By dislodging a microtubule-binding protein, the Aurora B kinase helps prevent sloppy connections between chromosomes and the mitotic spindle, Chan et al. suggest.

The KMN complex connects spindle microtubules to kinetochores, which is essential for chromosomes to separate during mitosis. To forge solid links between microtubules and the kinetochore, however, the KMN complex might need help from the Ska complex, but researchers aren't sure what recruits this latter group of proteins to the kinetochore.

Chan et al. found that two members of the KMN complex, Ndc80 and Mis13, bring the Ska complex to kinetochores. The mitotic kinase Aurora B inhibited this association by phosphorylating the Ska complex, thereby bumping it from kinetochores. Cells expressing a nonphosphorylatable Ska complex formed incorrect kinetochore–microtubule attachments and took longer to complete mitosis.

The researchers propose a division of labor during microtubule attachment. Early on in mitosis, microtubules promiscuously connect to and disconnect from kinetochores. If the Ska complex fastens to the KMN complex at this stage, it will land close to Aurora B and be phosphorylated, causing it to drop off. Once the KMN complex establishes a correctly oriented connection between a microtubule and a kinetochore, the Ska complex can land without being phosphorylated because tension from the spindle pulls KMN and the Ska complex away from Aurora B. Ska can then seal the kinetochore–microtubule link. Delaying the Ska complex's arrival until microtubules are correctly attached might prevent it from clamping weak attachments in place.

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J. Cell Biol.