gradients by FRET sensors
The detection of RanGTP-regulated gradients by FRET sensors
The gradients were detected by FRET sensors (described in Kalab et al. 2002) in interphase and mitotic Xenopus egg extracts.

(Images reproduced with the permission of Science)
Group Picture (09-10)
Group Picture (09-10)
Keisuke Hasegawa, Petr Kalab and Sung Jin Ryu

This page was last updated on 10/20/2010.