Stay tuned for some importin news about spindle assembly

Abstract

The Ran GTPase was originally characterized as a factor required for the transport of macromolecules in and out of the nucleus during interphase. In cooperation with the cytoplasmic GTPase-activating protein Ran-GAP and the nuclear GDP–GTP exchange factor (GEF) RCC1, Ran orchestrates binding and release of cargo proteins from nuclear import and export receptors (known as importins and exportins). During M phase, RCC1 is bound to mitotic chromosomes, and Ran is involved in regulating microtubule polymerization and spindle assembly. While it was known that Ran-GTP could elicit spontaneous aster formation when added to Xenopus extracts, its lack of association with microtubules suggested that its action relied upon one or more downstream effectors. Now, work by several laboratories has shown that, in a process highly analogous to nuclear transport, Ran-GTP causes release of microtubule-stabilizing factors from inhibitory complexes with importins 1 Gruss O.J et al. Ran induces spindle assembly by reversing the inhibitory effect of importin α on TPX2 activity. Cell. 2001; 104: 83-93 Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar , 2 Nachury M.V et al. Importin β is a mitotic target of the small GTPase Ran in spindle assembly. Cell. 2001; 104: 95-106 Abstract Full Text Full Text PDF PubMed Scopus (319) Google Scholar , 3 Wiese C et al. Role of importin-β in coupling Ran to downstream targets in microtubule assembly. Science. 2001; 291: 653-656 Crossref PubMed Scopus (273) Google Scholar .