Bub1 is essential for assembly of the functional inner centromere.

During mitosis, the inner centromeric region (ICR) recruits protein complexes that regulate sister chromatid cohesion, monitor tension, and modulate microtubule attachment. Biochemical pathways that govern formation of the inner centromere remain elusive. The kinetochore protein Bub1 was shown to promote assembly of the outer kinetochore components, such as BubR1 and CENP-F, on centromeres. Bub1 was also implicated in targeting of Shugoshin (Sgo) to the ICR. We show that Bub1 works as a master organizer of the ICR. Depletion of Bub1 from Xenopus laevis egg extract or from HeLa cells resulted in both destabilization and displacement of chromosomal passenger complex (CPC) from the ICR. Moreover, soluble Bub1 controls the binding of Sgo to chromatin, whereas the CPC restricts loading of Sgo specifically onto centromeres. We further provide evidence that Bub1 kinase activity is pivotal for recruitment of all of these components. Together, our findings demonstrate that Bub1 acts at multiple points to assure the correct kinetochore formation.

The Nup107-160 nucleoporin complex is required for correct bipolar spindle assembly.

The Nup107-160 complex is a critical subunit of the nuclear pore. This complex localizes to kinetochores in mitotic mammalian cells, where its function is unknown. To examine Nup107-160 complex recruitment to kinetochores, we stained human cells with antisera to four complex components. Each antibody stained not only kinetochores but also prometaphase spindle poles and proximal spindle fibers, mirroring the dual prometaphase localization of the spindle checkpoint proteins Mad1, Mad2, Bub3, and Cdc20. Indeed, expanded crescents of the Nup107-160 complex encircled unattached kinetochores, similar to the hyperaccumulation observed of dynamic outer kinetochore checkpoint proteins and motors at unattached kinetochores. In mitotic Xenopus egg extracts, the Nup107-160 complex localized throughout reconstituted spindles. When the Nup107-160 complex was depleted from extracts, the spindle checkpoint remained intact, but spindle assembly was rendered strikingly defective. Microtubule nucleation around sperm centrosomes seemed normal, but the microtubules quickly disassembled, leaving largely unattached sperm chromatin. Notably, Ran-GTP caused normal assembly of microtubule asters in depleted extracts, indicating that this defect was upstream of Ran or independent of it. We conclude that the Nup107-160 complex is dynamic in mitosis and that it promotes spindle assembly in a manner that is distinct from its functions at interphase nuclear pores.

Crm1 is a mitotic effector of Ran-GTP in somatic cells.

The Ran GTPase controls multiple cellular processes, including nuclear transport, mitotic checkpoints, spindle assembly and post-mitotic nuclear envelope reassembly. Here we examine the mitotic function of Crm1, the Ran-GTP-binding nuclear export receptor for leucine-rich cargo (bearing nuclear export sequence) and Snurportin-1 (ref. 3). We find that Crm1 localizes to kinetochores, and that Crm1 ternary complex assembly is essential for Ran-GTP-dependent recruitment of Ran GTPase-activating protein 1 (Ran-GAP1) and Ran-binding protein 2 (Ran-BP2) to kinetochores. We further show that Crm1 inhibition by leptomycin B disrupts mitotic progression and chromosome segregation. Analysis of spindles within leptomycin B-treated cells shows that their centromeres were under increased tension. In leptomycin B-treated cells, centromeres frequently associated with continuous microtubule bundles that spanned the centromeres, indicating that their kinetochores do not maintain discrete end-on attachments to single kinetochore fibres. Similar spindle defects were observed in temperature-sensitive Ran pathway mutants (tsBN2 cells). Taken together, our findings demonstrate that Crm1 and Ran-GTP are essential for Ran-BP2/Ran-GAP1 recruitment to kinetochores, for definition of kinetochore fibres and for chromosome segregation at anaphase. Thus, Crm1 is a critical Ran-GTP effector for mitotic spindle assembly and function in somatic cells.